Photo of flood(118k)
The NAWQA program is focusing on the occurrence and distribution of nutrients, suspended sediments, and pesticides in the Nation's water resources. Record flooding caused by tropical storm Alberto provided a unique opportunity to measure these constituents during extreme hydrologic conditions. Water samples were collected from several locations affected by the flood, but the most frequent data collection occurred at the Flint River at Newton (19 samples) and the Ocmulgee River at Lumber City (8 samples), the most downstream sites on these rivers that were accessible throughout the flood. The purpose of this more intensive monitoring was to assess changes in water quality at these points for the duration of the flood. Samples were analyzed by the USGS National Water Quality Laboratory for suspended sediment and to identify and quantify concentrations of nutrients and 84 pesticides that have been commonly used for weed and insect control throughout the United States.
During the period July 3-7, 1994, tropical storm Alberto moved northward from the Gulf of Mexico, across the Florida panhandle, and into parts of southwestern and central Georgia, southeast Alabama, and northwest Florida. Parts of central and southwestern Georgia received as much as 28 inches of rainfall during the storm. Americus received about 21 inches of rain during a 24-hour period on July 6 (data from U.S. Bureau of Commerce, National Weather Service).
The storm generated 7 inches of runoff (700 billion gallons) in the Flint River basin upstream of Newton, and 4.6 inches of runoff (410 billion gallons) in the Ocmulgee River upstream of Lumber City. Runoff and flooding of urban and agricultural areas can result in transport of industrial chemicals, untreated sewage, and agricultural chemicals in floodwaters, and subsequent deposition of sediment and transport of contaminants downstream. Contaminants in floodwaters can render the water unsuitable for drinking and aquatic life. Deposition of sediment and associated contaminants in floodplains and river mouths, or estuaries, may impair crop production, aquatic habitats, and fisheries.
Much of the topsoil eroded during intensive rainfall and flooding in the Flint and Ocmulgee River basins probably was redeposited in extensive floodplains within the basins. Even so, 110,000 and 83,000 tons of suspended sediment were transported from the Flint and Ocmulgee River basins, respectively. The suspended sediment that was transported from the basins was very fine-grained material, comprised mostly of silt and clay. Suspended sediment yields for the Flint and Ocmulgee Rivers upstream of the sample locations were similar, at 60 and 51 pounds per acre, respectively.
In floodwaters, total nitrogen concentrations were lower, and total phosphorus concentrations were higher than median concentrations in samples collected prior to the flood (figs. A and B). Nitrate concentrations were about 1% of EPA's drinking-water standard of 10 milligrams per liter. Nutrient loads in floodwaters were higher in the Flint River than the Ocmulgee River, probably because intensive rainfall from tropical storm Alberto caused runoff and flooding in large areas of cropland in the Flint River basin and in mostly forested and urban areas in the Ocmulgee River basin (see fig. C). Floodwaters in the Flint and Ocmulgee Rivers transported more than half the mean annual load of phosphorus and organic nitrogen, and a lesser part of the mean annual load of nitrate.
Water samples collected from the Flint River at Newton illustrate (A) decreases in nitrate concentrations and increases in suspended organic nitrogen, and (B) suspended phosphorus at discharges above flood stage.
Much of the nitrogen load during the flood was in the form of organic nitrogen generally derived from decaying leaf litter from floodplains and fields, rather than nitrate derived from sources such as fertilizer. Much of the phosphorus load was associated with suspended sediment, not soluble forms associated with fertilizer.
Fourteen herbicides, five insecticides, and one fungicide commonly used in agricultural or urban areas were detected in floodwaters. Thirteen of these pesticides were present in trace concentrations in the Apalachicola River just upstream of Apalachicola Bay. Figures D and E show concentrations and loads for pesticides that were measured in water samples throughout much or all of the flood. Other pesticides present at trace concentrations in one or more samples were: bentazon, metribuzin, norflurazon, trifluralin, 2,4-D, fonofos, malathion, and chlorothalonil. All pesticide concentrations were well below EPA standards and guidelines for drinking water. However, concentrations of the insecticides chlorpyrifos, carbaryl, and diazinon approached or exceeded recommended guidelines for protection of aquatic life.
The highest pesticide concentrations for the herbicides shown occurred 4 to 6 days prior to the peak discharge on the Flint River at Newton.
The cumulative flood loads of seven herbicides and the insecticide diazinon were very low relative to the annual use in Georgia (11 million pounds of herbicides and insecticides).
From U.S. Geological Survey Water-Resources Investigations Report 94-4183
By D.J. Hippe, D.J. Wangsness, E.A. Frick, and J.W. Garrett