Based on these results we propose a working hypothesis for the occurrence of As in this aquifer. Chemical weathering reactions driven by oxic, mildly acidic groundwater over thousands of years resulted in the mass transfer of As from reduced forms in the interior of primary minerals of these glacial outwash sediments to As(V) adsorbed onto the hydrous Fe and Al oxide and silicate particles that comprise the coatings on sediment-grain surfaces. Adsorbed As(V) responds readily to changes in groundwater chemistry. Sewage-derived phosphate, transported away from the sewage-effluent disposal beds, caused desorption of As(V) resulting in concentrations up to 0.07 µM in the suboxic zone of the sewage plume. Deeper in the sewage plume, biodegradation of organic compounds transported away from the disposal beds resulted in reductive dissolution of Fe oxides associated with the coatings; As(V) was reduced to As(III) either by As-respiring microorganisms or abiotic reactions. Reductive dissolution of hydrous Fe oxides likely contributed to causing As concentrations in excess of the 0.13 µM MCL by releasing As not otherwise desorbable by phosphate, such as As occluded in Fe oxides, and by decreasing the solid-phase concentration of Fe oxides, which have a higher affinity for adsorption of As, especially As(III), than other constituents of the coatings. The combination of reduction of As(V) to As(III) and reductive dissolution of Fe oxides has been shown to be a likely cause of elevated As concentrations at other sites.^{2,54,55,92,93,94}

Further research remains to be done to test this hypothesis, but the results have implications for management of shallow, unconfined, quartz-sand aquifers like the one on Cape Cod. As a shallow, unconfined aquifer, water quality is readily influenced by human activities on the land surface. The hydrous Fe and Al oxide and silicates on surfaces of quartz and feldspars, which dominate the mineralogy of the sediments, react with groundwater mainly through adsorption–desorption reactions.^{14,15,35,59,69,71,95} The comparatively low level of reactivity of these minerals provides high-quality drinking water¹³ but it also provides only a limited capacity for taking up contaminants and buffering against changes in groundwater chemistry. Arsenic occurs naturally and its presence, even at low concentrations, can render groundwater unfit as a source of drinking water. Maintaining an aquifer as a sustained source of drinking water requires preventing the development and spreading of conditions, like those described here, favorable to the mobilization of As. Given the dominance of adsorption reactions to the overall reactivity of the sediments with As and other groundwater contaminants, improved understanding of the chemical properties of the nanoparticulate material that comprises the coatings on sediment grain surfaces will also contribute to managing these types of aquifers as a sustained source of drinking water.