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, and local agencies to closely examine water supplies for contamination with even trace amounts of arsenic. Arsenic has been observed in groundwater in association with petroleum-impacted aquifers at a variety of sites. During anaerobic biodegradation of hydrocarbons, Fe(III)-reducing microorganisms can couple the reduction of solid phase Fe(III)-oxides to soluble Fe(II) with the oxidation of petroleum compounds. As the Fe(III) is reduced,, metals and nutrients that are sorbed onto the Fe(III) oxide surfaces may be released to solution. As a consequence, arsenic, which has a strong sorption affinity for Fe(III) oxides, can be released to groundwater. An investigation has been conducted at a petroleum-contaminated site where elevated levels of dissolved arsenic were detected. The investigation involved a comprehensive soil sampling program to determine the potential source of the arsenic and evaluate the potential transport of the dissolved arsenic using the geochemical equilibrium speciation model MINTEQA2 and the transport model VLEACHSM. Results of the investigation indicated that the observed dissolved arsenic was associated with an isolated area of elevated arsenic soil concentrations. The results suggest that as the soil arsenic mass is depleted in this area, sorption will rapidly increase as a result of a highly nonlinear sorption isotherm. As the dissolved mass enters the aquifer, the high arsenic sorption affinity of mineral oxides causes the dissolved mass to rapidly re-sorb to the aquifer material. The investigation results indicate that arsenic mobilization and transport are localized phenomena and that small localized dissolved arsenic plumes may be characteristic of many petroleum-contaminated aquifers which contain arsenic-bearing Fe(III) oxides.