What Idol Immersion Puts Into a River: Plaster, Lead Paint, and the Fish Downstream

Plaster of Paris is calcium sulphate hemihydrate. When it enters water, it does not dissolve the way salt does. It absorbs water, hardens briefly, then breaks into fine particulate matter that suspends in the water column and settles into the riverbed. A study published in the journal Environmental Monitoring and Assessment found that after Ganesh Chaturthi immersions in the Mula-Mutha river in Pune, total dissolved solids spiked by over 400 percent in the days immediately following the festival. The river bottom becomes layered with calcium sulphate sediment that smothers benthic organisms, the invertebrates, insect larvae, and small crustaceans that live in and on the riverbed and form the base of the aquatic food chain.

The paint is a separate problem. Traditional idol paints used in mass-manufactured idols commonly contain lead, mercury compounds, cadmium, and chromium-based pigments. These are not decorative choices, they are cheap, bright, and stable. When the idol breaks apart in water, these pigments leach directly into the water column. Lead in particular binds to suspended particles and sinks, concentrating in sediment at levels that persist for years after a single immersion event.

Fish absorb heavy metals through three routes: their gills, their digestive tract, and direct skin contact. Lead interferes with the synthesis of haemoglobin, the protein that carries oxygen in the blood, this is as true in Labeo rohita (rohu) as it is in humans. A 2018 study in the journal Ecotoxicology and Environmental Safety documented elevated lead concentrations in the muscle tissue of fish sampled from rivers in Maharashtra and Uttar Pradesh following immersion festivals, with levels in some samples exceeding WHO safe limits for human consumption by a factor of three.

Mercury, where present in idol paints, is more insidious. Inorganic mercury in water is converted by anaerobic bacteria in sediment into methylmercury, the organic form that bioaccumulates. Each step up the food chain concentrates it further. A small fish absorbs it. A larger predatory fish eats many small fish. A river bird, an otter, a human eating hilsa or rohu from a downstream stretch, each is receiving a more concentrated dose than the one before. This process, called biomagnification, means the fish most prized for eating are often the most contaminated.

Catfish are bottom feeders. They spend their lives in direct contact with sediment, the exact layer where plaster particulate and lead-bound particles accumulate. Species like Clarias batrachus (the walking catfish, called magur in Hindi) and Mystus seenghala are particularly exposed. Their feeding behaviour means they ingest contaminated sediment directly while foraging.

Freshwater mussels and clams, common in the Ganga, Godavari, and Krishna river systems, are filter feeders. They draw water through their bodies continuously, concentrating whatever is suspended in it. A mussel living downstream of a major immersion site can accumulate heavy metals at concentrations hundreds of times higher than the surrounding water. These mussels are harvested and eaten, often by communities with no awareness of the contamination upstream.

Juvenile fish are disproportionately vulnerable. Their gill surface area relative to body size is larger, their detoxification systems are immature, and their nervous systems are still developing. Lead exposure at critical developmental stages causes permanent neurological damage in fish, stunted growth, impaired schooling behaviour, and reduced reproductive success. A single immersion season can affect the recruitment of an entire year class of fish in a stretch of river.

Rivers have some capacity to dilute and flush pollutants, but plaster of Paris and heavy metals test that capacity in specific ways. Plaster sediment is not biodegradable. It does not break down into harmless compounds over time, it simply sits, compacting and reducing the oxygen content of the sediment layer, a condition called hypoxia. Benthic organisms suffocate. The invertebrate population collapses. And because those invertebrates are what juvenile fish eat, the fish population follows.

The Central Pollution Control Board has flagged idol immersion as a significant source of heavy metal loading in rivers including the Yamuna, Ganga, and Hussain Sagar lake in Hyderabad. Hussain Sagar is an instructive case: a 2010 survey found arsenic, lead, and cadmium concentrations in its sediment at levels classified as severely contaminated under international benchmarks, with idol immersion identified as a primary contributor alongside industrial discharge.

Some municipal bodies have introduced immersion tanks, large concrete pools where idols can be submerged separately from open water bodies, with the settled material collected and disposed of as solid waste. Uptake has been slow. The logistics of moving a community's ritual practice to a tank are real, and the emotional weight of a river immersion, the sense of return to something larger, is not nothing. But the fish in the river are not abstract either.

India celebrates Ganesh Chaturthi, Durga Puja, and dozens of regional festivals involving idol immersion across a single calendar year. The number of idols immersed annually runs into the tens of millions. Even if a fraction of those idols are made with plaster of Paris and synthetic paint, and surveys by organisations including the Centre for Science and Environment suggest the majority of mass-manufactured idols still are, the cumulative load on river systems is not a seasonal event. It is a chronic one.

The fish don't experience it as a festival. They experience it as a sudden change in water chemistry, a smothering of the riverbed, a slow accumulation in their tissue of something their biology was never designed to process. The rohu pulled from the Yamuna in October and sold at a market in Delhi is carrying the chemistry of what went into the river two weeks before. The person who buys it has no way of knowing that. The fish had no way of avoiding it.