Evaluation of Arsenic Levels and Speciation in Groundwater Within Pleasanton, CA

Elevated arsenic concentrations were detected in groundwater monitoring wells and a test well boring beneath the western portion of the Livermore Valley Basin in Pleasanton, California during regular sampling events in 1997. The source of the arsenic was not known, but was suspected from saturated sediments surrounding the wells. Arsenic species arsenite, As (III), has greater toxicity and mobility than arsenate, As (V). The prevalent arsenic species within the sediment and/or groundwater was not known. Forty two soil samples were collected at approximately 50 foot intervals from two wells/borings reporting elevated arsenic concentrations (HOP-8 and HOP-9) and from two control wells in which arsenic was not detected (TH-9 and TH-11). The soil samples were sieved and representative portions of the silt and clay fractions were analyzed for particle size using a laser particle size analyzer (LPSA). Sieve and LPSA results of the soil samples revealed semi-confining to confining layers at depth within HOP-8, and HOP-9, but not within TH-9 or TH-ll. Laboratory analysis of the 42 soil samples reported elevated arsenic concentrations (up to 15 mg/kg) within soil samples collected from HOP-8 at 400 to 500 ft below ground surface (bgs). Groundwater geochemical data was available for HOP-8 and computer analysis of the inorganic water geochemistry using WATEVAL estimated pE values between 5 and 10, which were easily converted to Eh values between 0.3V and 0.5V. These values were plotted on Eh-pH diagrams, from which it was concluded that the arsenic species within the soil and groundwater was predominately arsenate, As (V). Based on the findings of this study, groundwater within semi-confined to confined aquifer zones under mildly reducing, anaerobic methanogenic conditions is most likely responsible for releasing adsorbed arsenic within iron oxides into groundwater solution. Elevated arsenic levels detected within the subsurface soil samples, mean grain size analysis, low concentrations of dissolved sulfate and dissolved iron coupled with estimated pE values, validate these findings. Although arsenate, As (V) was determined as be the dominating arsenic species within soil and groundwater samples collected at 450 ft within HOP-8, the soil samples used for this study were exposed to the atmosphere during storage at Zone 7. It is possible that the dominate arsenic species in groundwater in HOP-8 from 400 to 500 ft may be arsenite, As (III) since the soil and groundwater samples were exposed to the atmosphere during sampling and storage.