Is Water Quality Getting Better or Worse? And Does It Matter?

A new USGS study evaluates water-quality trends in streams and rivers in a health-based context.

Suppose the concentration of a chemical constituent in a stream has increased or decreased over the past decade—is that change relevant?

A new USGS study tackles that question by assessing the direction of trends—upward or downward—for seven common chemical constituents in water and comparing the magnitudes of the concentrations to drinking-water benchmarks and nutrient criteria. Trends for ammonia, chloride, nitrate, sulfate, total dissolved solids, total nitrogen, and total phosphorus in more than 300 U.S. streams and rivers were evaluated for the period 2002–2012. Constituent concentrations were compared to established water-quality benchmarks such as drinking-water standards, non-health-based guidelines for taste, color, and odor, and nutrient criteria for aquatic life.

Concentrations of the seven constituents rarely increased or decreased enough to cross a benchmark during 2002–2012. In other words, there was no notable change in concentrations of these constituents—for better or worse—relative to benchmarks.  Concentrations of ammonia, nitrate, sulfate, chloride, and total dissolved solids tended to be below drinking-water benchmarks.  For those constituents with upward trends in concentrations that are approaching a benchmark, conditions are not worsening rapidly. 

Concentrations of total nitrogen and total phosphorus exceeded recommended nutrient criteria for aquatic life at most sites. At sites where nitrogen and phosphorus concentrations are decreasing and approaching their aquatic-life benchmarks, it is estimated that it will take 15 to 25 years, respectively, for these constituents to fall below the benchmark, assuming the current rate of change continues.

Identification of trends can help water-resource managers and policy makers determine whether current protective management actions should be continued, or whether additional or alternate management practices may be necessary.

For additional information on trends, or for data and methods used, contact Lori Sprague (lsprague@usgs.gov).