Multilayer Interactions in Groundwater Aquifers: Field Measurements and Numerical Modelling

Due to the climatic variability and scares surface water resource, the reliance on groundwater is still a common practice in Sultanate of Oman to meet the present and future water demand. This makes management of groundwater aquifers challenging. The aim of this study is to understand the effect of pumping on different aquifer formations in the Al-Khoud lower catchment area. Accordingly, six groundwater data loggers were installed in six monitoring wells in collaboration with the Ministry of Regional Municipalities and Water Resources (MRMWR). By using these data loggers, groundwater levels were measured every two hours since 22 June 2016. Furthermore, six field visits were conducted to collect manual observations, which are used for verifying records from data loggers. Moreover, long-term groundwater level data from MRMWR were also utilized. Groundwater extraction rates were obtained from the Public Authority of Electricity and Water Supply (PAEW) in 48 wells. Steady-state and transient groundwater-flow models were developed using MODFLOW software considering three aquifer layers; upper gravel, clayey gravel and cemented gravel. Residual errors in model calibration of 11 observation wells in July 2016 were below 1 m with RMSE of 0.50 m. Sensitivity analysis indicates water-levels are more sensitive to the groundwater inflow, water abstraction and horizontal hydraulic conductivity. Direct groundwater recharge effect was found to be the least significant due to urbanization associated with imperviousness in the study area. Vertical hydraulic conductivity is less sensitive compared to horizontal hydraulic conductivity. Water level observations and simulated results reveal 0.6 m to 1.5 m head differences between three layers at two well locations. Model analysis found that groundwater pumping, hydraulic conductivity and vertical anisotropy were the governing parameters affecting the hydraulic head differences. Transient model was calibrated with daily pumping rates disaggregated from monthly dataset. Specific yield of the layers was found to be 0.15 at the upper gravel, 0.08 at clayey gravel and 0.05 at cemented gravel. Model shows that the smaller specific yield and higher pumping rate collectively contribute for significant water-levels fluctuations. Sensitivity analysis indicates that the screen depths of pumping wells and discharge rates affects the water level variations in different layers. Effect of groundwater pumping in PAEW well zone near to the Al-Khoud dam dissipate towards the coast. In contrast, local pumping from private wells found to have a significant effect on groundwater level variations in the downstream area.