USGS

Water Resources of Colorado

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.

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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

 

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