URANIUM AND RADIUM IN THE SABKHA ENVIRONMENT: NEW CLUES FOR INTERPRETING THE ORIGIN OF HYPERSALINE BRINES

The coastal area of Abu Dhabi, United Arab Emirates, is the classic site for investigations into the formation of marine salt flats, or sabkhat. Historically, three models for the development of hypersaline water in this environment have been proposed, calling upon a number of processes and sources: 1) simple seawater flooding and evaporation, 2) seawater and local groundwater inflow, mixing and evaporation, and 3) seawater inflow, deep-basin brine upflow, mixing and evaporation. Uranium and radium have been analyzed in sabkha groundwater and underlying Tertiary formations of coastal Abu Dhabi in an attempt to identify the sources of solutes in sabkha brine.

Water samples were collected from sabkha sediment along a transect perpendicular to the shoreline, from the high-tide line to several kilometers inland. Wells were installed at six locations along the transect. At the two locations nearest the Arabian Gulf, brine of high uranium content (up to 16 ug/L), uranium activity ratios (UAR) of seawater value (1.15), and low radium-226 activity were found, all typical of modern seawater. The remaining four landward locations of the transect contained waters of more variable composition. In general these waters had low uranium concentrations, UAR values higher than that of seawater (up to 1.62), and high radium-226 activities, all of which are similar to oil field deep-basin brines found in the underlying Tertiary section.

Water and drill cuttings from wells penetrating several hundred feet below the sabkha sediments were also collected and analyzed for uranium and radium. UAR values of many solid phase samples were not at secular equilibrium (1.000), indicating re-crystallization of the formations immediately below the sabkha sediments. This is the result of water-rock interactions occurring within the last several thousand years, supporting the concept of upward flow of brine to the sabkha.

Results support the model requiring seawater inflow, deep- basin brine upflow, mixing and evaporation in the genesis of sabkha brine.