Characterizing the Transport of Dissolved Arsenic Associated with Petroleum-Impacted Aquifers

摘要

The recent lowering of the U.S. arsenic drinking water standard from 50 to 10 μg/L is driving federal, state,rnand local agencies to closely examine water supplies for contamination with even trace amounts of arsenic.rnArsenic has been observed in groundwater in association with petroleum-impacted aquifers at a variety ofrnsites. During anaerobic biodegradation of hydrocarbons, Fe(III)-reducing microorganisms can couple thernreduction of solid phase Fe(III)-oxides to soluble Fe(II) with the oxidation of petroleum compounds. Asrnthe Fe(III) is reduced,, metals and nutrients that are sorbed onto the Fe(III) oxide surfaces may be releasedrnto solution. As a consequence, arsenic, which has a strong sorption affinity for Fe(III) oxides, can bernreleased to groundwater.rnAn investigation has been conducted at a petroleum-contaminated site where elevated levels of dissolvedrnarsenic were detected. The investigation involved a comprehensive soil sampling program to determine thernpotential source of the arsenic and evaluate the potential transport of the dissolved arsenic using therngeochemical equilibrium speciation model MINTEQA2 and the transport model VLEACHSM. Results ofrnthe investigation indicated that the observed dissolved arsenic was associated with an isolated area ofrnelevated arsenic soil concentrations. The results suggest that as the soil arsenic mass is depleted in this area,rnsorption will rapidly increase as a result of a highly nonlinear sorption isotherm. As the dissolved massrnenters the aquifer, the high arsenic sorption affinity of mineral oxides causes the dissolved mass to rapidlyrnre-sorb to the aquifer material. The investigation results indicate that arsenic mobilization and transport arernlocalized phenomena and that small localized dissolved arsenic plumes may be characteristic of manyrnpetroleum-contaminated aquifers which contain arsenic-bearing Fe(III) oxides.