U.S. Geological Survey Scientific Investigations Report 2009-5268
Trends in Water Quality in the Southeastern United States, 1973–2005
Previous Regional Trend Studies in the Southeast
Studies of water-quality trends in the Southeast prior to 1970 were limited by the lack of consistent long-term data collection. A review of the literature for studies in which trends in water quality in Southeastern States were evaluated shows a complex record of change in many constituents. The following studies are limited primarily to those in which the results from multiple sites were analyzed using regression or seasonal Kendall’s tau analysis, and are presented to establish the state of knowledge about water-quality trends in the Southeastern United States.
In an international review of trend assessments of nutrients, Heathwaite and others (1995) concluded that historical water-quality data for North America are not abundant and that comprehensive data collection suitable for trend assessment only began in the 1960s. NASQAN was cited as one of the important networks for trend analysis.
Harned (1982) used discharge-weighted concentrations and regression to examine trends (1955–80) in the Neuse River (North Carolina). Increases in potassium and sulfate concentrations were observed, possibly as a result of increasing wastewater-treatment plant effluent and increased inputs from atmospheric precipitation. Trends in nutrient concentrations were not detected.
In a review of data for the Yadkin–Pee Dee River system (North Carolina), Harned and Meyer (1983) found a general pattern (1960–80) of a peak followed by a decline of potassium, calcium, magnesium, and dissolved solids; and a similar pattern of peak and decline in specific conductance. The decrease in concentrations after 1975 may have been a result of improvements in municipal and industrial wastewater treatment. The observed increase in sulfate and decrease in pH may have been related to increases in atmospheric inputs. Nitrate concentrations increased from 1945 to 1972 and remained stable from 1972 to 1980. Suspended-sediment concentrations dropped substantially (1950–80) as farmland reverted to forest.
Wells and Schertz (1983) conducted a detailed review of trends in dissolved-solids concentrations at 515 individual NASQAN sites. The results for individual sites were not summarized regionally.
Smith and Alexander (1983) reported increasing trends in sulfate and decreases in alkalinity at several USGS hydrologic benchmark sites in the Southeast from 1965 to 1980. They suggested that the trends were related to regional changes in acid deposition.
Buell and Grams (1985) examined five USGS hydrologic benchmark sites in Georgia for trends (1968–84). The results were mixed, showing both increasing and decreasing trends, which indicated possible basin water-quality responses to agricultural and urban land uses. A decrease in alkalinity at a forested site was suggested to be a result of change in atmospheric deposition.
Crawford (1985) reviewed trends at a site in the Cape Fear River basin (North Carolina; 1955–80) and reported increasing dissolved solids, potassium, sodium, magnesium, sulfate, and chloride concentrations, and increasing specific conductance. Nitrate concentrations increased, and silica concentration and pH decreased. The observed trends were positively associated with population changes and manufacturing employment, and negatively associated with harvested cropland, indicating an overall transition of the area from agricultural to urban and suburban land uses. Increasing dissolved-ions concentrations were correlated with increasing population and manufacturing employment, and with decreasing cropland acreage (Crawford and Harned, 1986). Variation in annual fertilizer sales was not significantly associated with measures of water quality.
Smith and others (1987a, b) examined over 300 sites of the NASQAN and National Water Quality Surveillance System (NWQSS; U.S. Environmental Protection Agency, 1976) for trends (1971–81), including 48 sites in the Southeastern United States. In general, chloride, sulfate, nitrate, and phosphorus concentrations increased across the Southeast, and sediment concentration trends were mixed. Smith and others (1987a, b) reported a correlation of increases of atmospheric deposition of nitrate with increasing nitrate concentrations in streams in the Eastern United States.
Stanley (1988) examined Pamlico River estuary (North Carolina) data for trends (1967–86). Stanley detected decreasing pH, decreasing nitrogen concentrations, and increasing chlorophyll-a concentrations in the estuary. He suggested that the increase in chlorophyll-a was associated with less severe light limitation on algal growth as a result of reduced suspended-sediment loads.
Harned and Davenport (1990) examined data for trends (1979–88) at 296 sites on streams draining to the Albemarle-Pamlico estuary in North Carolina. They reported a general decrease in nitrogen and phosphorus concentrations except in the Pamlico River estuary, and increases in estuarine chlorophyll-a concentrations. The Pamlico River is affected by an adjacent phosphate mining operation. The pH generally increased regionally except in the Pamlico River. Suspended-solids concentrations decreased in the system. Harned and Davenport correlated annual median constituent concentrations with annual basin-characteristic variables. Annual variation in harvested cropland was associated with dissolved-solids concentrations in the Neuse River estuary, total fertilizer use was associated with annual median dissolved-oxygen concentrations in the Pamlico River, and annual corn acreage was associated with annual phosphorus concentrations in the Pamlico River.
Lettenmaier and others (1991) evaluated 403 NASQAN sites for trends (1978–87) and reported a general pattern of increasing nitrate and dissolved-solids concentrations and increasing pH in the Southeast. Total phosphorus concentrations in the region generally decreased or were unchanged.
Lettenmaier and others (1991) suggested associations of increasing calcium, magnesium, and potassium concentrations with increasing urban area. Total nitrogen and total phosphorus trends were positively associated with population density and amount of pasture; sulfate concentration trends were associated positively with forest area and negatively with farm area, urban area, fertilizer amounts, and population density. Because the associations of trends with explanatory variables were few in number, however, they concluded that the results of the analysis were inconclusive.
Smith and others (1993) reviewed national USGS data for trends (1980–89) in dissolved oxygen, fecal coliform bacteria, dissolved solids, nitrate, total phosphorus, and suspended-sediment concentrations. Smith and others (1993) concluded that water quality improved only modestly nationally during the 1980s, with no distinct pattern evident in the Southeast.
Wangsness and others (1994) reported increases (1980–93) in phosphorus concentrations for the Chattahoochee River upstream from Atlanta, Georgia, and a 50-percent decrease in phosphorus load from 1988 to 1993 downstream from Atlanta, probably due to restrictions put on the use of phosphate in detergents and improved wastewater treatment.
Lynch and others (1995) examined trends (1980–92) in precipitation chemistry at 58 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites in the United States, including 11 sites in the Southeast. The Southeast sites showed decreasing sulfate, calcium, and magnesium concentrations, which were part of a national decline in major-ion concentrations in precipitation. Precipitation chemistry is an important driver of streamwater-quality trends.
Dunn (1996) evaluated trends (1972–93) using NASQAN nutrient data for 37 streams across the Southeast discharging into the Gulf of Mexico. Long-term increases in total nitrogen were detected at 19 sites, with decreases at 7 sites. Increases in total phosphorus were detected at 7 sites, and decreases were detected at 11 sites.
Aulenbach and others (1996) examined trends at 15 small research basins across the United States with collocated precipitation-quality monitoring to link precipitation trends with streamwater-quality trends. Four sites in the Southeast were included in the study, in which major ions, ammonium, and nitrate concentrations were examined. Association of stream chemistry with precipitation chemistry was evident only for chloride concentrations.
Peters and others (1997) examined nutrient trends (1970–95) at several sites along the Chattahoochee River near Atlanta, Georgia. Total phosphorus decreased from the late 1980s to 1995 primarily as a result of wastewater-treatment plant improvements and a 1990 phosphate-detergent ban. High variability in phosphorus concentrations was noted in urbanizing basins in the Atlanta area after 1993. Lower ammonia-nitrogen concentrations and increases in nitrate plus nitrite concentrations (1970–95) were probably a result of wastewater-treatment plant nitrification of ammonia to nitrate.
Childress and Bathala (1997) evaluated data from 34 stream and reservoir sites in the Raleigh–Durham (North Carolina) area for trends (1983–95). A decline in total phosphorus after 1988 coincided with wastewater-treatment plant improvements and removal of phosphates from detergent. Total nitrogen concentrations were found to be generally stable, with organic nitrogen concentrations decreasing and nitrate concentrations increasing, possibly reflecting changes in waste-treatment processes. In spite of the stable or decreasing nutrient-concentration trends and stable sediment concentrations, chlorophyll-a concentrations in area reservoirs increased.
Mast and Turk (1999) reviewed data from the USGS Hydrologic Benchmark Network sites for trends (1963–95), including 12 sites in the Southeast. Because the network was designed for detection of trends resulting from atmospheric deposition, most of the sites were located in basins with limited human activities. The trend results were mixed. Trends were observed (both increasing and decreasing) at most sites in sodium and chloride, but no consistent regional pattern was evident. Nitrite plus nitrate and sulfate increases in Cataloochee Creek (North Carolina) are consistent with earlier results reported by Lynch and others (1995).
Bricker and others (1999) reported results of the National Estuarine Eutrophication Assessment in which estuarine data were examined to develop a eutrophication index and to rank the eutrophication of 138 estuaries across the country. The assessment goal was to characterize the “spatial domain, severity, duration, frequency, and past trends of 16 eutrophication-related conditions in estuaries in the contemporaneous United States.” Of the 44 estuaries identified nationally with a high index of eutrophication, high nutrient input was identified as an important factor in only 6 estuaries. High retention of nutrients was indicated as an important factor in 25 of the 44 eutrophic estuaries. Eight of the highest-ranking eutrophic estuaries located in the Southeast are projected to become more eutrophic by 2020 based on population projections. The sites of concern include the Pamlico, Neuse, and New Rivers in North Carolina; the St. Johns River, Charlotte Harbor, Caloosahatchee River, and Sarasota, Tampa, and Choctawhatchee Bays in Florida; and Perdido Bay in Alabama. Eutrophic conditions in the Southeast were expressed primarily in chlorophyll-a concentrations, low dissolved oxygen, and nuisance and toxic algal bloom problems.
Hoos and others (1999) reviewed data for trends (1985–93) at 11 sites in the lower Tennessee River basin (Tennessee, Alabama, and Kentucky) using regression analysis. Concentrations of total ammonia and total nitrogen decreased in about half of the sites. Reduction of ammonia in wastewater effluent was associated with decreasing ammonia concentrations in the basin.
Nilles and Conley (2001) examined trends (1981–98) in precipitation chemistry for the United States. Precipitation chemistry trends may be reflected in streamwater-quality trends. Decreasing trends in concentrations of sulfate (North Carolina, South Carolina, Georgia, Florida, and Tennessee) and calcium (North Carolina, Georgia, Alabama, Florida, and Tennessee); and increasing trends in concentrations of nitrate (Alabama) and ammonium (North Carolina, South Carolina, and Florida) were observed. Nitrogen (NOX) emission controls were suggested as a reason for the lack of increase of nitrate concentrations in these States in spite of increased vehicular traffic and power production. Industrial particulate controls and dust-reduction regulations were cited as a possible cause for the widespread calcium reductions. The sulfate reductions are consistent with the reported 10–25 percent acid-deposition reductions in the eastern United States (1981–98; National Acid Precipitation Assessment Program, 1998).
A review of nitrogen and phosphorus trends for the Alabama and Tombigbee Rivers (Alabama, Mississippi, Tennessee, and Georgia) by McPherson and others (2003) included data from 1978 to 2001. The analysis revealed a continued general decrease in nitrate in the Alabama River from 1997 to 2001. Decreases in row-crop agricultural activities and improved wastewater treatment were suggested as possible causes for the nitrate decline.
Harned and others (2004) examined data from 18 basins in the Mobile River basin (Alabama, Mississippi, Tennessee, and Georgia; 1970–97). Decreasing total-nitrogen trends were observed in the Tombigbee and Alabama Rivers (1975–97) and in the Black Warrior River (1980–95). A general pattern of increasing total nitrogen concentrations was noted from 1975 to 1987 followed by a decrease, possibly as a result of improved wastewater-treatment processes. Total phosphorus decreased generally from 1972 to 1996, but increased from 1988 to 1996 at three sites on the Etowah River in Georgia, probably as a result of urban development. Multiple-regression analysis indicated a distinct association between annual total phosphorus concentration, and agricultural row crops and farm-animal populations.
Alexander and Smith (2006) reexamined national NASQAN data from 250 sites (1975–94) for total nitrogen and total phosphorus trends to assess the probability of change in trophic conditions. Forty-four percent of the sites examined for total phosphorus and 37 percent of the sites examined for total nitrogen had decreasing concentrations, and only 3 percent of the sites examined for total phosphorus and 9 percent of the sites examined for total nitrogen had increases. Alexander and Smith (2006) determined that trophic state improved at 25 percent of the sites and worsened at less than 5 percent of the sites, although half of the sites were still classified as eutrophic in 1994. They detected more decreasing trends than increasing trends in suspended sediment by a margin of 3 to 1. Six out of 41 Southeastern sites had increasing trends in total nitrogen concentrations. Alexander and Smith (2006) inferred that improved wastewater treatment from 1970 to 1995 contributed to reductions in nutrient concentrations nationally. The reduction of phosphate-detergent use is reflected by the decreasing trends in total phosphorus concentrations. Fertilizer use peaked nationally in 1981 and varied considerably afterwards. Livestock-manure use peaked in 1980, declined, and then increased to 1980 levels again by 1997. Alexander and Smith (2006) noted the complexity of associating trends to potential causes in part because of lag times in the movement of nutrients from sources to the streams. An examination of nutrient sources and nutrient loads in streams for the Southeast incorporating methods of source routing is under investigation (Hoos and others, 2008).
Spruill and others (2006) detected decreasing trends in sediment concentrations (1974–2004) in two Piedmont streams in North Carolina. They suggested that decreases in cultivated land, improved land management, and improved wastewater treatment were possible causes.
Sprague and others (2008) examined trends (1993–2004) in total phosphorus, total nitrogen, and nitrate concentrations for rivers of the United States. Thirty-nine percent of the 26 sites in the Southeast showed decreasing trends in total phosphorus, and 8 percent showed increasing trends. Eight percent of the 26 sites showed increasing total nitrogen trends, and 8 percent showed decreasing trends. No significant regional nutrient trend patterns were detected.
In general, previous assessments of trends for the Southeast indicated: