Risk of nitrate contamination in ground waters of the United States-- A national perspective

This map was designed to present patterns of risk for nitrate contamination at
large regional or national scales. Knowing where and what type of risks to
ground water exist can alert water-resources managers and private users of the
need to protect water supplies. By targeting regions with the highest risk of
nitrate contamination, resources can be directed to areas most likely to
benefit from pollution-prevention programs and long-term monitoring.

Nitrate contamination of ground water occurs in patterns based on "input"
factors (population density and nitrate contribution from fertilizer, manure,
and atmospheric deposition) and "aquifer vulnerability" factors (soil drainage
characteristic and woodland/cropland ratio in agricultural areas). Areas with
high nitrogen input, well-drained soils, and low woodland to cropland ratio
have the highest potential for contamination of ground water. The patterns of
risk for nitrate contamination of ground water were compiled in a national map
as described in Nolan and others (1997). The digital version of the national
map is included here.

The risk patterns still are being refined as better data sets become available,
and this map should not be considered to be the "final" version of the
patterns.

Procedures_Used:

Soil drainage characteristic was determined from data on soil hydrologic group
in the STATSGO (State Soil Geographic) data base (U.S. Soil Conservation
Service, 1994a,b). The STATSGO categorical values for hydrologic group were
converted to numbers in order to generalize (by area-weighted average) the
hydrologic group for each soil map unit. The threshold level of 2.5 was
determined; soils with a value less than 2.5 were considered to be well-drained
soils (similar to the STATSGO hydrologic group values of A or B) and soils with
a value greater than or equal to 2.5 were considered to be poorly-drained soils
(similar to the STATSGO hydrologic group values of C or D). The original map
unit level data was converted from a coverage (vector data) to a grid in order
to facilitate overlaying the data layers using ARC/INFO GRID processing. The
grid was divided into 2 groups, those cells having a hydrologic group value
less than 2.5 and those cells having a hydrologic group value greater than or
equal to 2.5.

The extent of woodland and cropland in agricultural areas was compiled from the
1992 Census of Agriculture (U.S. Bureau of the Census, 1995) in which the acres
of woodland and cropland are compiled by county. The woodland to cropland ratio
was calculated by dividing the acres of woodland by the acres of cropland. The
ratio represents the extent of woodland in relation to cropland in agricultural
areas of the Southeast, where dilution, denitrification, and vegetative uptake
likely attenuate nitrate concentration in ground water. This county level
information was converted from a coverage to a grid in order to facilitate
overlaying the data layers. The grid was divided into 2 groups, those cells
having a woodland/cropland ratio less than 0.3 and those cells having a
woodland/cropland ratio greater than or equal to 0.3.

Population density was determined from 1990 population data compiled by census
block group (U.S. Bureau of the Census, 1991) and was calculated for each
census block group by dividing the number of people in the block group by the
land area of the block group. This census block group information was converted
from a coverage to a grid in order to facilitate overlaying the data layers.
The grid was divided into 2 groups, those cells having greater than 386 people
per square kilometer and those cells having less than or equal to 386 people
per square kilometer.

Nitrogen loading was determined by combining nitrogen input from fertilizer
(Battaglin and Goolsby, 1994), manure (Smith and others, 1997), and atmospheric
deposition (Smith and others, 1997). This county level nitrogen data was
converted from a coverage to a grid in order to facilitate overlaying the data
layers. The grid was divided into 2 groups, those cells having greater than
2,100 kilograms per square kilometer and those cells have less than or equal to
2,100 kilograms per square kilometer.

These four grids were combined into one output grid. The sixteen values from
the input grids were consolidated into four risk categories for nitrate
contamination of ground water in the conterminous United States:

Group 4 -- high -- nitrogen loading greater than 2,100 kilograms per square
kilometer or population density greater than 386 people per square kilometer;
hydrologic group less than 2.5 and woodland to cropland ratio less than 0.3.

Group 3 -- moderately high -- nitrogen loading greater than 2,100 kilograms per
square kilometer or population density greater than 386 people per square
kilometer; hydrologic group greater than or equal to 2.5 or woodland to
cropland ratio greater than or equal to 0.3.

Group 2 -- moderately low -- nitrogen loading less than or equal to 2,100
kilograms per square kilometer and population density less than or equal to 386
people per square kilometer; hydrologic group less than or equal to 2.5 and
woodland to cropland ratio less than 0.3.

Group 1 -- low -- nitrogen loading less than or equal to 2,100 kilograms per
square kilometer and population density less than or equal to 386 people per
square kilometer; hydrologic group greater than or equal to 2.5 or woodland to
cropland ratio greater than or equal to 0.3.

The data set is provided in ascii format as well as exported ARC/INFO GRID
format. In the ascii file, only the field called "S1" is included. The field
called "VALUE" is not included. The projection file needed for the ascii format
of the data set is:

Projection ALBERS Datum NAD27 Zunits NO Units METERS Spheroid CLARKE1866 Xshift
0.0000000000 Yshift 0.0000000000 Parameters 29 30 0.000 /* 1st standard
parallel 45 30 0.000 /* 2nd standard parallel -96 0 0.000 /* central meridian
23 0 0.000 /* latitude of projection's origin 0.00000 /* false easting (meters)
0.00000 /* false northing (meters)

The retrospective database for nutrients in ground water and surface water is
available through the internet at
"http://water.usgs.gov/nawqa/nutrients/datasets/retrodata.html".

The current version of the map also appears in Nolan and others (1998), which
is available through the internet at
"http://water.usgs.gov/nawqa/wcp/index.html".