Assessment of deep aquifer development and its implications in granitic terrain using isotopes and chemistry

Abstract

This study is conducted in the Precambrian granitic terrain, to demonstrate the presence of the multi-aquifer system and examine the inter-communications between them. Distinct hydrochemical characters of the groundwater from shallow (~ 100 m) and deep (~ 400 m) wells, signifies their independent and unconnected nature initially. Hydrochemistry of deep groundwater is approximately constant (either Ca-Na-Cl or Ca-Na-Cl-SO4 type of water) during January 2015 to June 2016. Repeated carbon-14 measurements during the same period show the 14C activity of about 35 to 85 pMC (residence time about 3 to 8 ky BP). However, as a result of excess rainfall during 2016, hydrochemical facies of deep groundwater changed initially to Na-Ca-SO4-Cl and subsequently stabilised at Ca-Na-HCO3-Cl type with a drastic reduction in Cl and increased HCO3 and NO3 concentrations, while the 14C activity turned out to be to 100 pMC (modern age). These changes are attributed to ingression of fresh water into the deep aquifer, after paleo-groundwater depleted due to prolonged drought conditions along with the over-exploitation of limited potential deep aquifer. These drought and over-exploitation situations potentially improved the migration potential of fresh water to the deep aquifer and led to enhancing the groundwater recharge during the excess rainfall years. This brings a new perspective to the hydrogeological dynamics between shallow and deep groundwater. A conceptual model is proposed to explain the observed phenomenon. This study also suggests that, under climate driven drought conditions, deep aquifers could act as emergent groundwater resource to meet the water demands amid population growth.