DETERMINATION OF HYDROCHEMICAL PROCESSES AND CLASSIFICATION OF HYDROCHEMICAL FACIES FOR THE SAKAKAH AQUIFER, NORTHEASTERN SAUDI-ARABIA

The Sakakah Aquifer is a hydrostratigraphical unit that is being explo ited in the northeastern part of Saudi Arabia (Sakakah region). This a rea has received increasing attention since 1965, when economic reserv es of phosphorite deposits were discovered. The population of the area , as well as demand for groundwater, has increased since then. Therefo re, hundreds of groundwater wells were drilled, tapping the Sakakah Aq uifer and other water bearing units. Groundwater of the Sakakah Aquife r was found to range in quality between fresh (potable) water and rela tively saline water (TDS exceeds 3000 ppm). in the present study, seve ral hydrochemical techniques were used to classify water type facies a nd to determine the hydrochemical processes controlling the water qual ity variations. Firstly, the classification of water type facies was d one through the use of Durov diagrams. Secondly, the hydrochemical pro cesses were determined by observing the water quality changes down-gra dient. This is done by constructing hydrochemical cross-sections. Thir dly, the stoichiometric balance was checked by utilizing the x-y relat ionships between the major dissolved constituents.The Sakakah groundwa ter was classified into three distinct facies. Water type I is a calci um bicarbonate water with Na becoming important. This water type is be lieved to be recharge water of recent origin. Water type ill is relati vely saline sodium chloride water. This water is believed to be fossil water of old origin residing in the interstices. Water type II repres ents a mixture of the previous two types. Several hydrochemical proces ses have an influence on the Sakakah Aquifer groundwater facies. The m ost influential of these processes are mixing and ion-exchange. Oxidat ion occurs in the: low salinity zone, as O enriched recharge water rea ches the aquifer through fault planes and fracture zones. Therefore, t he structural complexity of the study area also plays an important rol e in controlling water quality. Finally, reduction reactions may occur in isolated cases and do not represent a trend of any kind. (C) 1998 Elsevier Science Limited.