Managed Aquifer Recharge in Saline Groundwater Zones

Abstract

Managed aquifer recharge is a cost-effective method for water resource management and environmental protection. Artificial recharge, along with the pumping of salt water, is also an effective way of mitigating saltwater intrusion in coastal aquifers. The Aquifer Storage and Recovery (ASR) through artificial recharge can be used for sustaining freshwater supply in the salt-affected aquifers. The recovery efficiency (RE) is used to quantify the efficiency of the recovered freshwater from the recharged aquifers targeted during the ASR method. Previous studies are primarily based on the injection of fresh water and studying their behaviour in the subsurface storage and movement. The effect of depth of release of fresh water in the subsurface within the saline aquifers and injection rate impacts are not investigated thoroughly for quantifying its impact on RE. This study aims to understand aquifer recharge in saline zones through numerical modelling for optimizing the depth of release of fresh water in the saline aquifers. COMSOL Multiphysics is used here to model the recharge dynamics of freshwater pocket in the salt affected groundwater zones. For numerical modelling, four different injection rates viz 5 mL/min, 15 mL/min, 30 mL/min and 45 mL/min were considered, and their associated RE was simulated. The movement of freshwater when released at different depths within the aquifer, viz top, centre and bottom was also simulated. Laboratory experiments were also conducted under controlled conditions using a setup of a 3-D tank to quantify the RE within a homogenous and isotropic sand aquifer for different scenarios of the freshwater release depths. The results showed that RE starts decreasing with an increase in depth from the top of the aquifer having high porosity by keeping the pumping well and recharge well at the same location. The RE was found to be reduced by 37%, 25% and 17%, respectively, by increasing depth from 15 cm to 45 cm from the aquifer top. The obtained results of RE were qualitatively compared with the salt concentration of the water samples collected at different locations within the tank aquifer setup. The analysis showed that as the depth of release increases, the buoyancy effect also increases, which; resulted in the upward movement of the freshwater at a faster rate, causing the reduction in RE with depth in highly porous media. This study will be helpful in a better understanding of the ASR in the saline zones and provide an improved method to enhance Recovery Efficiency of the injected freshwater.