Abstract
Groundwater stored in the deep seated sedimentary aquifers is the most important source of water supply. The lack of sufficient groundwater recharge and the overdependence on groundwater might lead to unavailability of this precious natural resource if proper management practices are not adopted. Finite difference modelling using the MODFLOW program was carried out in the east of Riyadh city to simulate the groundwater level conditions under different abstraction scenarios. The simulated aquifer system combines the Wasia and Biyadh aquifer (composed of sandstone) and Aruma aquifer (limestone) which lies between 24 degrees 30'00"25- 30'00" N and longitudes 47 degrees 00'00"-48 degrees 00'00" E. The transmissivity and storage coefficient values of Biyadh aquifer are 7.0x10(-3) to 7.0x10(-2) m(2)/day and 3.7x10(-4) to 9.4x10(-4) respectively. The transmissivity and storage coefficient values of Wasia aquifer ranges from 6.7x10(-3) to 8.5x10(-2) m(2)/day and 2x10(-4) to 2.3x10(-4) respectively. The model calibration involved altering the values of model input parameters to match field conditions within certain acceptable limits to forecast the aquifer response over a period of 35 years (2015-2050). The modelling grid consisted of 20 and 24 columns with the grids spacing of 4 km for the small grids and 6 km for large grids. The results showed that though the Wasia aquifer was productive, it showed a large decline in water levels if water abstraction continued at the present rate. If the existing trends of groundwater withdraw continues; the piezometric heads in Wasia and Biyadh aquifers will decline by the year 2050. A reduction in 25% of the existing groundwater pumping rate in the well field will minimize the rate of groundwater decline in the aquifer to a considerable extent.