Spatial and Temporal Variation of Dissolved Arsenic Concentrations in the Intermediate Aquifer of the Lahontan Valley, Nevada

A study was conducted in the intermediate-depth alluvial aquifer (50-500 ft) of Lahontan Valley, Nevada, to determine if there is temporal and spatial stability of dissolved arsenic concentrations, and if chemical mechanisms commonly associated with arsenic mobility are responsible for any trends observed. This aquifer is an alkaline and reducing environment that is chemically distinct from hydrologic units above and below it. Five wells previously observed to have substantial concentrations of both As(V) and As(III) were chosen to represent a groundwater flow-path and were sampled monthly from February 2004 - August 2005 . Arsenic concentrations were observed to be statistically unchanged for the sampling period at each well, as were the majority of other chemical constituents and physical characteristics. Spatially, total arsenic activities [AsT] increased nearly seven-fold along the flow-path, and the ratio of the concentration of arsenic as arsenite to total arsenic ([As(III)]/[AsT]) decreased. The concentration of total arsenic correlated with dissolved organic carbon (DOC), [HCO3-], and distance along flow-path (p = 0.04, 0.01, 0.02, respectively).NETPATH 2.0 and Visual MINTEQ 2.3 geochemical models were employed to model geochemical reactions controlling potential arsenic release to the aquifer using both field -measured and modeled electric potential (Eh) values. Models show minor desorption of AsO43- from hydrous ferric oxides (HFO) along the flow-path (~0.5%), and to a lesser degree with increased pH. H3AsO3 desorbed to a greater degree along the flow-path according to models, with little pH dependence. Arsenic out-competed phosphate for sorption sites according to the models. Competition from other ligands was also likely. Model analysis indicated that reductive dissolution of HFO did not occur at field-measured Eh and average pH for each well. However, under MINTEQ-computed Eh values that were considerably lower, HFO reductive dissolution was shown to occur, potentially releasing arsenic to groundwater. Further research into the strong correlation of AsT with DOC and HCO3- is suggested