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NAWQA RETROSPECTIVE DATABASE FOR NUTRIENTS IN GROUND WATER AND SURFACE WATER
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The National Water-Quality Assessment (NAWQA) Program is designed to describe the status and trends in the quality of the Nations ground- and surface-water resources and to provide a sound understanding of the natural and human factors that affect the quality of these resources. The retrospective database is a compilation of historical water-quality and ancillary data collected before NAWQA Study Units initiated sampling in 1993. 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).