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.