Solute recycling: An emerging threat to groundwater quality in southern India?

Groundwater from crystalline aquifers is abstracted at large rates for paddy irrigation in southern India resulting in widespread over-exploitation of the resource. Detailed field studies at watershed scale have shown that basin closure is happening (i.e., groundwater contribution to base flow has stopped) and irrigation return flow can contribute to as much as half of the aquifer recharge. Studies in other semi-arid regions have shown that irrigation return flow, through a process known as solute recycling, can contribute significantly to aquifer salinisation. To evaluate the impact of this process in the southern India context, a lumped reservoir model has been designed in order to simulate long-term trends of piezometric levels and solute concentrations at watershed scale. The model is applied to the well studied watershed of Maheshwaram (53km²), located 40km South of Hyderabad. It can reproduce qualitatively watershed-average groundwater levels and chloride concentrations inferred since 2001 that shows a progressive buildup. Simulation of the period 2010–2044 indicates that forecasted reservoir concentrations are very sensitive to aquifer mixing efficiency. In the case of complete mixing, base flow that activates after rainy years may export significant solute mass and level off aquifer concentration to acceptable levels. In the more realistic case of incomplete mixing, diluted base flow will export less solute and progressive solute mass buildup continues throughout the simulation period to end up with concentrations close to the ones that makes water no longer suitable for irrigation. Final aquifer concentrations may become even higher with scenarios that accelerate the lowering of the water table such as higher pumping rates, decrease in daily rainfall or increase in daily evaporation. These simulations show that solute recycling may have a significant negative impact on groundwater quality in southern India, especially in aquifers located in semi-arid hard-rock areas where the main source of irrigation is provided by groundwater.