Shallow ground water chemistry of arsenic, fluorine, and major elements: Eastern Owens Lake, California

摘要

Owens Lake in SE California became essentially dry by the 1920s after the Los Angeles Aqueduct was constructed and diversion of water from the Owens River began. Frequent dust storms at Owens Lake produce clouds of efflorescent salts which present human health hazards as a result of their small particle size and elevated concentrations of As and SO_4. This study was conducted to characterize the evolution of major elements in ground water in eastern Owens Lake and to examine the factors controlling the concentrations of dissolved As and F. Evapoconcentration of shallow ground waters at the lakebed surface produces high pH, high alkalinity brines with major ion compositions that are consistent with those predicted by the Hardie-Eugster Model. Evaporite minerals identified in the surface salts using XRD were halite (NaCl), thenardite (Na_2SO_4), trona (Na_3H(CO_3)_2·2H_2O), pirssonite (Na_2Ca(CO_3)_2·2H_2O), and nesquehonite (MgCO_3·3H_2O). Significant correlations between both As and F with Li in shallow ground waters indicate that As and F are not partitioned into surface salts until very high salinities are reached (> 9.0 m). Leaching experiments show that As and F can be readily released from lakebed salts when exposed to natural precipitation. Conservative behavior of As and F results from the high pH values and low Ca activities of shallow ground waters that contribute to: (1) redox stability of As(V) even at moderately reducing conditions, (2) a decrease in the adsorption affinities of As and F to mineral surfaces, (3) undersaturation with respect to fluorite (CaF_2(s)).