Occurrence, Distribution, And Speciation of Arsenic in the Southern High Plains Aquifer System

Significant levels of arsenic have been detected in the groundwater of the Southern High Plains. The potential sources include atmospheric deposition, the use of agricultural defoliants and natural subsurface geochemical interactions. To identify the source of arsenic, groundwater and soil samples were collected from sites spread over 18 counties in the Southern High Plains. Total arsenic and its inorganic species were quantified along with commonly occurring and related cations and anions such as iron, manganese and sulfate. Correlation studies were conducted to understand the variation of arsenical species with related parameters. A geochemical modeling tool, MINTEQ was used to predict the speciation of arsenic and compare these results with lab analyses. The distribution of arsenic in the soil profiles tested indicated a positive correlation with depth. The highest concentrations were found close to the water table while the upper soil layers had low to non-detect concentrations. In the groundwater samples, arsenic concentration and speciation varied significantly between sites. As (III) was found to be the dominant species in over 80% of the samples. MINTEQ speciation forecasts compared favorably with a majority of the groundwater analyses. Very little evidence of atmospheric deposition exists and the sources of arsenic are likely anthropogenic land sources in the shallow subsurface and natural geologic processes in the deeper subsurface.