During the last few decades, demand for electricity has increased enormously. Due to the inherent problems associated with nuclear and hydroelectric power generation, thermal power plants with coal as a source of energy are gaining importance even in non-coal producing regions.

Fly ash, a by-product of coal combustion, is prevalently disposed by wet-sluicing from electrostatic precipitators to on- or off-site ponds. This system disposes of ash as a slurry which is pumped into a natural or constructed basin where ash settles and supernatant is drained into a water body such as a river or a lake. Many elements present in coal get preferentially incorporated in and around fly ash particles during combustion. Most of these elements are soluble in water and thus become available to the biological systems.

The impact of wet disposal of the by-products from a 200 MW capacity I.P. thermal power station on the limnology of the river Yamuna, Delhi was studied. The power station uses about 4000 tons of bituminous grade-F coal which has about 40% solid residue content. A two-year field survey of the seasonal variation in the limnology of the impacted and non-impacted segments of the river and the ash ponds reveal that conductivity, TDS, DO, hardness, sulphates, nitrite and elements viz., Al, Sb, Bi, Cd, Cr, Co, Li, Mn, Mo, K, Si, and Zn increased significantly in the impacted waters. Also the phyto- and zooplankton density and diversity was reduced and the composition of various groups was drastically changed at the impacted site.

Laboratory studies were carried out on the physico-chemical characterization of fly ash, solubility behaviour of components associated with fly ash,ß and the effect of fly ash leachates on representative phyto- and zooplanktonic organisms in order to assess the potential impact of fly ash effluents on river water quality. Fly ash leachates adversely affected the growth parameters of plankton with the degree of toxicity related to the concentration of the leachate, the duration of exposure, and the test organism.

This study clearly suggests that fly ash effluent is a potential environmental hazard which can have far reaching adverse effects on the receiving water.