Hidden Metals and Brackish Waters: Study Maps Groundwater Health in Kerala’s Karamana Basin

Beneath the palm-fringed coastal plains of Thiruvananthapuram, the water that millions of people depend on is telling a complicated story. A new study of the Karamana River Basin in Kerala, Southern India, has combined classical hydrochemistry, heavy metal analysis, and a battery of multivariate statistical tools to build one of the most complete pictures yet of groundwater quality in this small but densely used basin. The research, published in the journal Discover Geoscience, analyzed twenty-two groundwater samples drawn from open wells and bore wells across the 702 square kilometer basin, and the results reveal a resource that is broadly fit for drinking but shadowed by pockets of salinity and worrying concentrations of trace metals.

The Karamana River rises at Chemmunji Mottai, a 1,717 meter peak in the Western Ghats, and flows through the coastal plains of Thiruvananthapuram district before emptying into the Arabian Sea. Classified among the world’s small mountainous rivers, it drains a landscape dominated by agriculture, with plantation crops such as coconut, rubber, plantain, and tapioca covering roughly 64 percent of the basin, particularly in its middle and lower reaches. Forest survives mainly within the Peppara reservoir catchment, while the Aruvikkara watershed hosts intensive farming and expanding settlements. Because the basin supplies a substantial share of the drinking and domestic water for Thiruvananthapuram city, understanding what controls its groundwater chemistry is not an academic exercise but a matter of public health.

The geological setting matters enormously here. The basin sits on a complex crystalline basement of Archaean to Proterozoic age, dominated by the Khondalite Group, chiefly garnetiferous-biotite-sillimanite gneiss, interspersed with pyroxene granulite bands that often contain graphite, along with granites, gneisses, and intrusions of gabbro, dolerite, pegmatite, and quartz veins. Quaternary deposits of coastal sands, pebble beds, and river alluvium mantle the Mio-Pliocene sediments along the coast. Rainfall is equally variable, averaging 2,600 millimeters per year across the basin but ranging from less than 1,400 millimeters in the southwest to more than 4,200 millimeters in the northeast. Despite Kerala’s reputation for abundant rain, acute drinking water shortages strike every summer, making groundwater the critical buffer between wet and dry seasons.

The field team measured pH, total dissolved solids, and temperature on site with handheld meters, recorded GPS coordinates at every well, and carried samples to the laboratory for determination of the major dissolved ions: calcium, magnesium, sodium, potassium, bicarbonate, chloride, sulfate, and nitrate. Ionic balance errors were kept within plus or minus five percent, a standard quality check in hydrochemistry. Trace elements, including iron, chromium, nickel, cadmium, cobalt, copper, and zinc, were measured in unfiltered samples using flame and graphite furnace atomic absorption spectrophotometry. Spatial distribution maps for every parameter were then generated with inverse distance weighting interpolation in a geographic information system, allowing the researchers to visualize how water chemistry changes as one moves from the highland headwaters toward the coast.

The physicochemical results paint a picture of generally mild but spatially uneven conditions. Groundwater temperatures ranged from 26 to 30 degrees Celsius, and pH values spanned 6.3 to 8.0 with an average of 7.26, indicating slightly acidic to mildly alkaline water, a condition the authors attribute to natural buffering processes combined with anthropogenic influences and possible seawater intrusion near the coast. Total dissolved solids ranged from 73 to 373 milligrams per liter, within drinking water norms. Electrical conductivity, however, told a more troubling story: applying a standard classification of water by dissolved solids, only about 56 percent of the samples qualified as fresh, while more than 43 percent fell into the brackish category, a striking signal of spatial heterogeneity likely driven by coastal proximity, saline encroachment, and dissolved solid inputs from human activity.

Among the major ions, sodium emerged as the dominant cation, followed by calcium, potassium, and magnesium, while bicarbonate was the most abundant anion, ahead of chloride, sulfate, and nitrate. This sodium-bicarbonate dominance is a classic fingerprint of silicate weathering in crystalline terrain, where feldspar minerals slowly dissolve and release sodium while consuming carbon dioxide. To decode the facies, the team used the Piper trilinear diagram, which plots cations and anions on two triangular fields and classifies water types on a central diamond. The samples spread across several cationic groups, including sodium-rich, sodium-magnesium, and sodium-calcium-magnesium waters, and anionic groups ranging from bicarbonate to mixed bicarbonate-chloride types, confirming that no single process governs the basin’s chemistry.

Two further graphical tools sharpened the interpretation. The Gibbs plot, which relates the ratio of sodium plus potassium to calcium against total dissolved solids, placed the great majority of samples squarely in the rock-water interaction field, indicating that mineral weathering, not precipitation or evaporation, is the primary engine of ion enrichment, though a few samples drifted toward the evaporation field. The Wilcox plot, which graphs electrical conductivity against percent sodium to judge irrigation suitability, showed that most samples fall in the very good to good range, with sodium adsorption ratios between 30 and 70 percent and conductivity mostly below 1,000 microsiemens per centimeter, meaning salinity and sodicity hazards for agriculture remain limited even where drinking quality is marginal.

The trace metal results are where the study raises its most serious flags. Cadmium averaged 0.018 milligrams per liter, chromium and cobalt each averaged 0.052, copper 0.023, iron 0.040, nickel 0.037, and zinc 0.062, with zinc showing the widest spread, from 0.004 to 0.319 milligrams per liter, a variability the authors link to localized pollution sources. Several samples exceeded drinking water standards for these metals, and the researchers note that prolonged exposure to cadmium, chromium, and nickel can pose health risks. They also highlight the role of iron and manganese hydrous oxides, which form in the aquifer and act as carriers, sorbing and transporting other heavy metals such as zinc, cobalt, nickel, and copper through the groundwater system. While the conventional parameters, pH, TDS, and the major ions, generally complied with World Health Organization and Bureau of Indian Standards guidelines, the metal data demanded a more cautious verdict on drinkability.

To synthesize all of this, the team computed a weighted arithmetic Water Quality Index, assigning each parameter a weight from one to five based on its importance to drinking water quality, converting concentrations into quality ratings against WHO standards, and summing the sub-indices. The resulting map shows most of the basin in the intermediate quality range of 44 to 72, with better water, scoring 15 to 44, around several wells and a clear decline in quality toward the middle and southern parts of the basin. Critically, a handful of sites near wells designated GW4, GW13, GW17, GW18, GW19, and GW20 scored above 100, falling into the very poor to unsuitable category. The authors identify agricultural runoff, domestic sewage, and localized industrial activity as the principal contamination sources behind these hotspots.

The statistical backbone of the study came from correlation analysis, principal component analysis, and hierarchical cluster analysis, and together they disentangled natural from human influences. TDS showed strong positive correlations with all the major ions, confirming dissolved solids as the master variable of ionic composition, with moderate links to nickel and other trace elements suggesting that salinity and metal mobilization are connected. PCA extracted three components explaining 48.6, 14.8, and 11.3 percent of the variance respectively: the first, loaded heavily on the major ions, reflects natural processes of rock-water interaction, silicate weathering, and evaporation; the second, loaded on sulfate, cadmium, and fluoride, points toward anthropogenic inputs; and the third, loaded on cobalt, nickel, cadmium, and zinc, reinforces the human contamination signal. Cluster analysis of the twenty-two samples using Ward’s method produced three distinct geochemical groups that corroborated the PCA picture. The overall message is one of cautious reassurance paired with vigilance: the Karamana basin’s groundwater remains broadly usable, but its brackish fringes and metal-laden hotspots demand continuous monitoring, targeted treatment, and land-use management if this vital coastal aquifer is to keep supplying a growing city sustainably.

Subject of Research: Hydrogeochemical and multivariate statistical assessment of groundwater quality in the Karamana River Basin, Kerala, Southern India

Article Title: Multi-faceted approach for comprehensive groundwater quality assessment of Karamana river basin in Kerala, Southern India

Article References: Shanmugasundharam, A., Anto, A. M., & Krishna, R. S. A. (2026). Multi-faceted approach for comprehensive groundwater quality assessment of Karamana river basin in Kerala, Southern India. Discover Geoscience, 4(1), Article 318. https://doi.org/10.1007/s44288-026-00677-1

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00677-1

Keywords: groundwater quality, hydrogeochemistry, Karamana River Basin, Kerala, heavy metals, Water Quality Index, principal component analysis, seawater intrusion, silicate weathering, trace elements, drinking water standards, coastal aquifers