U.S. Geological Survey Scientific Investigations Report 2009-5268
Trends in Water Quality in the Southeastern United States, 1973–2005
Summary and Conclusions
Previous assessments of trends in the Southeast produced an amalgam of overlapping studies that often shared data from several key data-collection networks. In general, the results of these studies indicated
Water-quality trends that were identified included
Water-quality trends were correlated with population changes, manufacturing employment, fertilizer sales, cropland acreage, harvested cropland, atmospheric deposition, population density, pasture area, forest area, farm area, urban area, fertilizer amounts, row crops, and farm-animal populations.
As part of the USGS NAWQA P rogram, water-quality data for 334 streams in eight States of the Southeastern United States were assessed for trends from 1973 to 2005. Forty-four USGS sampling sites were examined for trends for multiple periods within 1973–2005 in the physical properties of pH, specific conductance, and dissolved oxygen, and in concentrations of dissolved solids, suspended sediment, chloride, sodium, sulfate, silica, potassium, carbon, total nitrogen, total ammonia, total ammonia plus organic nitrogen, dissolved nitrite plus nitrate, and total phosphorus. The data used for this analysis are available in Staub and others (2009).
An additional 290 sites from the USEPA STORET database were tested for trends in total nitrogen and total phosphorus concentrations for the 1975–2004 and 1993–2004 periods. The seasonal Kendall test or Tobit regression was used to detect monotonic trends.
The pH increased at many of the sites in the Southeast from 1975 to 1985. Fewer trends are apparent for the period 1993–2004. Decreases in pH (greater acidity) were observed in western North Carolina and in areas of the Coastal Plain of South Carolina and Georgia. Harned and Davenport (1990) reported regional increases in pH in coastal North Carolina. Lettenmaier and others (1991) reported increasing trends in pH from 1978 to 1987 in the Southeastern United States.
Trends in major dissolved constituents and nutrients indicate dynamically changing stream chemistry in the Southeast. The evolution of stream chemistry over time is complex, and its relation to changes in the landscape is multivariate.
Specific conductance, an indicator of dissolved ions in water, generally has been increasing in the Southeast over the last 30 years, but with fewer increases during the 1993–2004 period. Long-term increasing trends were detected at 62 percent of the NWIS sites. The increases may be indicative of greater volumes of wastewater discharging to streams as a result of population growth. Increasing specific conductance trends in North Carolina were reported by Harned (1982), Harned and Meyer (1983), and Crawford (1985). Crawford and Harned (1986) reported association of increasing dissolved-ion concentrations with increasing population and manufacturing employment, and with decreases in cropland acreage. Lettenmaier and others (1991) reported a general national pattern of increasing dissolved-solids concentrations from 1978 to 1987 and an association of increasing calcium, magnesium, and potassium with increasing urban area. In addition, declines (1980–92) in sulfate, calcium, and magnesium concentrations in precipitation (Lynch and others, 1995) indicate that the sources for the concentration increases are not atmospheric and are evidence that trends in dissolved-ion concentrations in streams may be influenced by land-surface sources or wastewater inputs.
Trends in specific conductance generally are reflected by similar trends in alkalinity, calcium, chloride, sodium, dissolved solids, and hardness. This result is not surprising because of the interrelated nature of the constituents . S pecific conductance is an indicator of the amount of dissolved constituents in water; calcium is one of the principal cations that causes hardness in water, and dissolved solids are derived primarily from salts. However, the many associations of trends reinforce the observation of a general increase in concentrations of dissolved constituents in many streams of the Southeast over the last 30 years.
Ammonia concentrations generally have decreased in recent years. This decrease may be associated with more effective wastewater treatment. Few data are available for ammonia plus organic nitrogen. The observed trends reflect the trends for ammonia concentrations. The pattern of nitrite-plus-nitrate trends was inverse to that observed for ammonia. As a result of improved wastewater-treatment processes during 1985–95, ammonia was converted to nitrate before being discharged to streams. Peters and others (1997) and Childress and Bathala (1997) also reported declining ammonia and organic nitrogen concentrations and increasing nitrate concentrations, probably a result of changes in municipal wastewater-treatment processes.
Long-term total nitrogen decreases were detected at 49 percent of the sites tested in the Southeast. Decreasing trends observed from 1975 to 1995 are not apparent during 1993–2005. Dunn (1996) reported many increases (1972–92) in total nitrogen at sites along the Gulf Coast. Childress and Bathala (1997) reported stable total nitrogen concentrations in central North Carolina. Hoos and others (1999) reported decreases in total nitrogen for the lower Tennessee River basin. Alexander and Smith (2006) reported that 6 of 41 S outheastern sites had increasing trends in total nitrogen concentrations. Harned and others (2004) noted a pattern of increasing total nitrogen during 1975–87 in the Mobile River basin, Alabama, followed by a decrease and suggested it was possibly a result of improved municipal wastewater-treatment processes. However, most of the sites with sufficient data for trend analyses were limited to a few States in the Southeast—North Carolina, South Carolina, Georgia, and a small area of Florida. The recent total nitrogen trend results showed regional clusters of increasing and decreasing trends, and North Carolina’s trends were predominately decreasing. Nitrogen concentrations in South Carolina have generally declined over the long term (1975–2004) with much of the apparent improvement occurring during 1985–95, and more recent (1994–2004) mixed trends. A cluster of sites in Florida also show recent increases in total nitrogen concentration.
Long-term decreasing trends in total phosphorus were detected at 56 percent of the sites tested, and recent decreasing trends were detected at 23 percent of the sites. The overall pattern of trends in the Southeast that are evident from examination of the NWIS and STORET data reflect the effects of statewide phosphate-detergent bans and improvements in wastewater treatment that were implemented during 1972–99. Georgia, which implemented a phosphate-detergent ban in 1989, had many sites with increases in total phosphorus concentrations during the 1985–95 period but many more sites with decreasing concentrations for the 1993–2004 period. South Carolina, which implemented a ban in 1992, had several sites where increasing trends were detected during 1975–85 but many sites with decreasing trends during the 1985–95 and 1993–2004 trend-test periods. Decreasing long-term phosphorus trends also were reported by Lettenmaier and others (1991), Smith and others (1993), Wangsness and others (1994), Harned and others (2004), and Alexander and Smith (2006). Recent trend results showed clusters of increased total phosphorus for North Carolina, the Suwannee River in Florida, and decreasing trends in Georgia and South Carolina. Litke (1999) reviewed the history of national phosphorus controls. Phosphate detergent bans were implemented in Florida in 1972, in North Carolina and Virginia in 1988, in Georgia in 1989, and in South Carolina in 1992. Tennessee and Alabama had no restrictions on phosphate detergents. By 1999 all S tates but Tennessee had implemented phosphorus limits in wastewater-treatment plants.
Few trends were detected for sediment. This may be because of the limited number of sites with long-term suspended-sediment data and because the samples collected tend to be representative of low flows with low sediment concentrations. A program to detect suspended-sediment trends would require a design that included sampling during storm events and at high flows.
Multiple-regression analyses of water-quality constituents and physical properties with landscape variables indicate that when atmospheric inputs and agricultural practices change, water quality changes. Multiple-regression models do not demonstrate causation, and the relations suggested are complex. Selection of the variables for nitrogen fertilizer, atmospheric nitrogen deposition, and corn and tobacco harvests for 10 of the 22 multiple regression models reflects the importance of anthropogenic influences on many constituents. The selection of both crop and, in particular, beef cattle population density for 18 of the 22 models provides evidence that agricultural land-use practices and streamwater quality are connected.
Missing from the analyses were measures of change in many important landscape variables. We do not know how regional wastewater inputs from municipalities and industry change with time, or how urbanization and other land use change over time because data are not collected and compiled with the frequency or detail necessary to assess these trends. We do not know how conservation land-management practices change because the data necessary to assess these trends have not been collected. Further, the ancillary data that currently are available are usually collected at the county scale, are collected infrequently, or are unavailable in many States.
The support for long-term data collection, with quality-controlled laboratory analysis, and with sampling protocols and laboratory-method history has declined during 1973–2005 in the Southeast. Regional coverage by the current long-term data sites is patchy. The conclusions of this study are limited by the clustering of the sites in a few areas of several States, which requires qualification of conclusions about regional patterns in trend results.
Future trend assessments in the Southeast could be more effective if a network was developed with the long-term objective of providing coordinated, consistent, quality-assured water-quality data collection from representative basins across the region. Data collection could include measurement of in-stream chemical water quality and biology, with the additional objective of extensive monitoring of changing basin characteristics to fully assess constituent sources and to quantify landscape change. Substantial evidence is needed to establish causation. A better assessment of the relation between changes in the landscape and changes in water quality is needed to understand trends in water quality, to evaluate whether resource-management strategies are working, and to effectively manage environmental resources in the Southeast.
Several USGS scientists assisted with data compilation and analysis. Rodney R. Knight and Connor J. Haugh assisted with the trend analysis of southeastern sites. Phillip Jen provided help with NWIS data calculations. Anne B. Hoos provided the STORET datasets. David Lorenz provided assistance with trend-analysis programs.