Doug Burns, Watersheds Research Section, U.S. Geological Survey, Troy, New York

The U.S. Geological Survey has been monitoring stream chemistry and flow at four small watersheds in the Catskill Mountains of New York since the early 1990s; data at some of these streams extends back to the 1980s. These streams are located in the highest elevation terrain in this region where thin soils, steep slopes, and resistant bedrock with low base cation availability combine to provide little neutralization to acid deposition. The monitored streams span a pH range of about 4.8 to 6.3 and an acid-neutralizing capacity (ANC) range of 30 to -25 µeq/L at baseflow, and all four stream reach negative ANC values during high flow with elevated aluminum concentrations. The pH of precipitation in this region has been increasing by about 0.01/yr since the early 1990s according to data from the Biscuit Brook NTN site (NY68). This increase in precipitation pH is broadly consistent with trends at other NTN sites in the Northeast, and is driven largely by decreases in sulfate concentrations as a result of implementation of Title IV of the Clean Air Act Amendments of 1990. Two previous studies of trends in stream-water chemistry at these four sites have found a pattern of increasing pH and ANC and decreasing sulfate concentrations during 1992 – 2001, and 1992 - 2003. These trends are consistent with the decreasing trends in precipitation acidity in this region, and the trends were expected to persist as precipitation acidity continued to decrease during 2003 – 2006. Instead, the trends in pH and ANC are no longer significant over the period 1992 - 2006, as nitrate concentrations have increased during 2004 – 2006 to nearly unprecedented values. These recent increases in stream nitrate are believed to be related to defoliation by the Forest Tent Caterpillar that was observed during this period, and is known to have greatly affected the Catskills during 2004 – 2006. Previous studies have found that insect defoliation diminishes uptake of nitrogen by trees and the frass provides a source of readily nitrified organic matter to the forest floor. These results indicate that regional recovery of stream chemistry has thus far been so slight, that disturbances to the nitrogen cycle can disrupt the recovery trend. Disturbances such as that by defoliating insects can delay stream recovery for at least several years, and possibly longer if the intensity and duration of attack is great enough to
cause significant tree mortality.