WATER QUALITY OF THE APALACHICOLA-CHATTAHOOCHEE-FLINT AND OCMULGEE RIVER BASINS RELATED TO FLOODING FROM TROPICAL STORM ALBERTO; PESTICIDES IN URBAN AND AGRICULTURAL WATERSHEDS; AND NITRATE AND PESTICIDES IN GROUND WATER

By Daniel J. Hippe, David J. Wangsness, Elizabeth A. Frick, and Jerry W. Garrett

ABSTRACT

This report presents preliminary information on water quality in the Flint, Apalachicola, and Ocmulgee River basins during record flooding caused by tropical storm Alberto, July 1994. It also compares the types and concentrations of pesticides present in surface waters draining predominantly agricultural and urban watersheds, and presents preliminary information on the occurrence of nitrate and commonly used pesticides in the shallow ground water associated with agricultural land use in the southern Apalachicola-Chattahoochee- Flint (ACF) River basin.

During the period July 3-7, 1994, tropical storm Alberto as much as 28 inches of rain fell onto parts of southwestern and central Georgia and southeastern Alabama causing record flooding on the Flint and Ocmulgee Rivers and several of their tributaries. Much of the topsoil eroded during intensive rainfall and flooding probably was redeposited in extensive floodplains within the river basins. The suspended sediment transported from the basins was comprised mostly of silt- and clay-sized material. Total nitrogen concentrations were lower, and total phosphorus concentrations were higher than median concentrations in samples collected prior to the flood. Much of the nitrogen load was in the form of organic nitrogen generally derived from organic detritus, rather than nitrate derived from other sources, such as fertilizer. Floodwaters transported a large part of the mean annual load of total phosphorus and organic nitrogen and a lessor part of the mean annual load of nitrite plus nitrate. Fourteen herbicides, five insecticides, and one fungicide commonly used in agricultural and urban areas were detected in floodwaters of the Flint, Apalachicola, and Ocmulgee Rivers. Concentrations of nitrate nitrogen and detected pesticides were below EPA standards and guidelines for drinking water. However, concentrations of the insecticides chlorpyrifos, carbaryl, and diazinon approached or exceeded guidelines for protection of aquatic life.

Water-quality samples were collected at nearly weekly intervals from March 1993 through April 1994 from one urban and two agricultural watersheds in the ACF River basin, and the samples analyzed for 84 commonly used pesticides. More pesticides were detected and at generally higher concentrations in water from the urban watershed than the agricultural watersheds. A greater number of pesticides were detected throughout much of the year in the urban watershed than the agricultural watersheds. Median concentrations of all pesticides detected in water from each watershed were below EPA drinking-water standards and guidelines. However, median concen- trations of the insecticides chlorpyrifos and diazinon exceeded guidelines for protection of aquatic life.

Thirty-eight wells were installed in surficial aquifers adjacent to and downgradient of farm fields within agricultural areas of southwestern Georgia and adjacent areas of Alabama and Florida. Four reference wells were installed in forested areas to represent background water-quality conditions. The surficial aquifers were selected for sampling rather than deeper, regional aquifer systems because they are the uppermost water- bearing zones and are more susceptible to contamination. Even though regional aquifers are generally used for domestic- and public-water supplies, and for irrigation, degradation of water quality in the surficial aquifers serves as an early warning of potential contamination of regional aquifers. Nitrate concentrations were less than 3 mg/L as N (indicating minimal effect of human activities) in water from about two-thirds of the wells in agricultural areas. Water from the remaining agricultural wells had elevated nitrate concentrations in one or two samples, probably the result of human activity, and nitrate concentrations in two of these wells exceeded the EPA drinking-water standards. Water samples from eight agricultural wells had pesticide concentrations above method detection limits, but maximum concentrations were below EPA drinking-water standards or guidelines.