Trends in Sulfate Deposition and Stream Water Base Cation Concentrations in the Catskill Mountains of New York

Michael R. McHale1*, Douglas A. Burns1 and Angelika Winner2

The Catskill Regional Long-Term Monitoring Network includes four USGS gaging stations where stream water quality samples are collected through the range in flow. The stations are located in the Catskill Mountains in southeastern New York which receive some of the highest rates of acid deposition in the United States. The purpose of the network is to measure changes in stream water quality in response to reductions in acid deposition mandated by Title IV of the Clean Air Act Amendments of 1990. There was a significant decrease in sulfate concentrations of 1.15 microequivalents per liter per year from 1991 to 2005 in wet-only precipitation at the National Atmospheric Deposition Program station in the Biscuit Brook watershed. In both soil water and stream water sulfate decreased at about twice that rate. There were no significant trends in nitrate concentration in precipitation, soil water or stream water. Reduction in magnesium, rather than calcium, appeared to balance the reduction in sulfate concentration in stream water. There were significant decreasing trends in magnesium at all sites and those trends were stronger during low flow. Silica also showed significant decreasing trends in stream water concentration that were stronger during low flow. Furthermore, annual mean concentrations of magnesium and silica were strongly correlated with each other (r2=0.83) and with sulfate (r2=0.74 for magnesium; r2=0.84 for silica). These results suggest that the decrease in sulfate deposition may be causing a decrease in the weathering rate in these watersheds which are underlain by horizontally bedded sedimentary bedrock consisting of sandstone and conglomerate with some interbedded shale and siltstone. A decrease in the soil and bedrock weathering rate could significantly delay ecosystem recovery from acid deposition, as models suggest that recovery is highly sensitive to changes in the weathering rate.

1 U.S. Geological Survey, 425 Jordan Road Troy, N.Y. 12180, ph: 518-285-5675
2 University of Freiburg