The study identified trends between 1992 and 2012 in concentrations of nutrients, sulfate, and salinity at 633 river and stream sites in the conterminous U.S. and related them to land use in the context of water-treatment practices and regulatory actions.

Upgrades to wastewater treatment systems are likely responsible for the significant decrease in concentrations of the nutrients nitrogen and phosphorus in urban streams. Reductions in these nutrients are promising because nutrients contribute to algal blooms and impair drinking-water quality. In contrast, changes in nitrogen concentrations in agricultural areas were small and inconsistent, and in agricultural areas more sites had increases in phosphorus concentration than decreases.  These results suggest that efforts to reduce nutrients in agricultural areas have not been as successful as in urban areas. The nitrogen species ammonium, however, decreased broadly across all land uses.  Ammonium can be highly toxic to aquatic organisms and is associated with wastewater, manure, and atmospheric deposition.  The broad reduction in ammonium concentrations indicate that efforts to reduce these sources of ammonium have largely been successful. 

Efforts to reduce sulfur emissions through the 1990 Clean Air Act also appear to have had some success. Concentrations of sulfate, a by-product of atmospheric sulfur emissions that can make stream water more acidic, decreased in many of the stream and river sites included in the study.

A striking trend in the dataset was the substantial increase in salinity—measured as specific conductance—in streams in urban and agricultural areas, and areas with a mix of the two. “The issue of increasing salinity in rivers is gaining attention as a worldwide problem because it is linked to loss of stream biodiversity,” commented Edward Stets, lead author of the study. Elevated salinity also is of concern because it can cause metals to be released streambed sediments and other environmental sources, and from water-distribution facilities and plumbing, which can impair drinking-water quality and cause lead and copper contamination of tap water.

Citation:  Landscape drivers of dynamic change in water quality of U.S. rivers. Stets, E.G., Sprague, L.A., Oelsner, G.P., Johnson, H.M., Murphy, J.C., Ryberg, K., Vecchia, A.V., Zuellig, R.E., Falcone, J.A., and Riskin, M.L. Environmental Science and Technology, https://dx.doi.org/10.1021/acs.est.9b05344.