The stable-isotopic composition of water and sulfate from the Raudhatain and Umm Al Aish freshwater fields, Kuwait

Abstract

The desert plain of eastern Arabia slopes gently northeast over Kuwait where it is interrupted only by low hills, depressions, escarpments and shallow wadis. These wadis carry occasional runoff and drainage is mainly internal. Rainfall is variable and averages 100 mm yr. −1 most of it falling during intermittent winter thunderstorms. Average winter lows are 9°C and summer highs 45°C Evaporation potential far exceeds precipitation. High evaporation and low rainfall lead to the formation of a getch layer: a duricrust of gypsum and/or calcite within the desert sands. Underlying Kuwait Group sediments (450 m of sand, gravel and limestone) and Dammam Formation (200 m of dolostone) are the main aquifers in Kuwait. They are underlain by the Rus Formation (125 m of anhydrite) which is an aquiclude. Lenses of fresh groundwater (salinity 200–1400 mg 1 −1) occur in the depressions of Raudhatain and Umm Al Aish. Brackish and ultimately salty waters occur in the Kuwait Group and Dammam aquifers. Stable-isotopic analyses of rainwater show a deuterium excess of 10–20%. and reflect influences from both the Indian Ocean and the Mediterranean. Raudhatain and Umm Al Aish samples also lie about meteoric water lines (on a δ 180 vs. δ D plot) with averages: δ 18O = −3 ± 1%., δ D = −10%. and deuterium excess 17%o. They represent rainwater recharge during flooding; no effects of surface evaporation can be detected. In contrast, the brackish paleowaters of the Kuwait Group and Dammam Formation have much lower δ 180- (−4.5%.) and δ D- (−35%.) values and are modified meteoric water from a cool wet period in eastern Arabia 11,000–60,000 yr. B.P. No isotopic evidence can be seen for the mixing of the fresh and brackish waters. The Umm Al Aish waters (salinity 200–500 mg 1 −1 are more dilute than Raudhatain (up to 1400 mg 1 −1 where greater draw-off may have caused some intrusion of brackish water. The isotopic compositions of sulfate in the Raudhatain and Umm Al Aish waters (δ 34S = +15.3–16%. and δ 180 = +13.6–15%.) group away from Arabian Gulf seawater and the Rus anhydrite. The source of the sulfate appears to be rainwater-sulfate plus sulfate leached from the getch.