Abstract
Ground water with arsenic concentrations greater than the U.S. Environmental Protection Agency drinking water standard exists throughout much of the CALFED solution area. These high concentrations are of con-cern from the standpoint of both existing water supply and development of conjunctive use projects. Much is known about arsenic mobility in ground water subject to different hydrologic and geochemical conditions. However, some important knowledge gaps exist that limit the ability to design water supply projects that could prevent arsenic mobilization or promote arsenic removal from ground water. A few well studied sys-tems could provide a much better understanding of methods for preventing or eliminating high arsenic problems. Within the context of the examination of a few detailed field studies, some important research needs include: 1.) Determining the significance of metal-bridging aqueous complexes involving inorgan-ic arsenic and natural organic matter, 2.) In the con-text of in situ remediation, determining whether of metal oxides. Little is known about the quantitative significance competition of inorganic arsenic with other inorganic aqueous species in natu-ral systems. Experiments should be conducted with actual aquifer materials, as the effects of aging on arsenic desorption in laboratory studies are quite sig-nificant. 3.) Devise methods to detect and quantify rates of oxidation/reduction reactions of arsenic that are carried out by microorganisms at ambient concen-trations of arsenic and under in situ conditions. The findings from detailed field studies have the potential for greatly reducing the cost of meeting the new drinking-water standard for arsenic. The research would benefit a broad constituency.
Highlights
An evaluation of the human health effects of arsenic in drinking water (National Research Council 1999; National Research Council 2001), along with estimated compliance costs (Federal Register 2001), resulted in lowering the arsenic drinking-water standard from 50 to 10 μg/L
The microbiology of arsenic appears to be mostly confined to microorganisms that either gain energy from the redox changes associated with the transitions between As(III) and As(V), or those that carry out these reactions for the purpose of detoxification
Development of water resources projects could be more efficient if the arsenic content of ground water was well known
Summary
An evaluation of the human health effects of arsenic in drinking water (National Research Council 1999; National Research Council 2001), along with estimated compliance costs (Federal Register 2001), resulted in lowering the arsenic drinking-water standard from 50 to 10 μg/L. Large regions of the continental United States have ground water arsenic concentrations that greatly exceed the U.S. EPA drinking water standard, especially California and other states in the southwest (Welch et al 2000), including areas where conjunctive use is being considered or already practiced (Saracino-Kirby 2000). Arsenic concentrations in ground water exceed the current standards throughout much of the CALFED solution area (Figure 1). Evaporative concentration may be a contributing factor leading to high arsenic concentrations in areas such as the desert basins in southeastern California and the southern San Joaquin Valley (Fujii and Swain 1995; Welch and Lico 1998). Equilibrium generally is not obtained between arsenic and other redox couples commonly present in ground water
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