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Georgia Water Science Center Publications

Are Farmers Contaminating the Shallow Ground Water?

Map of the study area.

The U.S. Geological Survey (USGS) is working to address this difficult question. As part of the National Water-Quality Assessment (NAWQA) Program, water samples were collected from 38 shallow ground-water wells adjacent to farm fields in the southwestern part of the Apalachicola-Chattahoochee-Flint (ACF) River basin in the summer of 1993 and the spring of 1994. Preliminary analyses indicate that nitrate concentrations in 2 of 38 wells exceeded the EPA standard of 10 milligrams per liter (mg/L) for drinking water. Pesticides were measured in 18 wells at concentrations well below EPA standards for drinking water. Although there should be concern for the presence of pesticides in ground water, the trace concentrations are not believed to pose immediate health risks.

In 1990, about 2.64 million people lived in the ACF River basin, 60 percent of those in the Atlanta area. Most agricultural land in the northern part of the ACF River basin is used for pastures and, to a lesser extent, poultry production, while most agricultural land in the southern part of the basin is used for row crops and orchards.

One of the tasks of the NAWQA program is to design and implement a water-quality assessment of surface and ground water as it relates to various land uses. A part of the study is designed to determine the chemical quality of shallow ground water that underlies agricultural land-use areas in the southern basin. Thirty-eight monitoring wells were installed in the surficial aquifers adjacent to and downgradient from farm fields. Four reference wells were installed in forested areas to represent background water-quality conditions.

The depth to the water table in all the monitoring wells ranged from 3 to 67 feet below land surface. Water samples were collected from most wells during summer 1993 and again during spring 1994 to represent low and high water-table conditions, respectively. The samples were analyzed for nutrients, pesticides, volatile organic compounds, major ions, organic carbon, and select radionuclides. This pamphlet presents information on the occurrence of nitrate and commonly used pesticides in the shallow ground water associated with agricultural land use in southwestern Georgia and adjacent areas of Alabama and Florida.

Explanation is below Surficial aquifers were selected for sampling rather than deeper aquifer systems that are generally used for domestic- and public- water supplies and irrigation because surficial aquifers are the first water-bearing zones, and are more susceptible to contamination. Therefore, a measure of water quality in surficial aquifers may serve as an early warning of potential contamination of deeper aquifer systems.

Filtered water samples collected throughout the NAWQA program were analyzed by the USGS National Water Quality Laboratory to identify and quantify concentrations of nutrients and 84 pesticides commonly used throughout the United States. The term pesticide refers to groups of organic compounds designed to control pests such as weeds (herbicides), insects (insecticides), fungi (fungicides), and so forth. Many of these pesticides are commonly used for weed and insect control in the ACF River basin. However, the laboratory methods do not include some commonly used pesticides, such as paraquat, methanearsonate, glyphosate, DSMA, MSMA, and several chlorophenoxy herbicides. Therefore, it should not be assumed that these pesticides are not applied in the basin, or are not present in the water resource.

Pesticides in ground water at concentrations that exceed standards can be harmful to human health. Sources of pesticides are generally from applications made to farm fields, orchards, in and around buildings, lawns and gardens, golf courses, parks, and roadways. No pesticides were measured at concentrations that exceeded EPA standards for drinking water, however, alachlor and atrazine were within 28 and 14 percent, respectively, of the maximum contaminant level (MCL). Six pesticides were measured at trace concentrations in 16 agricultural wells (see figure and table). The herbicides listed are four of the top five herbicides applied to agricultural land in the ACF River basin. Although there should be concern for the presence of pesticides in ground water, the trace concentrations found in surficial aquifers are not believed to pose immediate health risks.

The maximum contaminant level (MCL) is the maximum concentration allowed in drinking water according to EPA drinking water standards. The lifetime-health advisory (HA) level is the maximum concentration in drinking water that would not cause adverse human-health effects, excluding cancer (based on a 150-pound adult consuming about 2 quarts of water per day for a period of 70 years).

High concentrations of nitrate in ground water can be toxic to infants and warm-blooded animals that drink the water. Sources of nitrate are generally from fertilizer, human and animal wastes, and atmospheric deposition. Nitrate concentrations in 2 of 38 agricultural wells exceeded the EPA standard of 10 mg/L for drinking water (see figure). Concentrations in 11 of the 38 wells ranged from 3 to 10 mg/L, indicating probable human influence. Twenty-five of the 38 agricultural wells and four reference wells had background concentrations of nitrate that were less than 3 mg/L.

Pesticides and nitrates used on agricultural lands appear to be finding their way into the shallow ground water. However, surficial aquifers are generally not used for drinking water, and concentrations are generally well below levels that pose health risks.

No pesticides were detected in measurable concentrations in ground water from four reference wells. Six pesticides and two breakdown products of atrazine and DDT were measured in 18 agricultural wells, but the concentrations were well below EPA drinking-water standards and are not believed to pose immediate health risks (see table).

Nitrate concentrations in ground water from four reference wells indicated no human influence. Twenty-nine percent of agricultural wells were influenced by human activities, but nitrate concentrations were below EPA drinking-water standards. Only five percent of agricultural wells had nitrate concentrations that exceeded the MCL of 10 mg/L.

The full report can be ordered from USGS. You may order online, or call or write:

U.S. Geological Survey
1770 Corporate Drive
Suite 500
Norcross, GA 30093
(678) 924-6700

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