Cadmium and Lead Push a Kerala River Beyond Safe Drinking Limits

A river in the southern Indian state of Kerala that was once so polluted by industrial discharges that local factories were shut down is still carrying a heavy load of toxic metals, according to a new open-access study published in Discover Geoscience. Researchers led by N. P. Chandni and Sushmitha Baskar of Indira Gandhi National Open University, working with colleagues at the DRDO Industry Academia Centre of Excellence at Bharathiar University, assessed surface water and riverbank soils from thirteen sites along the downstream stretch of the Chaliyar river in Kozhikode district. Their integrated analysis, combining pollution indices, ecological and biological risk metrics, and human health exposure modelling, paints a detailed picture of a river system in which cadmium and lead concentrations far exceed drinking water standards, even decades after the major industrial sources were closed.

The Chaliyar flows through the districts of Wayanad, Malappuram and Kozhikode before draining into the Arabian Sea, and its lower reaches pass through urbanised and industrially influenced terrain. The team sampled thirteen sites spanning upstream, midstream and downstream reaches, covering roughly 27 kilometres of river, during the post-monsoon season in February 2023. Water was collected in sterile polyethylene bottles and soils in airtight bags, transported at 4 degrees Celsius, and digested with reverse aqua regia before analysis by inductively coupled plasma optical emission spectrometry. Quality control was rigorous: calibration curves showed correlation coefficients of 0.99, spike recoveries fell between 92 and 105 percent, and every sample was analysed in triplicate. Seven metals were targeted: arsenic, cadmium, chromium, copper, nickel, lead and zinc.

The headline finding concerns lead and cadmium in the water column. Lead concentrations ranged from 1.75 to 1.93 milligrams per litre, roughly one hundred times the Indian drinking water limit of 0.01 milligrams per litre, while cadmium measured between 0.05 and 0.1 milligrams per litre against a permissible limit of just 0.003. Arsenic exceeded permissible limits at every sampling transect, and zinc at two sites was more than twice the acceptable drinking water standard. Yet when the same waters were judged against irrigation criteria, only lead remained within bounds, with a baseline of 5 milligrams per litre. The practical implication is stark: the water is unsuitable for drinking but still usable for irrigation, a distinction the authors quantify through the Heavy Metal Evaluation Index, which scored between 202.94 and 241.13 against drinking standards, placing all samples in the high pollution category, but only 10.95 to 21.93 against irrigation standards, indicating low pollution for that use.

The riverbank soils told a more nuanced story. Lead concentrations of 30 to 59 milligrams per kilogram exceeded the average shale baseline of 20, and cadmium exceeded its 0.3 milligram per kilogram baseline at every location, with a mean of 0.654. Zinc surpassed its baseline at three sites. The geoaccumulation index, which compares measured concentrations against background values while correcting for natural lithogenic variation, placed most metals in the unpolluted class, but cadmium reached the moderately polluted category at sites 7, 12 and 13, with index values ranging from 0.152 to 1.152 across the study area. Contamination factors reinforced the pattern: cadmium registered considerable contamination at those same three sites, lead showed moderate contamination everywhere, and arsenic, chromium, copper and nickel remained in the low contamination range throughout.

Cadmium emerged as the dominant ecological threat. The Ecological Risk Index, which weights contamination factors by metal-specific toxic response factors, ranged from 60.17 to 124.47 across locations, a mild risk status overall, but the individual contribution of cadmium reached the high-risk category at four sites. The authors attribute this to a combination of wastewater discharge, small-scale industrial activity and agricultural runoff, with possible geogenic contributions from the region’s charnockite, granite gneiss and quartzite geology, which naturally host trace levels of these elements. Biological risk, assessed with the mean effect range median quotient, was less alarming: 61.5 percent of soil samples fell into the slightly toxic category and the rest were non-toxic, with nickel rather than cadmium acting as the dominant contributor to that particular metric.

The human health assessment is where the findings become most consequential. Using standard exposure equations with a 70-kilogram adult, a 2.2 litre per day water ingestion rate and a 70-year exposure duration, the team calculated hazard indices for ingestion and dermal pathways. For surface water, ingestion hazard indices ranged from 22.5 to 31.4, far above the threshold value of 1 that marks acceptable non-carcinogenic risk, with lead the single largest contributor at hazard index values of 15.7 to 17.4. Dermal hazard indices for water ranged from 3.49 to 5.07, also above 1 but substantially lower. In contrast, the riverbank soils posed negligible non-carcinogenic risk, with all hazard indices orders of magnitude below 1 for both pathways.

Carcinogenic risk calculations for arsenic, chromium and lead revealed a more troubling picture. For chromium and lead in surface water, ingestion-based cancer risks exceeded the 10 to the minus 4 upper bound of the acceptable range at every location, with chromium values reaching 7.07 times 10 to the minus 3, meaning a potential cancer incidence approaching seven cases per thousand exposed individuals under the study’s assumptions. Dermal risks were lower but still elevated for chromium. In soils, ingestion risks for chromium reached 4.11 times 10 to the minus 5, within the acceptable band, while dermal soil risks were negligible. The consistent pattern, that ingestion dominates over skin contact as the exposure route of concern, aligns with findings from other Indian river systems such as the Gomti and with international studies, and it matters practically because the downstream Chaliyar supports active fishing communities whose residents depend on the river daily.

To translate contamination into actionable numbers, the researchers applied a heavy metal toxicity load model that estimates the percentage of each metal that would need to be removed to bring concentrations back to baseline. For surface water judged against drinking standards, lead required a modelled 99 percent removal at every site, while cadmium needed 80 to 90 percent removal. In soils, cadmium required 40 to 70 percent removal and lead 33 to 65 percent. The authors point to bioremediation as a promising strategy, citing prior work on a cadmium-resistant Klebsiella planticola strain that precipitates cadmium sulfide, alongside options such as phytoremediation, biochar amendment and clay mineral immobilisation.

Statistical source apportionment added spatial texture to the picture. Principal component analysis of the water data, with two components explaining 61.81 percent of variance, linked arsenic, copper and nickel to a widespread anthropogenic factor consistent with urban wastewater, agricultural runoff and industrial activity, while cadmium and zinc loaded on a separate component suggesting a partly geogenic zinc source. For soils, two components explained 82.47 percent of variance, with lead and chromium dominating the first factor, pointing to mixed natural and human origins. Hierarchical cluster analysis grouped upstream, midstream and downstream water sites into distinct clusters reflecting their metal profiles, whereas soil clusters mixed locations from different reaches, indicating heterogeneous bank contamination.

The authors are candid about limitations: the study relies on a single post-monsoon sampling campaign, so seasonal variability, which monsoon dynamics can strongly influence, remains unquantified, and thirteen sites confined to the Kozhikode stretch cannot represent the entire basin. Nevertheless, the work establishes a valuable baseline and a set of management priorities, from hotspot identification and source control through stricter effluent regulation and improved waste management, to long-term monitoring, riverbank vegetation restoration and the development of local geochemical background values. For a river whose industrial polluters were closed in the late 1990s, the persistence of cadmium and lead at levels that threaten both ecosystems and human health is a sobering reminder that shutting a factory does not clean a river; legacy contamination, changing land use, sand mining and diffuse runoff continue to shape the water that flows to the Arabian Sea.

Subject of Research: Trace metal contamination and associated ecological and human health risks in the surface water and riverbank soils of the Chaliyar river basin, Kerala, India

Article Title: Assessment of trace metal contamination and associated health risks in the Chaliyar river basin, Kerala

Article References: Assessment of trace metal contamination and associated health risks in the Chaliyar river basin, Kerala. (n.d.). https://doi.org/10.1007/s44288-026-00769-y

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00769-y

Keywords: heavy metal pollution, Chaliyar river, cadmium contamination, lead contamination, water quality, riverbank soil, ecological risk assessment, carcinogenic risk, health risk assessment, Kerala, geoaccumulation index, bioremediation