Evaluation of the Saq aquifer groundwater resources in Al Ula, Saudi Arabia

Regional sandstone aquifers in arid areas provide prolific water resources that can meet human water demands, but their sustainability is at risk where recharge is very low and significant withdrawals occur. The Saq sandstone aquifer in the Al Ula area (Saudi Arabia) has been investigated through a field survey that included groundwater level measurement, groundwater sampling and analysis, crop inventory mapping and irrigation practice enquiries to assess groundwater abstractions. A groundwater model has been built to assimilate the collected data and investigate the aquifer response under various abstractions scenarios to meet future agricultural and domestic water demand. Since the 1960’s, water levels have dropped by 5–20 m within the Al Ula Valley, with maximum water level drawdowns reaching 30–40 m in Al Ula City. Present water level drawdown rates range from 0.5 to 2 m year^–1 in the agricultural areas of the valley. Less than half of the sampled groundwater is in compliance with regulatory guidelines for drinking water purpose, and high groundwater nitrate concentrations are observed in agricultural areas. Predictive scenarios of decreasing abstractions suggest a reduction in the water level decline by 2050 but do not reverse the current decreasing trend. The estimated recharge is much lower than groundwater abstraction rates and current withdrawals are effectively mining the groundwater reserves. Further challenges arise from a water quality standpoint, due to the presence of radionuclide activities (mostly ²²⁸Ra) in the aquifer above the WHO guidelines.