The Paravur aquifer is a natural laboratory for testing proximity-based vulnerability assumptions. The municipality covers 16.19 square kilometres, is surrounded on three sides by water — Paravur Lake, the Edava-Nadayara Kayal and the Ithikkara River — and hosts roughly 8,000 open dug wells, most reaching just 6 to 15 metres into a shallow unconfined alluvial aquifer. That aquifer consists of 3 to 8 metres of fine-to-medium sand containing 8 to 15 percent clay, predominantly montmorillonite-kaolinite assemblages, overlying laterite bedrock. Hydraulic conductivity of 5 to 15 metres per day allows seawater to migrate rapidly during the dry season, while the clay fraction sets the stage for cation exchange reactions that can chemically mask or mimic intrusion.

The research team divided the transect into three zones — Core (0–3 km), Mid (3–6 km) and Outer (6–8.7 km) — and collected 50, 25 and 25 samples respectively, allocating density proportionally to intrusion risk and well distribution. Every tenth sample was analysed in duplicate, yielding a mean relative percentage difference of 3.2 percent for major ions, and ionic balance errors fell within ±5 percent for all samples after re-analysis. Field parameters were measured with a multiparameter probe, and major ions were determined at an environmental monitoring laboratory following APHA standards, with samples stored at 4 °C and analysed within 48 hours.

Bulk salinity behaved conventionally. Mean total dissolved solids fell from 504.61 milligrams per litre in the Core Zone to 302.35 in the Mid Zone and 184.06 in the Outer Zone, a 63.5 percent reduction coast to inland. Electrical conductivity followed the same trajectory, from 354.18 to 123.32 microsiemens per centimetre. But pH ran in the opposite direction, rising from acidic coastal values of 5.76 to alkaline inland values of 7.27 — a 1.51-unit increase over 8.7 kilometres that contradicts the saline, alkaline signature seawater would normally impose at the shoreline. Statistical tests confirmed these gradients were genuine: Kruskal-Wallis and ANOVA analyses produced p-values below 0.001 for pH, sodium, conductivity, TDS and sulphate, with Cohen’s d effect sizes for Core-Outer comparisons reaching 2.54 for pH and 2.87 for sodium.

The process-based indicators told the inverted story. The chloride-to-sodium ratio, which in seawater stands at 1.81, increased systematically from 0.78 at the coast to 1.62 inland, with several inland wells within 10 percent of seawater stoichiometry. The chlorinity index climbed from 55.07 to 79.83 percent, crossing the threshold that indicates substantial seawater influence. Most strikingly, the Base Exchange Index, which distinguishes salinisation (negative values) from freshening (positive values), reversed from +2.01 meq/L in the Core Zone to −0.15 meq/L in the Outer Zone. Piper trilinear diagrams corroborated the pattern: coastal wells plotted as mixed, sulphate-enriched facies with acidic pH, while 88 percent of inland wells plotted squarely in the Na-Cl seawater field with alkaline pH and negligible sulphate.

Principal component analysis quantified the dual processes at work. On eight standardised variables, PC1 explained 52.3 percent of the variance with strong loadings on chloride (0.89), sodium (0.87) and electrical conductivity (0.83), a pattern consistent with conservative seawater mixing. PC1 scores declined coast to inland, mirroring the salinity gradient. PC2 explained 18.8 percent with a strong loading on BEX (0.81) alongside calcium (0.72) and magnesium (0.68), consistent with cation exchange. Mean PC2 scores shifted from positive at the coast to negative inland, and the calculated Exchange Contribution Index rose from 31.2 percent in the Core Zone to 42.7 percent in the Outer Zone — evidence that exchange reactions become the dominant control on water chemistry with distance from the sea.

The sulphate story adds an anthropogenic twist. Coastal wells averaged 123.53 milligrams per litre of sulphate, far above the 17.8 milligrams per litre expected from conservative seawater mixing at those chloride levels. With Paravur’s population density of roughly 6,214 persons per square kilometre and no centralised sewerage, domestic wastewater discharged through septic tanks and soak pits readily infiltrates the shallow aquifer, and sulphide oxidation and protein mineralisation in septic effluent are well-documented sulphate sources. The authors recommend isotopic analysis (δ34S and δ18O of sulphate) in future work to partition sources definitively, noting that nitrate measurements — absent from this study — would also have traced sewage contamination.

The irrigation assessment revealed what the authors call a freshening-contamination paradox. On conventional indices, the water looks broadly suitable: 96 percent of samples met the magnesium hazard criterion, 93 percent met the permeability index criterion, and 100 percent met the residual sodium carbonate criterion. Yet Kelly’s Ratio, which compares sodium to the sum of calcium and magnesium, exceeded the critical threshold of 1 in 91 percent of all samples — 100 percent in the Core Zone, 88 percent in the Mid Zone and 76 percent in the Outer Zone. Mean Kelly’s Ratio values of 2.15, 1.62 and 1.32 across the zones indicate sodium concentrations 1.3 to 2.2 times higher than divalent cations, a sodicity imbalance that threatens soil structure and permeability. Correlation analysis mechanistically linked forward cation exchange (negative PC2) to elevated sodium hazard (r = −0.58, p < 0.001).

What mechanism could deliver seawater signatures inland while the coast freshens? The authors propose three candidates, all requiring geophysical confirmation. Buried paleo-channels, mapped regionally by the Geological Survey of India, could act as high-permeability conduits bypassing the shoreline, analogous to documented systems in coastal Bangladesh and the Nile Delta. Variable laterite thickness of 2 to 6 metres could create differential vertical connectivity, with fractured inland laterite permitting upward seawater leakage. And semi-confined deeper horizons could enable density-driven inland migration, with well screens selectively sampling freshwater at the coast and saline water inland. Similar inverted patterns have been reported in the Mar del Plata aquifer in Argentina and the Gaza coastal aquifer, suggesting that a non-trivial share of the world’s heterogeneous coastal aquifers may behave this way.

The practical implications are stark. Monitoring networks concentrated near the shoreline, relying on electrical conductivity, TDS and chloride alone, would systematically miss the zones of greatest agricultural risk in aquifers like Paravur’s. The authors recommend extending networks inland regardless of measured salinity, coupling process-based indicators such as BEX and the chloride-to-sodium ratio with sodium-specific irrigation indices like Kelly’s Ratio, and refusing to classify low-salinity water with positive BEX as

Subject of Research: Assessing the dynamics and severity of seawater intrusion in a vulnerable coastal aquifer: an integrated hydrogeochemical facies and ionic ratios approach in Paravur, India

Article Title: Assessing the dynamics and severity of seawater intrusion in a vulnerable coastal aquifer: an integrated hydrogeochemical facies and ionic ratios approach in Paravur, India

Article References: Kani, M. K., Biju, G. N., & VS, V. (2026). Assessing the dynamics and severity of seawater intrusion in a vulnerable coastal aquifer: an integrated hydrogeochemical facies and ionic ratios approach in Paravur, India. Estuarine Management and Technologies, 3, 201-232. https://doi.org/10.3897/emt.3.198905

Image Credits: AI Generated

DOI: 10.3897/emt.3.198905

Keywords: Assessing, dynamics, severity, seawater, intrusion, vulnerable, coastal, aquifer, integrated, hydrogeochemical, facies, ionic