By Stephen R. Hinkle and Danial J. Polette
USGS Water-Resources Investigations Report 98-4205, 28 pages, 6 figures, 4 tables, 1 appendix, 1 plate
The spatial distribution of arsenic concentrations in ground water of the Willamette Basin was assessed by analyzing data from 728 historical and project sites. Concentrations of arsenic ranged from < 1 to 2,000 mg/L (micrograms per liter). Concentrations in 58 (8.0 percent) of the samples exceeded the U.S. Environmental Protection Agency (USEPA) current Maximum Contaminant Level (MCL) of 50 mg/L.
Regionally, the distribution of arsenic concentrations appears to be primarily related to aquifer geology. High arsenic concentrations (concentrations exceeding the USEPA current MCL) are widespread in ground water in bedrock areas in south-central and eastern Lane County, and in eastern Linn County. High concentrations of arsenic also are occasionally detected in ground water in the Tualatin Basin. High arsenic concentrations in bedrock areas in Lane and Linn Counties appear to be associated with two regionally extensive associations of rocks: (1) the Fisher and Eugene Formations and correlative rocks, and (2) the undifferentiated tuffaceous sedimentary rocks, tuffs, and basalt. These associations of rocks include extensive volumes of silicic (rhyolitic) volcanic rocks, which commonly are associated with high concentrations of arsenic. High arsenic concentrations in the Tualatin Basin are associated with alluvial deposits. At a regional scale, well depth does not appear to be a useful predictor of arsenic concentration in the Willamette Basin.
Temporal variability in arsenic concentrations was characterized through quarterly sampling at 17 wells. Temporal variability ranged from very little to variation by nearly a factor of three. No obvious correlation with seasons was apparent. One possible explanation for observed temporal variability in arsenic concentrations is that the pumping history of the wells prior to sampling may have been variable. Variability in pumping history may have led to different sources of water being pumped, and thus, differences in water chemistry. The observed temporal variability in arsenic concentrations was generally small relative to spatial variability in the basin. Although the temporal variability in arsenic concentrations was small at most sites sampled in this study, it may be necessary to collect water samples under a variety of well-use conditions and seasons to adequately characterize arsenic concentrations when wells used for drinking water are being sampled.
Water samples from five domestic wells were analyzed for arsenic species, and two additional analyses for arsenic species in ground water from the Willamette Basin were available in the literature. Arsenite was the predominant species of arsenic in six of these seven samples. Arsenite is more toxic than arsenate, and arsenite also is more difficult to remove from drinking-water supplies.
Regional patterns of arsenic occurrence in the Willamette Basin indicate that the sources of arsenic in ground water are not human related. Arsenic-containing metal oxides, volcanic glass in volcanic rocks of rhyolitic to intermediate composition, and clays are likely sources. Evolution of high-arsenic ground water in bedrock areas of Lane and Linn Counties may be controlled in part by arsenic adsorption/desorption reactions that commonly control arsenic mobility. One or more of the following factors likely affect arsenic adsorption/desorption reactions in parts of these areas: (1) high pH, (2) presence of competing anions, and (3) occurrence of reducing conditions. For alluvial ground water of the Tualatin Basin, (1) presence of competing anions and (2) occurrence of reducing conditions may be important controlling factors in arsenic adsorption/desorption reactions. Dissolution of iron oxides, with subsequent release of adsorbed and (or) coprecipitated arsenic, also may play an important role in arsenic mobility in ground water of the Tualatin Basin.
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