Monsoon Rains Drive Toxic Metal Spikes in Chennai’s Drinking Water Reservoir, Study Finds

A new study of Chembarambakkam Lake, one of the most important freshwater reservoirs supplying the city of Chennai in Tamil Nadu, India, has revealed that the annual monsoon does far more than refill the lake. According to research published in the journal Environmental Monitoring and Assessment, the post-monsoon period brings sharply elevated concentrations of toxic heavy metals, including chromium, nickel, lead and iron, into both the water column and the sediments of the reservoir. The findings suggest that seasonal runoff from the surrounding catchment, combined with the resuspension of contaminated sediments, acts as a powerful delivery mechanism for pollutants that pose measurable risks to both aquatic ecosystems and human health.

The research team, led by Ayyandurai Mani and Mathiyazhagan Narayanan of the Saveetha Institute of Medical and Technical Sciences in Chennai, together with colleagues from the institute’s medical college laboratories, set out to track how nine toxicologically significant metals behave across seasons. The elements examined were arsenic, lead, cadmium, chromium, nickel, copper, zinc, iron and aluminium, all of which were selected because they persist in the environment, have the potential to bioaccumulate in living organisms, and serve as sensitive indicators of aquatic pollution. By sampling water and sediment across different seasons and multiple stations, the researchers were able to build a picture of how the lake’s contamination profile shifts with the monsoon cycle.

The seasonal signal was unmistakable. Concentrations of chromium, nickel, lead and iron were substantially higher in the post-monsoon season, a pattern the authors attribute to increased runoff across the catchment, greater inflow of contaminated water into the lake, and the physical stirring up of bottom sediments as inflows surge. This mechanism matters because sediments act as long-term repositories for heavy metals. When storms and flood pulses disturb the lake bed, metals that had been locked away can be released back into the water column, effectively giving the reservoir a second, delayed dose of pollution long after the original sources discharged their loads.

To quantify the ecological dimension of the problem, the team applied a suite of established sediment contamination indices, including the geoaccumulation index, the contamination factor, the degree of contamination and the potential ecological risk index. These metrics, which compare measured metal levels against background geological values and weight each element by its toxicity, told a consistent story: iron and cadmium emerged as the most significant contributors to ecological risk. Particularly striking were the pre-monsoon degree-of-contamination values, which indicated severe contamination at multiple sampling locations even before the rains arrived, suggesting that the lake carries a substantial legacy burden that the monsoon then redistributes and amplifies.

On the water quality side, the researchers calculated the heavy metal pollution index and the heavy metal evaluation index, two widely used composite measures that aggregate the concentrations of multiple metals relative to permissible limits. The results showed that many of the sampling stations fell into the moderate to highly contaminated categories during the post-monsoon period. For a reservoir that feeds directly into Chennai’s drinking water supply, this seasonal deterioration is more than an academic concern. It implies that the very months in which the lake collects the water it will store for the dry season are also the months in which its metal load peaks.

The most consequential part of the study, however, is its human health risk assessment. Using standard exposure models that estimate the intake of contaminants through water consumption, the team computed hazard quotients for non-carcinogenic effects and total carcinogenic risk values for each metal. The analysis revealed that exposure to chromium, nickel and arsenic produced non-carcinogenic hazard index values exceeding 1, the conventional threshold above which adverse health effects cannot be ruled out. More alarmingly, the calculated carcinogenic risks surpassed the widely used upper limit of one in ten thousand, meaning the estimated lifetime cancer risk from these exposures exceeds the level typically considered acceptable by regulatory agencies.

Children emerged as the most sensitive population group in the assessment. Because children consume more water relative to their body weight and their developing physiology makes them more vulnerable to toxic insults, the risk metrics for this group were consistently the highest. This finding echoes a growing body of international research showing that heavy metal contamination in drinking water sources disproportionately threatens the youngest members of exposed communities, with potential consequences ranging from developmental impairment to elevated lifetime cancer risk.

The study’s emphasis on seasonality carries a broader lesson for freshwater management in monsoon-influenced regions. Climate-induced hydrological variability is intensifying around the world, and the authors note that these shifting patterns, together with growing anthropogenic pressures, strongly affect the transport and bioavailability of heavy metals in freshwater ecosystems. In other words, the monsoon is not simply a dilution event that flushes pollutants away; in catchments loaded with industrial, agricultural and urban residues, it can act as a pump, mobilising contaminants from land and sediment into the water that millions of people depend on. Understanding this seasonal dynamic is therefore essential for maintaining ecosystem integrity and reducing health risks.

Chembarambakkam Lake is not an isolated case. Comparable studies from reservoirs and lakes across South Asia and beyond, from Dhaka to Lake Nasser in Egypt, have documented similar patterns of metal enrichment in water and sediment, with seasonal hydrology playing a decisive role in when and how contamination peaks. What sets the new work apart is its integrated approach, combining sediment contamination indices, water quality indices and formal human health risk calculations within a single seasonal framework. This holistic design allows managers to see not just where contamination is worst, but when it is worst and who is most at risk.

The practical implications are clear. Monitoring programmes that sample only once a year, or only during the dry season, risk missing the post-monsoon spikes that the study identifies as the most hazardous period. The authors’ results underscore the need for seasonal surveillance of drinking water reservoirs, targeted control of catchment runoff, and remediation of metal-laden sediments before they can be resuspended. As climate change alters the intensity and timing of monsoon rainfall across South Asia, the findings from Chembarambakkam Lake offer a timely warning: the health of a city’s water supply may hinge on what the rains stir up.

Subject of Research: Seasonal heavy metal contamination and associated ecological and human health risks in a tropical freshwater reservoir

Article Title: Seasonal variations in heavy metals concentration in water and sediment and associated ecological and human health risk assessment in Chembarambakkam lake, Tamil Nadu, India

Article References: Mani, A., Kolandhasamy, P., Loganathan, P., & Narayanan, M. (2026). Seasonal variations in heavy metals concentration in water and sediment and associated ecological and human health risk assessment in Chembarambakkam lake, Tamil Nadu, India. Environmental Monitoring and Assessment, 198(10), Article 1058. https://doi.org/10.1007/s10661-026-15899-6

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15899-6

Keywords: heavy metals, Chembarambakkam Lake, water quality, sediment contamination, monsoon, ecological risk, human health risk, carcinogenic risk, pollution indices, freshwater reservoir, Tamil Nadu, seasonal variation