Assessment of groundwater pollution from ash ponds using stable and unstable isotopes around the Koradi and Khaperkheda thermal power plants (Maharashtra, India)

The impact on local water resources due to fly ash produced in the Koradi and Khaperkheda thermal power plants (district of Nagpur, Maharashtra — India) and disposed in large ponds at the surface was assessed through the study of environmental variation of ratios of stable and unstable isotopes. Analyses of oxygen and hydrogen isotopes suggest scarce interaction between the water temporarily stored in the ponds and the groundwater in the study area. Data also highlight that the high salinity of groundwater measured in the polluted wells is not due to evaporation, but to subsequent infiltration of stream waters draining from the ponds to the local aquifer. 87Sr/86Sr values, when associated with Sr/Ca ratios, demonstrate the dominant role of waste waters coming from tens of brick kilns surrounding the pond sulfate pollution. Uranium isotopic analyses clearly show evidence of the interaction between groundwater and aquifer rocks, and confirm again the low influence of ash ponds. A new conceptual model based on the study of the isotopes of radium is also proposed and used to estimate residence times of groundwater in the area. This model highlights that high salinity cannot be in any case attributed to a prolonged water–rock interaction, but is due to the influence of untreated waste water of domestic or brick kiln origin on the shallow and vulnerable aquifers. •Ash ponds have wide environmental and social impact in India.•Isotope ratios can be used as tracers for possible pollution of groundwater.•Isotopes of O, H, Sr, U and Ra have been used to investigate the area of Koradi.•Salinity of groundwater is not due to fly ash but linked to local brick kilns.•A model for the residence time of water based on Ra isotopes is described.