[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.

The sudden, catastrophic release of gas from Lake Nyos on 21 August 1986 caused the deaths of at least 1700 people in the northwest area of Cameroon, West Africa. Chemical, isotopic, geologic, and medical evidence support the hypotheses that (i) the bulk of gas released was carbon dioxide that had been stored in the lake's hypolimnion, (ii) the victims exposed to the gas cloud died of carbon dioxide asphyxiation, (iii) the carbon dioxide was derived from magmatic sources, and (iv) there was no significant, direct volcanic activity involved. The limnological nature of the gas release suggests that hazardous lakes may be identified and monitored and that the danger of future incidents can be reduced.

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Leachate from municipal landfills can create groundwater contaminant plumes that may last for decades to centuries. The fate of reactive contaminants in leachate‐affected aquifers depends on the sustainability of biogeochemical processes affecting contaminant transport. Temporal variations in the configuration of redox zones downgradient from the Norman Landfill were studied for more than a decade. The leachate plume contained elevated concentrations of nonvolatile dissolved organic carbon (NVDOC) (up to 300 mg/L), methane (16 mg/L), ammonium (650 mg/L as N), iron (23 mg/L), chloride (1030 mg/L), and bicarbonate (4270 mg/L). Chemical and isotopic investigations along a 2D plume transect revealed consumption of solid and aqueous electron acceptors in the aquifer, depleting the natural attenuation capacity. Despite the relative recalcitrance of NVDOC to biodegradation, the center of the plume was depleted in sulfate, which reduces the long‐term oxidation capacity of the leachate‐affected aquifer. Ammonium and methane were attenuated in the aquifer relative to chloride by different processes: ammonium transport was retarded mainly by physical interaction with aquifer solids, whereas the methane plume was truncated largely by oxidation. Studies near plume boundaries revealed temporal variability in constituent concentrations related in part to hydrologic changes at various time scales. The upper boundary of the plume was a particularly active location where redox reactions responded to recharge events and seasonal water‐table fluctuations. Accurately describing the biogeochemical processes that affect the transport of contaminants in this landfill‐leachate‐affected aquifer required understanding the aquifer's geologic and hydrodynamic framework.