Mineral Precipitation and Trace Oxyanion Behavior during Evaporation of Saline Waters

Evaporation basins used for the disposal of agricultural drainage waters in central California contain elevated trace element levels that pose hazards to groundwater quality and wildlife visiting the ponds. A study was conducted to evaluate the solution dynamics of mineral-forming elements and trace oxyanions (U, Mo, and V) during evaporation of saline waters whose chemical compositions evolve according to two distinct chemical divides, and to characterize the evaporite minerals formed from the complete evaporation of these waters. The alkali and alkaline earth metals exhibited nonconservative behavior, forming evaporite minerals such as bloedite, calcite, aragonite, gypsum, halite, thenardite, and trona. Molybdenum behaved nonconservatively, while V exhibited conservative behavior that did not differ whether V was initially added as V(IV) or V(V). Uranium displayed conservative behavior under conditions of low U concentrations and high alkalinities. Nonconservative behavior was observed for U, however, under higher U concentrations and low alkalinities. We conclude that V and U in waters with alkalinities >10 mmol_c L^-1 will not accumulate in evaporation pond minerals. In ponds with low alkalinity, U will partition to a solid mineral phase.