Coastal Aquifer Under Watch: Eight-Year Study Finds Alibag Groundwater Still Safe but Slowly Salinizing

Along the palm-fringed coastline of Alibag Tehsil in Maharashtra’s Raigad district, the water beneath the ground tells a quiet but consequential story. A new eight-year assessment of the region’s groundwater has found that, despite measurable signs of progressive mineralization and localized seawater influence, the aquifer that sustains hundreds of thousands of people remains suitable for both drinking and agriculture. The study, published in Discover Geoscience by Rajat Bar and Anargha Amit Dhorde of Nowrosjee Wadia College in Pune, offers one of the most integrated pictures yet of how a humid coastal aquifer system responds to the combined pressures of geology, the sea, and human activity.

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