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Water-Resources Investigations Report 2003-4182

Comparison Between Agricultural and Urban Ground-Water Quality in the Mobile River Basin, 1999-2001

U.S. Geological Survey Water-Resources Investigations Report 2003-4182, 38 pages (Published 2003)

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

By James L. Robinson

This report is available online in pdf format: USGS WRIR 2003-4182 (5 MB)

ABSTRACT

Cover of SIR 2003-4182.

The Black Warrior River aquifer is a major source of public water supply in the Mobile River Basin. The aquifer outcrop trends northwest - southeast across Mississippi and Alabama. A relatively thin shallow aquifer overlies and recharges the Black Warrior River aquifer in the flood plains and terraces of the Alabama, Coosa, Black Warrior, and Tallapoosa Rivers. Ground water in the shallow aquifer and the Black Warrior River aquifer is susceptible to contamination due to the effects of land use. Groundwater quality in the shallow aquifer and the shallow subcrop of the Black Warrior River aquifer, underlying an agricultural and an urban area, is described and compared.

The agricultural and urban areas are located in central Alabama in Autauga, Elmore, Lowndes, Macon, Montgomery, and Tuscaloosa Counties. Row cropping in the Mobile River Basin is concentrated within the flood plains of major rivers and their tributaries, and has been practiced in some of the fields for nearly 100 years. Major crops are cotton, corn, and beans. Crop rotation and no-till planting are practiced, and a variety of crops are grown on about one-third of the farms. Row cropping is interspersed with pasture and forested areas. In 1997, the average farm size in the agricultural area ranged from 196 to 524 acres. The urban area is located in eastern Montgomery, Alabama, where residential and commercial development overlies the shallow aquifer and subcrop of the Black Warrior River aquifer. Development of the urban area began about 1965 and continued in some areas through 1995. The average home is built on a 1/8 - to 1/4 - acre lot.

Ground-water samples were collected from 29 wells in the agricultural area, 30 wells in the urban area, and a reference well located in a predominately forested area. The median depth to the screens of the agricultural and urban wells was 22.5 and 29 feet, respectively. Ground-water samples were analyzed for physical properties, major ions, nutrients, and pesticides. Samples from 8 of the agricultural wells and all 30 urban wells were age dated using analyses of chlorofluorocarbon, sulfur hexafluoride, and dissolved gases. Ground water sampled from the agricultural wells ranged in age from about 14 to 34 years, with a median age of about 18.5 years. Ground water sampled from the urban wells ranged in age from about 1 to 45 years, with a median age of about 12 years. The ages estimated for the ground water are consistent with the geology and hydrology of the study area and the design of the wells.

All of the agricultural and urban wells sampled for this study produce water from the shallow aquifer that overlies and recharges the Black Warrior River aquifer, or from the uppermost unit of the Black Warrior River aquifer. The wells are located in the same physiographic setting, have similar depths, and the water collected from the wells had a similar range in age. Statistically significant differences in groundwater quality beneath the agricultural and urban areas can reasonably be attributed to the effects of land use.

Ground water from the agricultural wells typically had acidic pH values and low specific conductance and alkalinity values. The water contained few dissolved solids, and typically had small concentrations of ions. Some of the agricultural ground-water contained concentrations of ammonia, nitrite plus nitrate, phosphorus, orthophosphate, and dissolved organic carbon in concentrations that exceeded those typically found in ground water. Pesticides were detected in ground water collected from 25 of the 29 agricultural wells. Nineteen different pesticide compounds were detected a total of 83 times. Herbicides were the most frequently detected class of pesticides. The greatest concentration of any pesticide was an estimated value of 1.4 microgram per liter of fluometuron.

The Wilcoxan rank sum test was used to determine statistically significant differences in water quality between the agricultural and urban ground water. Ground water from the agricultural and urban areas had a similar range of values for most physical properties, major ions, and age. Ground water from the agricultural area contained statistically greater concentrations of magnesium than ground water from the urban area. Fewer pesticide compounds were detected in the agricultural ground water than in the urban ground water. The total number of pesticide detections also was less in the agricultural ground water than in the urban ground water. Atrazine was the only manmade compound detected frequently enough in both data sets to allow statistical comparison. There was no statistical difference in atrazine concentrations between ground water from the agricultural and urban areas.

The Spearman rho and Kendall tau tests were used to test for statistically significant covariance among selected constituents in the agricultural ground water and for crop type. The concentration of nitrite plus nitrate increased as the concentration of magnesium and the number of pesticides detected increased. This correlation is attributed to land application of nitrogen-based fertilizer to enhance yield, crushed limestone and dolomite (which contain magnesium) to raise the acidic pH of the soil, and to the application pesticides. No correlation was found between crop type and ground-water quality, the concentrations of pesticides, or the number of pesticides detected in the agricultural ground water. This lack of correlation probably is a result of such practices as crop rotation and the simultaneous cultivation of a variety of crops.

CONTENTS

Abstract

Introduction

Purpose and Scope

Acknowledgments

Environmental setting of the Mobile River Basin

Description of the study area

Hydrogeology of the study area

Study design and methods

Land use and land cover

Monitoring well network

Water-quality samples

Quality-assurance and quality-control procedures

Graphical and statistical methods

Chemical and physical properties of the shallow sediments

Soil characteristics

Sediment core analyses

Lithology of the shallow sediments

Shallow aquifer properties

Ground-water quality

Physical properties

Major ions

Nutrients and dissolved organic carbon

Pesticides

Age of ground water

Relations among ground-water quality, ground-water age, and land use

Summary

References

Appendixes


REPORT AVAILABILITY

This report is available online in pdf format: USGS WRIR 2003-4182 (5 MB)
To view the PDF document, you need the free Adobe Acrobat® Reader installed on your computer.

Suggested citation:

Robinson, James L., 2003, Comparison Between Agricultural and Urban Ground-Water Quality in the Mobile River Basin, 1999-2001: U.S. Geological Survey Water-Resources Investigations Report 2003-4182, 38 p.

For more information, contact the Alabama Water Science Center.

USGS Alabama Publications Alabama Water Science Center

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