Simulation of groundwater recharge from an aquifer storage recovery well under shallow water-table condition

A finite-element groundwater flow model, HYDRUS 2D, was used to simulate drawup and drawdown of piezometric pressure heads in the aquifer storage recovery cycles of varying buffer storage volumes and residence times in a highly brackish, semi-confined aquifer under shallow water-table condition. Physical flow region implemented in HYDRUS 2D involved a soil profile of 500 m width and 69 m depth, with an exocentric elliptical cavity of 1 m radius at 54 m depth. No flux boundary was given at the soil surface, at the bottom and at the lateral sides of the flow region. Saturated hydraulic conductivity was estimated through inverse modelling technique using experimental pressure head time pairs during the first aquifer storage recovery cycle from a piezometer and an observation well. High regression coefficient (0.96), low sum of squared differences of predicted and experimental mean (1.12), and low root mean square error (RMSE; 0.81) between predicted and experimental pressure heads while calibrating the first ASR cycle indicate the high level of accuracy of estimating field-saturated hydraulic conductivity. Modelling efficiency was higher in the piezometer (99.86) than in the observation well (94.57). Overall, the HYDRUS 2D model performed well for simulating drawup and drawdown with RMSE values ranging from 0.26 to 1.29 and modelling efficiency ranging from 94.57 to 99.9 during validation in the second to sixth ASR cycles. Radial influencing zone increases with buffer storage volume with a mean value of 122 m during recharge, indicating that the next tube well should be installed at least 122 m away from the existing aquifer storage recovery well.