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Trends in Stream Water Quality in the Southeastern United States

Eos Trans. American Geophysical Union, vol. 88, no. 52, Fall Meeting Supplement
by Douglas A. Harned, Erik L. Staub, Kelly L. Peak, Kirsten C. Tighe, and Silvia Terziotti

Abstract

As part of the U.S Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program water-quality data for 253 streams in 8 states of the Southeastern United States were assessed for trends from 1973-2005. Forty-three USGS sampling sites were examined for trends over multiple periods within 1973-2005 in measures 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.  An additional 210 sites from the U.S. Environmental Protection Agency STORET database were tested for trends in total nitrogen and total phosphorus concentrations over the 1975-2004 and 1993-2004 periods. The seasonal Kendall test or Tobit regression was used to detect monotonic trends.

Concentrations of dissolved constituents have increased in many streams in the Southeast over the last 30 years. Specific conductance, an indicator of dissolved ions in water,  increased in the Southeast  in 26 USGS sites over the long term, but showed fewer increases in the 1993-2004 period. The pH increased at 11 of the 43 USGS sites in the Southeast from 1975 to 1985. Fewer trends in pH are apparent for 1993-2004.

Concentrations of phosphorus in streams in the Southeast have decreased over the last 35 years coinciding with phosphate detergent bans and improvements in waste-water treatment that were implemented beginning in 1972. Sixteen of the 17 long-term trends in phosphorus concentrations detected at the 43 USGS sites were decreasing. Twenty-seven of the 32 long-term (1975-2004) trends detected in total phosphorus concentrations at the 210 STORET sites were decreasing.

Nitrogen trends the Southeast are mixed. Decreasing trends in total nitrogen observed at USGS sites from 1975 to 1995 are not apparent during 1993-2004. Of the 18 recent (1993-2004) trends in total nitrogen detected at the 210 STORET sites, 13 were increasing trends. Ammonia concentrations decreased from 1973-2005 at 11 of the 43 USGS sites, and decreased in the more recent 1993-2004 period at 12 USGS sites. Nitrite plus nitrate concentrations decreased from 1973-2005 at 9 of the 43 USGS sites, and increased at 6 sites.

A comparison of the USGS sites with multiple trend results for nitrate, ammonia plus organic nitrogen, ammonia and total nitrogen showed few associations. Trends in total nitrogen were directly related to changes in total ammonia plus organic nitrogen trends but not nitrate. This suggests that the principal change in stream nitrogen chemistry has been driven by a change in ammonia and organic nitrogen concentrations in streams, a change largely brought about by improved municipal waste-water treatment.

A long-term water-quality and landscape trends-assessment network for the Southeast is needed to assess how water-quality changes over time in response to variation in population, agricultural, wastewater, and landscape variables. Understanding change in stream chemistry over time and its relationship with changes in population, agriculture, and waste-treatment technology in the Southeast will assist planners and water-resource managers in developing water-resource protection strategies.

Harned, D.A., Staub, E.L, Peak, K.L., Tighe, K.C., and Terziotti, S., 2007, Trends in Stream- Water Quality in the Southeastern United States, Eos Trans. American Geophysical Union, vol. 88, no. 52, Fall Meeting Supplement, Abstract GC13A-0954.