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waters. As an example, filling these areas with dredge spoils raises questions about the release of contaminants and changes in wetland-habitat values. The USGS is participating in the restoration of a 350-acre tract of land on San Pablo Bay by monitoring the development of tidal channels and flow patterns, changes in geotechnical and geochemical properties of preexisting and new sediment (dredge spoils), and sedimentation patterns within the restored wetlands and adjacent areas. The USGS also is mapping wetlands distributions by using remotely sensed image data and monitoring physical processes (including currents, wind, and waves) that alter wetlands and adjacent shallows; by quantifying the distribution and elevations of the shoreline; and by developing models that characterize the physical forces acting upon wetlands.

Information Transfer

he USGS is developing new tools and procedures to make existing and newly

developed information about the San Francisco Bay Estuary and Delta readily available. The accessibility of existing spatial data scattered among several USGS laboratories will be increased by the creation of a World Wide Web information interface. World Wide Web navigators, such as Mosaic, will enable any user with Internet access to browse and retrieve information relevant to the San Francisco Bay and Delta (including text, pictures, maps, and animations) and interact directly with USGS experts.

Long-term USGS monitoring and research programs in San Francisco Bay provide needed data, information, interpretations, and assessments that contribute to the work of other Federal and State agencies. This linkage of science and decisionmaking ensures the coordination of activities necessary for effective environmental resource management.

Frederic H. Nichols has participated in USGS studies of San Francisco Bay since 1972.

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Issues such as hazardous wastes, environmental degradation, population growth, soil contamination and erosion, water quality and adequate water supplies, and atmospheric changes are of paramount importance today as the Nation and the world wrestle with the impacts of human activity on the natural environment. Understanding the conditions and functions of environmental systems and the factors that are changing them is an important function of the USGS scientific mission. Policymakers and resource managers need to have scientific information on both natural and impacted environments in order to recognize and mitigate adverse effects on these systems from human activities and to develop strategies accommodating natural environmental variation. Improving the technical basis for maintaining the environmental systems that sustain and improve the quality of human life, as the articles in this section discuss, is part of the ongoing work of the USGS.

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Nitrate in the South Platte River Alluvial Aquifer, Colorado

he South Platte River alluvial aquifer

between Denver and Greeley, Colo., covers an area of about 75 square miles. The predominant land use in the area is irrigated agriculture, which has changed the chemistry and hydrology of the alluvial aquifer in at least two important ways. First, the infiltration of irrigation water has resulted in a buildup of dissolved nitrate in ground water in the aquifer. Second, it has increased the amount of discharge from the aquifer to the South Platte River. One objective of the study was to determine if naturally occurring processes in the aquifer reduce nitrate concentrations in the water prior to its discharge to the South Platte River, thereby decreasing the effect of irrigated agriculture on water quality in the river.

The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program began in 1991 and describes the status of and trends in the quality of the Nation's surface- and ground-water resources. The program provides a thorough understanding of the natural and human factors that affect the quality of those resources. NAWQA plans to conduct investigations in

60 study areas that represent a variety of geologic, hydrologic, climatic, and geographic conditions throughout the Nation. The South Platte River Basin study unit, which is in parts of Colorado, Nebraska, and Wyoming, was among the first 20 study units in which work was begun in 1991. The South Platte River alluvial aquifer is the most productive aquifer in the basin; most of its water is used to irrigate overlying cropland. The aquifer consists of unconsolidated

One objective of the study was to determine if naturally occurring processes in the aquifer reduce nitrate concentrations in the water prior to its discharge to the South Platte River, thereby decreasing the effect of irrigated agriculture on water quality in the river.

clay, silt, sand, and gravel and ranges in thickness from about 15 to 60 feet and in width

20 40 KILooners

Boundary of alluvial aquifer study area

South Platte River

WYOMING

40 MILE's

Alluvial aquifer study area in the South Platte River Basin.

45

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Relative changes in nitrate concentrations and nitrate nitrogen isotoperatios in samples of ground water from the recharge and discharge areas.

Median concentrations of nitrate in water in the recharge area for indicated river reaches. Median concentrations of nitrate were calculated for samples collected from 1992 through 1994. Mass-balance-estimated nitrate concentrations in the groundwater discharge are for samples collected in April 1994.

Microbial denitrification is a bacterial process that converts nitrate (NO3) to nitrogen gas (N2). Nitrogen gas is a harmless end product and is the major component in the air we breath.

For more information on nitrates in the
South Platte River alluvial aquifer, contact
Peter McMahon at:
Telephone: (303) 236–4882
Internet: pmcmahongpmcdcolka.cr.
usgs.gov

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Long-term agricultural activity on land that overlies the South Platte River alluvial aquifer has resulted in a buildup of dissolved nitrate in water in the recharge area. Nitrate concentrations in ground water from the recharge area near Greeley ranged from less than 0.1 to greater than 45 milligrams per liter as nitrogen; the median concentration was 26 milligrams per liter as nitrogen. The maximum contaminant level for nitrate is 10 milligrams per liter as nitrogen in drinking water as established by the U.S. Environmental Protection Agency. Measurement of the stable nitrogen-isotope ratios of the nitrate indicated that the nitrate was derived from animal waste—an interpretation that is consistent with the long-term practice in the area of fertilizing fields with animal manure from feedlots.

Nitrate concentrations in water from the discharge area near the South Platte River were substantially lower than concentrations in water from the recharge area. Nitrate concentrations in ground water from the discharge area ranged from about 2 to 30 milligrams per liter as nitrogen; the median nitrate concentration was 6 milligrams per liter as nitrogen. The decrease in nitrate concentrations between the recharge and the discharge areas indicated that nitrate concentrations were attenuated along flow paths in the aquifer.

Microbial denitrification in aquifer sediments in the discharge area was at least partly responsible for the decrease in nitrate concentrations between the recharge and discharge areas. For example, concentrations of dissolved oxygen were high in water from the recharge area. Denitrifying activity is inhibited in the presence of oxygen; therefore, nitrate persists in water from the recharge area. In contrast, dissolved oxygen was absent or at very low concentrations in water in the discharge area, which allowed denitrification in sediments in discharge areas. The stable nitrogen isotope ratios of nitrate in the ground water increased (became more enriched in the heavy isotope *N compared with the light isotope *N) as the nitrate was transported from the recharge area to the discharge area. At the same time, the ground water became enriched in dissolved nitrogen gas, which is the major product of the denitrification reaction. Increases in nitrate nitrogen isotope values and nitrogen gas concentrations along with decreases in nitrate concentrations further indicated that denitrification in sediments in the discharge area was at least partly responsible for reducing nitrate concentrations in the ground water before its discharge to the river.

A mass balance based on nitrate concentrations in surface-water inflows and outflows indicated that the concentration of nitrate in ground water that discharges to the South Platte River in the study area was much less than what would be predicted on the basis of nitrate concentrations in the water from the recharge area. Differences between groundwater nitrate concentrations in the recharge area and those in the discharge area were observed elsewhere along a 250-mile segment of the alluvial aquifer from north of Denver to Julesburg, Colo.; these differences indicate that denitrification may have occurred in discharge areas throughout the alluvial aquifer in Colorado. Despite the effect of denitrification in reducing nitrate

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