Concentrations of naturally occurring arsenic in ground water vary regionally due to a combination of climate and geology. Although slightly less than half of 30,000 arsenic analyses of ground water in the United States were 1 μg/L, about 10% exceeded 10 μg/L. At a broad regional scale, arsenic concentrations exceeding 10 μg/L appear to be more frequently observed in the western United States than in the eastern half. Arsenic concentrations in ground water of the Appalachian Highlands and the Atlantic Plain generally are very low ( 1 μg/L). Concentrations are somewhat greater in the Interior Plains and the Rocky Mountain System. Investigations of ground water in New England, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that arsenic concentrations exceeding 10 μg/L are more widespread and common than previously recognized. Arsenic release from iron oxide appears to be the most common cause of widespread arsenic concentrations exceeding 10 μg/L in ground water. This can occur in response to different geochemical conditions, including release of arsenic to ground water through reaction of iron oxide with either natural or anthropogenic (i.e., petroleum products) organic carbon. Iron oxide also can release arsenic to alkaline ground water, such as that found in some felsic volcanic rocks and alkaline aquifers of the western United States. Sulfide minerals are both a source and sink for arsenic. Geothermal water and high evaporation rates also are associated with arsenic concentrations 10g/L in ground and surface water, particularly in the west.

[1] Recharge rates of nitrate (NO 3 À ) to groundwater beneath agricultural land commonly are greater than discharge rates of NO 3 À in nearby streams, but local controls of NO 3 À distribution in the subsurface generally are poorly known. Groundwater dating (CFC, 3 H) was combined with chemical (ions and gases) and stable isotope (N, S, and C) analyses to resolve the effects of land use changes, flow patterns, and water-aquifer reactions on the distributions of O 2 , NO 3 À , SO 4 = , and other constituents in a two-dimensional vertical section leading from upland cultivated fields to a riparian wetland and stream in a glacial outwash sand aquifer near Princeton, Minnesota. Within this section a ''plume'' of oxic NO 3 À -rich groundwater was present at shallow depths beneath the fields and part of the wetland but terminated before reaching the stream or the wetland surface. Groundwater dating and hydraulic measurements indicate travel times in the local flow system of 0 to >40 years, with stratified recharge beneath the fields, downward diversion of the shallow NO 3 À -bearing plume by semiconfining organic-rich valley-filling sediments under the wetland and upward discharge across the valley and stream bottom. The concentrations and d 15 N values of NO 3 À and N 2 indicate that the NO 3 À plume section was bounded in three directions by a curvilinear zone of active denitrification that limited its progress; however, when recalculated to remove the effects of denitrification, the data also indicate changes in both the concentrations and d 15 N values of NO 3 À that was recharged in the past. Isotope data and mass balance calculations indicate that FeS 2 and other ferrous Fe phases were the major electron donors for denitrification in at least two settings: (1) within the glacial-fluvial aquifer sediments beneath the recharge and discharge areas and (2) along the bottom of the valley-filling sediments in the discharge area. Combined results indicate that the shape and progress of the oxic NO 3 À plume termination were controlled by a combination of (1) historical and spatial variations in land use practices, (2) contrast in groundwater flow patterns between the agricultural recharge area and riparian wetland discharge area, and (3) distribution and abundance of electron donors in both the sand aquifer and valley-filling sediments. The data are consistent with slow migration of redox zones through the aquifer in response to recharging oxic groundwater during Holocene time, then an order-of-magnitude increase in the flux of electron acceptors as a result of agricultural NO 3 À contamination in the late twentieth century, to which the redox zone configuration still may be adjusting. The importance of denitrification for NO 3 À movement through formerly glaciated terrains should depend on the source areas and depositional environments of the glacial sediments, as well as geomorphology and recent stream-valley sediment history.

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