The research team set out to close a persistent gap in the literature. Most earlier investigations of groundwater quality have examined drinking suitability, irrigation suitability, or hydrochemical characteristics in isolation, rather than weaving them together into a single framework. For Alibag, a coastal taluka of roughly 510 square kilometers bounded by the Arabian Sea to the west, the Amba River to the north, and the Kundalika River to the south, no comprehensive study had simultaneously evaluated hydrochemical characteristics, spatial variability, and contamination sources for both domestic and agricultural uses. The new work addresses that gap by combining hydrochemical analysis, spatial mapping, and suitability evaluation across the entire tehsil.

Methodologically, the study rests on secondary data drawn from India’s official monitoring apparatus: the Central Ground Water Board and the Groundwater Surveys and Development Agency. Records from 17 groundwater monitoring wells, collected during the pre-monsoon seasons of 2014, 2018, and 2022, formed the backbone of the analysis. Thirteen wells had data for all three years, while four additional wells at Alibag, Borli, Kihim, and Navedar Kolgaon entered the dataset only in 2022. The parameters examined spanned the classic hydrogeochemical toolkit: pH, electrical conductivity, total dissolved solids, total hardness, the major cations calcium, magnesium, sodium, and potassium, and the major anions bicarbonate, chloride, sulfate, nitrate, and fluoride.

To translate these raw measurements into a verdict on drinkability, the researchers employed the weighted arithmetic Water Quality Index, a method originally proposed by Horton in 1965 and later modified by Brown and colleagues. The approach assigns each parameter a unit weight inversely proportional to its recommended drinking water standard, meaning that constituents with lower permissible limits and higher health significance, such as fluoride and nitrate, exert greater influence on the final score. Quality ratings for each parameter are calculated relative to standards set by the Bureau of Indian Standards and the World Health Organization, then summed into a single index. Scores below 25 denote excellent water, 25 to 50 good, 50 to 75 poor, 75 to 100 very poor, and anything above 100 is deemed unsuitable for drinking.

The results were reassuring on that front. Every sample from 2014 and 2018 fell into the excellent category, and by 2022 approximately 53 percent remained excellent while the remaining 47 percent had slipped into the good class. Not a single sample crossed into the poor, very poor, or unsuitable ranges across the entire study period. That slight decline in 2022, the authors note, tracks closely with localized increases in electrical conductivity, dissolved solids, hardness, and major ions at several coastal locations, a signature consistent with creeping salinity rather than industrial or sewage contamination.

The chemical detail behind the index reveals the aquifer’s trajectory. pH values ranged from 6.26 to 8.20, with averages between 7.51 and 7.90, indicating water that stayed neutral to slightly alkaline throughout. Electrical conductivity, a proxy for dissolved ionic load, climbed from a 2014 range of 112 to 1516 microsiemens per centimeter to 206 to 1878 by 2022, with the highest values concentrated in northern and coastal zones. Total dissolved solids followed the same path, rising from 71 to 955 milligrams per liter to 132 to 1202, with elevated readings around Poynad, Kude, Nagaon, Vadhav BK, and Navedar Kolgaon. Total hardness, spanning 55 to 620 milligrams per liter, exceeded desirable limits at several sites including Bhilji, Poynad, Kude, Nagaon, Valvade Shahabaj, and Vadhav BK, marking water that ranges from hard to very hard.

The cation chemistry reinforces the picture of active rock-water interaction. Calcium concentrations rose from 10 to 104 milligrams per liter in 2014 to 22 to 200 in 2022, exceeding desirable limits at Nagaon, Vadhav BK, and Alibag in the final year. Magnesium peaked early, with values up to 87.5 milligrams per liter in 2014 before declining, though localized enrichment persisted in northern and western areas. Sodium climbed steadily from less than 20 milligrams per liter to as much as 160 by 2022, concentrated in coastal zones, while potassium, negligible in the early years, jumped to 33 milligrams per liter at a few northwestern sites in 2022, a pattern the authors attribute to marine influence together with agricultural and anthropogenic inputs. Among the anions, bicarbonate rose sharply from 18 to 189 milligrams per liter in 2014 to 110 to 476 in 2022, signaling enhanced carbonate weathering, while chloride reached 347 milligrams per liter at Dapoli in 2022. Sulfate, nitrate, and fluoride, by contrast, remained safely below permissible limits throughout, ruling out widespread contamination from fertilizers, wastewater, or fluoride-bearing minerals.

Two graphical techniques pinned down the processes driving this chemistry. Piper trilinear diagrams showed that in 2014 roughly 62 percent of samples belonged to the calcium-magnesium-bicarbonate facies, indicative of temporary hardness, with the rest classified as calcium-magnesium-chloride-sulfate type. By 2018 every sample had converged on the calcium-magnesium-bicarbonate type, and by 2022 about 29 percent had shifted toward a mixed facies, a drift the researchers link to localized seawater mixing and ion exchange. Gibbs diagrams, which plot ionic ratios against total dissolved solids, placed nearly all samples squarely in the rock-dominance field, confirming that mineral weathering and rock-water interaction, rather than atmospheric precipitation or evaporation, are the principal engines of groundwater evolution in Alibag.

For agriculture, the team deployed five complementary indices: the Sodium Adsorption Ratio, Residual Sodium Carbonate, Percentage Sodium, Magnesium Hazard, and Permeability Index. The news was largely favorable. SAR values ranged from just 0.034 to 2.847, far below the excellent threshold of 10, indicating negligible risk of soil sodicity. Residual Sodium Carbonate placed nearly all samples in the safe category, with only one sample each from Poynad in 2014 and Kihim in 2022 falling into the marginal range. Percentage sodium stayed below 50 for every sample, classifying the water as good for irrigation. The exceptions emerged with Magnesium Hazard, which exceeded recommended limits in about 61 percent of samples in 2014 before improving to 85 percent suitability in 2018 and 88 percent in 2022, and the Permeability Index, which placed most samples in the moderately suitable Class II, with roughly 23 percent unsuitable in 2014 improving to just a handful of restricted sites by 2022. The authors conclude that magnesium enrichment and soil permeability, not sodium, are the principal constraints on irrigation in the tehsil, and that a few areas still require treatment before agricultural use.

The broader message is one of cautious vigilance. Groundwater in Alibag is governed predominantly by natural geogenic processes, with localized seawater intrusion and human activity contributing to a gradual, measurable mineralization trend along the coast. The study’s authors acknowledge limitations, including the absence of seasonal sampling within years, a monitoring network that may not capture local-scale heterogeneity, and the exclusion of trace elements, heavy metals, pesticides, and microbiological contaminants. Yet the direction of travel is clear: salinity-related parameters are rising in coastal zones, and the researchers argue that periodic monitoring, regulation of groundwater abstraction, and sustainable management practices are essential to prevent future deterioration. For a region where 215 villages dot the landscape and roughly 25 sit directly on the sea’s edge, the aquifer’s slow chemical drift is a warning worth heeding while the water is still, by every index applied, good to drink.

Subject of Research: Groundwater quality assessment for drinking and irrigation suitability in a coastal aquifer in Alibag Tehsil, Maharashtra, India

Article Title: Assessment of groundwater quality for drinking and irrigation purpose of Alibag Tehsil, Raigad district, Maharashtra

Article References: Bar, R., & Dhorde, A. A. (2026). Assessment of groundwater quality for drinking and irrigation purpose of Alibag Tehsil, Raigad district, Maharashtra. Discover Geoscience, 4(1), Article 299. https://doi.org/10.1007/s44288-026-00671-7

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00671-7

Keywords: groundwater quality, hydrogeochemistry, water quality index, coastal aquifer, seawater intrusion, irrigation suitability, drinking water, Maharashtra, Alibag, SAR, Piper diagram, Gibbs diagram

The work addresses a technology that has quietly become one of the most promising defenses against seawater intrusion: the underground physical barrier, sometimes called a subsurface dam or cutoff wall. These are impermeable structures built into the aquifer itself, typically from the ground surface down to the impermeable basement, designed to block or slow the landward migration of seawater while allowing freshwater to pool behind them on the inland side. Japan has built such structures for decades in the Ryukyu Arc, and interest has surged worldwide as rising seas and growing coastal populations intensify pressure on shallow groundwater. Yet, as the authors note, most previous analyses have treated barriers in two dimensions only, examining cross-sections perpendicular to the coast and ignoring what happens along the barrier’s length.

That simplification matters because real barriers are finite. They have a measurable length running parallel to the shoreline, and water can flow around their ends. A barrier that looks impregnable in a two-dimensional cross-section may leak saltwater around its flanks, and a well placed near its edge may behave very differently from one placed at its center. The new study tackles this three-dimensional reality head-on. Dewandel and colleagues derived steady-state semi-analytical solutions for the shape of the sharp freshwater–seawater interface in a sloping coastal aquifer containing a finite-sized, impermeable rectangular barrier that fully penetrates the aquifer, with or without a pumping well operating nearby.

The technical machinery behind the solutions combines two classical ideas with one modern numerical trick. First, the authors adopt the Dupuit–Forchheimer assumption, which treats groundwater flow as essentially horizontal in aquifers whose lateral extent greatly exceeds their thickness, and the sharp-interface approximation, which treats fresh and saline water as immiscible fluids separated by a discrete boundary rather than a gradual mixing zone. These assumptions, long the backbone of coastal aquifer analysis dating back to the Ghyben–Herzberg principle and Strack’s single-potential formulation, reduce a fiendishly complex variable-density flow problem to a tractable one. Second, to handle the flow disturbance created by the rectangular barrier, the team employed the Method of Fundamental Solutions, a boundary-type numerical technique in which the solution is built from a superposition of analytical point sources, or image wells, whose strengths are adjusted to satisfy the boundary conditions on the barrier’s faces.

A crucial innovation is the inclusion of an inland boundary flow condition in all of the solutions. Many earlier analytical treatments assumed either constant hydraulic head or an infinite aquifer inland, which obscures the role of the natural hydraulic gradient that drives freshwater toward the sea. By specifying the inland flux, the new solutions explicitly account for both the regional hydraulic gradient and the slope of the aquifer’s impermeable basement. This makes them applicable to realistic sloping coastal settings rather than idealized flat ones, and it allows the researchers to capture how the ambient groundwater flow interacts with the barrier and the pumping stress. Separate solutions are provided for unconfined aquifers, where the water table is free to rise and fall, and for confined aquifers, where the saturated thickness is fixed by overlying impermeable layers.

Because the saturated thickness of an unconfined aquifer changes wherever the water table is drawn down or mounded, the underlying flow equations are nonlinear. The authors discuss two linearization strategies for handling this. The first, an h-linearization, assumes the deviation of the saturated thickness from its undisturbed value is small, which is reasonable when the barrier and well sit near the coastline, since the sea acts as a constant-head boundary that limits water-table fluctuations. The second, an h²-linearization, relaxes that restriction and remains valid as long as the thickness change does not exceed half the undisturbed thickness. In the more general formulation, a linearization constant is refined through successive iterations until the strength coefficients of the image wells converge, a process the authors note can take from an hour to several hours of computation depending on barrier size, though the simplified near-coast version runs almost instantly.

Validation came from comparison with spatially distributed numerical modeling, including simulations with the USGS MODFLOW 6 code and with published three-dimensional results from earlier barrier studies. Across test cases spanning different hydraulic conductivities, aquifer thicknesses, hydraulic gradients, barrier lengths and well positions, the semi-analytical interface predictions tracked the numerically simulated salinity distributions closely, both in cross-sections through the center of the barrier–well system and along its edges. The agreement held for confined conditions as well, and the authors reproduced the behavior seen in independent published simulations of finite-length barriers placed at varying distances from the shoreline, lending confidence that the new equations capture the essential physics despite their simplifying assumptions.

The practical payoff of the study is a set of design rules that emerge directly from the mathematics. The analysis shows that the maximum pumping rate a well can sustain without drawing seawater into its capture zone, the so-called maximum safe or critical pumping rate, increases both with the length of the barrier and with the barrier’s distance from the coast. In other words, a longer wall shields more of the aquifer’s freshwater lens, and a wall set farther inland leaves more room for a well to operate on the protected side before the interface toe reaches it. The solutions also quantify how much the barrier improves on the no-barrier baseline, expressing the gain in safe yield as a function of barrier dimensions, well placement and aquifer properties, information that is invaluable when weighing the considerable cost of constructing an underground cutoff wall against the water it secures.

Because the equations rest on the Dupuit–Forchheimer and sharp-interface assumptions, the authors are careful to position them as a first-order tool rather than a replacement for detailed simulation. They deliberately exclude dispersion-driven mixing at the interface, aquifer heterogeneity and transient effects, all of which a full three-dimensional variable-density model can represent. But that is precisely their strength. Where a numerical model may take hours to set up and run, the semi-analytical solutions evaluate in seconds, making them ideal for the preliminary design stage: screening dozens of candidate barrier lengths, positions and well configurations, and identifying the handful of options worth subjecting to rigorous numerical analysis. The authors also provide a corrected formulation incorporating an empirical density correction factor to account for the mixing that real interfaces exhibit, tightening the link between the idealized sharp interface and observed salinity profiles.

The timing of this work could hardly be better. The growing trend of saltwater intrusion, driven by over-pumping, reduced recharge and sea-level rise, threatens coastal agriculture and drinking water supplies on every inhabited continent, and the coastal groundwater squeeze between rising seas and intensifying demand is now recognized as a defining water-management challenge of the century. Tools that let engineers quickly and reliably size a subsurface barrier, place a well at a safe distance and estimate the sustainable yield of the protected freshwater body could turn a promising but underused technology into standard practice. With this study, the French team has extended the analytical toolkit for coastal aquifer management from the flat, two-dimensional world of textbook theory into the sloping, finite, three-dimensional world where real barriers and real wells actually operate.

Subject of Research: Semi-analytical modeling of the freshwater–seawater interface controlled by underground barriers and pumping wells in sloping coastal aquifers

Article Title: Steady-state semi-analytical solutions for assessing the shape of the freshwater–seawater interface induced by a finite-sized underground physical barrier and a pumping well in a sloping coastal aquifer

Article References: Dewandel, B., Gouy, A., & Frissant, N. (2026). Steady-state semi-analytical solutions for assessing the shape of the freshwater–seawater interface induced by a finite-sized underground physical barrier and a pumping well in a sloping coastal aquifer. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03177-1

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03177-1

Keywords: seawater intrusion, coastal aquifer, underground barrier, subsurface dam, freshwater–seawater interface, pumping well, semi-analytical solutions, Method of Fundamental Solutions, Dupuit–Forchheimer, hydrogeology, groundwater management, safe pumping rate

Cork City Island is a low-lying urban area of just 1.2 square kilometres, wedged between the North and South Channels of the River Lee in southwest Ireland. The city stands on land that was once a braided network of river channels and wetlands, drained and embanked during the 1700s and 1800s. Beneath the surface lies the Lee Buried Valley aquifer, a deposit of glacial gravels 20 to 50 metres thick with hydraulic conductivities between 4.4 and 4.8 thousandths of a metre per second. Because these gravels transmit water so readily, groundwater beneath the city responds almost immediately to both tidal surges and heavy fluvial discharge, making Cork an ideal natural laboratory for studying how coastal aquifers behave under tidal forcing.

The research team installed automated water-level loggers in monitoring wells across the island, recording groundwater levels at 15-minute intervals. They then applied spectral analysis, using the Fast Fourier Transform to isolate the dominant tidal constituents, and cross-correlation analysis to measure how long each pressure wave takes to travel from the river into the aquifer. The dominant signal proved to be the principal lunar semidiurnal tide, the M2 constituent, with a period of roughly 12.4 hours. Tidal efficiency, the ratio of groundwater fluctuation amplitude to the tidal amplitude in the river, ranged dramatically across the island, from a near-total 98.93 percent at well MW6 to a heavily damped 11.70 percent at well BH03, while time lags varied from just over two minutes to nearly 222 minutes.

These measurements revealed a fundamental asymmetry in how the aquifer responds. In the eastern and central sectors, groundwater is squeezed simultaneously by tidal waves from both the North and South Channels, an interference that reduces attenuation and allows elevated hydraulic heads to persist far into the aquifer interior. In the western sector, where tidal influence from the South Channel terminates at the Gillabbey weir, only the North Channel exerts tidal forcing, and the tidal signal decays steeply: from 78 percent efficiency just 22 metres from the river to under 12 percent at 300 metres. Excluding one anomalous well, the propagation delay increased linearly with distance at roughly 0.46 minutes per metre, a remarkably uniform rate that speaks to the aquifer’s overall connectivity.

To convert these observations into aquifer properties, the team applied the classical Jacob–Ferris analytical framework, which describes how a tidal pressure wave diffuses inland through a confined aquifer, losing amplitude exponentially and accumulating phase lag. Under ideal conditions, diffusivity estimated from amplitude attenuation should match diffusivity estimated from time lag. In practice, near-river wells showed large discrepancies, quantified by a slope factor that deviated sharply from unity. The culprit, the researchers argue, is riverbed clogging: a low-permeability layer at the river–aquifer interface that absorbs amplitude and delays the signal before it even enters the aquifer. By analysing well-to-well pairs instead, the team isolated the true matrix diffusivity, obtaining values of about 9.25 square metres per second in the eastern domain and 7.15 square metres per second in the west, with a lower local bound near 0.5 square metres per second.

These field-derived parameters were then fed directly into a three-dimensional transient groundwater model covering the entire island, with a computational mesh of more than 18,000 elements per layer representing the made ground, the estuarine aquitard, and the gravel aquifer. Calibration and validation against ten monitoring wells produced root mean square errors mostly below 0.15 metres, and critically, the model maintained its accuracy when applied to independent validation periods. By constraining the model’s parameter space with the analytical diffusivity range of 0.5 to 9 square metres per second, the researchers reduced the problem of parameter non-uniqueness, in which many combinations of conductivity and storage produce identical tidal responses.

The predictive scenarios reveal a strikingly nonlinear threat. Groundwater heads first exceed ground level at an estuarine stage of about 1.6 metres above Ordnance Datum, but up to 2.6 metres the flooded area remains below 7 percent of the island, confined to riverbanks and low-lying depressions. Above roughly 2.8 metres, however, isolated flooded patches begin to merge into continuous inundated zones. Between 3.4 and 3.8 metres, the flooded fraction leaps from about 25 to 28 percent up to 55 to 60 percent, with the steepest expansion occurring in that final interval. Under the extreme 4.0-metre scenario, between 66.6 and 70.6 percent of the study area could experience groundwater heads above the surface.

That 2.8-metre threshold is not a distant hypothetical. Current mean tidal levels in Cork already sit near 2.0 metres, observed maxima reach 2.6 metres, and compound events combining high tides with heavy rainfall can push stages to 3.0 to 4.0 metres. Mean sea level in Cork Harbour has already risen 41 centimetres since 1842, progressing at 2.2 millimetres per year, and global projections suggest 0.3 to 1.1 metres of additional rise by 2100. The study also found that predictive uncertainty grows with the hazard: the gap between the most and least vulnerable parameter scenarios peaks at 5.21 percent of the island’s area at the 3.8-metre stage, meaning that the very conditions that produce the worst flooding are also those where subsurface variability matters most.

The research also exposed weaknesses in previous risk zoning. Historical classifications based on discrete monitoring points had designated much of the island as having low tidal influence, yet the new continuous mapping shows that tidal efficiencies of 80 to 90 percent extend well beyond the zones where they were expected, with some wells in nominally low-influence areas recording efficiencies above 95 percent. The authors suggest that buried remnants of Cork’s historic braided river network, including culverted waterways beneath the modern street grid, may act as preferential flow paths, hydraulic superhighways that carry tidal signals deep into areas that appear disconnected from the river. Leakage through the confining aquitard and thin peat interbeds within the gravels add further complexity that point measurements alone cannot capture.

The implications stretch far beyond Cork. More than two billion people live in coastal areas, many on low-lying ground above permeable aquifers, and groundwater flooding can strike before or without any marine inundation, saturating basements, undermining foundations, and overwhelming stormwater and sewer networks from below. The integrated framework developed here, in which field-measured tidal responses constrain a numerical model whose outputs feed spatial vulnerability mapping, offers a transferable template for other estuarine cities. The researchers also validated a rapid-assessment equation that predicts groundwater levels directly from river stage, tidal efficiency, and time lag, achieving near-perfect correlation in the eastern wells, and they propose machine learning as a next step for the more complex western domain. As seas continue to rise, the study’s central message is sobering: the flood of the future may not arrive over the seawall, but quietly from the ground beneath our feet.

Subject of Research: Tidal groundwater propagation and sea-level-rise-induced groundwater flooding in a coastal urban aquifer

Article Title: Integrated assessment of tidal groundwater propagation and sea-level rise-induced groundwater flooding in a coastal urban site

Article References: Soboh, M., & O’Shea, M. (2026). Integrated assessment of tidal groundwater propagation and sea-level rise-induced groundwater flooding in a coastal urban site. Results in Engineering, 32, Article 113263. https://doi.org/10.1016/j.rineng.2026.113263

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113263

Keywords: groundwater flooding, sea-level rise, coastal aquifer, tidal propagation, Cork City, hydraulic diffusivity, Jacob-Ferris, numerical modelling, urban hydrogeology, River Lee, climate risk, flood vulnerability

The study draws on an unusually rich dataset: seven years of continuous monitoring, from 2016 to 2023, covering groundwater levels, water temperature, hydrochemistry, and land surface deformation across Tongzhou’s unconsolidated sedimentary aquifer system. Such systems, built up from layers of sand, silt, and compressible clay deposited over millennia, are typical of coastal plains worldwide and are notoriously vulnerable to over-abstraction. When water is pumped from confined aquifers faster than it can be replenished, pore pressure in the aquifer skeleton drops, the soft clay layers above and between the sandy units compact, and the ground surface sinks. Reversing that process is far harder than causing it, which is why the Tongzhou findings have attracted attention among hydrogeologists.

To untangle the causes behind the observed changes, the team deployed two sophisticated statistical tools. The first was generalized additive modeling, a flexible regression framework that can capture nonlinear relationships between groundwater variables and their potential drivers without imposing a rigid mathematical form. The second was wavelet transform coherence analysis, a signal-processing technique that examines how two time series co-vary across different frequencies and over time, revealing not only whether variables are linked but whether one responds to the other with a delay. This lag-detection capability proved crucial, because groundwater systems rarely react instantaneously to changes in pumping or recharge; pressure signals propagate slowly through low-permeability clay layers, and chemical adjustments can take even longer.

The headline result is unambiguous: since abstraction limits were implemented, groundwater levels in both confined aquifers have risen significantly. That recovery set off a cascade of secondary effects. Chloride concentrations, a key indicator of salinity, have declined across the monitored system. Land surface dynamics have flipped from sinking to rising, with the rebound gradually tending toward stabilization over time. Groundwater temperature, by contrast, remained relatively stable throughout the monitoring period, a detail that turned out to be scientifically telling rather than uninteresting.

By applying their statistical toolkit to all of these variables simultaneously, the researchers identified groundwater levels as the dominant controlling factor of the entire groundwater environment. Changes in hydrochemistry and in land surface deformation were both lagged responses to level variations, meaning the chemistry and the ground surface did not simply track pumping in real time but adjusted after a delay as pressure changes propagated through the aquifer-aquitard sequence. This finding carries practical weight for water managers: it implies that the benefits of conservation measures unfold over years, and that monitoring programs must be designed to capture these delayed responses rather than judging policy success on immediate outcomes alone.

The temperature story is particularly striking. While groundwater temperatures in many urbanizing regions are warming due to the urban heat island effect, geothermal anomalies, and heat pump discharge, Tongzhou’s confined groundwater temperatures were found to be governed primarily by the geothermal gradient, the natural increase in temperature with depth, with only limited influence from human disturbances. In other words, the deep aquifer system has remained thermally insulated from the surface changes happening above it. That stability provides a useful baseline: any future thermal anomaly in these aquifers would stand out clearly against a naturally controlled background, making temperature a sensitive early-warning indicator for emerging human impacts.

Salinity dynamics revealed a more complex picture. Confined Aquifer I, the shallower of the two confined units, still carries high chloride concentrations as a legacy of historical seawater intrusion, a reminder that coastal aquifers can store the chemical fingerprints of past marine transgressions long after the inciting event. Yet even in this compromised aquifer, recovery is underway. As groundwater levels rose, chloride concentrations declined, and the team determined that during the recovery process the chloride behavior was controlled by water-rock interactions, the slow chemical exchanges between groundwater and the mineral grains of the aquifer matrix. This suggests that freshening is not merely a matter of flushing salty water out but involves ongoing geochemical equilibration as the flow regime reverses, a process that models of coastal aquifer restoration need to account for.

Perhaps the most visually dramatic finding concerns the land itself. The study attributes the observed changes in surface elevation mainly to the rebound of overlying compressible clay layers as groundwater levels in the deep aquifers recovered. The physics follows classical consolidation theory, first formulated by Karl Terzaghi a century ago: when pore water pressure in a clay layer falls, effective stress on the mineral skeleton rises and the layer compresses; when pore pressure recovers, some of that compression is elastic and can be recovered, allowing the surface to rise. The Tongzhou data show this rebound in action, with the land tending toward stability as the system equilibrates. For low-lying coastal cities already contending with sea-level rise, every centimeter of prevented or reversed subsidence matters, and the study demonstrates that subsidence can be at least partially reversible when abstraction is curtailed early enough.

The broader significance of the work extends well beyond one district on the Jiangsu coast. Coastal plains across Asia, Africa, and the Americas face the same triple threat of over-pumping, saltwater intrusion, and land subsidence, often compounded by climate change and urban growth. Many studies have documented the damage; far fewer have tracked, with high-resolution monitoring and rigorous causal analysis, what recovery actually looks like. The Tongzhou case shows that a managed aquifer system can respond coherently to policy intervention: levels rise, chemistry freshens with a lag, the ground rebounds, and the thermal regime stays steady. It also shows that the response is orchestrated by a single master variable, groundwater level, which therefore serves as the most effective lever and the most informative monitoring target.

For groundwater managers, the practical lessons are concrete. First, abstraction restrictions work, but their full benefits materialize over multi-year timescales as lagged responses play out through the aquifer system. Second, chemical recovery from historical salinization is governed by water-rock interactions and will be gradual, so expectations should be calibrated accordingly. Third, land rebound tied to clay-layer recovery offers a quantifiable measure of policy success, one that can be tracked with satellite-based deformation measurements as well as ground instruments. Fourth, stable temperatures governed by the geothermal gradient provide a clean reference against which future anthropogenic thermal impacts, from heat pump fields to urban warming, can be detected. As the authors note, these findings improve understanding of groundwater evolution in unconsolidated sedimentary aquifer systems and provide a scientific basis for groundwater management in coastal regions. In an era when roughly half of humanity lives within reach of the sea, the quiet recovery unfolding beneath Tongzhou’s fields and streets is a story worth watching, and perhaps emulating.

Subject of Research: Spatiotemporal evolution of groundwater levels, hydrochemistry, temperature, and land subsidence in a coastal aquifer system under abstraction restrictions

Article Title: The spatiotemporal evolution and controlling factors of the groundwater environment: Insights from long-term monitoring of Tongzhou District, China

Article References: Dong, L., Wu, J., Zhang, J., Luo, Z., & Li, Z. (2026). The spatiotemporal evolution and controlling factors of the groundwater environment: Insights from long-term monitoring of Tongzhou District, China. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03167-3

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03167-3

Keywords: groundwater, hydrogeology, coastal aquifer, land subsidence, saltwater intrusion, chloride, water-rock interaction, groundwater management, China, wavelet coherence, generalized additive model, aquifer recovery

The team measured the full suite of physicochemical parameters, major ionic species, and selected trace metals using standard field and laboratory methods. What they found was a groundwater system under pressure from both natural geogenic processes and human activity. In the pre-monsoon period, salinity exceeded acceptable limits in 54.16 percent of samples, while sodium and chloride concentrations breached thresholds in 58.33 percent. Heavy metals told an even starker story: iron surpassed the standards set by Bangladesh’s Department of Environment in 75 percent of samples, manganese in 91.67 percent, potassium in 71.17 percent, and arsenic in 66.67 percent. These are not marginal exceedances confined to a few unlucky wells; they describe a broad regional pattern of contamination affecting the majority of the water sources tested.

After the monsoon, the pattern shifted but did not improve. Salinity exceedance dropped to 43.48 percent of samples, suggesting that monsoon recharge dilutes some of the salt burden, yet sodium concentrations exceeded limits in 65.22 percent of samples and chloride in 47.83 percent. More alarmingly, arsenic exceedance jumped to 95.65 percent of post-monsoon samples, while iron and manganese remained problematic in 86.96 percent and 73.91 percent respectively, and potassium in 60.87 percent. The monsoon, rather than flushing the aquifer clean, appears to mobilize or concentrate certain contaminants, particularly arsenic, in ways that make post-monsoon water in many wells less safe than the same wells produced months earlier.

To understand why, the researchers turned to the classical tools of hydrogeochemistry. Integrated analysis of the water’s ionic composition indicated that groundwater evolution in the district is strongly controlled by mineral dissolution and water-rock interaction, the slow chemical conversation between infiltrating rainwater and the sediments it percolates through. Ratios of sodium to chloride below one pointed to chloride enrichment driven by evaporation and localized saline-water influence, a signature consistent with coastal intrusion and concentration effects. Positive values of the chloro-alkaline indices revealed the occurrence of reverse ion exchange, a process in which calcium and magnesium in the water swap places with sodium held on clay mineral surfaces, further reshaping the water’s chemistry as it moves through the aquifer matrix.

The study’s methodological novelty lies in how it translates all this chemistry into a single, defensible measure of drinkability. Rather than relying on a conventional water quality index, which weights every parameter equally regardless of how much it actually varies, the team employed an entropy-based water quality index, or EWQI. Rooted in Shannon’s information theory, entropy weighting assigns greater influence to parameters that show the most variability and uncertainty across samples, letting the data itself decide which contaminants matter most. In the pre-monsoon season, 20 percent of samples rated as excellent and 35 percent as good, but a striking 30 percent fell into the extremely poor category. After the monsoon, excellent samples rose to 30 percent, yet extremely poor samples climbed to 43 percent, meaning that while some wells improved, the worst wells got worse, widening the gap between safe and unsafe water sources across the district.

Because water quality classification is inherently fuzzy, a sample is rarely purely good or purely bad, the researchers also deployed a fuzzy inference system coupled with three-dimensional response surface analysis. This modeling approach, borrowed from control engineering, allows partial memberships in multiple quality categories simultaneously and can probe how pairs of contaminants jointly influence the overall index. The analysis revealed that interactions between iron and potassium, and between manganese and arsenic, were strongly associated with variations in the EWQI. In other words, the degradation of water quality in Lakshmipur is not simply the sum of individual contaminants but emerges from combinations of metals acting together, a finding that has direct implications for which wells should be prioritized for treatment or abandonment.

Independent pollution indices corroborated the EWQI results. The heavy metal pollution index, the heavy metal evaluation index, the degree of contamination, and the nitrate pollution index all confirmed widespread heavy metal contamination across the sampled wells. The convergence of four separate indices, each constructed differently, on the same conclusion strengthens the case that metal contamination in the district’s aquifers is systemic rather than incidental. Given that arsenic enrichment in the alluvial aquifers of the Bengal delta has been documented for decades, the new study adds Lakshmipur to the map of districts where the problem persists and, in the post-monsoon window, intensifies.

The most consequential part of the analysis concerns human health. Using standard exposure models, the team calculated hazard quotients for non-carcinogenic effects of the detected contaminants and aggregated them into a hazard index. The results showed that children face greater non-carcinogenic risks than adults, with hazard index values exceeding the safe threshold of one. A hazard index above one indicates that the combined exposure to contaminants through drinking water surpasses the level considered acceptable over a lifetime, and the fact that children exceed this threshold more readily reflects their higher water intake relative to body weight and their developing physiology. In practical terms, the water from a substantial fraction of wells in the district poses measurable health risks to the most vulnerable members of the community.

What emerges from the study is a call to action grounded in seasonal reality. The authors emphasize the need for regular seasonal groundwater monitoring, safe-well mapping so that households can identify which wells are safe at which times of year, treatment of metal-contaminated wells, and sustainable management of the coastal aquifer as a whole. Because water quality in Lakshmipur is not static, a well that tests clean in March may deliver arsenic-laden water in November, monitoring programs that sample only once a year risk systematically missing the worst conditions. The integration of entropy weighting, fuzzy logic, pollution indices, hydrogeochemical interpretation, and health risk assessment in a single framework offers a template that could be applied to other coastal districts of Bangladesh and to deltaic aquifers worldwide facing similar combinations of salinity, geogenic metals, and human pressures.

The researchers also chart a path forward. Future work, they note, should include long-term seasonal observations, denser sampling networks, and formal uncertainty and sensitivity analyses to sharpen groundwater quality assessments under changing climatic and anthropogenic conditions. As sea levels rise and extraction intensifies across the Ganges-Brahmaputra-Meghna delta, the pressures on aquifers like Lakshmipur’s will only grow. This study demonstrates that with the right analytical toolkit, it is possible to see not just whether groundwater is contaminated, but when, where, and why, knowledge that coastal communities will need in increasing measure as the seasons continue to reshape the water beneath their feet.

Subject of Research: Seasonal hydrogeochemistry, water quality, and health risks of coastal groundwater in Lakshmipur District, Bangladesh

Article Title: Hydrogeochemical evaluation and entropy-driven water quality assessment of Lakshmipur District’s groundwater, Southeastern Bangladesh

Article References: Hasan, M. M., Haque, K. E., Ahmed, S., Hasan, M. N. I., & Hasan, M. M. (2026). Hydrogeochemical evaluation and entropy-driven water quality assessment of Lakshmipur District’s groundwater, Southeastern Bangladesh. Environmental Geochemistry and Health, 48(15), Article 616. https://doi.org/10.1007/s10653-026-03501-4

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03501-4

Keywords: groundwater, Bangladesh, coastal aquifer, arsenic, heavy metals, salinity, entropy water quality index, fuzzy inference, health risk assessment, hydrogeochemistry, water pollution, seasonal monitoring

The research team, led by Yaneth A. Bustos-Terrones of the Instituto Tecnológico de Culiacán, analyzed a decade of groundwater quality data, from 2012 to 2022, collected at seven monitoring wells operated through Mexico’s National Water Quality Monitoring Network, RENAMECA, run by the National Water Commission. The wells were deliberately placed in the most environmentally relevant sectors of the aquifer: downstream of a dam surrounded by farmland, near villages and ranches, in zones of intensive cultivation, and in the center of the city of Angostura. Because the aquifer is coastal and its water table sits just 2 to 12 meters below the surface, the shallow groundwater it holds is exceptionally exposed to whatever happens on the land above it.

The headline numbers are striking. Average total dissolved solids in the aquifer measured 1,008 milligrams per liter, and electrical conductivity averaged 1,606 microsiemens per centimeter, both exceeding World Health Organization guideline values for drinking water. Total hardness averaged 488 milligrams per liter, a level dominated by calcium and magnesium salts, and in some individual samples dissolved solids climbed as high as 3,000 milligrams per liter, with hardness approaching 1,500. Sodium, at 181 milligrams per liter, hovered uncomfortably close to its 200 milligram guideline limit, a concern both for drinking water and for the long-term health of irrigated soils. By contrast, nutrients told a more moderate story: nitrate averaged 9.63 milligrams per liter, below the permissible limit, though individual outliers reached 30 milligrams per liter in areas of concentrated fertilizer use.

What makes the study scientifically compelling is its dissection of where all that salt comes from. Using ionic ratio diagrams, Piper plots, and a battery of multivariate statistical tools, the researchers showed that the aquifer’s fundamental chemistry is set by natural geogenic processes: the dissolution of silicate minerals and carbonate rocks, together with ion exchange as water moves through the alluvial sands, gravels, silts, and clays that fill the basin to depths of over 200 meters. Samples plotted squarely in the silicate weathering field on magnesium-to-sodium versus calcium-to-sodium diagrams, while bicarbonate ratios pointed to a strong carbonate contribution from dissolving calcite and dolomite. In other words, the water was always destined to be mineral-rich. But agriculture, the study concludes, is amplifying these natural processes dramatically.

The mechanism is a feedback loop familiar to hydrogeologists but rarely documented with such temporal depth. Intensive irrigation pushes water through the soil profile, leaching dissolved salts and fertilizer-derived ions downward into the shallow aquifer. During the dry season, evaporation concentrates what remains, while return flows from over-irrigated fields redistribute ions across the landscape. The result is a salinity pattern that does not simply track drought, as it would in an untouched system, but responds in complex, non-linear ways to the interplay of recharge, evapoconcentration, and human water use. During the drought years of 2021 and 2022, the researchers recorded dissolved solids between 408 and 456 milligrams per liter and conductivity between 1,413 and 1,514 microsiemens per centimeter, evidence of seasonal mineralization even as external inputs declined.

Seasonality adds another layer of drama. During the rainy months from June to October, nutrients and fecal coliforms spike as runoff carries fertilizers, organic matter, and wastewater into the shallow groundwater. In 2014, a wet year, nitrate peaked at 14.26 milligrams per liter and total phosphorus at 0.285 milligrams per liter, a signature of agricultural leaching in full swing. During the dry season, microbial contamination recedes but salts concentrate. The aquifer, in effect, breathes with the climate, inhaling contaminants with each wet season and exhaling mineralized water through every drought.

The spatial contrasts are equally revealing. The well near the Eustaquio Buelna reservoir, in the heart of farm country, recorded the worst chemistry of all: total hardness of 1,090 milligrams per liter and electrical conductivity of 3,625 microsiemens per centimeter, alongside the highest organic carbon and nutrient loads. A well in an intensive farming zone showed fecal coliform counts of 211 per 100 milliliters, while the most alarming microbial reading came from a site surrounded by ranches and cropland, which in 2020 registered a staggering 24,196 coliforms per 100 milliliters, an episodic contamination event the authors attribute to livestock or domestic wastewater. Urban wells, by contrast, showed lower salinity but persistent, if localized, microbiological contamination, a fingerprint of untreated domestic discharges.

For irrigation, the picture is mixed but increasingly precarious. Applying the classification scheme of the United States Salinity Laboratory, the team found that 67 percent of the water remains suitable for irrigation, with moderate salinity and low sodium adsorption risk. But roughly 17 percent of samples fell into the poor category, restricting use for salt-sensitive crops, and 16 percent, all from the well near the reservoir, rated very poor, with salinity and sodium hazards high enough to degrade soil structure and suppress crop yields over time. The Water Quality Index, which synthesizes ten parameters weighted by their health significance, confirmed that most of the aquifer still rates good to moderate, but with a deteriorating core where dissolved solids climb and quality collapses.

The broader lesson extends far beyond Sinaloa. Mexico officially recognizes 653 aquifers, and a considerable number suffer overexploitation or quality decline; the Mocorito River Aquifer, where extraction already exceeds recharge, is a case study in what happens when agricultural intensification, drought, and shallow coastal geology converge. The authors argue that protecting such systems demands sustained long-term monitoring, optimized fertilizer application, sustainable irrigation practices, and protection of recharge zones, supplemented by isotopic tracing techniques such as nitrogen-15 and oxygen-18 to distinguish fertilizer-derived nitrate from wastewater sources. As climate change tightens its grip on northwestern Mexico, bringing longer dry seasons and less reliable recharge, the study’s central warning resonates: groundwater quality is not just a function of what we dump into aquifers, but of how hard we make them work, and the salt accumulating beneath Mexico’s breadbasket is the receipt for a decade of that pressure.

Subject of Research: Hydrogeochemical processes controlling groundwater quality in the Mocorito River Aquifer, a tropical agricultural region of northwestern Mexico

Article Title: Analysis of hydrogeochemical processes regulating groundwater quality in a tropical agricultural landscape of northwestern Mexico

Article References: Bustos-Terrones, Y. A., Mendoza-Aguilar, O. A., Loaiza, J. G., Rangel-Peraza, J. G., Ramirez-Pereda, B., Kurniawan, T. A., Estrada-Manjarrez, J., & Rojas-Valencia, M. N. (2026). Analysis of hydrogeochemical processes regulating groundwater quality in a tropical agricultural landscape of northwestern Mexico. Environmental Earth Sciences, 85(15), Article 387. https://doi.org/10.1007/s12665-026-13104-y

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13104-y

Keywords: groundwater, hydrogeochemistry, salinization, agriculture, aquifer, water quality index, Mexico, Sinaloa, irrigation, drought, multivariate statistics, coastal aquifer