ARSENIC SPECIATION AND MOBILITY IN GROUNDWATER, ANCHORAGE, ALASKA

There are over 11,000 private drinking water wells in Anchorage, AK that draw water from either bedrock or glacial drift aquifers that is used as primary sources of drinking water. In 2004 several of these wells were analyzed for total As concentrations and many were identified as having As concentrations above the EPA drinking water standard of 10 ug/L. Seven wells located in west and south Anchorage (As > 50 ug/L) and one well located on the UAA campus (As > 10 ug/L) were selected for a more specific investigation of the occurrence and speciation of As (3/5) and the influence of seasonality and redox on the mobility of As. Each of these wells was sampled on a monthly basis beginning May 2007. In-situ measurements of static water level, DO, pH, T, and SC were made at each site. Alkalinity was measured by titration in the field and Fe (2) and total Fe were measured calorimetrically beginning July, 2007. Dissolved inorganic As species were separated in the field using anion-exchange chromatography to measure As (3). Major anions were determined by IC and all elemental concentrations were determined by ICP-MS. As (5) was determined by difference from a bulk dissolved analysis and the anion exchange sample. TOC was determined by standard methods. The static water level in the wells ranges from 9.7-97.9 ft. pH ranges from 7.70-8.83, DO is generally < 0.15 mg/L. TOC ranges from < 1.0 mg/L to 48 mg/L. Dissolved As concentrations are < 1.0 ug/L to 116.7 ug/L in May and < 1 ug/L to 73.1 ug/L in June 2007. In general, dissolved As concentrations increase as the static water level decreases and have a positive correlation with alkalinity, TOC, Cl, dissolved Fe and Mn and a negative correlation with pH and dissolved Zn. H2S was detected in one well at 0.04 mg/L and this same well has between 1.7-8.2 mg/L sulfate possibly indicating that sulfate reduction is not a major factor in the groundwater system. Phosphate was detected at levels below detection up to 1.97 mg/L. Initial measurements indicate that As (3) appears to be the dominant As species in the groundwater. Based on field analysis from July 2007 Fe (2) is the dominant Fe species in these waters which may be evidence for Fe reducing bacteria affecting redox conditions and the mobility of As.