Probability of Detecting Atrazine/Desethyl-atrazine and Elevated Concentrations of Nitrate in Ground Water in Colorado
by Michael G. Rupert
Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 02-4269, 35 p., 18 figs.
This document also is available in pdf format: 
WRIR 02-4269 (6,091 KB)
(Requires Adobe Acrobat Reader)
Abstract
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 and desethyl-atrazine (a breakdown product of atrazine) 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 and(or) desethyl-atrazine (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 using a geographic information system to produce a data set 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 maps were constructed.
Table of Contents
Abstract
Introduction
Background
Purpose and Scope
Acknowledgments
Climate, Geology, and Water Use
Methods of Investigation
Compilation of Anthropogenic, Hydrogeologic, and Ground-Water Quality Data
Anthropogenic and Hydrogeologic Data
Ground-Water Quality Data for Model Calibration
Ground-Water Quality Data for Model Validation
Statistical Methods and Regression Models
Selecting the Most Significant Land-Cover Buffer Size
Estimating the Probability of Detecting Atrazine/Desethyl-atrazine and Elevated Concentrations of Nitrate in Ground Water
Development of Atrazine/Desethyl-atrazine Model
Development of Nitrate Model
Validation of the Atrazine/Desethyl-atrazine and Nitrate Models
Construction of Atrazine/Desethyl-atrazine and Nitrate Probability Maps
Model Results
Use of the Atrazine/Desethyl-atrazine and Nitrate Models for Alachlor, Metolachlor, and Simazine
Appropriate Uses of the Probability Maps, and Suggestions for Improvements
Summary
References Cited
