Hydrogeological and quantitative groundwater assessnlent of the basaltic aquifer, northern harrat Rahat, Saudi Arabia

The Northern Harrat Rahat consists of 300m of basalt lavas covering some 2,000 km² to the south-east of AI-Madinah in western Saudi Arabia. Like many basalt sequences, the Rahat basalts form an important aquifer and groundwater resource. The aquifer has a saturated thickness of up to 60m and is made up of the weathered upper part of underlying basement, pre-basalt sands and gravels, and the fractured basalts. Since 1992, groundwater has been abstracted from the aquifer as part of the Al-Madinah water supply. To assess the potential of the aquifer, an assessment has been made based on pumping tests of about 70 wells. The hydraulic parameters have been shown to be highly variable, typical of the fractured domain. The aquifer contains good-quality water in storage, but recei ves limited recharge. Groundwater temperature anomalies indicate remnant volcanic activity locally. A numerical groundwater model has been constructed, which has been calibrated using limited groundwater head measurements, but with good abstraction records. Predictions of groundwater heads and the examination of several abstraction scenarios indicate that the aquifer can continue to support part of the Al-Madinah demand for the next several years, if certain well distributions are adopted. The predictions also show that the aquifer can only support the total demand of the city for a few days as a contingency resource.