Effects of urban development on nutrient loads and streamflow, upper Chattahoochee River Basin, Georgia, 1976–2001
Calhoun, Daniel L.
Frick, Elizabeth A.
Buell, Gary R.
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The use of secondary and tertiary treat-ment of wastewater and the treatment of stormwater in the Metropolitan Atlanta area has mitigated many of the water-quality problems associated with urbanization in the upper Chattahoochee River Basin. However, treat-ment has not reduced total nitrogen. The engineering of urban watersheds to efficiently convey stormwater run-off has succeeded in moderating flooding in urban areas; however, increased stormflow volumes and streamflow velocities have degraded the physical structure of some streams and reduced the baseflow component of total streamflow. Nationwide, urbanization is increasing the volume of wastewater that requires treatment. For this study, effluent discharge-monitoring data reported by wastewater-treatment plants (WWTPs) with outflows greater than 1 million gallons per day (Mgal/d) were analyzed in conjunction with instream nutrient con-centrations to estimate phosphorus and nitrogen loads for the Chattahoochee River. Phosphorus loads have de-creased from 1976 through 2001, primarily as a result of reductions in the use of phosphorus and improved removal processes at WWTPs. Ammonia nitrogen loads also have decreased during this period whereas nitrate nitrogen loads have increased. These changes result from increases in tertiary treatment that oxidizes ammonia to produce nitrate nitrogen. Effluent volume discharged from WWTPs has increased as population increased from 1976 through 2001. Hydrograph separations for streams tributary to the upper Chattahoochee River indicate that urbanization reduces baseflows, increases stormflows, and may reduce the assimilative capacity of the Chattahoochee River, making the task of restoring the designated uses of area streams more difficult.