Magnitudes, nature, and effects of point and nonpoint discharges in the Chattahoochee River basin, Atlanta to West Point Dam, Georgia

Abstract

During the period April 1975 to June 1978, the U.S. Geological Survey conducted a river-quality assessment of the Upper Chattahoochee River basin in Georgia. One objective of the study was to assess the magnitudes, nature, and effects of point and nonpoint discharges in the Chattahoochee River basin from Atlanta to the West Point Dam. On an average annual basis and during the storm period of March 1215, 1976, nonpoint-source loads for most constituents analyzed were larger than point-source loads at the Whitesburg station, located on the Chattahoochee River about 40 river miles downstream of Atlanta. Most of the nonpoint-source constituent loads in the Atlanta-to-Whitesburg reach were from urban areas. Average annual point-source discharges accounted for about 50 percent of the dissolved nitrogen, total nitrogen, and total phosphorus loads, and about 70 percent of the dissolved phosphorus loads at Whitesburg. During weekends, power generation at the upstream Buford Dam hydroelectric facility is minimal. Streamflow at the Atlanta station during dryweather weekends is estimated to be about 1,200 ftVs (cubic feet per second). Average daily dissolved-oxygen concentrations of less than 5.0 mg/ L (milligrams per liter) occurred often in the river, about 20 river miles downstream from Atlanta during these periods from May to November. During a low-flow period, June 1-2, 1977, five municipal point sources contributed 63 percent of the ultimate biochemical oxygen demand, 97 percent of the ammonium nitrogen, 78 percent of the total nitrogen, and 90 percent of the total phosphorus loads at the Franklin station, at the upstream end of West Point Lake. Average daily concentrations of 13 mg/L of ultimate biochemical oxygen demand and 1.8 mg/L of ammonium nitrogen were observed about 2 river miles downstream from two of the municipal point sources. Carbonaceous and nitrogenous oxygen demands caused dissolved-oxygen concentrations between 4.1 and 5.0 mg/L to occur in a 22mile reach of the river downstream from Atlanta. Nitrogenous oxygen demands were greater than carbonaceous oxygen demands in the reach from river mile 303 to 271, and carbonaceous demands were greater from river mile 271 to 235. The heat load from the Atkinson-McDonough thermoelectric 2 CHATTAHOOCHEE RIVER BASIN, GEORGIA powerplants caused a decrease in the dissolved-oxygen concentrations of about 0.2 mg/L. During a critical low-flow period, a streamflow at Atlanta of about 1,800 ftVs, with present (1977) point-source flows of 185 ft:Vs containing concentrations of 45 mg/L of ultimate biochemical oxygen demand and 15 mg'/L of ammonium nitrogen, results in a computed minimum dissolvedoxygen concentration of 4.7 mg/L in the river downstream from Atlanta. In the year 2000, a streamflow at Atlanta of about 1,800 ftVs with pointsource flows of 373 ftVs containing concentrations of 45 mg/L of ultimate biochemical oxygen demand and 5.0 mg/L of ammonium nitrogen, will result in a computed minimum dissolved-oxygen concentration of 5.0 mg/L. A streamflow of about 1,050 ftVs at Atlanta in the year 2000 will result in a dissolved-oxygen concentration of 5.0 mg/L if point-source flows contain concentrations of 15 mg/L of ultimate biochemical oxygen demand and 5.0 mg/L of ammonium nitrogen. Phytoplankton concentrations in West Point Lake, about 70 river miles downstream from Atlanta, could exceed 3 million cells per milliliter during extended low-flow periods in the summer with present pointand nonpointsource nitrogen and phosphorus loads. In the year 2000, phytoplankton concentrations in West Point Lake are not likely to exceed 700,000 cells per milliliter during extended low-flow periods in the summer, if phosphorus concentrations do not exceed 1.0 mg/L in point-source discharges. INTRODUCTION The traditional approach to improving stream-water quality has been to identify the point sources of discharge and require that the discharge from these sources meet specified standards. The approach has been modified by the passage of the Federal Water Pollution Control Act Amendments of 1972 (Public Law 92-500), which require the implementation of wastewater treatment plans for the control of constituent loads from both point and nonpoint sources on an areawide basis. Studies have indicated that storm runoff from nonpoint sources may contain organic wastes, nitrogen, phosphorus, trace metals, and sediment in sufficient quantities to be a significant contributor to the degradation of water quality in the receiving stream. The BOD (biochemical oxygen demand) load from urban runoff in a Durham, N.C., basin was estimated by Bryan (1970) to be equal to the BOD load from the basin's WTF (wastewater treatment facility), which operated at secondary treatment levels. Bryan (1974) also reported that 0.74 (ton/mi2 ) /yr (ton per square mile per year) of lead was discharged into the 1.67 mi 2 (square miles) urban basin in Durham, N.C., and that the lead was associated with the suspended solids. Whipple (1970), in a study of three New Jersey river basins, reported that less than 39 percent of the total organic loading,