GIS Coverage for the National Water-Quality Assessment (NAWQA) Program Retrospective Database for Nutrients in Surface Water: Monitoring Locations

The retrospective database is a compilation of historical water-quality and
ancillary data collected before NAWQA Study Units initiated sampling in 1993.
This coverage contains the point locations of monitoring locations where
historical water-quality data was collected. 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 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.

Supplemental_Information:

The retrospective database includes over 22,000 surface-water samples. The
surface-water data are for samples collected during 1980-90 at sites that had a
minimum of 25 monthly samples. 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).

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. 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 surface-water sites is based on Anderson Level I
categories (Anderson and others, 1976). 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). 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.

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).

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.

Within the database, concentrations less than detection are reported as
negative values of the detection limit. Missing values are indicated by a
decimal point. (During processing of the tabular data, these decimal points
were replaced will NULL values; See Data_Quality_Information section.

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.

[Summary provided by the EPA.]