Relation of Land Use to Nutrient and Suspended-Sediment Concentrations, Loads, and Yields in the Upper Chattahoochee River Basin, Georgia, 1993-98
Frick, Elizabeth A.
Buell, Gary R.
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This report describes the effects of various land uses on fluvial transport of nutrients (nitrogen, phosphorus, and organic carbon) and suspended sediment during different hydrologic conditions within the upper Chattahoochee River basin from 1993 to 1998. Fluvial transport is discussed within the context of nutrient and suspended-sediment concentrations and load and yield estimates. Monthly and stormflow water-quality samples were collected and analyzed by the U.S. Geological Survey at four tributary streams to the Chattahoochee River: West Fork Little River (predominant land use, poultry and livestock production), Sope Creek (suburban), Peachtree Creek (urban), Snake Creek (silviculture). Monthly water-quality samples also were collected at two sites on the Chattahoochee River, one upstream and one downstream from Metropolitan Atlanta. Stormflow concentrations were significantly larger than basefiow concentrations for dissolved and suspended organic nitrogen, dissolved and suspended organic carbon, total phosphorus, and suspended sediment at all tributary sites and for ammonia at poultry and livestock production, suburban, and urban sites-thus enhancing constituent transport during periods of stormwater runoff. Nitrate concentrations were largest in baseflow samples at the poultry and livestock production site, indicating relatively large baseflow contributions of nitrate from ground water. Nutrient and suspended-sediment concentrations were significantly larger in the Chattahoochee River downstream than upstream from Atlanta. Treated wastewater effluent was the primary source of elevated nitrate concentrations in the Chattahoochee River downstream from Metropolitan Atlanta. Temporal and spatial patterns in the annual loads of nutrients and suspended sediment from the four tributary watersheds and from the Chattahoochee River sites upstream and downstream from Metropolitan Atlanta indicate that specific land uses within each of these basins exert a dominant control on the variations in loads. Alterations to basin hydrology resulting from development exert the dominant control on loads in the urban and suburban watersheds. Although mean-annual discharge at the urban site was 2.5 times larger than at the suburban site, mean-annual loads were typically 2.5 to 4 times larger at the urban site. Agricultural practices exert the dominant control on loads in the watersheds with predominantly poultry and livestock production and silvicultural land use. Although mean-annual discharge at the poultry and livestock production site was 40 percent smaller than at the silviculture site, mean-annual loads of nitrogen and phosphorus were about 3.6 times larger at the poultry and livestock production site. As a result of nonpoint runoff from intensive urban land uses and point-source discharges from wastewater-treatment facilities in Metropolitan Atlanta, Chattahoochee River loads of nitrogen, phosphorus, organic carbon, and suspended sediment were 8.5, 14, 3.7, and 5.3 times larger, respectively, at the site downstream than the site upstream from Metropolitan Atlanta. Three of the four tributary watersheds (urban, suburban, and silviculture) discharge to the Chattahoochee River between the sites upstream and downstream of Metropolitan Atlanta and drain 14 percent of the intervening area. However, on an annual basis, only 4 to 7 percent of the nitrogen increase was accounted for by loads from these tributaries, thus indicating the importance of point source contributions of nitrogen in this reach. By contrast, 12 to 37 percent of the suspended sediment was accounted for by loads from these tributaries, thus indicating the importance of nonpoint sources of suspended sediment. Among the tributary sites, the largest annual nutrient yields were from the tributary site with intensive poultry and livestock production within its watershed-ammonia, 0.52 tons per square mile (tons/mi2); nitrate, 3.2 tons/mi2; dissolved organic nitrogen, 0.66 tons/mi2; suspended organic nitrogen, 4.3 tons/mi2; orthophosphate, 0.25 tons/mi2 total phosphorus, 1.6 tons/mi2; suspended organic carbon, 14 tons/mi2; and suspended sediment, 1,600 tons/mi2. Surface runoff was the primary source of elevated yields of all constituents except nitrate. Sewer overflows in the predominantly urban watershed were the likely source of the largest dissolved organic carbon (8.8 tons/mi2) yields estimated among the six sites studied and elevated yields of ammonia and organic nitrogen. The predominantly silvicultural watershed had the smallest annual yields-sometimes by as much as an order-of-magnitude lower than the other tributary watersheds. From 1993 to 1995, yields of nitrogen and carbon constituents were approximately two to three times larger and yields of suspended sediment and total phosphorus were about six times larger at the Chattahoochee River site downstream from Metropolitan Atlanta than at the upstream site. Point-source discharges of treated sewage effluent from 10 major municipal wastewater treatment plants account for much of the increase in ammonia and nitrate and some of the increase in total phosphorus yields. Nonpoint sources account for most of the increase in suspended-sediment yields and some of the increase in total-phosphorus and suspended-organic carbon yields.