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Field work was carried out during the years 1971-73 to investigate the hydrology of the Abu Dhabi sabkhas with the purpose of determining (1) the source of subsurface water inducing the diagenesis of Holocene sediments and (2) the directions and rates of hydrologic movements. The ionic ratios of Cl/Br and K/Br and the stable isotope contents of the subsurface brines of the sabkha separated them into three distinct categories according to their origin: (1) coastal sabkha zone, of evaporated marine waters from supratidal flooding (a) daily near the coast from the lagoon and (b) occasionally farther inland from the open sea; (2) intermediate sabkha zone, a mixture of marine waters with meteoric groundwaters which are isotopically altered by capillary evaporation and/or diage esis, that is, the oxygen-18 content increases while the deuterium content remains relatively constant; and (3) continental sabkha zone, of meteoric groundwater with variable isotopic composition as a result of evaporation and sporadic addition of rainwater.
The intermediate sabkha zone is the site of extensive diagenesis, precipitation of gypsum and anhydrite, and formation of dolomite. Unusual winter storms in conjunction with spring tides produce high supratidal flooding in the intermediate zone by open seawater. The groundwater table rises nearly to the surface. Floodwaters dissolve and transport away interstitial salts, which are carried seaward surficially or downward through the aquifer at an average rate of 11 cm/year. Subsequent secular evaporation tends to lower the groundwater table and induce Darcy flow under a vertical hydraulic gradient of evaporative pumping, that is, upward movement of water through the saturated zone to replace water lost by capillary evaporation. An appreciable vertical groundwater gradient is induced by the presence of a cemented crust which serves as an aquiclude about 1 to 2 m below the surface.
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