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Saltwater intrusion, strategies to mitigate the impacts of swi, critical knowledge gaps, conclusions, acknowledgements, data availability statement, potential management practices of saltwater intrusion impacts on soil health and water quality: a review.

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Haimanote K. Bayabil , Yuncong Li , Zhaohui Tong , Bin Gao; Potential management practices of saltwater intrusion impacts on soil health and water quality: a review. Journal of Water and Climate Change 1 August 2021; 12 (5): 1327–1343. doi: https://doi.org/10.2166/wcc.2020.013

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Several studies have documented the multifaceted impacts of climate change and variability on agricultural and environmental sustainability, and social and economic development. Climate change and variability contribute to increased warmer conditions, increased frequency of heavy rain that accounts for an increasing proportion of total rainfall, extreme weather characterized by spatially variable cycles of drought and wetness, increased frequency of tropical storms/hurricanes, increased frequency of storm surges, and accelerated rate of sea-level rise (SLR). As SLR continues, it is expected that salinity due to saltwater intrusion (SWI) will impact soil health and agricultural production. As such, the significant threats of salinity necessitate more work to be done to better understand its impact on soil health and associated functional ecosystem processes. This is of even greater importance in areas such as South Florida where the surface and groundwater resources are hydrologically connected due to the shallow and highly permeable limestone soils. A better understanding of the impacts of salinity due to SWI on soil health is critical to design effective mitigation strategies. Healthy soil has multifaceted benefits to enhance agricultural productivity, i.e. regulates the flow of water; serves as a source and sink of nutrients; minimizes greenhouse gas emissions and provides optimal biological and chemical conditions for the transformation of nutrients into plant-available forms. Improved understanding of the processes and impacts of SWI on soil health will assist in guiding management decisions and policies to mitigate the impacts of SWI and salinity on agricultural soils. This review paper provides a comprehensive overview of the impacts of SWI and soil salinity on agricultural soil health and water quality.

Climate change poses a serious threat to agricultural land use and soil health.

Soil salinity affects soil–water–plant relationships.

Soil amendments (e.g., biochar, hydrogel, organic mulch, and gypsum) can be used to improve soil health.

Climate change impacts are multifaceted and integrated mitigation strategies should be adopted.

Salt water intrusion could result in increased nutrient solubility and transformation, which could lead to increased nutrient loading to freshwater resources.

Most of the large cities globally are located along the coastlines and more than 40% of the world population lives within 100 km of the coastline, while 10% of the population lives in coastal areas that are less than 10 m.a.s.l. ( UN 2017 ). Overall, there is strong evidence that changes in the sea level will impact a significant portion of the global population around the world, especially those in coastal regions ( Neumann et al. 2015 ; Kummu et al. 2016 ). By the year 2050, the global population residing in low elevation coastal zones is expected to increase significantly, which represents the most vulnerable population to sea-level rise (SLR) ( Kummu et al. 2016 ; UN 2017 ; Abadie 2018 ; Kulp & Strauss 2019 ). The most densely populated countries in coastal regions, such as Bangladesh with a large population in coastal areas, are extremely vulnerable to an accelerated rate of SLR ( Kulp & Strauss 2019 ). A report by Weeman & Lynch (2018) based on the findings of a study at the University of Colorado Boulder that evaluated 25 years of NASA and European satellite data concluded that the rate of global SLR in recent years is becoming alarming. The same study estimated that the SLR is expected to reach 0.65 m by 2100 if it continues at the current rate ( Weeman & Lynch 2018 ). Projected accelerations of SLR highlight the contribution of climate change and variability as the driving factors ( Horton et al. 2014 ; Olsson et al. 2015 ). The rate of climate variability during the recent years has been accelerated due to increasing concentrations of greenhouse gases (GHGs) in the atmosphere that contributed to an increase in the air and water temperatures, which in turn results in an increased rate of ice melting in Greenland and Antarctica thus contributing to accelerated SLR ( MacCracken 2001 ; Singh & Singh 2012 ; Mimura 2013 ; Pattyn et al. 2018 ; Overland et al. 2019 ). The accumulation of GHGs traps heat causing warming of the earth (US EPA 2015 ; Dahlman 2019 ; Butler & Montzka 2020 ). Oceans absorb more than 90% of the trapped heat ( Cheng et al. 2017 ). However, with the increase in temperature due to climate change and variability water volume in oceans rises, which is referred to as ‘thermal expansion’, contributing to increases in the rate of SLR ( Milly et al. 2003 ; Vermeer & Rahmstorf 2009 ; Meehl et al. 2012 ; Mimura 2013 ). Recent studies have shown that the rate of thermal expansion increased from 0.4 to 0.8 mm/year from 1970 to 2010 ( Rhein et al. 2013 ).

On the other hand, the carbon content in the ocean is about 50-fold greater than that in the atmosphere ( Sarmiento & Gruber 2002 ) and absorbs nearly 30% of anthropogenic CO 2 emissions ( Cicerone et al. 2004 ; Bates et al. 2012 ). Therefore, the ocean plays a major role in slowing down the rate of climate change ( Raven & Falkowski 1999 ; Bigg et al. 2003 ; UN 2017 ). The ocean also plays a major role in the global water cycle by representing the majority of the total water exchange, as evaporation and precipitation, between the atmosphere and the earth's surface ( Schmitt 1995 ; Bengtsson 2010; Charette & Smith 2010 ). Climate change and variability are accelerating the rate of SLR, which in turn causes storm surge and flooding that impacts the livelihood ( Awal et al. 2016 ) and salinization of water and soils ( Figure 1 ).

Areas threatened by saltwater intrusion. Reproduced with permission from Ou de Essink et al. (2010) .

Currently, most of the focus and efforts are concentrated on mitigating the impacts of climate change on coastal regions due to storm surge, land subsidence, and loss of aquatic habitat ( Craft et al. 2009 ; Ardon et al. 2016 ; Rosen 2017). Increases in SLR, due to climate change and variability, and associated risks of flooding, saltwater intrusion (SWI), and salt buildup in soils and water are major concerns for human habitat, sustainable development as well as soil health, and crop productivity ( Craft et al. 2009 ; Singh & Singh 2012 ; Mimura 2013 ; Rosen 2017 ). The net negative effects of SWI are on food security due to the salinization of water and soils ( Pezeshki et al. 1990 ; Arslan & Demir 2013 ; Dasgupta et al. 2015 ; Szabo et al. 2016 ; Litalien & Zeeb 2020 ). Salinization of water and soils affects the availability of freshwater and suitability of arable land for crop production as most crops are sensitive to high salt contents in irrigation water and/or in soils ( Habibullah et al. 1999 ; Wichelns & Qadir 2015 ; Butcher et al. 2016 ; Szabo et al. 2016 ; Khanom 2016 ; Chen & Mueller 2018 ; Majeed & Muhammad 2019 ). In addition, SWI can mobilize nutrients from the soil by influencing soil chemistry, which in turn become sources of eutrophication of the adjacent freshwater bodies as well as coastal waters impacting marine life ( DeLaune & Smith 1985 ; Portnoy & Giblin 1997 ; Williams et al. 2014 ; Neubauer et al. 2019 ; Tully et al. 2019 ). Guha & Panday (2012) used the groundwater model to predict the impact of SLR on salinity and groundwater levels, chloride concentrations, and groundwater discharge into canals in Miami-Dade and Broward counties. Their predictions showed that SLR of 0.6–1.2 m resulted in a groundwater head increase in some areas of 4–15%; however, the effect was more significant on the average relative chloride concentrations which increased 100–600% in some wells.

Currently, an estimated 1.1 × 10 9 ha land, which accounts for 7% of the world's total land area, is impacted by soil salinity and/or sodicity ( Yensen 2008 ; Wicke et al. 2011 ). Poor management of irrigation, particularly using brackish water with inadequate freshwater for leaching salt buildup from the root zone, is contributing to increasing soil salinity ( Connor et al. 2008 ; Wichelns & Qadir 2015 ). About 33% of globally irrigated land is impacted by salinization ( Munns 2005 ; Chaves et al. 2009 ). In addition, salinity negatively impacts plant physiological processes, such as photosynthesis and respiration, which in turn reduces plant growth and production ( Minhas et al. 2020 ; Tomaz et al. 2020).

This review paper mainly focuses on available research on the impacts of soil and water salinization due to SWI on soil health indicator parameters, nutrient transformation and transport, and GHG emissions. In addition, this review summarizes the potentials of selected soil amendments as potential mitigation strategies of soil and water salinity in agricultural soils.

In coastal areas, under normal conditions, freshwater moves towards the sea; therefore, it prevents SWI of coastal aquifers ( Barlow & Reichard 2010 ; Beebe et al. 2016 ; Memari et al. 2020 ). The interface between the freshwater and saltwater is a diffuse layer, generally referred to as the ‘zone of dispersion’ where freshwater and saltwater are mixed in the aquifer deep below the land surface ( Abd-Elhamid et al. 2020 ; Sowe et al. 2020 ) ( Figure 2 ).

Mechanisms of SWI. Reproduced from Prinos (2013) .

Major factors causing SWI are (i) excessive pumping of groundwater; (ii) reduced recharge of groundwater; (iii) changes in land use such as man-made canals for draining the land for urban and/or agricultural uses; (iv) SLR; and (v) increased frequency of storm surges, due to climate change, that floods the land surface with the seawater part of it may be discharged into the canal and eventually drains into the fresh aquifer system ( Chen et al. 2020 ; Maliva 2020 ; Siegel 2020 ; Zhu et al. 2020 ).

As SLR continues to accelerate during the recent years, much of the attention has been on SWI impacts on groundwater and the threat it poses to the freshwater resources in coastal regions. However, limited attention is given to the impact of the landward movement of saltwater by the surface and/or groundwater connections, which can influence the salinity of the agricultural soils and, in turn, the impact of crop production in the coastal zones ( Herbert et al. 2015 ; Bhattachan et al. 2018 ). The impact of SLR is also on the salinization of drainage ways, including natural as well as man-made canals and ditches, which in turn impact the soils in areas adjacent to these waterways as well.

Land-use changes such as the conversion of wetlands for cultivation often result in lowering the elevation of the landscape and more inland water connection through drainage ditches to maintain the soil suitable for cultivation ( Alshammari et al. 2020 ; Sathiya Bama et al. 2020). These changes will contribute to an increased risk of salinization of inland surface water and the soil. This impact will accelerate as the climate crisis enhances the risk of drought and storms in the coastal zone and inland regions where an extensive network of artificial drainage canals exists as a part of land-use change and development.

SWI is a critical issue in Florida, which heavily depends on freshwater aquifers for drinking water and other uses including agriculture and landscaping. Due to high chloride concentration in seawater (∼19,000 mg/L), SWI of the slightest scale could result in higher chloride concentration in freshwater greater than the EPA's 250 mg/L recommendation for drinking water use ( Stumm & Morgan 1981 ). South Florida is surrounded by seawater on three sides, where seawater is found in deep rock layers below the freshwater aquifer. The impact of SWI is very significant for freshwater management as pumping increases to meet the water needs for the increasing population and other water uses.

Effects of salinity on GHG emission

SLR results in two most important factors in coastal forested wetlands, i.e. soil moisture or wet/dry cycles and salinity. These changes, in turn, affect the biogeochemistry of the soil, as well as GHG emissions. Liu et al. (2017) examined the changes in soil moisture levels and salt concentration (0, 1, and 5 parts per thousand as sodium chloride (NaCl)) on GHG emissions and dissolved organic carbon (DOC) characteristics of forested wetlands. Their results showed that carbon (C) and nitrogen (N) cycling in these soils were impacted more by water levels than salinity. Wet and dry cycles of the soil decreased the DOC production and total methane (CH 4 ) emissions. CO 2 emission was linearly related to DOC; therefore, it indicates that DOC is the main factor that determines the availability of CO 2 which can be subject to emission.

Other studies have also shown that an influx of seawater into wetlands increased CO 2 production ( Marton et al. 2012 ; Chambers et al. 2014 ). Similarly, Steinmuller & Chambers (2018) reported that 5 and 15 parts per thousand salt content in water increased CO 2 emission from wetland soils.

However, the mechanisms and magnitude of the salt-induced loss of soil carbon (C), as CO 2 , CH 4 , or DOC, from wetland soils remain less understood.

Maucieri et al. (2017) investigated the effects of salinity, biochar amendment, and different soil wetting levels on GHG emission from a semi-arid Australian soil. They reported that the major effect was associated with soil wetting with an increase in water holding capacity from 25 to 100% resulting in a 170% increase in CO 2 emission, while CO 2 emission was decreased by 19 and 28% at 5 and 10 dS ( decisiemens ) per meter salinity, respectively, compared with that in the control, i.e. no salinity treatment.

Nitrous oxide (N 2 O) emission from agricultural soils accounts for 30% of global anthropogenic N 2 O emissions ( Mosier 1994 ). However, some estimates place this proportion as high as 80% ( Beauchamp 1997 ) with a high level of caution on lack of reliable data for high precision estimates N 2 O emission from agricultural soils. Soil water management appears to be the main factor determining the N 2 O emission from agricultural soils ( Cao et al. 2019 ). However, the precise relationship between water management, irrigation water salinity, and N 2 O emission is not clearly defined and hence needs more research.

Salinization has shown a varied impact on GHG emissions. Increasing salinity has been shown to increase N 2 O emission ( Low et al. 1997 ) and decrease CH 4 emission ( Marton et al. 2012 ), while Kontopoulou et al. (2015) reported no salinity effects on CH 4 , CO 2 , and N 2 O emissions.

Wei et al. (2018) reported that N 2 O emission was greater from soil irrigated by 5 mg/L saline water when compared with irrigation water with 2 and 8 mg/L salt. They studied N 2 O emission at 0 and 120 kg N/ha treatments. Across both N rates, N 2 O emission decreased by 22–40% with irrigation using saline water at 2 or 8 mg/L, when compared with that irrigation using freshwater. For a similar comparison using saline irrigation water at 5 mg/L, N 2 O emission increased by 58–88%. Surprisingly, a substantial increase in N 2 O emission occurred at the intermediate salinity level (5 mg/L) compared with that of low and high salinity levels (2 and 8 mg/L, respectively) in irrigation water.

A salinity level of 0.4–8.0 dS/m in irrigation water increased N 2 O emission from a cotton field without nitrogen fertilizer under drip irrigation ( Zhang et al. 2016 ). Other researchers ( Inubushi et al. 1999 ) reported that N 2 O emission and irrigation water salinity were variable in different soils, indicating that soil properties influence determination of N 2 O emission and irrigation water salinity interactions. Yet, other studies have shown that salinity decreased N 2 O emission ( Tsuneda et al. 2005 ; Maucieri et al. 2017 ).

Methane is the most potent GHG; therefore, it plays a significant role in the climate crisis. The global warming potential of CH 4 is 25-fold greater than that of CO 2 ( IPCC 2007 ). A better understanding of factors impacting CH 4 emission is important to develop management practices to mitigate CH 4 emission that would slow the rate of the climate crisis. CH 4 emission from soils is primarily influenced by the depth of water table, temperature, salinity, and soil organic carbon. Olsson et al. (2015) reported that CH 4 emission increased at the high and constant water table and at high temperatures. Therefore, any factors, such as rising sea level and seawater intrusion inland, which influence the rising water table, increase CH 4 emission. However, the CH 4 emission rate decreased as the salinity level increased above 18 parts per thousand.

Effects of salinity on nutrient transformation and transport

Human activities, including expansion in industrialization and intensive agricultural production systems, will continue to impact the eutrophication of water bodies. Furthermore, climate change and variability may continue to play a major role in eutrophication due to greater water temperature, increased outflow of freshwater, and nutrients ( Rabalais et al. 2009 ).

SWI may impact soil biogeochemical processes that lead to nutrient export into coastal zones, which in turn contribute to eutrophication. Steinmuller & Chambers (2018) , in their study on three wetland soils from Florida, have shown that 5 and 15 parts per thousand salt content in water increased CO 2 production as well as P and NH 4 concentrations in porewater. This suggests increased risk of phosphorus (P) and nitrogen (N) transport from coastal soils into the sea, which contributes to increased eutrophication. Herbert et al. (2015) also reported that SWI impacted soil biogeochemical processes as well as vegetation communities, habitat, and microbial community.

Some of the reasons for increased nutrient transport from coastal soils into marine ecosystems are: (i) loss of the wetland's capacity to retain or denitrify N due to the introduction of brackish water into the wetlands ( Craft et al. 2009 ; Chambers et al. 2011 ); (ii) as the wetlands become brackish, plant uptake of N decreases due to reduced plant growth as a result of sulfide toxicity and/or increased salt stress; therefore, increased residual N can be subject to transport ( Cormier et al. 2013 ); (iii) the influx of seawater with high concentrations of cations such as Na + , Mg 2+ , and Ca 2+ exchanges NH 4 + from the exchange sites in wetlands ( Giblin et al. 2010 ; Weston et al. 2010 ), which makes NH 4 + subject to transport from inland into sea resulting in eutrophication; (iv) an increase in SO 4 2− due to SWI promotes organic matter mineralization in wetland sediments resulting in an increased release of NH 4 + from organically bound N ( Lamers et al. 1998 ); soil water intrusion to inland wetlands can increase dissolved NH 4 + concentration as a result of microbial processes that inhibit nitrification ( Joye & Hollibaugh 1995 ) and/or reduction of NO 3 to NH 4 + ( Gardner et al. 2006 ). However, these findings are based on the studies conducted on soil cores or small plots; therefore, there is a need for landscape- and ecosystem-scale studies to understand fully the above processes and determine the mechanisms and extent of nutrient transformation and transport from inland to the sea.

Impact of SWI on freshwater quality in Florida

Along the western and eastern coasts of Florida, there is clear evidence of SWI gradually impacting the freshwater aquifers. Nearly 90% of the drinking water supply in South Florida comes from underground aquifers. The soils in South Florida are extremely porous due to limestone subsoils, therefore, they are favorable for an increased rate of SWI. Most crops are sensitive to high salinity leading to a decline in crop growth and production.

The data from a sea-level monitoring station in Key West have shown that the mean sea level has risen about 228 mm over the last 100 years. This is slightly greater than the global rise of sea level, which is 170–210 mm ( Hagemann 2016 ). Future sea-level projections compared with the base level in 1992 show that the average sea-level rise of 154–254 mm is expected by 2030, 356–864 mm by 2060, and 787–2,057 mm by 2100 ( Hagemann 2016 ). The rising sea-level increases flooding risks to coastal regions, which also includes agricultural lands. To some extent, this risk is managed by an extensive network of canals to provide adequate drainage. The canal water levels are monitored regularly to evaluate the potential for rising groundwater levels as a result of SLR.

The Central and South Florida Water Management system was built in the 1950s without the knowledge of current and future accelerated rates of SLR. However, since 2008 SFWMD has been rigorously working on developing short- and long-term strategies to manage the impacts of accelerated SLR.

As of 2011, about 1,200 km 2 of the mainland part of the Biscayne Bay was impacted by SWI ( Hagemann 2016 ). In an area of 24 km 2 , the saltwater front was mapped further inland than it was in 1995 ( Hagemann 2016 ). The studies have shown that saltwater intruded along the base of the aquifer.

Blanco et al. (2013) evaluated temporal and spatial changes in chloride concentrations in shallow and deep groundwater in Southwest (Collier and Lee counties) and Southeast (Broward and Miami-Dade counties) Florida between 1985 and 2000. Their study provided clear evidence of the impact of SWI on the quality of freshwater aquifer in South Florida. The same study reported that chloride concentration in Southwest Florida was increased from 132 to 230 mg/L for shallow wells (0–43 m), while for deep wells (greater than 43 m) the increase was from 392 to 447 mg/L ( Figures 3 and 4 ).

Chloride concentrations in groundwater in Southwestern Florida; deep wells (>43 m), for 1985 and 2000; and changes from 1985 to 2010. Reproduced from Blanco et al. (2013) .

Chloride concentrations in groundwater in Southwestern Florida; shallow wells (<43 m) for 1985 and 2000; and changes from 1985 to 2000. Reproduced from Blanco et al. (2013) .

Similarly, in Southeast Florida, chloride concentrations in shallow wells (less than 33 m) increased from 159 to 470 mg/L and in deep (greater than 33 m) wells from 1,360 to 2,050 mg/L during the same period (1985–2000) ( Figures 5 and 6 ). Based on the US EPA guidelines on allowable chloride concentration in drinking water, the authors estimated that between 1985 and 2000 Southwest and Southeast Florida lost about 12–17% of the freshwater resource due to SWI.

Chloride concentrations in groundwater, shallow (less than 33 m), and deep (greater than 33 m) wells, measured in 1985 and 2010 in Southeastern Florida. Reproduced from Blanco et al. (2013) .

Changes in groundwater chloride concentrations from 1985 to 2010 in Southeastern Florida: (a) shallow (less than 33 m) and (b) deep (greater than 33 m) wells. Reproduced from Blanco et al. (2013) .

In Florida, with the accelerated rate of SLR in recent years, in part due to climate change and variability, and increased pumping of groundwater for irrigation and meeting other competing needs due to the growing population, addressing the impact of SWI on freshwater resources and on soil health is a pressing priority.

The most effective technique to mitigate salt buildup in the soil from any source is to drain the salts by using fresh quality water so that the excess salts are drained below the rooting zone to overcome negative effects of salt concentration on root growth, which in turn affects the above-ground portion of the plants. This may sound a very simple and effective technique but may not be feasible in all cases. The availability of good quality freshwater in the vicinity of saline soils is often a major bottleneck, because over time the available water in the saline soil zone may also be impacted by salt; therefore, using brackish water for leaching the saline soil will not be effective in leaching salts. In addition, some of the soils in the saline soil zone may not be well drained, which will limit the leaching of salts from the root zone. This is the limitation in soils with hardpan and/or shallow water tables. In this review paper, we summarize the potential and limitations of selected soil amendment techniques in mitigating effects of soil salinity.

Several studies in different parts of the world have documented the beneficial effects of gypsum amendment in amelioration of soil salinity, thereby increased a yield response of different crops ( Shainberg et al. 1989 ; Ilyas et al. 1997 ; Hanay et al. 2004 ; Silveira et al. 2008 ; Gharaibeh et al. 2009 ; Niazi et al. 2009 ; Makoi & Verplancke 2010 ) or gypsum plus manure ( Smith 2009 ; Cha-um et al. 2011 ; Kahlon et al. 2012 ; Murtaza et al. 2013 ). In addition to its positive impact on soil salinity, gypsum amendment of agricultural soils has been demonstrated to have multiple benefits, such as (i) improving soil structure and water infiltration; (ii) mitigating aluminum toxicity; (iii) decreasing swelling of clays; (iv) excellent source of plant-available calcium and sulfur to those soils deficient in those nutrients; (v) decreasing bulk density; (vi) helping to mitigate phosphorus runoff from soils; (vii) preventing soil erosion, and (viii) increasing N fertilizer efficiency and availability of soil potassium (K).

Gharaibeh et al. (2009) conducted a study in Jordan to demonstrate the reclamation of a sandy clay loam soil using gypsum that was a byproduct of phosphate fertilizer, generally called ‘phosphogypsum’ (PG), which has very similar properties to those of the mined gypsum. They used moderately saline (EC = 2.16 dS/m) irrigation water for leaching of Na + from gypsum-amended soil. They used PG rates at 5, 10, 15, 20, 25, and 32 Mg/ha. Their results showed that 32 Mg/ha PG amendment resulted in a 96% reduction in total Na + in the soil and increased soil hydraulic conductivity (6.8 mm/h) compared with that of non-gypsum-amended soil (5.2 mm/h).

The application of PG at the rate of 32 Mg/ha is expected to require more cost for fuel and labor for transportation and spreading the material. However, PG is a waste byproduct during phosphate fertilizer production from rock phosphate, and the phosphate fertilizer industry is interested in the potential use of this byproduct because of extra costs and environmental concerns involved in storing PG. However, it is necessary to ensure that the specific PG to be used as a soil amendment is free of other contaminants. In addition, gypsum is also reported to have some negative impacts on nutrient solubility and leaching. Gypsum could negatively affect P, Fe, and Mn solubility; and nutrient uptakes by plant roots and thereby plant growth ( Elrashidi et al. 2010 ). Similarly, gypsum-amended soils showed a reduction in exchangeable K and Mg ( Syed-Omar & Sumner 1991 ).

Sulfuric acid or elemental sulfur

The ultimate mechanism of mitigation of salinity between sulfuric acid and elemental sulfur is the same. However, elemental sulfur has a lag period of up to 4–6 weeks or longer in cooler soils for the microbial transformation of elemental sulfur into sulfuric acid, which is also dependent on soil temperature, humidity, and soil aeration. The additional benefit of this amendment is the reduction of soil pH, which promotes plant uptake of several nutrients.

Organic mulch

Unlike the other amendments described above, there is a need to find low cost locally available materials especially for use in developing countries, which have seen an increase in soil salinity problems due to poor irrigation management. Soil mulching is an age-old technique in practice for a very long time in agriculture for improving soil's physical and chemical properties, minimizing soil erosion, reducing extreme fluctuations in soil temperature, and improving soil nutrient availability. Mulching also reduces soil evaporation losses, thereby improving water-use efficiency, reducing excess salt buildup, and minimizing salinity effects on plant growth and production ( Zhang et al. 2008 ; Abou-Baker 2011 ; Dong 2012 ; Shirish et al. 2013 ).

Moniruzzamani & Shamim (2015) used rice straw to evaluate the effects of different mulching techniques to mitigate soil salinity and sweet gourd yield response in Bangladesh. They compared: (i) mulching at the bottom of the plow layer of the pit; (ii) top of the soil surface of the pit; (iii) combination of (i) and (ii); and (iv) no mulch. The EC measurement 75 days later was significantly lower in mulching at the bottom and top of the soil surface of the pit (4.98 dS/m) when compared with that of no mulch (8.82 dS/m). The former treatment resulted in significantly greater yield (18.5 Mg/ha) compared with that in the rest of the treatments. Yield from no mulch treatment was only 10.6 Mg/ha. Their results distinctly showed the beneficial effects of mulching to reduce salinity, thereby increasing the yield.

Al-Dhuhli et al. (2010) evaluated the effectiveness of date palm leaves and plastic mulch in reducing soil evaporation and maintaining favorable soil temperature for sorghum production in Oman using two irrigation water salinity levels (3 and 6 dS/m). They concluded that date palm leaves and mulch were more effective than plastic mulch in reducing salt accumulation in the soil, soil water loss by evaporation, and soil temperature, and in turn resulted in greater sorghum yield. This study also confirmed that salt accumulation can be reduced by mulching by minimizing the evaporation water loss from the soil.

Taufiq et al. (2017) investigated the effects of rice straw mulch (3.5 Mg/ha) and other amendments, including 5 Mg/ha gypsum, or manure and combination of gypsum and manure, on amelioration of soil salinity and peanut yield response in Indonesia on soil with 12 dS/m salinity. Mulching and 5 Mg/ha gypsum decreased soil EC, but peanut yield did not increase. They also reported favorable responses with mulching plus 750 kg/ha sulfur amendment.

Biochar is a charcoal-type material produced by ‘pyrolysis’ of biomass, of different origin, i.e. heating at a temperature typically between 300 and 700 o C under oxygen-deprived conditions. Although biochar can be produced from biomass of any type, the focus is on the use of agricultural wastes such as straw, rice husk, peanut hull, yard clippings, or tree trimmings. This provides an economically and environmentally sound way to recycle these wastes. Biochar is high in C content; therefore, it has been investigated as an amendment on agricultural soils for carbon sequestration and to improve soil properties. Recently, there is also an interest in exploring the benefits of biochar amendment to alleviate the negative effects of saline soils to improve crop production and quality.

High-specific surface area, CEC, and microporosity are the properties of biochar which are responsible for enhancing the soil productivity when biochar is amended to agricultural soils ( Thies & Rillig 2009 ). These properties of biochar are responsible for increased retention of water and nutrients in poor soils amended with biochar. Furthermore, biochar can also absorb heavy metals, pesticides, and other contaminants ( Rhodes et al. 2008 ; Beesley et al. 2010 ; Kookana 2010 ; Beesley et al. 2011 , p. 201; Buss et al. 2012 ).

Jin et al. (2018) conducted a pot experiment to evaluate the effects of biochar amendment (15, 30, and 45 g/kg) on a saline-sodic soil (pH 8.53; CEC = 23.91 μS/m; clay content = 35.6%) and evaluated sodium accumulation in rice plants as well as yield and quality. Biochar amendment significantly decreased Na + accumulation, while increasing rice yield even at the lowest rate of biochar amendment.

She et al. (2018) reported that the biochar amendment reduced the negative effects of saline irrigation water; therefore, it was beneficial to increase tomato production by mitigating the negative effects of salt stress. They also reported the highest rates of photosynthesis and respiration at the highest rate of biochar amendment but using non-saline irrigation water. Biochar amendment adsorbed Na ions, while released Ca, K, and Mg ions into the soil solution hence increased their availability to plants under saline conditions. Once again, the question is the economic feasibility of the biochar amendment. The lowest rate they studied was 45 Mg/ha.

Thomas et al. (2013) investigated the effects of two rates (5 and 50 Mg/ha) of biochar, made from Fagus grandifolia sawdust prepared at 378 °C, on the response of two herbaceous plants Abutilon theophrasti and Prunella vulgaris under induced salt stress condition created by adding 30 g/m 2 NaCl. The salt-induced mortality of seedlings was completely mitigated only at 50 Mg/ha biochar treatment, but not at the lower rate (5 Mg/ha). However, biochar treatments did not affect photosynthetic carbon gain, water-use efficiency, or chlorophyll fluorescence in both plant species compared with the control. The authors claim that the mitigation of salt stress by the biochar amendment was due to salt sorption. However, this favorable response was only observed at a 50 Mg/ha application rate, which is quite high and could be less feasible for large-scale field applications due to high input costs, and thus net returns may be very marginal or not possible. Therefore, it is important to assess the economic feasibility of using biochar as a soil amendment given the costs of biochar production through pyrolysis as well as labor and fuel costs for its application.

Hydrogels are synthetic polymers, produced by chemical stabilization of hydrophilic polymers in a tri-dimensional network ( Montesano et al. 2015 ), with the ability to absorb a large amount of water, and therefore called ‘superabsorbers’. In the past, agricultural application has been mostly as a soil amendment to absorb and retain water and subsequently make it available to plants, therefore facilitating increased water conservation and water-use efficiency ( Horie et al. 2004 ; Zohuriaan-Mehr & Kabiri 2008 ). Recently, attention has been expanded to investigate the role of hydrogels to mitigate salt stress on various crops ( Sayed et al. 1991 , p. 199; Szmidt & Graham 1991 ; Wang et al. 2004 ; Dorraji et al. 2010 ). The major limitation concerning the use of synthetic polymers as soil amendments has been lack of and/or slow degradation of these polymers resulting in prolonged-lasting of these residues in the soil, raising concerns about negative environmental and health impacts ( Mooney 2009 ; Timilsena et al. 2015 ). To overcome the above limitation and to maintain sustainable agricultural practices, i.e. to minimize waste residues in our soil and water resources, the development of polymers using biodegradable biomass-based raw materials has been investigated. The emphasis has been on using lignin or agricultural residues to develop water-insoluble hydrogels. Lignin is abundantly available as a residue of biorefinery processes or pulp industry ( Wang et al. 2016 ). Agricultural biomass residues that can be used include bagasse or sorghum, which are abundantly available in most places. Agricultural residues can be used to produce a hydrogel with high cationic exchange capacity and swelling characteristics that could reduce salinity risks. Furthermore, this hydrogel and gypsum can be used together to achieve maximum absorption of Ca 2+ and reduce the impacts of Na + and Cl − ions. Further research is needed to provide additional data on the appropriate rate of hydrogel amendment to mitigate the negative effects of salinity on plant growth and production under a range of salinity stresses.

Soil salinization due to poor irrigation management is a global issue. In addition, soil salinity SWI due to SLR is becoming a major threat that impacts agricultural production and natural resources. Addressing soil salinization is of significant importance to global food and water security. However, there is still a critical knowledge gap on how soil salinity alters the chemistry, soil health, nutrient transport, and GHG emissions in agricultural soils. Such knowledge is extremely critical to develop strategies to mitigate these negative impacts.

The major knowledge gap related to climate change and SWI is a lack of clear understanding with some degree of certainty on the extent of the problem. Current and future projections both in areal coverage and severity are not well known.

The availability of reliable data on soil health indicator parameters, water quality, and other ecosystem processes remains a major issue. For example, data on chloride concentrations, which is an indicator of water salinity, are scarcely available. Chloride concentration data are required to predict the trends in salinity changes in groundwaters. Guha & Panday (2012) , from their modeling work on SWI, indicated that the lack of accurate and adequate measured chloride data needed to calibrate and validate models was a major bottleneck.

Salt accumulation can be leached below the crop root zone using freshwater (rainfall or irrigation). However, climate change and variability cause extreme weather events such as cycle droughts and flooding. Erratic and often deficit rainfall in some areas is inadequate to leach salts below the root zone and thus groundwater is used to meet freshwater needs. Therefore, a better understanding of the mechanisms and negative impacts of SLR and SWI on the freshwater resources, soil health, nutrient transformation and transport, and GHG emissions is a global priority to develop best management practices to mitigate these negative impacts to safeguard food and water security. However, our understanding of the potential of management practices such as soil amendments, leaching the salts from the crop root zone, also needs to be enhanced so that alternative management strategies are in place to mitigate the negative impacts of SWI on agricultural production and soil health.

Another area where the knowledge gap exists is the possibility of genetically improving crops for increased salt tolerance to keep salt-affected lands under production. Similarly, there is only a limited understanding of hydraulic and physical management strategies to mitigate SWI.

With projected increases in global SLR, much of the attention with regards to its impact has been on the SWI of groundwater and the threat it poses to livelihoods and freshwater resources coastal regions. However, limited attention is given to the impact of the landward movement of saltwater, through the surface and/or groundwater connections, on the salinity of the soil, overall soil health, and agricultural productivity. Land-use changes such as the conversion of wetlands to agricultural areas due to population pressure and urbanization will result in a lower groundwater level which in turn would result in more water flowing inland from the sea. These changes will contribute to increased risk of salinization of inland surface water and the soil, as the SLR is expected to continue at an accelerated rate into the future. This impact will be severe as the climate crisis enhances the risk of drought and storms in the coastal and inland regions with increased groundwater pumping to meet crop water requirements. The impact of soil salinity on soil health and water quality, and overall agricultural productivity and thereby food and water security could be significant. More research is needed to better understand the scale of the impacts and develop integrated management practices to mitigate the negative impacts. Research should involve different scales and employ multi-pronged approaches including monitoring of changes in major indicators of soil health such as soil organic matter, microbial biomass, soil bulk density, water infiltration, aggregate stability, soil pH, and status of major nutrients such as N and P. Furthermore, there is a compelling need to understand the role and mechanism of different management strategies to mitigate the negative effects of SWI on soil salinity and crop productivity. The potential management strategies include soil amendments such as engineered biochar, salt repelling hydrogel, gypsum, and organic mulch. Improved understanding of the processes and impact of soil salinity on soil health will assist in designing best management practices to mitigate the impacts of soil salinity on agricultural soils and crop productivity.

This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award no. 2020-67019-31163. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.

All relevant data are included in the paper or its Supplementary Information.

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• Published: 05 October 2021

Saltwater intrusion impacts

• Graham Simpkins 1  

Nature Reviews Earth & Environment volume  2 ,  page 740 ( 2021 ) Cite this article

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• Cryospheric science
• Geomorphology

In Arctic regions, coastal permafrost is threatened by the collective influence of rising temperatures, sea level rise and erosion, impacting greenhouse gas emissions, infrastructure and ecological functioning. While direct inundation and erosion are common impacts of sea level rise, saltwater intrusion can also occur, affecting freezing points and, thus, permafrost thaw. These impacts, however, remain little quantified.

Saltwater intrusion, a little-examined impact of sea level rise in high latitudes, therefore has a marked influence on projections of coastal permafrost change. While these effects are demonstrated in a mathematical model and represent a good first step, more research is required to ascertain intrusion impacts via field work and in more physically constrained models.

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Original article

Guimond, J. A. et al. Saltwater intrusion intensifies coastal permafrost thaw. Geophys. Res. Lett. https://doi.org/10.1029/2021GL094776 (2021)

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• College of Engineering

A more efficient way to turn saltwater into drinking water

by Anna Boyle

Although 71% of the world’s surface is covered by seawater, four billion people live under conditions of severe water scarcity at least one month of the year. Researchers are working on a honeycombed-patterned membrane, only a few atoms thick, that can desalinate water using less energy than existing methods.

• Where mechanics meets artificial intelligence

Water scarcity is a major problem across the world. “It affects every continent,” says Amir Barati Farimani , an assistant professor of mechanical engineering . “Four billion people live under conditions of severe water scarcity at least one month of the year. Half a billion people live under severe water scarcity all year.” 

Yet even as people struggle without access to safe drinking water, there are oceans of undrinkable water right outside their doors. “71% of the world’s surface is covered by seawater,” Barati Farimani says. “So this is a very interesting contradiction.”

In order to combat this problem, Barati Farimani has focused his research on water desalination. This is the process in which salty seawater can be transformed into fresh water. 

There are many ways to desalinate water, but one of the most effective is membrane desalination. In this method, water is pushed through a thin membrane with tiny holes. The water flows through the pores, but the salt ions can’t, leaving only fresh water on the other side.   

Source: College of Engineering

A conductive metal-organic framework (MOF) is a new type of membrane for water desalination.

In his latest research, Barati Farimani explores the potential of a new type of membrane, called a metal-organic framework (MOF). “These membranes consist of both the metal center and organic compound,” Barati Farimani says. The organic compound and metal connect in a pentagonal pattern, leaving a hole in the center that serves as a pore. “If you look at them, they are like a honeycomb,” Barati Farimani adds.

There are a couple of reasons why the framework is more effective. First, it’s incredibly thin. It’s a few atoms thick, which means there’s very little friction as the water molecules pass through the pores.  

Additionally, the placement of the pores helps with permeation. “When you don’t have adjacent pores, there’s a huge pressure from the wall on the molecules,” Barati Farimani says. This makes the desalination process less efficient. To understand why, just imagine pouring water into a funnel. The water moves more slowly through the hole at the end because it’s pushed against the walls and forced through a small space.

The MOF, on the other hand, has multiple adjacent pores. “There’s no pressure from the wall side,” Barati Farimani says. “And that gives them this opportunity to pass more easily through the pore.” Imagine pouring water through a strainer this time—it moves much more quickly, because it has multiple exit points it can escape through. 

Finally, the MOF has more structural integrity than other materials. In most materials, scientists have to drill tiny holes in order to create the needed pores, which limits the amount that can be created per surface area. “If you want to make a lot of pores, graphene or MoS 2 can’t do that,” Barati Farimani says. “Structurally they can’t hold the pressure.” 

But thanks to its honeycomb structure, MOF is intrinsically porous. This allows a higher ratio of pores to surface area. It also saves on time and energy, since the pores don’t need to be drilled, or even adjusted in size.

The differences between the MOF and other typical membranes are notable, both in terms of how quickly water passes through and how many ions are rejected. And that’s just looking at a simulation of a few pores. A desalination plant can have billions of pores, raising its efficiency exponentially. “In the scale of a large operation, it would be huge,” Barati Farimani says. “Even a slight increase in efficiency would mean a huge leap.” 

We need to provide fresh water for many underprivileged people. That’s our mission—to make it so energy efficient that we have water desalination everywhere. Amir Barati Farimani , Assistant Professor , Mechanical Engineering

Barati Farimani’s article on his research was published in Nano Letters , a monthly peer-reviewed scientific journal published by the American Chemical Society. It adds to a growing conversation about water desalination and represents an important step forward in the field.  

In addition to the world of academics, Barati Farimani hopes that his research can make an impact in people’s lives. “We need to provide fresh water for many underprivileged people, like in Africa or other places,” he says. “Basically that’s our mission—to make it so energy efficient that we have water desalination everywhere.”

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ORIGINAL RESEARCH

Current and future salinity intrusion in the south-western coastal region of bangladesh.

• 1 Climate Change and Sustainable Development Policies at the University of Lisbon, Nova University of Lisbon, Lisbon, Portugal
• 2 Universitat de València, Valencia, Spain
• 3 University of East Anglia, Norwich, United Kingdom
• 4 Department of Anthropology, University of Chittagong, Chittagong, Bangladesh
• 5 Department of Geography, University of Valencia, Valencia, Spain
• 6 Department of Physics, Faculty of Sciences, University of Lisbon, Lisbon, Portugal
• 7 Institute of Social Sciences, University of Lisbon, Lisbon, Portugal

The southwestern coastal regions of Bangladesh (SWCRB) are highly exposed to saltwater intrusions brought about through cyclones and storm surges. These salinity intrusions are contributing to soil and water salinity in the coastal areas. This study aimed to determine the impact of these salinity intrusions on the quality of water and soil in three vulnerable coastal areas. In this investigation, water and soil samples were collected and analysed for pH, electrical conductivity (EC) and other trace elements. The analysis found many of the parameters to be higher than the recommended values. The study found that in soil samples there was a significant correlation between OM and ECe dS/m, as well as K and TN; and a highly significant correlation between TN and OM. This study further examined the historical salinity data at low and high tides to determine any patterns occurring alongside storm surges and cyclones. Water salinity statistics were obtained from the three locations of the Bangladesh Water Development Board (BWDB), which neighbours the study area. A Digital Evaluation Model (DEM) predicts the salinity induced by storm gushes in the corresponding impacted zones. Lastly, the study compared projections for future storm surges at current and predicted sea levels. Potential storm gushes circumstances from 1 to 9 m can impact up to 33% of the nation and 97% of the Shyamnagar Upazila. The occurrence of cyclone-related storms will increase and make cultivation and settlement in the region difficult. The predicted sea-level rises and saltwater contamination will intensify the adverse effects of salinity.

Introduction

Bangladesh is a country that is highly susceptible to soil and water salinization due to its geographical location. The reason for Bangladesh’s susceptibility to salinization is due to the ever-present occurrence of cyclones and storm surges annually across the country. Storm surges are disturbances on the sea surface caused by extreme events such as cyclones where wind drag occurs and pressure drops ( Alam et al., 2017 ; Cochran et al., 2019 ; Kim et al., 2019 ). These storm surges can last from several hours up to several days and can encroach up to 10 m inland, bringing with them salinization through saltwater intrusions ( Dasgupta et al., 2015a ; ibid). Saltwater intrusions in the southwestern coastal region of Bangladesh are having devastating consequences on water resources, agriculture and human health. Soil and water resources are essential to life on Earth, having a serious bearing on food and water security, biodiversity, climate, and human well-being ( McBratney et al., 2014 ; Bannari and Al-Ali, 2020 ). Saline soils are mainly found in arid and semi-arid regions, where evapotranspiration surpasses rainfall. However, they are also found in coastal districts due to seawater infiltration and inundation by coastal tides ( Karmakar et al., 2016 ). Bangladesh is especially vulnerable to saltwater intrusions, as the country has a vast area of low altitude near the coast and is often subject to tropical cyclones ( Brammer 2014 ; Faneca et al., 2015 ). Saltwater intrusions have detrimental impacts on land, by increasing the soil and surface water salinity ( Rahman et al., 2011 ). Salt-affected soils also harm the bioavailability of plant nutrients such as N, P, K, Ca, or Mg ( Fageria et al., 2011 ; Daliakopoulos et al., 2016 ; Alam et al., 2017 ). Also, the growth potential of legume crops is hampered by saline soil ( SRDI, 2012 ; Alam et al., 2017 ). The high salt content in soil roots is the main obstacle to the intensification of crop production.

The southwestern coastal area of Bangladesh, in the Bay of Bengal, experiences frequent tropical cyclones and their associated storm surges; that flood areas with saline water ( Dasgupta et al., 2010 ). Bangladesh was hit by 154 cyclones between the years 1877 and 1995, many of them included storm surges that went more than 7 m inland ( Dasgupta et al., 2014 ). More recently between the years 2000–2020, there have been eight major cyclones, including Cyclone Sidr in 2007 and its associated storm surge, that affected around 3.45 million people ( Hossain and Mullick 2020 ). Approximately 37% of arable seaside land is currently impacted by fluctuating degrees of soil salinity due to these surges ( Dasgupta et al., 2014 ). Furthermore, 1.02 million hectares (about 70%) of this arable land on the coast is affected by varying degrees of soil salinity in general. The increase in soil and water salinity causes problems within the coastal ecological setting, affecting the cultivation of crops, thus decreasing food security and increasing the shortage of drinking water by significantly reducing the quality of freshwater ( Brammer 2014 ). This situation can potentially subject more than 20 million people to the harmful effects of excess salt through food and water resources ( Haldar et al., 2017 ). According to the Intergovernmental Panel on Climate Change (IPCC) report, saltwater intrusion in low-lying coastal areas, river deltas and estuaries has increased, leading to salinization of groundwater, surface water and soil resources ( Oppenheimer et al., 2019 ). Excessive groundwater Mining has lowered the groundwater level, and rising sea levels have caused seawater to invade coastal aquifers from the ocean, leading to long-term salinization in southwestern Bangladesh ( Salehin et al., 2018 ). During the dry season, the flow of the lower Ganges becomes low, and seawater pushes inland saltwater into rivers and canals, through vertical filtration or infiltration into nearby land, resulting in salinization of groundwater and soil, which lasts until the onset of the rainy season ( Lam et al., 2021 ; Salehin et al., 2018 ). Floods and storm surges caused by severe tropical cyclones such as Sidr (2007) and Aila (2009) are also responsible for the long-term salinization of soil and surface water (Kabir et al., 2016; Salehin et al., 2018 ). This affects agricultural activities such as plant germination, biomass production and yield, and people’s livelihoods ( Lam et al., 2021 ).

On the Bangladesh Southwestern coast, shrimp and prawn farming has become widespread ( Ahmed and Diana 2015 ). These farms can be for freshwater and saltwater shrimp with saltwater shrimp farming, increasing salinity, especially in the southern coastal areas ( Rahman et al., 2013 ). Farmers raise shrimp on their land in saline aquaculture ponds that can contribute to groundwater and soil salinity. Freshwater shrimp farms are, on the other hand, at risk due to saltwater intrusions increasing gher salinity and poisoning the shrimp ( Ahmed and Diana 2015 ). The total area of land affected by salinity in Bangladesh in 1973 was 83.3 million hectares; by 2000, it had risen to 102 million hectares, and by 2009, it reached 105.6 million hectares ( Brammer 2014 ).

Another important driving factor increasing soil and water salinization over the previous half-century is climate change ( Daliakopoulos et al., 2016 ; Gorji et al., 2019 ). This is because soil and water are closely associated with the atmospheric and climatic schemes through carbon, nitrogen and hydrological rotations. Therefore, the changing climate will impact soil and water processes. Low-lying semi-arid and arid areas are even further exposed to soil and water salinity due to declining groundwater quality and rainfall shortages ( Kurylyk and MacQuarrie 2013 ). In these areas, irrigation-based cultivation is indispensable, despite causing the salinization of soil and water which can lead to land degradation ( Baumhardt et al., 2015 ). Following rigorous irrigation, soil salinity affects around 40–45% of the Earth’s land and leads to immense economic harm to a universal extent ( Oo et al., 2013 ). One such example is in the Jaffna Peninsula in Sri Lanka where 32.8% of the land and 45% of paddy land have been impacted by salt ( Gopalakrishnan and Kumar 2020 ). The global average surface temperature in the latter part of the current century (2081–2100) is forecasted to go beyond 1.5–2°C ( IPCC, 2014 ; Talukder et al., 2018 ). Around 70% of the global coastal areas are predicted to undergo remarkable sea level rise. Climate change impacts would lead to the increased river and groundwater salinity in Bangladesh’s Southwestern coastal regions by 2050 ( Brammer 2014 ). At least 2.9 million poor people are affected by a drinking and irrigation water deficit in the region ( Bannari and Al-Ali, 2020 ).

Due to the threat of increasing salinity, there is a need to study the effects of soil and water salinity in the southwestern coastal region of Bangladesh in the Shyamnagar Upazila, Shatkhira district, where it has caused significant negative effects on crops, fish and livestock production. This study area is adjacent to the low-lying plains at the confluence of the Ganges, Brahmaputra and Meghna (GBM) rivers which cover over 80% of Bangladesh ( Yu et al., 2016 ). These regions at the coast usually have a mean elevation ranging between one and 4 m above mean sea level (MSL) ( Rashid 1977 ). Slight variations in the tidal levels can cause sea water to travel far inland, depending on the tidal conditions and freshwater upstream flows ( Dasgupta et al., 2015b ). These low lying areas make the country vulnerable to sea level rises and saline intrusions, as well as other potential extreme weather events ( Karim and Mimura 2008 ).

The main goal of this research is to analyse the geochemical properties of saltwater and its related compounds, the response to saltwater intrusion in river systems, and the possible areas affected by storm surges in the present and the hypothetical 2100 conditions. Similar studies have been conducted to investigate the effect of cyclones and storm water surges on disaster-prone areas. One such study was conducted in the same Shyamnager Upazila district in Bangladesh ( Shaibur et al., 2019 ). In the above-mentioned study, water samples were collected from ponds, pond sand filters and deep tube wells in the Buri Goalini and Gabura unions. The authors found that water from the pond sand filters was fit for drinking purposes according to the World Health Organisation (WHO), but water from the ponds and deep tubes were saline and not suitable for drinking purposes. The study by Shaibur et al. (2019) reflects the impacts that extreme weather events have had on the freshwater supplies in the coastal regions of Bangladesh. It is no surprise that years of saltwater intrusions have harmed the country however little is still known about the impacts on the region’s soil and the long-term response to these events. It is important to consider the impact on the soil as well as on the water as these 2 are highly linked and influenced by each other in natural systems. Furthermore, assessing the response to these salinization events over the past decades can assist in pattern identification in how water and soil responds. Recognising these patterns can help the people and governments of disaster-prone areas to better prepare for the after-effects of such extreme weather events, particularly in the agricultural sectors.

Therefore, the objectives of this study are: 1) to assess the primary water and soil parameter results through laboratory tests; 2) to assess the geochemical composition, abundance and effects of saltwater related compounds in soil and water samples in Shyamnagar Upazila; 3) to analyse the relationship of the response to saltwater intrusion at three stations along the Betna-Kholpetua River using historical data and 4) to create a map of the areas commonly affected by storm surges under current conditions and predicted 2100 conditions for Bangladesh and the Shyamnagar Upazila regions.

Methodology

The investigation was carried out at the Buri Goalini, Munshigonj and Gabura Unions under the Shyamnagar Upazila of Satkhira District, situated between 22°36′ and 22°24′ north latitudes and between 89°00′ and 89°19′ east longitudes ( BBS, 2011 ). The 2011 Bangladesh census identifies a population base of 318,254 in Shyamnagar alone whilst 10% of the nationwide population, 14 million people, reside in the South-western coast ( Szabo et al., 2016 ; Efretuei 2016 ; ibid). The main rivers of the region are the Kobadak, Sonai, Kholpatua, Morischap, Raimangal, Hariabhanga, Ichamati, Betrabati, Kalindi and Jamuna. Usual rainfall is 1, 68 mm with a day-to-day temperature fluctuating from 21 to 30°C. The yearly comparative moisture fluctuates between 7 and 80%. The standard deviation of annual precipitation around this expanse differs from 334.0 to 586.3 mm, ( Kabir and Golder 2017 ) previously reported average maximum and minimum temperatures of 27.7 and 15.6°C during the dry season (November–February), 33.2 and 23.3°C during pre-monsoon season (March–May), and 31.7 and 25.5°C during monsoon season (June–October) for the South-western region of Bangladesh ( Mukul et al., 2019 ).

The investigated region displays even topography, where the maximum expanse remains within 1 m from sea level. The soils are classified as histosols and are described as having grey, slightly calcareous, loamy soils on river banks and grey or dark grey, non-calcareous clays, mainly containing silts, in the extensive land area across the water bodies ( Islam et al., 2012 ; Gorji et al., 2019 ; ibid). Histosols form when organic matter is generated fast than it is decomposed, this often occurs in areas prone to flooding, but not freezing, where there is poor drainage ( Brady and Weil 2008a ). Saline and exceedingly saline zones enclose approximately 3336.67 and 241.4 ha, corresponding to 41.46 and 36.6% of all the land of the study region ( SRDI, 2012 ). The pH level usually varies from 5.4 to 7.44 ( Islam et al., 2017 ; Shaibur et al., 2017 ).

Generally, Shyamnagar Upazila soils are highly dominated by silty clay and silty-clay loam texture (fine-textured and plastic in nature) where the presence of clay particles is much ( Shaibur et al., 2017 ). Clay particles are mainly negatively charged which attract and adsorb positively charged particles to their surface (ibid) Also, lateral movement of water occurs in clay textured soil and that’s why waterlogging conditions develop. When these waterlogging conditions are saline, soil salinity develops ( Naher et al., 2011 ). Cations responsible for salt production, such as Ca 2+ , K + , Na + , and Mg 2+ , in the soil, get adsorbed by clay particles. Islam et al. (2012) stated that the salinity level of the soil in Shyamnagar Upazila varies from moderate to high. Cation exchange capacity (CEC) of clay or clay loam type soils lie within 15–30 meq/100 g soil or above ( Penn and Camberato 2019 ).

Naher et al. (2011) found that the cation exchange capacity (CEC) of the Shyamnagar Upazila soil varies from 12.0 to 27.6 meq/100 g soil. Also, Shaibur et al. depicted that the status of bicarbonate (HCO 3− ), sodium (Na), magnesium (Mg) and sulfur (S) were within 366–793 ppm, 7.50–13.50 ppm, 5.11–6.01 meq/100 g soil and 264–431 ppm respectively ( Shaibur et al., 2017 ). Bangladesh’s coastal zones have undergone key alterations around the previous five decades, mainly due to recurrent and varied natural catastrophes with direct and indirect bearings on land assets and their various uses ( Rasel et al., 2013 ). The land is despoiled and disappearing due to the impact of increasing salinity, flooding of low-lying swampy land, deluges, and land attrition due to involuntary and chaotic land exploitation by the inhabitants. This intense variation in land exploitation and alteration of the agricultural system has impeded normal crop production during the year (ibid).

The three most vulnerable unions, Gabura, Burigoalini and Munshiganj located in the Shyamnagar Upazila were the focus areas of this study ( Figure 1B ). A total of 18 soil samples and 29 water samples were collected in these areas. Data was collected during rainy season in Bangladesh besides the rainy season in Bangladesh coincides with the summer monsoon season (June to mid-October), this season’s rainfall accounts for 75–80% of the total rainfall the country’s annual rainfall ( Ahmed and Kim 2003 ).

FIGURE 1 . (A) The geographical location of the river delta salinity stations in the southwestern coastal area of the Bay of Bengal, [ Humanitarian Open Street Map Team (2021) HOTOSM Bangladesh (southwest) Waterways. Volunteered geographic information. Retrieved on November 22, 2021 from: https://data.humdata.org/dataset/hotosm_bgd_southwest_waterways ]. (B) Along with the three Unions from where the data was collected. [Landsat-8 image courtesy of the U.S. Geological Survey; OCHA ROAP (2020) Bangladesh—Subnational Administrative Boundaries. Bangladesh Bureau of Statistics (BBS). Retrieved on November 22, 2021 from: https://data.humdata.org/dataset/administrative-boundaries-of-bangladesh-as-of-2015 ]. (C) Soil Salinity Bangladesh [Soil Resource Development Institute (2010) Saline Soils of Bangladesh. SRMAF Project, Ministry of Agriculture. Retrieved on November 22, 2021 from: https://hub.arcgis.com/maps/9628788cedd7460abfe9261b3dbb74b7/about ].

Soil and Water Sampling

Soil and water samples were collected between the 10th and 13th of July 2019 at the three unions. Samples were collected in bottles that were shaken overnight with 20% nitric acid and rinsed with deionized water to remove internal and external contaminants. Soil samples were collected using a manual auger at a depth of 10–15 cm deep ( Soil Survey Staff, 2014 ).

Eighteen soil samples were collected within these areas from agricultural fields, ponds, riverbanks, and shrimp ghers. The coordinates and land use information for the soil samples are displayed in Table 1 and the water samples are displayed in Table 2 . Photographs of the sites are also shown in Figures 2 , 3 for soil and water samples respectively.

TABLE 1 . Areas where soil samples were collected and GPS coordinates with land use coded according to sampling source.

TABLE 2 . Water sample collection areas and GPS location with land use coded according to sampling source.

FIGURE 2 . Sample of soil collection areas.

FIGURE 3 . Water sample collection areas.

After the samples were collected, all samples were sent to the laboratory of the Soil Resources Development Institute (SRDI) within 21 h and kept in a refrigerator at a temperature below four degrees Celsius ( Soil Survey Staff, 2014 ). Analysis of the soil and water samples was conducted from July 14 to August 21, 2019, at the SRDI. Soil samples were analyzed for pH, electrical conductivity (EC), calcium (Ca), organic matter (OM), total nitrogen (TN), potassium (K) and phosphorous (P). Water samples were analyzed for pH, EC, total dissolved solids (TDS), sodium (Na), bicarbonate (HCO 3 ) and chloride (Cl).

Twenty-nine water samples were also collected in these areas from tube wells, pond sand filters, and rainwater harvesting tanks. The coordinates and land use information for the soil samples are displayed in Table 2 and photographs of the collection sites are shown in Figure 3 below.

Soil and Water Sample Analysis

The major chemical constituents of soil and water and their quality factors were analysed using standard methodologies. Standard saturation paste method was used to determine the ECe where 350 g air-dried soil was taken for each sample to prepare the saturated paste, left it in room temperature for 24 h for equilibrium then the saturated paste extracts were collected by subsequently using Buchner funnel and applying suction ( Rhoades, 1996 ; McBratney et al., 2014 ; Soil Survey Staff, 2014 ). In this research ECe soil sample was measured in saturated paste as ECe, dS/m at 25°C and for the EC water sample was measured as EC, dS/m at 25°C respectively. The EC was measured by means of the Jenway EC meter from the extracts (ibid). pH was measured using the Jenway pH metre as described by Tan (2005) . The salt content (Total Dissolved Solids) was calculated using the following formula ( Sparks 2003 ):

The Olsen Sodium Bicarbonate test was used to determine soil phosphorus. Soil potassium and sodium were determined separately by flame emission spectrophotometer (Jenway Model: PEP-7), using potassium and sodium filters, respectively, as outlined by Jackson (1973) . Chloride and bicarbonate contents were determined by the titrimetric method described by Jackson (1973) . The sample was determined by the Kjeldahl method described by Bremner (2009) and organic carbon was analysed using Walkley and Black (1934) wet oxidation method ( Ghosh et al., 1983 ; ibid).

Additionally, historical data for electrical conductivity (EC) and chloride concentration was obtained from the Bangladesh Water Development Board (BWDB) for the South-western coastal region with data ranging from 1968 to 2019. Data from the BWDB was available from 3 stations: Kalaroa (SW 23; 2001–2018), Elarchar (SW 254.5; 2001–2018), Benarpota (SW 24; 1980–2018). The EC and chloride data from the BWDB was averaged on yearly basis for low and high-water levels. This data was plotted in Microsoft Office Excel 16 as the levels of chloride and EC at high and low tide over time in years.

Statistical Analysis

The results of the soil and water samples were analysed using Pearson’s correlation coefficient using the computer software IBM SPSS 25. Pearson’s correlation coefficient is a linear correlation model using two sets of data that produces a value, r , by accounting for the covariance and standard deviations within the data sets. The r vale produced indicts how highly one dataset is correlated or influenced by another with high values being more significant. The standard Pearson’s r is calculated as follows:

Standard deviation and other general calculations were also conducted. A standard linear regression, using historical data, was created to determine the R 2 value for chloride and EC at low and high tide for the Kalaroa, Elarchar, and Benarpota stations. This was done to identify variables that correlate with each other to find statistical patterns that might reveal evidence of the physical mechanism underlying them. Most calculations were conducted in Microsoft Excel 16 unless otherwise stated.

Cyclonic Storm Surge Analysis

To calculate the areas affected by different seawater surges, three different storm surge heights were used, 1.5, 5.25, and 9 m as the minimum, mean and maximum surge height ( Huq and Shoaib, 2013 ). A 90-m resolution Digital Terrain Model retrieved from ( Jarvis et al., 2008 ) with a 1-m resolution altitude data were used. However, due to the 1-m altitude resolution of our data, the numbers were rounded to 1, 5, and 9 all of the areas within 1-m, 5-m and 9-m of mean sea level in the DEM were used to estimate the respective affected areas. The 1-m sea-level change was calculated using the same process, 1, 5, and 9-m surge 1-m above the current sea levels, similar to the methodology used in the study by Karim and Mimura (2008) . This 1-m sea-level rise projection were taken from (ibid) where a study puts the sea-level change in 2100 between 30 and 100 cm, as well as the IPCC’s RCP8.5 estimating a rise of 52–98 cm in sea level by 2100, based on Mukul et al. (2019) . Due to the constrained nature of the data resolution, a 1-m sea-level rise was used. Then, each affected area was counted by converting the raster DEM pixels into a vectorised matrix and selecting the pixels with the desired elevation values of 1 m, the sum of pixels from 1 to 5-m for the 5-m surge, and the sum of the pixels from 1 to 9 for the 9-m surge scenarios ( Jarvis et al., 2008 ). This same technique was used with the 1-m sea-level rise scenario but taking the values from 1 m above, so by taking the pixel count from 2 m for the 1-m surge, the sum of 2–6 pixels for the 5-m surge, and the sum of 2–10 pixels for the 9-m surge. The pixel count then was multiplied by 8100 to calculate the area in square metres, and then divided by 1,000,000 to get the square kilometres ( Huq and Shoaib, 2013 ; ibid).

Soil Analysis

The results of the soil analysis are displayed in Table 3 above.

TABLE 3 . Some selected chemical properties and land use of soil samples collected from Gabura, Burigoalini and Munshiganj sites.

ECe values of the soil samples are shown in Table 3 , where the electrical conductivity of the soil samples varies between 1 and 32.7 dS/m. Again, the above results show high salinity statuses for all three respective sampling areas with more than 95% of the samples havilo1ng an ECe dS/m value higher than 8 dS/m across all the sampling areas. Soil salinity is often based on the direct measure of the electrical conductivity, soils with an ECe above 4 dS/m of which most of the samples were; furthermore some scientists have recommended that soils with an ECe above 2 dS/m be classified as saline as most crops will be harmed by salinity above ECe 2 dS/m ( Ghosh et al., 1983 ; Sparks 2003 ; Brady and Weil 2008b ). This will also have a large impact on agricultural production as only some salt-tolerant varieties of crops may produce a satisfactory yield in these areas ( Alam et al., 2017 ; Van Tan and Thanh, 2021 ).

The highest pH values found for Gabura, Burigoalini and Munshiganj were 8.8, 6.8, and 8.4 whilst the lowest pH values were 6.5, 6.6, and 5.1, respectively. The results indicate that most of the areas of Gabura and Burigoalini have higher soil pH levels (mildly to strongly alkaline) as opposed to the neutral range ( Ahmad and Rahman 2010 ). However, lower pH values were found in the Munshiganj site soils. The results confirm that agricultural and livestock production in Gabura and Burigaolini has been seriously affected due to the high pH levels ( Alam et al., 2017 ). Salinity is known to have an impact on the pH of a soil, as salinity increases, pH decreases, in other words, the soil becomes more acidic. This serves to explain the lower pH soils having high salinity or TDS ( Ghosh et al., 1983 , Sparks, 2003 ).

Organic Matter (OM), nitrogen (N) and potassium (K) showed statistical significance in their sample abundance ( Table 4 ). Most of the samples displayed a potassium concentration that exceeded the recommended 2.0 m eq /100 g range and could be detrimental to certain species. The reason for this high levels of K could be due to the alternating wetting and drying of the soils that can be brought about during extreme weather events, it is known that alternating wetting and drying periods enhances the K fixation in the soil, making it more available ( Weil and Brady, 2017 ). Above this threshold, luxurious potassium intake in plants, retains elevated levels causing slow plant growth, furthermore excess K is toxic for human and animal consumption ( He and Chen 2013 ; Alam et al., 2017 ). The abundance in total nitrogen (TN) and organic matter was observed for all the samples in all study areas ( Xue and An 2018 ). Inundated soil is rich in organic matter and nitrogen content, which explains the proportionally higher rates of these two variables, especially in water-filled soils such as ponds and ghers ( Fulton et al., 2011 ). External organic and non-organic additions may change the ratio of total nitrogen and available nitrogen, as nitrogen may be unavailable to plants in certain compounds ( Bingham and Cotrufo 2015 ). The deficiency of phosphorous (P) in the soil samples is visible in all of the study areas, this is to be expected as high organic matter in soils leads to lower levels of phosphate fixation ( Weil and Brady, 2017 ). However, there is a higher concentration of phosphorous in shrimp ghers as compared to agricultural lands, likely due to biological excretions by the shrimp. This is a problem, as the phosphorous concentrations are too low for agricultural production, which has the potential to lead to phosphorus deficiency in plants, resulting in the need to use phosphorous fertilizers ( Lawlor et al., 2004 ; Sharma et al., 2013 ). The low phosphorous in these soils is likely due to the high level of organic matter in the soil which has likely arisen due to the nature of heavy flooding in the area, soils that are not aerated tend to have higher OM contents as less decomposition occurs, this in turn also reduces the soils P fixation capacity ( Weil and Brady, 2017 ). Similarly, calcium (Ca) concentrations in all the samples are way above the recommended 5–10 m eq /100 g soil ( Van Tan and Thanh, 2021 ). This excess of calcium inhibits seed germination and reduces growth rates dramatically ( Lawlor et al., 2004 ). A statistical correlation was conducted between the soil nutrients with soil pH, electrical conductivity (EC) and organic matter (OM) was conducted and the results are shown in Table 4 .

TABLE 4 . Pearson correlation coefficient of soil nutrients with soil pH, ECe and organic matter (SOM).

The Pearson’s correlation coefficient study in Table 4 showed a significant positive correlation between ECe dS/m and OM (0.555*), OM and TN (0.752**) and OM and K (0.560*). Other correlations were found to be nonsignificant, indicating no influence between the parameters. This correlation study illustrated that organic matter concentration is positively correlated with electrical conductivity (EC), total nitrogen (TN) and potassium (K) and vice versa. The correlation with TN is expected as OM is essentially dead plant and animal material which tend to be mainly made of carbon, nitrogen, phosphorous, and hydrogen. Similarly the correlation between OM and K is expected as an increase in OM increase the CEC of soils, making K fixation higher ( Weil and Brady, 2017 ). The correlation between OM and ECe dS/m is interesting as an increase in ECe dS/m directly reduces the decomposition of OM in the soil ( Iranmanesh and Sadeghi 2019 ). A decrease in OM decomposition will also result in a decrease of available P which further explains the results in Table 3 . In summary, the soils in the area are high in OM due to the nature of flooding in the area; the flooding in the area also increases the overall TDS and in turn ECe dS/m of the soil. The increase in ECe dS/m in the soil will decrease OM decomposition which will further decrease P availability ( Ghosh et al., 1983 ). This will continue in cycles with every extreme weather event.

Further comparisons were conducted between pH and phosphorous (P) and are graphically displayed in Figure 4 below.

FIGURE 4 . Statistical relationship between Phosphorous (ppm) and pH. The abundance of Phosphorous is greater within the neutral area of pH despite not having any correlation (R 2 = 0.039).

Comparing phosphorous content with pH shows that at a pH of 7.0–8.3, samples had P content greater than 12 ppm ( Jackson, 1973 ). At higher and lower pH values (pH > 8.3; pH < 7.0) the P content of the samples was mostly below 10 ppm. However, no significant correlation exists between these two variables, indicating either the absence of coupled physical processes. In soil, both acidic and alkaline conditions increase phosphorus concentrations, which explains their relatively low abundance outside the neutral pH range. Thus, maximum P availability occurs at near-neutral pH, confirming the findings of Penn and Camberato (2019) . However, the input of synthetic phosphorous in agriculture and shrimp farms may skew the results, so no statistically significant relationship can be found. This means that if pH was to fluctuate above or below a neutral pH, it will rapidly be threatened phosphorous levels.

Water Analysis

The results of the water analysis are displayed in Table 5 below where the EC values varies from 0.1 to 41.3 dS/m. The results show that most of the samples have exceeded the permissible limit of salinity in the sampling areas, particularly in river water (>30 dS/m) ( Alam et al., 2017 ). Increased shrimp production using brackish water causes artificial salinity in the water and affects adjacent agricultural fields and drinking water reservoirs ( Mahmuduzzaman et al., 2014 ; World Health Organization, 2017 ). Continuous use of saline water can be a great threat to human consumption and irrigation.

TABLE 5 . Some of the selected chemical properties and land use of water samples collected from the Gabura, Burigoalini and Munshiganj sites.

As seen above, G1–G4 had much higher Na values than compared to the other soil samples in the area. Furthermore, six samples from the Buri Goalini area and one from the Munshigani union had similar high Na concentrations. This is a concern as high Na levels can cause sodicity in soil which makes soil less permeable and more prone to being flooded ( Sparks 2003 ). In turn, the soil in these flooded areas will develop higher organic matter concentrations (histosol formation).

Approximately 93% of the water samples showed a sodium (Na) concentration beyond the recommendation of 200 mg/L as well as chloride above 250 mg/L, these concentrations are unsuitable for drinking water ( World Health Organization, 2017 ). As these two elements are associated with salt, the samples containing NaCl are much higher in TDS than the standard values ( Tavakkoli et al., 2011 ; ibid). The results furthermore represent a direct connection between pH and bicarbonate (HCO 3− ). The higher the bicarbonate, the higher the pH (alkaline condition) (ibid). Most of the water samples from the Gabura and Burigoalini sites showed moderately to highly alkaline conditions exceeding the permissible limit for human consumption ( Islam et al., 2017 ). Saline water can also corrode metal pipes of supply water and increase metal concentration in drinking water which can pose a further threat to human health. Among the water samples, the concentration of bicarbonate (HCO 3- ) was highest at 384.3 mg/L for sample B-15 and lowest at 30.5 mg/L for sample B-11; despite this, none of the samples was above the standard limit of 600 mg/L ( World Health Organization, 2017 ). This trend follows a similar relationship with pH, which was verified by the very strong statistical correlation in Table 6 .

TABLE 6 . Pearson correlation coefficient of water nutrients with pH and EC.

The Pearson’s correlation coefficient study of water parameters in Table 6 illustrated a highly significant and positive correlation between pH and bicarbonate (0.79**), EC and Na (0.99**), EC and Cl − (0.91**) and Na and Cl − (0.89**) ( Tavakkoli et al., 2011 ). Other correlations were found to be non-significant. Overall, the correlation study of the study areas showed that in most cases, the concentrations of EC, Na, and Cl − were highly and positively correlated with each other ( Tavakkoli et al., 2011 ; Alam et al., 2017 ; Islam et al., 2017 ). On the other hand, pH showed a weak and negative correlation with EC and other parameter concentrations besides bicarbonate. The pH and bicarbonate show strong positive correlations, which are related to the bicarbonate buffer system ( Rhoades, 1996 ). A Pearson correlation was conducted between different soil and water parameters.

The Pearson correlation coefficient study between the soil and water parameters depicted a significant negative correlation between the ECe dS/m of soil and the EC of water (−0.47*); the ECe dS/m of soil and the Na of water (−0.50*) and a positive correlation between the OM of soil and the Na of water (0.50*) ( Akter et al., 2016 ; Alam et al., 2017 ). Other correlations, both positive and negative were found to be non-significant. Surprisingly, a negative but significant correlation is visible between the ECe dS/m of soil and the EC of water (−0.47*). This indicates that if water salinity increases, soil salinity decreases. This can be explained by the dissolution of salts from the soil into the water when it is waterlogged ( Islam et al., 2017 ). Other nonsignificant correlations are visible between chloride, calcium and potassium, which are all responsible for the increase of the salinity in soil and water by producing calcium chloride (CaCl 2 ) and potassium chloride (KCl − ) salts. However, these correlations are too weak to draw any conclusions.

Historical Patterns of Water Salinity

Historical data was collected and analysed overtime to try to determine and/or predict future patterns and correlations. The relationship between chloride and EC at different tide levels at the Benaporta station is shown in Figure 5 .

FIGURE 5 . Chloride concentrations (ppm) and electric conductivity (dS/m) at high and low tides in the Betna-Kholpetua river at Benarpota from 1980 to 2018. This graph shows a large increase in salinity and chloride concentrations in 1982, a smaller one in 2007, and another one from 2009 to 2011.

The Benaporta station shows a matching relationship between Cl − and EC concentrations at low and hide tides, but Cl − at high tide shows a more attenuated relationship ( Ahmad and Rahman 2010 ). At the same time, there was a massive spike in chloride in 1982, with EC levels reaching over 10,000 (dS/m), a smaller spike is visible from 2006 to 2007 with salinity levels of >3,000 dS/m and chloride levels of >1,000 ppm, and another spike from 2009 to 2011. Thereafter, a more intense increase in both salinity levels of 3,000 to 4,000 dS/m and chloride concentrations of 1,000 to 2,000 ppm, from 2009 to 2011. This increase is followed by minor spikes from 2012 to 2018.

The relationship between chloride and EC at different tide levels at the Benaporta station is shown in Figure 6 .

FIGURE 6 . Chloride concentrations (ppm) and electric conductivity (dS/m) at high and low tides in the Betna-Kholpetua River at Kalaroa from 2001 to 2018. This graph shows an increase of both chloride and EC from 2001 to 2003, followed by a more prolonged increase from 2006 to 2011, finally with a slight increase from 2013 to 2015.

The Kalaroa station clearly shows a spike of both chloride and EC from 2001 to 2003, with a salinity level of >4,000 dS/m and a chloride concentration of >1,000 ppm at high tides. These levels are similar to the same spike at Benaporta station. From 2006 to 2011, there is a massive increase in salinity ( Pal et al., 2016 ). The SIDR Disaster caused salinity intrusions in the south-western parts of Bangladesh–with levels fluctuating between 5,000 and 6,000 dS/m at high and low tides, and chlorine levels fluctuating between 2,000 and 3,000 ppm. These values are more than double the spike recorded between 2001 and 2005 and are higher than the same anomaly observed at Benaporta station. Finally, from 2013 to 2015 a minor spike can be seen, however, much weaker than the two others. Lastly, the relationship between chloride and EC at different tide levels at the Elarchar station is shown in Figure 7 .

FIGURE 7 . Chloride concentrations (ppm) and electric conductivity (dS/m) at high and low tides in the Betna-Kholpetua River at Elarchar from 2001 to 2018. An increase of both chloride and EC from 2001 to 2003, followed by a more prolonged increase from 2006 to 2011, finally with a slight increase from 2013 to 2015.

In Figure 7 , a trend similar to that observed at the Kalaroa station is repeated at Elarchar, where an increase in chloride and EC is observed from 2001 to 2003 with a salinity level of >8,000 dS/m and a chloride concentration of >2,000 ppm at high tides. Surprisingly, these values are twice as high as those seen in Kalaroa. Along with this, a massive increase in salinity, with levels well above 12,000 dS/m at high and low tides, and chlorine levels above 6,000 ppm is recorded from 2007 to 2011 ( SRDI, 2012 ; World Health Organization, 2017 ). These values are again twice as high as those recorded at the Kalaroa station. Table 7 reflects the correlations between the chloride concentrations at high tide between the three stations.

TABLE 7 . Correlations between Chloride concentration at low and high tides from the Bearpota, Kalaroa and Elarchar stations.

Every correlation is higher than R 2 = 0.2 except for the Benarpotas high tide with the Kalaroa’s high and low tide. This indicates that most data sets are affected quite similarly by the same environmental process. The same correlations were done using the EC data in Table 8 .

TABLE 8 . Correlations between EC concentration at low and high tides from the Bearpota, Kalaroa and Elarchar stations.

All the correlations had values greater than R 2 = 0.2, indicating that these data values respond in a similar way to the same environmental process.

The correlation analysis from Table 7 shows the chloride concentrations of the three stations at low and high tide, except for the correlation between Benarpota’s high tide and Kalaroa high and low tide data. Similarly, Table 8 shows that all had a significant correlation above R 2 = 0.2. Both tables show correlations above R 2 = 0.5. The two most predominant spikes from 2001 to 2003 and 2006/7 to 2011 are therefore statistically supported by these correlations. This means that the physical and environmental processes behind these spikes are affecting trends in salinity levels of the Betna-Kholpetua River in a similar and fairly uniformly manner.

Storm Surges and Sea-Level Rises

Figure 8 below displays two maps with projected impacts of potential storm surges at different distances inland in the Bay of Bengal. Figure 8A is at current sea level and Figure 8B is with a 1-m rise in sea level.

FIGURE 8 . Storm surge projections over the Bay of Bengal for areas affected by surges of 1, 5, and 9 m. (A) at current sea level. (B) at a 1 m rise in sea level.

These maps show that a potential storm surge of 9 m has the capacity, in both scenarios, to cover over a third of the country [Self-made with data retrieved from Jarvis et al. (2008) ]. The results from the different storm surge flooding extents with current sea levels indicate that even a small cyclone that creates a 1-m surge ( Dasgupta et al., 2014 ; Hossain and Mullick 2020 ) has the potential to affect 1,492 km 2 or about 1% of the area of Bangladesh and could reach the city of Khulna. However, a more severe cyclone creating a 5-m storm surge has the potential to flood 17,724 km 2 of the coastal area, or about 12% of the country’s territory, reaching the outskirts of Chittagong and Dhaka. Lastly, as a worst-case scenario, a very extreme cyclone, with a 9-m storm surge could flood an area of 48,491 km 2 or about a third of the country ( Karim and Mimura 2008 ; IPCC 2014 ), having a considerable impact on Dhaka and Chittagong.

Based on the maps above it is evident that the most vulnerable areas are the rivers. Many streams enter deep into Bangladesh’s territory, which means that even a moderate cyclone (1-m surge), can affect river environments and freshwater supplies far from the coast. These results may explain the correlations between the Kalaroa, Benarpota and Elarchar stations in Tables 7 , 8 , as these are within the range of a 1-m storm surge. Also, in Figures 5 – 7 , these stations reflect an increase in salinity over the years, where cyclones have led to an intrusion of saltwater ( IPCC 2014 ).

As seen in Figure 8B , the map indicates the same seawater surge scenario but based on the scenario of a projected 1-m rise of sea level, according to the IPCC’s RCP8.5 from Mukul et al. (2019) . This indicates that a 1-m surge can reach as far as the north-western border with India, because the surge can penetrate deep into the country through the channels of the rivers that run through it, covering 1.2%. A 5-m surge will cover a great part of the southwestern portion of Bangladesh and the surrounding floodplain areas of river systems with the potential to affect 17% of the country’s area ( IPCC 2014 ). An extreme 9-m surge event is predicted to cover more than one-third of the country and has the potential to affect the entire southern portion, having a considerable impact on all three major cities. Similarly, the Bangladeshi Sundarbans, will be highly affected, as the southwestern portion ( Dasgupta et al., 2014 ), will be mostly inundated by a 9-m water surge.

Figure 9 displays the maps from two different storm surge scenarios for the Shyamnagar Upazila region. Map A is a projection of the area affected by surges of 1, 5, and 9 m due to a cyclone or a tropical storm, with current sea levels. Map B shows the same area but with a 1-m rise in sea level, similar to Figure 8 .

FIGURE 9 . Results from two different storm surge scenarios for the Shyamnagar Upazila, map ( A) is a projection of the area affected by surges of 1, 5, and 9 m due to a cyclone or a tropical storm, with current sea levels. Map (B) shows the same area but with a 1-m rise in sea level. This map shows that a 9-m storm surge can cover most of the area (>90%) in the Upazila. Self-made with data retrieved from Jarvis et al. (2008) .

The maps above show that a 9-m storm surge can cover most of the area (>90%) in the Upazila [Self-made with data retrieved from Jarvis et al. (2008) ]. The first scenario shows that a 1-m surge will affect 79 km 2 of the area or around 6% of the Upazila, however, a 5-m water surge can affect more than half of the area, and a potential 9-m surge scenario will flood almost the entire area or 97% of the Upazila.

Comparatively, Figure 9B shows the same scenario, but with a baseline of 1 m above sea level, indicating that 5.6% of the area or 79 km 2 will be submerged by 2100 ( IPCC 2014 ). Here, the most significant change can be seen in the 1-m surge, as it will cover 9% of the area, mainly affecting a large part of the mainland around Shyamnagar town. The 5-m surge will now cover almost three-quarters of the Upazila, affecting areas in the southern Sundarbans that were only affected by a 9-m surge. Finally, a 9-m surge will cover less area compared to the current scenario but is expected to affect almost the entire Upazila.

Bangladesh is an extremely vulnerable nation due to its low topography, where half of the country lies within 5 m above sea level. Furthermore, Bangladesh is also extremely vulnerable to cyclonic and tropical storm activity, as displayed in the projection scenarios above. Based on several studies this research identifies that from 1891 to 2008, about 178 cyclones have hit the coast of Bangladesh ( Karim and Mimura 2008 ; Dasgupta et al., 2010 ). The deadliest cyclones in recent years were in 1970, 1982, 1991, Sidr in 2007 and Ayla in 2009, with several other cyclones flooding vast areas of the Bangladeshi coast ( Dasgupta et al., 2010 ; Dube 2012 ). Karim and Mimura (2008) indicated that about seven cyclones with surges of more than 4 m have occurred between 1970 and 2008. In summary, the Bangladeshi coast is affected by more than one cyclone each year and a cyclone with a moderate surge storm (>4 m) every 5.4 years. Based on this information, it is expected that more than 12% of the country is at a very high risk of saltwater flooding every 5.4 years, as well as 1–2% of the country being flooded by seawater every year. This also means that 64% of the total area of the Shyamnagar Upazila is expected to be flooded with saline water, and 5.6% annually.

At extreme scenarios, the highest prediction of a 9-m surge will have a lower periodicity but will not be uncommon. Zaman (2011) indicates that storm surges of these proportions are common in the literature, and cyclones with storm surges of 15-m have also been reported. In 1876, a cyclone with an associated 13-m surge hit the coast of Bangladesh, and a 10-m storm surge hit the coast in 1970 ( Dasgupta et al., 2010 ). Furthermore, a 10-m wave is expected to hit the coast every 20 years ( Dasgupta et al., 2014 ). For the Shyamnagar Upazila, this means that most of the area will be flooded with saline water every 20 years.

The same parameters will apply for storm surges with an estimated 1-m rise in sea level. The increase in the area flooded by sea rise compared to the current conditions depends on the topography of the land, which allows more water to pass inland, rather than an increase in the intensity of cyclones. Karim and Mimura (2008) note that the frequency of cyclones in November is increasing for the coast of Bengal. There has also been a clear increase in global cyclone activity over the last 35 years ( Dasgupta et al., 2010 ). Huq and Shoaib (2013) indicate that an intensification of wind speeds and cyclones is expected in the foreseeable future, due to increased ocean surface heat. This will increase the size and periodicity of associated seawater intrusion over Bangladeshi territory. This suggests that the above predictions for the area affected by cyclones underestimate the possible future predictions, which should be 1–3 m depending on the vulnerability of the area ( Dasgupta et al., 2010 ).

Furthermore, evidence of these seawater surges is recorded in the chemical archives within the Kalaroa, Benarpota, and Elarchar stations of the Bangladesh Water Development Board as a pattern of spikes or increases of both salinity and chloride. These spikes reflect the Sidr, Aila, and Nargis storms in 2007, 2008, and 2009 respectively. First, the 1982 spike, with EC levels reaching over 10,000 (dS/m), can be attributed to a storm surge in the same year, which caused heavy loss of life ( Dube 2012 ). Similarly, the 2007 to 2011 spikes can be attributed to storms Sidr in 2007 ( Pal et al., 2016 ) and Ayla in 2009 ( Rasheed et al., 2014 ; Haldar et al., 2017 ) that resulted in saltwater intrusion into the river system. Similarly, during 2008, ( Fritz et al., 2009 ) storm Nargis primarily affected the Myanmar deltaic region, but also the coast of Bangladesh, which can be considered as a link between the two spikes in 2007 and 2009 in Figures 5 – 7 .

The storm surges that create saltwater intrusion far into Bangladesh, permeate the soil and freshwater systems, such as rivers and ponds ( Brindha et al., 2014 ; Zahid et al., 2018 ). This is seen in Table 5 , as the river samples show a disproportionately high salinity value. The soil sample analysis shows very saline soils with high organic matter and low phosphate levels, this can lead to metal salt toxicity and phosphorous deficiency in plants, which will reduce agricultural yields. Shaibur et al. (2017) confirmed the salinity range of 4.21–8.02% in the Gabura and Burigoalini union soils and stated that these areas are in poor condition for crop and fish cultivation ( Fulton et al., 2011 ; Shaibur et al., 2017 ). High soil salinity prevents plants from getting essential nutrients as sodium competes with other nutrients such as K + , Ca 2+ ( Soil Survey Staff, 2014 ; Akladious and Mohamed, 2018 ). The water samples also revealed high levels of salinity, with most samples exceeding the recommended levels for human consumption. Not only did most of the water samples have a high level of salinity (TDS) they also had a high level of Na, this can lead to surrounding soils becoming not only saline but sodic as well ( Ghosh et al., 1983 ; Tavakkoli et al., 2011 ). Sodic soils are less permeable and thus encourage waterlogging and flooding in the regions they occur. This is not a problem in semi-arid or arid regions which are often very saline. This is because in arid regions soil particles are generally large and thus do not allow flooding ( Sparks 2003 ). The Bangladesh coastal regions are similar to the neighbouring Brahmaputa floodplain which consists mainly of smaller silt and clay particles. These soils are prone to flooding and an increase in solicit will worsen the problem. These Na levels also make water unsuitable for agricultural production and can also cause structural damage to people’s homes ( Tavakkoli et al., 2011 ; Shaibur et al., 2017 ). Not only is the water and soil unfavourable to crop-based agriculture, but it is also having a major effect on freshwater shrimp farmers as high salinity causes the death of shrimp ( Ahmed and Diana 2015 ). This has prompted discussions of relocation of prawn farms to more inland regions that are less affected by extreme weather events and sea-level rises (ibid).

High concentrations of sodium in water reduces its quality, causing high blood pressure in humans and limiting the absorption of nutrients in plants ( Kumar and Puri, 2012 ). Very high concentrations of chloride accelerate the corrosion rate of metal pipes, depending on the alkalinity of the water. This could lead to an increase in the concentration of metal in the water supply, posing further risks to human health ( Radelyuk et al., 2021 ).

The general trend shows an increase in potassium in the soil with increasing water salinity ( Table 9 ). This is in accordance with the findings by Alam et al. (2017) where salinity will increase the presence of certain ions within the soil, some of these beyond the scope of the study. This suggests that if salinity rises due to the rise in water surges, potassium levels will also rise, adding to the damage already done in agricultural areas ( Rasel et al., 2013 ). Water and soil salinity also have a negative correlation, indicating that salt stored and concentrated in dry soils due to evaporation may dissolve with future monsoons ( Sharma et al., 2013 ).

TABLE 9 . Pearson correlation coefficient analysis between soil and water parameters.

Eight water samples, all from pond water, as well as 2 soil samples, had pH values above 8.5. This water pH is unfit for human consumption and may have adverse effects on human health ( Kumar and Puri 2012 ). It creates a corrosive environment for metal pipes, which increases the concentration of dissolved metals in drinking water. According to Akter et al. (2016) , the mean pH in local drinking water in Bangladesh is slightly alkaline (pH of 7.4). Although our study reveals much more severe results, alkalinity did surpass the recommended level for human consumption.

Low levels of phosphorous in soil could be attributed to the precipitation of phosphorus in solution under alkaline conditions as well as the inability of organic matter to fix phosphates, in which the soils have a high organic matter content ( Weil and Brady, 2017 ; Alam et al., 2017 ). This means that if there are more occurrences of saltwater intrusion into cultivated land, alkalinity is expected to rise, which will further decrease the availability of P for plant uptake. The data from Table 3 shows that agricultural land is disproportionately depleted of P compared to other land uses, as this element acts as a plant fertilizer. On the other hand, shrimp ghers had the highest amount of phosphorus, which can be explained by the phosphorous leftover from fertilizers and food for shrimps ( Tan 2005 ).

Low water levels of flooding can reach far into Bangladeshi territory according as seen in Figure 8 . The hydraulic dynamics of the river system has a strong influence on surface and groundwater, exchanging the water mass between reservoirs ( Alam et al., 2017 ). Based on the results in Table 5 , saline water intrusions are canalized through rivers and channel systems. This suggests that salinity will be much more widespread than previously thought. There has been a 27% increase in salt-affected coastal areas and beaches between 1973 and 2009. This led to an increase in the amount of salt being added to the sea bed ( Jarvis et al., 2008 ; Alam et al., 2017 ). Rising sea levels, increasing the intensity of water surges their frequency will have a significant impact on places further inland ( Rahman et al., 2013 ; Dasgupta et al., 2014 ; Rasheed et al., 2014 ). In the next century, this rise in salinity, alkalinity, and related amounts of other geochemical chemicals may be predicted to be ubiquitous. Also, agriculture and permanent settlements may become unsuitable in the Shyamnagar Upazila region ( IPCC 2014 ).

Through analysis of the primary data, this study revealed that most of the water samples and more than 50% of the soil samples have moderately alkaline conditions. Also, the concentration of calcium (Ca) and potassium (K) in the soil and sodium (Na) and chloride (Cl − ) in the water was very high compared to permit limits for human consumption and agricultural cultivation. This study further revealed the deficient status of phosphorus (P) and total nitrogen (TN) in the soil of the study area which is directly an impact of a high EC that reduces the decomposition of organic matter. Eventually, one large storm surge is predicted along the coast, covering 12% of the region every 5 years, and one third every 20 years. More than half of the Shyamnagar Upazila, on the other hand, is affected every 5 years and almost entirely every 20 years. In the Betna-Kholpetua river station, small surges of 1 m are indicated as cyclonic surges, resulting in increased EC and chloride. They greatly contribute to the salinization of soil and water of the Southwestern coast of Bangladesh, which explains the very serious levels of salinity observed in this report. For the Shyamnagar Upazila, the salinity of most water and soil samples was well above the permissible standards for human use and agriculture. Similarly, extremely alkaline environments would impact natural soil biogeochemical processes that already promote scattered phosphorus precipitation, corrode pipes, and pollute drinking water and decrease crop size and yield.

The salt content of the soil may also adversely affect the supply of other vital nutrients such as K, Ca, and P, and thus impede the optimum cultivation of crops ( Fageria et al., 2011 ). In the short term, these problems are likely to be exacerbated by recurrent and significant saltwater intrusion, leading to an increase in salinity and alkalinity. In the near future, these problems are likely to worsen as climate change exacerbates the increase in storms in Bangladesh and increase the salinity and pH levels. It was found that not only do current conditions show that a third of the study area can be affected by major storms, but that this region will expand in the future as the frequency and periodicity of these events increase. The Shyamnagar Upazila is so affected that not only farming, but permanent human settlements could be inappropriate. This suggests a terrible future for Bangladesh, as increased salinity and alkalinity of water intakes and in soil for farming would directly impact millions of people through flooding.

The vision of sustainable climate and livelihood in Bangladesh can be achieved with saltwater supply through structural management such as coastal sluice schemes, barriers, sluices, and coastal areas, as well as non-structural management to change land use and other practices. Innovation and cultivation of different types of salt-tolerant crop varieties can also reduce food scarcity in the areas affected by salinity. Furthermore, a more detailed analysis is ultimately necessary to formulate the ambitious management model through various optional scenarios.

Data Availability Statement

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/ Supplementary Material .

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication. Conceptualization, MA; methodology, MA; software, MA and AC; validation, MA, AC, FDS, and SL; formal analysis, MA, and investigation, MA; resources, MA; FDS and SL; data curation, MA; writing—original draft preparation, MA writing—review and editing, MA, FDS, AC, and SL; visualization, MA and AC; supervision, FDS; AC and SL. All authors read and approved the final version of the manuscript.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This research is part of MA's PhD study, supported by authors listed in this paper.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontierspartnerships.org/articles/10.3389/sjss.2022.10017/full#supplementary-material

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Keywords: sea level rise, storm surges, Geochemical of salinity, Soil and water salinity, SWCRB

Citation: Ashrafuzzaman M, Artemi C, Santos FD and Schmidt L (2022) Current and Future Salinity Intrusion in the South-Western Coastal Region of Bangladesh. Span. J. Soil Sci. 12:10017. doi: 10.3389/sjss.2022.10017

Received: 07 August 2021; Accepted: 02 February 2022; Published: 21 March 2022.

Copyright © 2022 Ashrafuzzaman, Artemi, Santos and Schmidt. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Md Ashrafuzzaman, [email protected] , [email protected]

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"Maybe it's a friendly bear," he says.

Diana, another TikTok user , asked her sister-in-law what she would choose and was left speechless.

"I asked her the question, you know, just for giggles. She was like, 'You know, I would rather it be a bear because if the bear attacks me, and I make it out of the woods, everybody’s gonna believe me and have sympathy for me," she said. "But if a man attacks me and I make it out, I’m gonna spend my whole life trying to get people to believe me and have sympathy for me.'"

Bear vs. man debate stirs the pot, woman and some men at odds

The hypothetical has caused some tension, with some women arguing that men will never truly understand what it's like to be a woman or the inherent dangers at play.

Social media users answered this question for themselves, producing memes, spoken word poetry and skits in the days and weeks since.

So, what would you choose?

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How to Manage a Cross-Functional Team

• David Burkus

Start by getting clear on how you’ll communicate.

Cross-functional teams — those with people from different departments who have varied expertise — are becoming more common, as is the rise of project-based work arrangements.  Early in your career, you may even find that your first “real” leadership role is managing a newly formed, cross-functional team for a specific and short-term project. There are a few key actions new leaders can take to get their team off to a great start.

• Establish goals and roles. Each member has different knowledge, skills, abilities, and past experiences. As the team leader, it’s important to establish the scope of the project quickly, deliberate on the tasks required, and facilitate a discussion around who on the team would be best suited for different tasks. Resist the urge to assign tasks to whomever volunteers first, instead think who fits the task best.
• Set communication norms. Just as your team members will bring different knowledge, skills, and abilities, they’ll also bring their own unique work and communication preferences. Take some time during the initial project kickoff to discuss those work and communication preferences. Failure to establish collaboration norms tells teammates they can work and communicate however they want, making it more likely for messages to get missed.
• Build safety and candor. To generate ideas and solve problems, we need to lower inhibitions and create a culture where each member can be more honest and less guarded. To encourage this openness, new leaders often call for candor in meetings, but doing so before building psychological safety is a mistake. As a leader, it’s your responsibility to establish that level of safety. You can do this by signaling your own openness to feedback or expressing doubt in your own ideas.
• Celebrate milestones and small wins. Milestones don’t just track progress; they serve as an early warning signal that a pivot might need to be made. These milestones will also give the team early wins to celebrate and help them feel a sense of accomplishment. Celebrating small wins can boost morale and foster a sense of collective appreciation

A couple years into your career, you’ll find a few different opportunities to step up and practice your leadership skills. Sometimes it will be preparing and running a team meeting. Other times it may be volunteering for a new task the team requires.

• David Burkus is an organizational psychologist and best-selling author of five books, including Best Team Ever . For more information, visit his website .

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Journal of Materials Chemistry A

Bottom-up deposition lithium into 3d lithiophobic-lithiophilic host for long-life lithium metal anodes.

Rational structure design of a 3D host is one of the most promising strategies to achieve uniform Li deposition and inhibit uncontrolled growth of Li dendrites. Herein, a 3D-CTPA skeleton with lithiophobic-lithiophilic properties is used for 3D composite Li anode (3D-CTPA@Li) by electro-deposition. The resulting 3D-CTPA@Li symmetric cell shows a long lifespan of 2000 h at 1 mA cm−2 for 1 mAh cm−2. Additionally, the 3D-CTPA@Li/LiFePO4 full cell exhibits a long cyclic stability with 86.6% capacity retention after 500 cycles at 0.5C. The good electrochemical performance of 3D-CTPA@Li anode can be attributed to the unique features of 3D-CTPA skeleton, which results in dominant Li (110) facets for enhanced electrochemical kinetics. This is supported by XPS results, which shows an inorganic-rich SEI layer is formed on the surface of 3D-CTPA@Li anode. Due to the robust SEI layer with heterogenous electronic insulated/ionic conductive (Li3N/LiF) properties, which can effectively inhibit the reduction of Li+ and electrolytes at the top of 3D-CTPA@Li anode, as well as, serve as “ionic sieves” to realize uniform and fast Li+ transport through the interface. As a result, metallic Li preferentially nucleates/grows inside the 3D-CTPA pores spaces with a bottom-up Li deposition.

• This article is part of the themed collection: Journal of Materials Chemistry A HOT Papers

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• Supplementary information PDF (1789K)

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B. Huang, Z. Cai, T. Peng, Y. Tan, N. Zhang, W. Liu, H. Zhong and Y. Mai, J. Mater. Chem. A , 2024, Accepted Manuscript , DOI: 10.1039/D4TA02002B

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