USGS

 

U.S. GEOLOGICAL SURVEY

Scientific Investigation Report 2004-5118


Ground-Water Vulnerability to Nitrate Contamination at Multiple Thresholds in the Mid-Atlantic Region Using Spatial Probability Models

By Earl A. Greene, Andrew E. LaMotte, and Kerri-Ann Cullinan

 

This report is available as a pdf.

 

ABSTRACT

The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency’s Regional Vulnerability Assessment Program, has developed a set of statistical tools to support regional-scale, ground-water quality and vulnerability assessments. The Regional Vulnerability Assessment Program’s goals are to develop and demonstrate approaches to comprehensive, regional-scale assessments that effectively inform managers and decision-makers as to the magnitude, extent, distribution, and uncertainty of current and anticipated environmental risks. The U.S. Geological Survey is developing and exploring the use of statistical probability models to characterize the relation between ground-water quality and geographic factors in the Mid-Atlantic Region.

Available water-quality data obtained from U.S. Geological Survey National Water-Quality Assessment Program studies conducted in the Mid-Atlantic Region were used in association with geographic data (land cover, geology, soils, and others) to develop logistic-regression equations that use explanatory variables to predict the presence of a selected water-quality parameter exceeding a specified management concentration threshold. The resulting logistic-regression equations were transformed to determine the probability, P(X), of a water-quality parameter exceeding a specified management threshold. Additional statistical procedures modified by the U.S. Geological Survey were used to compare the observed values to model-predicted values at each sample point. In addition, procedures to evaluate the confidence of the model predictions and estimate the uncertainty of the probability value were developed and applied. The resulting logistic-regression models were applied to the Mid-Atlantic Region to predict the spatial probability of nitrate concentrations exceeding specified management thresholds. These thresholds are usually set or established by regulators or managers at National or local levels.

At management thresholds of 1 milligram per liter and 3 milligrams per liter as nitrogen, the probability of nitrate concentrations exceeding these levels is greater than 50 percent (0.50) throughout much of the Mid-Atlantic Region. This includes extensive areas throughout central Maryland, southeastern Pennsylvania, northwestern Pennsylvania, and the Delmarva Peninsula. In addition, extensive areas in North Carolina and Virginia also have high probabilities of nitrate concentrations in ground water exceeding management thresholds of 1 milligram per liter and 3 milligrams per liter. The mapped areas showing a high predicted probability of nitrate concentrations in ground water exceeding 1 milligram per liter and 3 milligrams per liter correspond to areas that are mapped as cultivated land cover and/or overlying carbonate rocks. At a management threshold of 10 milligrams per liter (corresponding to the U.S. Environmental Protection Agency standard for nitrate in drinking water of 10 milligrams per liter), the predicted probability of nitrate concentrations in ground water exceeding this level is low for most of the Mid-Atlantic Region, except for the Delmarva Peninsula, southeastern Pennsylvania, and areas mapped as carbonate rocks in Virginia, Maryland, and Pennsylvania.

 

TABLE OF CONTENTS

Abstract

Introduction

Purpose and scope

Previous investigations

Acknowledgments

Data description

Dependent variable

Explanatory (independent) variables

Logit model

Significance testing of â coefficients

Model goodness-of-fit

Statistical model development

Area of well influence

Single variable models

Multivariate model

Stepwise logistic regression

Multicollinearity and interaction terms

Evaluation of model performance

Calibration

Discrimination

Model validation

Confidence interval

Analysis of variable effects

Ground-water vulnerability to nitrate contamination in the Mid-Atlantic Region

Summary

References cited


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For more information about USGS activities in Maryland, Delaware and the District of Columbia contact:

 

Director
U.S. Geological Survey
Maryland, Delaware, D.C. Water Science Center
8987 Yellow Brick Road
Baltimore, MD 21237
Telephone: (410) 238-4200
Fax: (410) 238-4210

 

or access the USGS Water Resources of Maryland, Delaware, and District of Columbia home page at:  http://md.water.usgs.gov/.




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