FACTORS CONTROLLING GROUNDWATER QUALITY IN EASTERN UNITED ARAB EMIRATES: A CHLORINE-36 APPROACH

Chlorine-36 along with oxygen and hydrogen isotopes and select major ions are used to investigate the factors controlling groundwater quality in the eastern part of the United Arab Emirates (UAE). Two main aquifers occupy the study area: The Eastern Gravel Plain aquifer that occurs near the coastal area, and the inland Ophiolite aquifer. Previous studies suggest that groundwater, particularly in the Eastern Gravel Plain aquifer, is affected by seawater intrusion and evaporation. Samples of groundwater from the Eastern Gravel Plain aquifer (Cl >1000 mg/l) have R³⁶Cl values in the range of 13.3 to 17.4 x10^-15 while those from the Ophiolite aquifer (Cl <517 mg/l) have R³⁶Cl values in the range of 20 to 118 x10^-15. These values indicate the presence of bomb-produced 36Cl and are consistent with recent (post 1950) recharge of most of the samples. dD - d¹⁸O relationship plots on a typical evaporation line, which suggests evaporation of the recharging waters prior to infiltration. The R³⁶Cl values of the coastally- bound Eastern Gravel Plain aquifer are lower than the Ophiolite aquifer which suggests an additional input of chloride to this aquifer. Moreover, in most cases the chloride concentration is not balanced by the concentrations of other cations such as sodium, potassium, calcium or magnesium. Seawater intrusion is thus ruled out. Measurements of bromide concentrations support the contention that organically bound Cl is being released into this environment. Agricultural practices, which are extensive in this region, are a possible source. Also, sodium and silica concentrations provide evidence for the release of these elements by the weathering of host rocks.