APPLICATION OF AS-SULFIDE OXIDATION RATES TO THE EVALUATION OF SURFACE WATER QUALITY IN SEMI-ARID ENVIRONMENTS

Experiments were conducted on four arsenic-sulfide solids, orpiment, As2S3(am), realgar, and AsS(am), to determine rates of oxidative dissolution through a range of dissolved oxygen (DO) and pH values. One objective of our experimental approach was to determine the rates of release of arsenic from arsenic-sulfide solids within the neutral to alkaline pH range and oxidizing conditions that are characteristic of surface waters found in the semi-arid Western USA. Experimental methods will be generally described in this presentation because the details have been presented elsewhere previously. Experimental results generally show that the oxidative dissolution of arsenic-sulfide solids increases with increasing pH at a constant level of DO and with increasing DO concentrations at a relatively constant pH value. The results of our experimental work have been applied to sensitivity analyses of arsenic concentrations using a simple conceptual model of a surface water in which DO, pH and mineral surface reactivity are varied in a series of geochemical modeling calculations. Results of these sensitivity analyses show that within oxidizing, alkaline conditions, the rate of release of arsenic from arsenic-sulfide solids reaches the 10 ppb Maximum Contaminant Level (MCL) in less than a month assuming 100% surface reactivity. These “worst case scenario” results, compounded with the influence of evaporation in a semi-arid climate and decreased adsorption of dissolved arsenic species to hydrous ferric oxide surfaces at alkaline pH values, may lead to high arsenic concentrations in surface waters in contact with arsenic-sulfide minerals. Precipitation of soluble arsenic salts, such as scorodite, may control the upper limit of arsenic concentrations in surface waters, but likely may not control concentrations to within MCL.