Earthquake (M 5.1) induced hydrogeochemical and δ18O changes: Validation of aquifer breaching-mixing model in Koyna, India

Abstract

Hydrochemical parameters and oxygen-18 (δ18O) were measured in groundwater -from wells and spring waters from the active seismic area around Koyna-Warna reservoirs over a period of three years (2005-2007) to explore their relationship with seismicity. In comparison to average values observed later, hydrochemical data of one of the wells revealed increased concentrations of chloride (100 per cent), sulphate (300 per cent) and fluoride (700 per cent) while the δ18O value decreased contemporarily to the occurrence of an earthquake of magnitude 5.1 on 2005 March 14. Based on the hydrochemical and isotopic characteristics of different well waters in this area, we conceptualize a model of aquifer breaching and mixing of deep aquifer water with shallow aquifer water due to the earthquake. We estimated the fractions of two components of different aquifer waters mixed, using two conservative tracers, chloride and δ18O. The necessary mixing proportions of ~50 per cent are unprecedented and call for further work to understand the earthquake-related processes causing the observed hydrogeochemical and isotopic anomalies in the Koyna-Warna region. © 2010 The Authors Geophysical Journal International © 2010 RAS.