Raster dataset showing the probability of elevated concentrations of nitrate in ground water in Colorado, hydrogeomorphic regions and fertilizer use estimates included

This dataset is one of eight datasets produced by this study.
Four of the datasets predict the probability of detecting
atrazine and(or) desethyl-atrazine (a breakdown product of atrazine)
in ground water in Colorado; the other four predict the probability
of detecting elevated concentrations of nitrate in ground water in
Colorado. The four datasets that predict the probability of
atrazine and(or) desethyl-atrazine (atrazine/DEA) are differentiated
by whether or not they incorporated atrazine use and whether
or not they incorporated hydrogeomorphic regions. The four datasets
that predict the probability of elevated concentrations of nitrate
are differentiated by whether or not they incorporated fertilizer
use and whether or not they incorporated hydrogeomorphic
regions. Each of the eight datasets has its own unique strengths
and weaknesses. The user is cautioned to read Rupert (2003, Probability
of detecting atrazine/desethyl-atrazine and elevated concentrations
of nitrate in ground water in Colorado: U.S. Geological Survey
Water-Resources Investigations Report 02-4269, 35 p.,
http://water.usgs.gov/pubs/wri/wri02-4269/) to determine if he(she)
is using the most appropriate dataset for his(her) particular needs.
This dataset specifically predicts the probability of detecting
elevated concentrations of nitrate in ground water in Colorado with
hydrogeomorphic regions and fertilizer use included. The following
text was extracted from Rupert (2003).

Draft Federal regulations may require that each State develop a
State Pesticide Management Plan for the herbicides atrazine,
alachlor, metolachlor, and simazine. Maps were developed that the
State of Colorado could use to predict the probability of detecting
atrazine/DEA in ground water in Colorado. These maps can be
incorporated into the State Pesticide Management Plan and can help
provide a sound hydrogeologic basis for atrazine management in
Colorado. Maps showing the probability of detecting elevated nitrite
plus nitrate as nitrogen (nitrate) concentrations in ground water in
Colorado also were developed because nitrate is a contaminant of
concern in many areas of Colorado.

Maps showing the probability of detecting atrazine/DEA at or greater
than concentrations of 0.1 microgram per liter and nitrate
concentrations in ground water greater than 5 milligrams per liter
were developed as follows: (1) Ground-water quality data were overlaid
with anthropogenic and hydrogeologic data by using a geographic
information system (GIS) to produce a dataset in which each well had
corresponding data on atrazine use, fertilizer use, geology,
hydrogeomorphic regions, land cover, precipitation, soils, and well
construction. These data then were downloaded to a statistical
software package for analysis by logistic regression. (2) Relations
were observed between ground-water quality and the percentage of
land-cover categories within circular regions (buffers) around wells.
Several buffer sizes were evaluated; the buffer size that provided
the strongest relation was selected for use in the logistic regression
models. (3) Relations between concentrations of atrazine/DEA and
nitrate in ground water and atrazine use, fertilizer use, geology,
hydrogeomorphic regions, land cover, precipitation, soils, and
well-construction data were evaluated, and several preliminary
multivariate models with various combinations of independent variables
were constructed. (4) The multivariate models that best predicted
the presence of atrazine/DEA and elevated concentrations of nitrate
in ground water were selected. (5) The accuracy of the multivariate
models was confirmed by validating the models with an independent
set of ground-water quality data. (6) The multivariate models were
entered into a geographic information system and the probability
GRIDS were constructed.