Seasonal fluctuations and mobility of arsenic in groundwater resources, Anchorage, Alaska

摘要

There are approximately 12,000 private wells in the Municipality of Anchorage, Alaska, USA, that supply drinking water to thousands of homeowners. The results presented in this paper are from a study conducted to understand the speciation and seasonal fluctuations of As in the groundwater of Anchorage. A total of eight private drinking water wells were sampled from May to October, 2007, to determine inorganic species of As (III/V) and other physiochemical parameters of the groundwater. Arsenic concentrations above Environmental Protection Agency (EPA) drinking water standard of 10μg/L had been previously measured in all of these eight wells by the Municipality of Anchorage. Seven of the wells draw water from glaciofluvial aquifers and one well taps into the older bedrock aquifer on the mountain-side. In-situ measurements as well as groundwater samples were collected from each well and inorganic species of As (III/V) were separated in the field using anion exchange columns. Elemental analysis was conducted by inductively coupled plasma mass spectrometry (ICP-MS), major anions by ion chromatography (IC), and Fe (II) and total Fe were measured in the field with a portable spectrophotometer. The groundwater was neutral to basic ranging in pH from 7.6 to 8.8, dissolved O_2 was generally 0.1-1.0mg/L and specific conductance ranged from 300 to 1000μS/cm. Most of the groundwater is classified as Ca-Mg-HCO_3 and dissolved As concentrations ranged from below detection (~0.5μg/L) to 117μg/L with arsenite as the dominant species. Filtered and unfiltered water samples had less than 1% difference in As concentrations suggesting that most As occurs in the dissolved form. Arsenic concentrations were positively correlated with water levels indicating that the highest As concentrations occur in the aquifer during recharge events. Positive relationships with dissolved Fe and supersaturation with respect to secondary Fe oxides indicates that the As is likely associated with the Fe oxides that are partially dissociated under the dominating reducing conditions of the aquifers. There is also evidence of a positive relationship between As and SO42- which indicates that some of the As may be associated with the oxidation of Fe-bearing sulfides such as pyrite or arsenopyrite, however, this is thought to be a less important process for this system compared to Fe reduction. Most of the groundwater samples indicate supersaturation with respect to carbonates such as calcite, dolomite and siderite and there is a positive relation between As and HCO3- indicating that carbonate buffering is an important process in the groundwater geochemistry of As. In some cases homeowners filter their tap water with household water treatment systems which have a range of As removal effectiveness from about 25-100%.