Response of Ned Wilson Lake watershed, Colorado, to changes in atmospheric deposition of sulfate

The Ned Wilson Lake watershed responds directly and rapidly to changes in precipitation inputs of sulfate, which has important implications for effects of acid deposition on the aquatic system. Chemistry at three precipitation collection sites and three watershed sites (a pond, a lake, and a spring) has been monitored in and near the Flattops Wilderness Area in northwestern Colorado beginning in 1981–1983. Bulk snowpack concentration of sulfate in the watershed and volume-weighted annual mean concentration of sulfate in precipitation at two nearby sites generally decreased from 1981 to 1985, were small through 1987, and increased in 1988–1989. Changes in concentration of sulfate at the watershed sites are controlled by precipitation inputs. Responsiveness of the individual sites was dependent on their position along the hydrologic flow path. The fastest response was in the pond, which has a hydrologic residence time of less than 1 year; over 90% of the variance in concentration of sulfate in the pond was explained by changes in concentration in precipitation. The lake has a hydrologic residence time of 1 to 4 years; a regression model of the concentration of sulfate in the lake, as a function of the concentration in the lake during the previous year and the concentration in precipitation, explained 87% of the variance in concentration of sulfate in the lake. The hydrologic response time of the spring is unknown; it was not responsive to changes in concentration of sulfate in precipitation. The recent increase of sulfate concentration in precipitation and in the pond and lake is evidence for a rapid rather than a delayed response, which could not be determined when only a decreasing trend in sulfate concentration was reported in 1982–1987. Watersheds of this type are sensitive to acidification (acid-neutralizing capacity less than 60 μeq L⁻¹), and these results indicate conservative behavior of sulfate. This is important in predicting effects of future changes in atmospheric deposition, which could potentially be caused by anthropogenic emissions or climatic change.