National Water-Quality Assessment (NAWQA) Program
The retrospective database is a compilation of historical water-quality and ancillary data collected before NAWQA Study Units initiated sampling in 1993. Water-quality data were obtained by study-unit personnel from the U.S. Geological Survey (USGS) National Water Information System (NWIS), from records of State water-resource agencies, and from STORET, the U.S. Environmental Protection Agency (USEPA) national database. Ancillary data describing characteristics of sampled sites were compiled by NAWQA Study Units or obtained from national-scale digital maps. Mueller and others (1995) used this data to determine preexisting water-quality conditions in the first 20 NAWQA Study Units that began in 1991. Also, Nolan and Ruddy (1996) used the data to describe areas of the United States at risk of nitrate contamination of ground water.
The retrospective database includes over 10,000 ground-water samples and over 22,000 surface-water samples. The ground-water data are for samples collected during 1970-92 from wells that best represented land uses in the Study Units. These data are supplemented by data from the Delmarva Peninsula NAWQA pilot study and the USGS Toxic-Substances Hydrology Program. The surface-water data are for samples collected during 1980-90 at sites that had a minimum of 25 monthly samples.
Sampling dates at ground-water sites were reported in a variety of formats. Year of sampling is included in the retrospective database because it was reported most often by the various Study Units. Year of sampling also is convenient because some Study Units reported median constituent concentrations. If sampling date ranges for median values fell within a single year, then year of sampling was retained in the national data set for that sample.
Because sampling, preservation, and analytical techniques associated with these historical data changed during the period of record and are different for different agencies, reported nutrient concentrations were aggregated into the following groups: (1) ammonia as N, (2) nitrate as N, (3) total nitrogen, (4) orthophosphate as P, and (5) total phosphorus. For example, ammonia includes both ammonium ions and un-ionized ammonia. More information on methods used to aggregate constituent data is available in the report by Mueller and others (1995).
Ground-water nitrate concentrations below the method detection limit were reported in one of three ways, depending on the source of the data. Values below the detection limit either were set to zero, set equal to the detection limit, or set to one-half the detection limit. To overcome this limitation, nonparametric statistical methods were used to analyze the retrospective data. Nonparametric methods involve ranking data values by magnitude, and those having the same magnitude (e.g., those equal to the detection limit) are treated as "ties" and receive the same rank.
Much of the ancillary data, such as well and aquifer descriptions and land-use classification for surface-water drainage basins, were provided by NAWQA Study Units. Some ancillary data for ground-water well locations were evaluated at the national scale to maintain consistency between Study Units. Data evaluated at the national scale include land use, soil hydrologic group, nitrogen input to the land surface, and the ratios of pasture or woodland to cropland.
Land-use classification of ground-water and surface-water sites is based on Anderson Level I categories (Anderson and others, 1976). Land use was determined for most ground-water sampling sites from digital (GIRAS) data collected during 1970-1980 (Fegeas and others, 1983). Although land-use changes have occurred at some sites since the GIRAS data were collected, the data are relevant to current conditions because of the time required for water to move from the land surface to the underlying aquifer. Land use at surface-water sites was classified by NAWQA Study Unit personnel based on the Anderson Level I categories. Many surface-water sites were affected by mixed land uses, such as Forest and Agricultural, or Agricultural and Urban. Surface-water sites with very large drainage areas (greater than 10,000 square miles) were considered to be affected by multiple land uses, and were designated as Integrated land use. More detailed descriptions of the land-use categories in the retrospective database are given by Mueller and others (1995).
Soil hydrologic group was determined from digital maps compiled by the U.S. Soil Conservation Service (1993). The categorical values (A, B, C, and D) from the digital maps were converted to numbers to permit aggregation (Mueller and others, 1995). Ground-water sampling sites were assigned the mean hydrologic-group value of the soil mapping unit in which they were located. Surface-water sites were assigned the area-weighted mean for soil mapping units in the upstream drainage basin. Many surface-water sites did not have digitized basin boundaries available, so hydrologic group could not be evaluated.
Nitrogen inputs from atmospheric deposition, animal manure, and fertilizer application in the vicinity of ground-water sites were evaluated using national-scale digital maps, using Puckett's (1995) approach. Fertilizer and manure applications were estimated from national databases of fertilizer sales (U.S. Environmental Protection Agency, 1990) and animal populations (U.S. Bureau of the Census, 1989). Nitrogen input by atmospheric deposition was derived from data provided by the National Atmospheric Deposition Program/National Trends Network (1992).
Percentages of cropland, woodland, and pasture were obtained from the Census of Agriculture (U.S. Bureau of the Census, 1989) and used to calculate ratios of pasture to cropland and woodland to cropland for each county in which ground-water sampling sites were located.
Population data were obtained from the U.S. Bureau of the Census (1991). Total population in the upstream drainage was compiled for the surface-water data set, whereas population density in number of people per square kilometer was compiled for the ground-water data set.
Historical data can be of limited use in national assessments because of inconsistencies between and within agencies in database structure and format and in sample collection, preservation, and analytical procedures. For example, changes in sample collection and analytical procedures can cause shifts in constituent concentrations that are unrelated to possible changes in environmental factors. See Mueller and others (1995) for assumptions and limitations associated with the retrospective database.
Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E., 1976, A land use and land cover classification system for use with remote sensor data: U.S. Geological Survey Professional Paper 964, 28 p.
Fegeas, R.G., Claire, R.W., Guptill, S.C., Anderson, K.E., and Hallam, C.A., 1983, Land use and land cover digital data: U.S. Geological Survey Circular 895-E, 21 p.
Mueller, D.K., Hamilton, P.A., Helsel, D.R., Hitt, K.J., and Ruddy, B.C., 1995, Nutrients in ground water and surface water of the United States--An analysis of data through 1992: U.S. Geological Survey Water-Resources Investigations Report 95-4031, 74 p.
National Atmospheric Deposition Program (NRSP-3)/National Trends Network, 1992: Ft. Collins, CO, NADP/NTN Coordination Office.
Nolan, B.T., and Ruddy, B.C., 1996, Nitrate in ground waters of the United States--Assessing the risk: U.S. Geological Survey Fact Sheet FS-092-96.
Puckett, L.J., 1995, Identifying the major sources of nutrient water pollution: Environmental Science and Technology, v. 29, n. 9, p. 408-414.
U.S. Bureau of the Census, 1989, Census of agriculture 1987--Final county file: Washington, D.C., Data Users Services Division.
U.S. Bureau of the Census, 1991, Census of population and housing, 1990: Public Law 94-171 data for the United States: Washington, D.C., The Bureau.
U.S. Environmental Protection Agency, 1990, County-level fertilizer sales data: Washington, D.C., Office of Policy, Planning, and Evaluation, USEPA PM-221.
U.S. Soil Conservation Service, 1993, State soil geographic data base (STATSGO)--Data users guide: Miscellaneous Publication Number 1492, 88 p.