Determination of hydrochemical processes and classification of hydrochemical facies for the Sakakah Aquifer, northeastern Saudi Arabia

Abstract

The Sakakah Aquifer is a hydrostratigraphical unit that is being exploited in the northeastern part of Saudi Arabia (Sakakah region). This area has received increasing attention since 1965, when economic reserves of phosphorite deposits were discovered. The population of the area, as well as demand for groundwater, has increased since then. Therefore, hundreds of groundwater wells were drilled, tapping the Sakakah Aquifer and other water bearing units. Groundwater of the Sakakah Aquifer was found to range in quality between fresh (potable) water and relatively saline water (TDS exceeds 3000 ppm). In the present study, several hydrochemical techniques were used to classify water type facies and to determine the hydrochemical processes controlling the water quality variations. Firstly, the classification of water type facies was done through the use of Durov diagrams. Secondly, the hydrochemical processes were determined by observing the water quality changes down-gradient. This is done by constructing hydrochemical cross-sections. Thirdly, the stoichiometric balance was checked by utilizing the x-y relationships between the major dissolved constituents. The Sakakah groundwater was classified into three distinct facies. Water type I is a calcium bicarbonate water with Na becoming important. This water type is believed to be recharge water of recent origin. Water type III is relatively saline sodium chloride water. This water is believed to be fossil water of old origin residing in the interstices. Water type II represents a mixture of the previous two types. Several hydrochemical processes have an influence on the Sakakah Aquifer groundwater facies. The most influential of these processes are mixing and ion-exchange. Oxidation occurs in the low salinity zone, as O enriched recharge water reaches the aquifer through fault planes and fracture zones. Therefore, the structural complexity of the study area also plays an important role in controlling water quality. Finally, reduction reactions may occur in isolated cases and do not represent a trend of any kind.