Biogeochemical reactions in aquifers can mobilize naturally occurring arsenic (As) from sediments that are not enriched in As compared to its average abundance in crustal rocks^1,2 Aquifers with dissolved As concentrations significantly higher than 0.67 micromoles per liter (µM), which equals 50 micrograms per liter (µg/l), that supply large populations with drinking water have received considerable attention.^1,2,3 With the recent lowering of the maximum contaminant level (MCL) for As in drinking water in the United States to 0.13 µM (10 µg/l), interest in understanding the fate and transport of As in potential drinking-water supplies is likely to intensify.^4,5,6,7 In addition to its potential impact on human health, scientific interest in As in groundwater has been stimulated by the potential for using it to gain insight into oxidation-reduction reactions⁸ and, more recently, the role of microbial communities in As cycling.⁹ The ultimate source of naturally occurring As in most aquifers is likely to be sulfide minerals because the As content of these minerals greatly exceeds those of other rock-forming minerals that typically comprise aquifer sediments.¹ Arsenic in pyrite and other metal sulfide minerals exists in low oxidation states, primarily –1 or 0.¹⁰ In contrast, the dominant oxidation states of As in groundwater are As(III) (As in the plus 3 oxidation state) and As(V).¹ The biogeochemical reactions and hydrologic processes leading to the oxidation of As in primary sedimentary minerals to As(III) and As(V), as well as those reactions that influence the fate and transport of As(III) and As(V) are subjects of continued research.^1,11 Improved understanding of these processes can contribute to improved understanding of subsurface biogeochemistry and could provide important information for maintaining critical drinking water supplies.¹²

The objective of this paper is to analyze the chemical reactions influencing dissolved As concentrations and speciation in a shallow aquifer whose mineralogy is dominated by quartz, feldspars, and other silicate minerals. These minerals react with groundwater mainly through the mineral dissolution and precipitation reactions associated with chemical weathering, which tend to be slow, and, therefore, tend to impart low concentrations of solutes to the groundwater with which they are in contact.¹³ However, as a result of these weathering reactions, nanometer-size precipitates of iron (Fe) and aluminum (Al) oxides and silicates can form on the surfaces and interiors of primary mineral grains (e.g., Refs. 14,15,16). Arsenic released as a result of dissolution of pyrite and other sulfide minerals may be incorporated into these precipitates by adsorption or other processes. Arsenic adsorbed onto these precipitates could be desorbed and, therefore, mobilized in response to changes in chemical conditions. This was examined by determining dissolved As concentrations and speciation in different zones of an aquifer characterized by different chemical conditions owing to land-disposal of dilute sewage effluent to the aquifer. In addition, the results of a field experiment are presented in which the mobilization of As in uncontaminated groundwater above the sewage plume was examined by injecting phosphate, which is a strongly adsorbing anion capable of desorbing As through competition for adsorption sites. The study was carried out at a field site where previous investigations have made significant contributions to characterizing the site's hydrogeology,^{17,18,19,20,21,22,23} the biogeochemistry of the sewage plume,^24,25,26,27 and the chemical properties of the sediments.^15,28,29 Drawing on this literature in interpreting the results of our investigation, we discuss the impact of groundwater chemistry on As concentrations in the aquifer.