Results of a national water quality assessment indicate that nitrate is detected in 71% of groundwater samples, more than 13 times as often as ammonia, nitrite, organic nitrogen, and orthophosphate, based on a common detection threshold of 0.2 mg/L. Shallow groundwater (typically 5 m deep or less) beneath agricultural land has the highest median nitrate concentration (3.4 mg/L), followed by shallow groundwater beneath urban land (1.6 mg/L) and deeper groundwater in major aquifers (0.48 mg/L). Nitrate exceeds the maximum contaminant level, 10 mg/L as nitrogen, in more than 15% of groundwater samples from 4 of 33 major aquifers commonly used as a source of drinking water. Nitrate concentration in groundwater is variable and depends on interactions among several factors, including nitrogen loading, soil type, aquifer permeability, recharge rate, and climate. For a given nitrogen loading, factors that generally increase nitrate concentration in groundwater include well-drained soils, fractured bedrock, and irrigation. Factors that mitigate nitrate contamination of groundwater include poorly drained soils, greater depth to groundwater, artificial drainage systems, intervening layers of unfractured bedrock, a low rate of groundwater recharge, and anaerobic conditions in aquifers.

[1] A combination of chemical and dissolved gas analyses, chlorofluorocarbon age dating, and hydrologic measurements were used to determine the degree to which biogeochemical processes in a riparian wetland were responsible for removing NO 3 À from groundwaters discharging to the Otter Tail River in west central Minnesota. An analysis of river chemistry and flow data revealed that NO 3 À concentrations in the river increased in the lower half of the 8.3 km study reach as the result of groundwater discharge to the river. Groundwater head measurements along a study transect through the riparian wetland revealed a zone of groundwater discharge extending out under the river. On the basis of combined chemical, dissolved gas, age date, and hydrologic results, it was determined that water chemistry under the riparian wetland was controlled largely by upgradient groundwaters that followed flow paths up to 16 m deep and discharged under the wetland, creating a pattern of progressively older, more chemically reduced, low NO 3 À water the farther one progressed from the edge of the wetland toward the river. These findings pose challenges for researchers investigating biogeochemical processes in riparian buffer zones because the progressively older groundwaters entered the aquifer in earlier years when less NO 3 À fertilizer was being used. NO 3 À concentrations originally present in the groundwater had also decreased in the upgradient aquifer as a result of denitrification and progressively stronger reducing conditions there. The resulting pattern of decreasing NO 3 À concentrations across the riparian zone may be incorrectly interpreted as evidence of denitrification losses there instead of in the upgradient aquifer. Consequently, it is important to understand the hydrogeologic setting and age structure of the groundwaters being sampled in order to avoid misinterpreting biogeochemical processes in riparian zones.

A mass-balance budget of N cycling was developed for an intensive agricultural area in west-central Minnesota to better understand NO 3 -contamination of ground water in the Otter Tail outwash aquifer. Fertilizer, biological fixation, atmospheric deposition, and animal feed were the N sources, and crop harvests, animal product exports, volatilization from fertilizer and manure, and denitrification were the N sinks in the model. Excess N, calculated as the difference between the sources and sinks, was assumed to leach to ground water as NO 3 -. The budget was developed using ground water data collected throughout the 212-km 2 study area. Denitrification was estimated by adjusting its value so the predicted and measured concentrations of NO 3 -in ground water agreed. Although biological fixation was the largest single N source, most was removed when crops were harvested, indicating that inorganic fertilizer was the primary source of N reaching the water table. It was estimated that denitrification removed almost half of the excess NO 3 -that leached below the root zone. Even after accounting for denitrification losses, however, it was concluded that the ground water system was receiving approximately three times as much N as would be expected under background conditions. The N cycle of agro-ecosystems is by definition dominated by agricultural sources and sinks, the magnitude of which greatly exceeds natural cycles that evolved over many millennia. Agricultural inputs of N in the USA have increased 20-fold in the past 50 yr, and the most dramatic increases have taken place in the past 30 yr (Puckett, 1995). Vitousek (1994) has estimated that of the total N used by humans throughout history up to 1992, approximately half was applied from 1982 to 1992. Consequently, many agro-ecosystems and their associated ground water and surface water systems may still be receiving increasing N amounts.One of the least appreciated components of the alteration of the natural cycle is the dramatic increase in N inputs caused by increased cultivation of N fixing crops such as alfalfa (Medicago sativa L.), soybean [Glycine max (L.) Merr.], and other legumes. These crops have a much greater capacity to fix atmospheric N than natural vegetation. For example, alfalfa can fix approximately 218 kg N ha -1 yr -1 , whereas deciduous forests can fix only about 12 kg N ha -1 yr -1 (Keeney, 1979;Jordan and Weller, 1996). In some agro-ecosystems, legume fixation may even be the largest source of N. For example, Keeney (1979) estimated that for the state of Wisconsin, 2 N fixation by legumes exceeded inputs from commercial fertilizer by 2.5 times. Peterson and Russelle (1991) estimated that for the eight states of the U.S. Corn Belt, alfalfa alone contributed 1 × 10 9 kg of N compared with 4 × 10 9 kg from commercial fertilizer. It is important to consider this N source because it may contribute NO 3 -to ground water and surface waters either directly through N added to the soil or indirectly as the result of mineralization of plant re...