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Trends and Nutrient Loads for the Mobile River Basin, Alabama, Georgia and Mississippi, 1970-97

Douglas A. Harned

U.S. Geological Survey, 3916 Sunset Ridge Road, Raleigh, North Carolina 27607

       Databases for U.S. Geological Survey (USGS) surface-water sampling sites in the Mobile River Basin were reviewed for available water-quality data for the period 1970-97. Sites for which sufficient data were available were evaluated for long-term trends and nutrient transport was calculated for selected basins. The effort was undertaken as part of the National Water-Quality Assessment (NAWQA) Program.
       Data from USGS sampling sites having periods of continuous streamflow were examined for long-term trends with the Seasonal Kendall trend test. Trends and loads were estimated for total nitrogen at 15 sites and for total phosphorus at 14 sites. The Seasonal Kendall trend test adjusts for seasonal variability by using concentrations adjusted for the effects of streamflow with residuals from LOWESS (LOcally Weighted Sum of Squares) smoothed curves. Trends also were determined for sites without continuous data by using multivariate regression analysis.
       The Alabama River at Claiborne, Ala., and Tombigbee River below Coffeeville Lock and Dam near Coffeeville, Ala., stations were used to represent the most downstream locations in the Mobile River Basin. The station on the Tombigbee River at Gainesville Ala. represents water quality from the Mississippi area of the drainage basin. The trends identified for the Alabama River at Claiborne, and Tombigbee River near Coffeeville, Ala. included decreasing concentrations of total nitrogen, which suggest an overall reduction in the nitrogen contributions to Mobile Bay from the Mobile River from the mid-1970s to the present. The Alabama River also shows decreases in organic phosphorus (1979-94) and dissolved silica (1970-97). Total organic nitrogen increased at the Alabama River site from 1975 to 1985 but have declined since 1985. The Tombigbee River at Gainesville had similar declines in organic phosphorus (1975-96), and silica (1970-95). Specific conductance, alkalinity, and concentrations of dissolved solids, calcium, sulfate, chloride, and potassium increased at the Alabama River (1970-97) and Tombigbee River at Gainsville sites (1975-95), probably due to increases in point-source discharges.
       Total nitrogen and organic nitrogen concentrations have decreased (1985-95) in the Black Warrior River below Selden Dam near Eutaw, Ala., one of the major tributaries to the Tombigbee River. A generalized pattern of decrease in total nitrogen concentration is evident in streams in the Birmingham, Ala. area. Total phosphorus concentrations increased from 1970 to 1996 at three stations on the Etowah River in Georgia, probably due to urban development in the area. Other trends included increased concentrations from 1985 to 1995 in dissolved solids, chloride and potassium for the Black Warrior River; and decreased concentrations of total nitrogen (1985-96) and specific conductance (1985-95) in Sipsey Fork near Grayson Ala., and total nitrogen and nitrate in the Cahaba River near Centerville (1980-1997).
       Total nitrogen and total phosphorous loads were calculated using multivariate linear regression with bias correction by a minimum variance unbiased estimation method. The loads for the Tombigbee River and the Alabama River were third and fourth highest after the Mississippi and Atchafalaya River for major streams draining into the Gulf of Mexico. The total nitrogen load for the Tombigbee River is around 2 percent of the load and the phosphorus load is about 3 percent that of the Mississippi River. Future study of trends in land use and their relation to water-quality variation will allow a better understanding of the causal factors that drive water-quality trends in the Mobile River basin.