Durga Idol Immersion in the Hooghly: What River Monitoring Has Found

The West Bengal Pollution Control Board estimates that Kolkata and its satellite towns immerse more than 28,000 Durga idols in the Hooghly and its connected water bodies during the four days of Durga Puja visarjan. The largest single-day surge falls on Dashami, the tenth day of Navratri, when processions from every para converge on the ghats at Babughat, Nimtala and Ahiritola simultaneously. The idols range from household murti no taller than a child to pandal centrepieces that require cranes and flatbed trucks. Each one carries into the water whatever it was made of.

Traditional Durga idols from Kumartuli, the potters' quarter of north Kolkata, are built on a bamboo armature packed with straw and jute, skinned in clay taken from the banks of the Hooghly itself. This base material is largely benign. The problem is what goes on top. Synthetic paints containing cadmium, lead and chromium compounds are used for the vivid reds, yellows and greens that define the iconography. Plaster of Paris, which replaced clay for many commercial idols because it is cheaper and holds detail better, does not dissolve. It settles as sediment and raises the turbidity of the water for weeks. Thermocol and synthetic fabric used for the idol's clothing and ornaments add non-biodegradable bulk. Studies published by the National Environmental Engineering Research Institute have identified measurable spikes in heavy metal concentrations at immersion ghats in the 48 to 72 hours following Dashami, with lead and cadmium showing the sharpest post-immersion peaks in water samples drawn from mid-channel.

Dissolved oxygen is the single most direct measure of a river's capacity to support fish and invertebrates. The Hooghly is already a stressed river: industrial discharge from Howrah and Bhatpara keeps baseline dissolved oxygen levels in many stretches below the 5 mg per litre threshold that most freshwater fish require. West Bengal Pollution Control Board monitoring at Babughat in the days immediately following large immersion events has recorded further drops in dissolved oxygen, attributed to the decomposition of organic material from the straw, jute and clay. The effect is localised and typically recovers within a week in fast-flowing stretches, but slower backwaters and tidal creeks connected to the main channel show longer suppression. Hilsa, the Hooghly's most ecologically and economically significant fish, is a migratory species that uses the river as a spawning corridor. October sits at the edge of its upstream migration window, which means the immersion period and the fish's most vulnerable transit overlap.

The National Green Tribunal issued directions in 2017 requiring idol immersion to be conducted in temporary tanks or artificial ponds where possible, with the collected material disposed of separately rather than left in the river. Several Kolkata municipalities have installed immersion tanks at major ghats, and some of the largest community pujas now use them. Compliance is uneven. Smaller neighbourhood pujas and household idols continue to enter the river directly, and enforcement at the ghats during the immersion rush is logistically difficult. The shift toward clay-only, natural-pigment idols is real and growing, driven partly by NGT pressure and partly by a revival movement among Kumartuli artisans who argue the traditional method produces a better idol. But the transition is partial. Plaster of Paris idols remain the majority in the commercial market because they cost less and can be produced faster.

The Hooghly supports Gangetic river dolphins, at least three species of freshwater turtle, and a food web that runs from invertebrates through catfish and carp to the fishing communities of the Sundarbans delta. Heavy metal accumulation in sediment does not flush out the way dissolved oxygen recovers. Cadmium and lead bind to fine particles that settle in low-velocity zones and enter the food chain through bottom-feeding invertebrates. The immersion surge is one of several stressors the river carries, alongside year-round industrial effluent, sewage outfall and sand mining. Attributing any single ecological outcome solely to idol immersion would misread the data. What the monitoring does show is a measurable, repeated, time-stamped chemical signal that corresponds to a known source, which is more than can be said for many of the river's other inputs. That specificity is what makes it tractable.