Use of stable and unstable isotopes to study the effect of the disposal of fly ash on the pollution of shallow groundwaters: the example of the Koradi and Khaperkheda thermal power plants (Maharashtra, India)

An increasing amount of fly ash from thermal power plants is produced in India every year. Its disposal is generally done in ponds after it is mixed together in suitable proportion of water to make it in a slurry form. In this work we studied the impact on local water resources of fly ash produced in the Koradi and Khaperkheda thermal power plants (district of Nagpur, Maharashtra - India) where sludge is disposed in three large ponds established in an area where the population of many small villages uses groundwaters for drinking and domestic purposes. Here, groundwater locally exceeds the concentration limits recommended by the World Health Organization for Mg2+, Ca2+, NO3-, SO42-, Total Dissolved Solids (TDS) and for some minor elements like As, Mo, V and U. As a first step a new geological map of the study area has been prepared to understand the possible water-rock interactions. Then, an extensive geochemical survey of groundwater, stream water and fly ash was also carried out to identify the possible origin of the pollutants and to discriminate between geogenic and anthropogenic sources. Analyses carried out on oxygen and hydrogen isotopes suggest scarce interaction between the water temporarily stored in the ponds and groundwaters. Data also highlight that the high salinity measured in the polluted wells cannot be produced by evaporation and subsequent infiltration of stream waters draining from the ponds in the local aquifer. 87Sr/86Sr values, when associated with Sr/Ca ratios, point out the main role of industrial waters on the share of pollution due to sulphates in the villages of Masada, Khairi and Kawatha. Here, the analytical results suggest that a large part of the sulphates in the groundwater of the villages of Masada, Khairi and Kawatha originate from the infiltration of industrial water from tens of factories that mix fly ash with relatively high quantities of gypsum and lime for the production of bricks. Uranium isotopic analyses clearly show the evidences of the interaction between groundwaters and aquifer rocks and confirm again the scarce influence of sludge waters. A new conceptual model based on the study of the isotopes of radium was proposed and used to estimate the short residence times of groundwaters. This model highlights that high salinity can't be in any case attributed to a prolonged water-rock interaction but is probably due to the influence of untreated waste waters of domestic and/or industrial origin on the shallow and vulnerable aquifer. In addition, the interaction with the relatively U-rich Gondwana units, like Talchir formation, is probably the cause of the high concentration of this element. Results showed how the relatively high concentrations of Mo, As, B and F in circulating waters are linked to the leaching from fly ash, also pointing out a direct spatial correlation between the concentration of fluorides in the groundwater and their closeness to the ash ponds.