Why Clay Idols Dissolve at Different Rates: The Sediment and Oxygen Evidence From Immersion Sites

The word "clay" covers a family of minerals with very different structures. Idol makers across India work primarily with two: natural river clay, which is largely kaolinite, and processed clay bodies that contain montmorillonite (the dominant mineral in what potters call "black cotton soil"). Kaolinite has a rigid, two-layer crystal structure. Montmorillonite has a three-layer structure that absorbs water between its sheets and swells. When a kaolinite idol enters a river, the water penetrates slowly along grain boundaries. When a montmorillonite idol enters, the mineral lattice begins to expand almost immediately, the idol softens from within before it crumbles at the surface.
Particle size compounds this. River clay used in traditional Ganesh idols from the Konkan coast tends to be fine-grained and loosely packed. Commercially processed clay used in large Durga Puja idols from Bengal is often mixed with sand and straw to hold the form during firing or drying, those additives slow water infiltration and extend the time the idol holds its shape in water by several days.Within the first six hours of a clay idol entering still or slow-moving water, turbidity spikes sharply. A 2019 study published in Environmental Monitoring and Assessment measured total suspended solids in Hussain Sagar, Hyderabad, during Ganesh Chaturthi immersion and found TSS rising from a baseline of around 40 mg/L to over 320 mg/L at peak immersion points. That turbidity matters biologically: suspended clay particles scatter light, cutting photosynthetically active radiation at depth. Submerged aquatic plants and phytoplankton in the lower water column lose access to light within hours.
Turbidity from clay also differs from turbidity from organic runoff. Clay particles carry a negative surface charge and bind to dissolved metals and phosphorus already in the water. As they settle, they carry those compounds into the sediment, concentrating what was diffuse into a dense layer at the bottom.The idol disappears. The sediment does not. Benthic cores taken from Powai Lake in Mumbai after successive Ganesh Chaturthi seasons show a visible stratigraphy: thin, dense clay layers alternating with the lake's natural organic sediment. These clay layers are low in porosity. Oxygen from the water column diffuses into sediment at a rate governed by porosity, a dense clay cap effectively seals the organic matter beneath it from aerobic decomposition. Anaerobic bacteria take over. They produce hydrogen sulphide and methane rather than carbon dioxide, and dissolved oxygen in the water just above the sediment drops.
This is the mechanism behind the fish kills that recur in enclosed water bodies after large immersion events. The fish are not poisoned by the clay itself. They suffocate as dissolved oxygen near the bottom falls below the 4 mg/L threshold that most freshwater fish species require for normal respiration. Bottom-feeding species, catfish, loaches, freshwater prawns, are hit first because they live and feed in the zone of lowest oxygen.An unpainted clay idol in clean water is a mineralogy problem. A painted, decorated idol is an organic chemistry problem layered on top of it. Traditional natural pigments, ochre, turmeric, indigo, add modest organic load. Synthetic paints, particularly the oil-based enamels used on large commercial idols, introduce a secondary oxygen demand that operates on a different timescale than the clay dissolution itself.

As paint films detach from a dissolving idol, they float, sink, or adhere to sediment depending on density and surface chemistry. Acrylic and enamel fragments have been recovered in benthic samples from the Yamuna near Delhi's Chhath ghats. These fragments do not biodegrade on any ecologically relevant timescale. Plaster-of-Paris, still used in a significant fraction of commercial idols despite regulatory restrictions in several states, adds a separate problem: it releases calcium sulphate as it dissolves, raising sulphate concentrations in the water and feeding sulphate-reducing bacteria, the same bacteria that produce hydrogen sulphide and accelerate oxygen depletion.
The combination of a dense clay sediment cap, a paint-driven organic oxygen demand, and sulphate-fed anaerobic activity creates a depletion cascade that no single factor alone would produce.Immersion sites matter more than the idols themselves in determining ecological outcome. The Central Pollution Control Board's monitoring of designated immersion ghats found that rivers with flow rates above 0.5 metres per second showed dissolved oxygen recovery within 48 to 72 hours of immersion events, because turbulence re-aerates the water column and disperses settled sediment before a stable cap can form. Still water bodies, lakes, ponds, temple tanks, showed suppressed dissolved oxygen for 10 to 21 days post-immersion, with some enclosed tanks recording anoxic conditions at depth for over a month.

The biology of the site also primes the outcome. A water body already carrying high nutrient loads from agricultural runoff or sewage has less oxygen buffer to absorb the additional demand from idol dissolution. The idols that cause the most damage are not necessarily the largest or the most decorated, they are the ones that enter water bodies already running close to the edge of their oxygen capacity.
Clay breaks down. The question the sediment record is answering is what it breaks down into, and what the water was carrying before it arrived.