Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

This technical note also applies to the following indicators:

• Farmlands: Phosphorus in Farmland Streams
• Forests: Nitrate in Forest Streams
• Urban/Suburban: Nitrate in Urban Streams
• Urban/Suburban: Phosphorus in Urban Streams

This technical note supplements the technical note for

• Fresh Waters: Phosphorus in Lakes, Reservoirs, and Large Rivers

The Indicators

Nitrogen (N) and phosphorus (P) are chemical elements that serve as essential nutrients for plants and animals, but at excessive concentrations they can contaminate groundwater and streams. In surface waters they can promote excessive growth of algae (nitrogen typically causes blooms in coastal waters, whereas phosphorus more commonly causes blooms in freshwater systems), whose decay removes oxygen and threatens aquatic animals. At high concentrations, some forms of nitrogen (e.g., nitrate and ammonia) can be directly toxic to fish and create health problems for humans. In groundwater, excessive nitrate poses a threat to humans who drink from contaminated wells. Common forms of nitrogen that are readily available to plants for growth include nitrate and ammonia, and phosphate is the plant-available form of phosphorus. Sources include precipitation, dissolved natural minerals, farm and domestic fertilizers, discharges from septic systems, and effluents from sewage treatment plants.

Graphs for stream sites show mean-annual concentrations of dissolved nitrate plus nitrite or total phosphorus. Graphs for groundwater data are based on nitrate concentration in one sampling of each well. Data are reported as either parts per million (milligrams per liter) as nitrogen or parts per million (milligrams per liter) as phosphorus. The data are labeled “mean total nitrate” although the analytical method actually reports nitrate plus nitrite. This reporting convention is reasonable because except in highly polluted waters, nitrite levels are only a very small fraction of the total and can, therefore, be considered insignificant.

The Data

Data Source: The data were collected and analyzed by the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) program in 36 major river basins and aquifers distributed across the United States from 1992 to 1998. NAWQA samples watersheds with relatively homogeneous land use/land cover to better illuminate the effect of land use on water quality. For this report, data from watersheds where a single land use typically was predominant were used to characterize water quality conditions in farmlands, forests, and urban settings.

Nutrient data are from 15 to 25 samples collected annually at stream sites draining 105 agricultural, 38 urban and suburban, and 36 forested areas. Nitrate data were from samples collected at 1,190 wells in agricultural, 601 wells in urban and suburban, and 79 wells in forested areas. These data are summarized at http://water.usgs.gov/nawqa/. Note that the sites labeled “urban” in this analysis should overlap with the “urban and suburban lands” defined as the subject of this report (see pp. 181), but, since different definitions were used in the two efforts, this might not always be the case.

Information on the drinking water standard for nitrogen can be found at http://www.epa.gov/safewater/mcl.html#inorganic. Information on the 1986 phosphorus recommended goal for preventing excess algae growth can be found in EPA 440/5-86-001 (see references). Information on regional nutrient (phosphorus) criteria can be found at http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/.

For farmlands, extensive data have been collected from different farming systems at the watershed-level scale, from 1991 to 2000, that will become available shortly through the National Agricultural Library (http://www.nal.usda.gov/). These data will allow additional investigations of the effect of land use and specific farming practices on water quality.

Data Collection Methodology: All samples were collected and analyzed by USGS according to the overall NAWQA design (Gilliom et al. 1995). Stream water samples were collected using depth and width integrating techniques so that the sample is representative of the water flowing past the sampling point (Shelton 1994). Groundwater samples were collected primarily from monitoring wells and low-capacity domestic wells using procedures that resulted in a sample representative of water in the aquifer (Lapham et al. 1995). Methods employed for random selection of well locations for targeted land use are described by Scott (1989) and Squillace and Price (1996). Methods for sample preservation and processing can be found in Shelton (1994) for stream samples and in Koterba et al. (1995) for groundwater samples. Fishman (1993) and Patton and Truitt (1992) describe analytical methods used for nutrient constituents. Land use in the watersheds upstream of stream sampling points or in the vicinity of wells was characterized according to procedures described in Gilliom and Thelin (1997) and Koterba (1998), respectively.

Data Analysis: The data are highly aggregated and should be interpreted mainly as an indication of general national patterns. The data were collected and analyzed by NAWQA in 36 major river basins and aquifers distributed across the United States from 1993 to 1998. The watersheds and aquifers studied were selected to be generally representative of water and land use in each area. Because this is a national assessment, the percentage of targeted land use varies across the nation. For example, watersheds dominated by agricultural land varied from 10 to 99% as cropland and/or pasture; urban and suburban land varied from 6 to 100%; and forested land ranged from 61 to 100%. Water quality is affected by both the percentage of land use in a watershed and the proximity of that land use (as a source of contamination) to streams and rivers. For example, agricultural or urban/suburban land uses might exert a dominant influence on a stream or river, in spite of occupying a small percentage of land cover in the watershed, if these land uses are located in close proximity to the river or stream.

Data Quality/Caveats: Sampling sites were selected to be representative of specific land use types rather than locations where contamination was known or suspected. All samples were collected, processed, preserved, and analyzed using the same methods. Nutrient data were reviewed to identify outliers and inconsistent results by the teams who collected the samples and by a national team (Mueller 1998). Most data have been published by USGS in a series of technical reports focusing on specific study areas and in national summary results (see http://water.usgs.gov/nawqa/ for a list of reports).

Data Access: All data used in this document are summarized at http://water.usgs.gov/nawqa/.

References

Fishman, M.J. 1993. Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of inorganic and organic constituents in water and fluvial sediments. U.S. Geological Survey Open-File Report 93-125.

Gilliom, R.J., W.M. Alley, and M.E. Gurtz. 1995. Design of the National Water-Quality Assessment Program: Occurrence and distribution of water-quality conditions. U.S. Geological Survey Circular 1112.

Gilliom, R.J., and G.P. Thelin. 1997. Classification and mapping of agricultural land for National Water-Quality Assessment. U.S. Geological Survey Circular 1131.

Koterba, M.T. 1998. Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program: Collection, documentation, and compilation of required site, well, subsurface, and landscape data for wells. U.S. Geological Survey Water-Resources Investigations Report 98-4107.

Koterba, M.T., F.D. Wilde, and W.W. Lapham. 1995. Groundwater data-collection protocols and procedures for the National Water-Quality Assessment Program: Collection and documentation of water-quality samples and related data. U.S. Geological Survey Open-File Report 95-399.

Lapham, W.W., F.D. Wilde, and M.T. Koterba. 1995. Groundwater data-collection protocols and procedures for the National Water-Quality Assessment Program: Selection, installation, and documentation of wells, and collection of related data. U.S. Geological Survey Open-File Report 95- 398.

Mueller, D.K. 1998. Quality of nutrient data from streams and ground water sampled during 1993–95—National Water- Quality Assessment Program. U.S. Geological Survey Open- File Report 98-276.

Mueller, D.K., J.D. Martin, and T.J. Lopes. 1997. Quality-control design for surface water sampling in the National Water- Quality Assessment Program. U.S. Geological Survey Open- File Report 97-223.

Patton, C.J., and E.P. Truitt. 1992. Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory— Determination of total phosphorus by Kjeldahl digestion method and an automated colorimetric finish that includes dialysis. U.S. Geological Survey Open-File Report 92-146.

Scott, J.C. 1989. A computerized data-base system for land-use and land-cover data collected at ground-water sampling sites in the pilot National Water-Quality Assessment Program. U.S. Geological Survey Water-Resources Investigations Report 89-4172.

Shelton, L.R. 1994. Field guide for collecting and processing stream-water samples for the National Water-Quality Assessment Program. U.S. Geological Survey Open-File Report 94-455.

Squillace, P.J., and C.V. Price.1996. Urban land-use study plan for the National Water-Quality Assessment Program. U.S. Geological Survey Open-File Report 96-217. U.S. Environmental Protection Agency. 1986. Quality criteria for water-1986. U.S. Environmental Protection Agency Report EPA 440/5-86-001.