GEOCHEMICAL PROCESSES AFFECTING ARSENIC DISTRIBUTION IN ARID BASINS OF THE MOJAVE DESERT, USA

Surficial sediments in some arid basins are known to contain high arsenic (As) concentrations, whereas other nearby basins lack such apparent enrichments. Rock-type and ore mineralization are prominent in producing this contrast, but geochemical processes that vary among basins have a direct influence on active As redistribution to produce areas of enrichment. We propose that some As enrichments are the product of alkaline water formed in basins by geochemical evolution of groundwater. The high pH induces desorption of As from sediment surfaces. Subsequent evaporation of the resulting solution produces high As concentrations in surface crusts and within the vadose zone sediment. Such accumulations represent a hazard to ecosystem as well as human health. Dust emissions from the basin surfaces spread the labile arsenic. Bioleaching with simulated body fluids suggests that As bound to dusts from our study sites can be released and assimilated by the human body.

Water, sediment, and surficial salts from two geochemically distinct basins along the California-Nevada border of the Mojave Desert were sampled and chemically analyzed. The Franklin Lake playa along the Amargosa River is underlain by alkaline, saline, high-As groundwater (pH, 9.6; TDS, 100 g L^-1; As, 16 mg L^-1). Dispersed surface salts include halite, trona, burkeite, and thenardite. Franklin playa sediment underlain by shallow ground water (<1 m) has large arsenic enrichments in salts at the ground surface (100 ppm). Areas in the basin with deeper ground water (>1.5 to 4 m) have maximum water-soluble As enrichments (20 ppm) within the vadose zone sediment. In contrast to the Franklin Lake playa, the Mesquite playa basin is underlain by circumneutral, saline, low-As water (pH, 7.2; TDS, 200 g L^-1; As, <10 μg L^-1). Halite and gypsum are the common salts in this basin. Water extraction of surficial and vadose zone sediment from the Mesquite basin released little As (<0.1 ppm), but treatment with a pH 9.5 solution of carbonate-bicarbonate released 1.6 ppm (whole-sediment basis). This As release is attributed to desorption from ferric oxides that coat silt and clay. Some basins that lack areas of As enrichment likely have an ambient water composition that does not evolve to alkaline pH, allowing labile As to remain dispersed on the sediment.