Aeolian Transport of Natural Hexavalent Chromium to Ground Water of the Abu Dhabi Emirate, UAE

A ground-water resources evaluation of the Emirate of Abu Dhabi identified elevated values of hexavalent chromium (Cr+6) ranging from 30 to greater than 1000 μg/L, (micrograms per liter) over thousands of square kilometers. Unlike other elevated natural occurrences in ground water of alluvial valley-fill aquifer systems in Paradise Valley, Arizona, León Valley, Mexico, and the Mojave Desert, California, chromium in this aquifer system appears to have been transported from the source rock to the aquifer recharge area as eolian dust (clay sized particles). It is hypothesized that the Cr+3 is derived from weathering of chromium-rich pyroxenes and olivines present in the outcrops and alluvial fans from an ophiolite sequence of the adjacent Oman (Hajar) Mountains. Cr+3, present as a substitute ion in the mineral lattice, is released by weathering and oxidized to Cr+6 by reduction of Mn previously derived by weathering of the same rocks. Cr+6 is sorbed or co-precipitated on iron oxide coating of particles. When the surfaces of these particles are abraded either by wind or water they release fine micron size particles, that when dry, are easily transported over large areas by wind. The dust is deposited of the surface and then Cr+6 is mobilized by infiltrating water and transported into the underlying aquifer. Chromium analyses of rain, dust, and surface deposits are consistent with this model as is an electron probe analyses of minerals from the ophiolite.