Trends in stream quality in the continental United States, 1978–1987

Abstract

The nonparametric seasonal Kendall's test, and a related multivariate test were used to analyze 403 National Stream Quality Accounting Network (NASQAN) stations for trend for the period 1978–1987. Nominal sampling frequencies were quarterly or bimonthly. For all groups and individual constituents, trends were detected for only a minority of stations at the relatively liberal 10% significance level. The groups with the greatest percentage of trends were common ions (mostly upward) and nutrients and suspended sediment (mostly upward for total nitrogen and mostly downward for total phosphorus). In the pH and alkalinity group, the trends were dominantly upward; while this might be the result of reductions in atmospheric deposition, this hypothesis is clouded by the existence of uptrends in sulfate at many of the same stations. The trace metal analyses showed that the constituents with the greatest number of trends (dominantly downward) were arsenic and cadmium; most of the stations with downtrends were in the major population areas of the east and midwest. Additional exploratory analyses of possible relationships between trends and land use and population did not give strong evidence of possible causation.