The role of nitrate in the acidification of streams in the Catskill Mountains of New York

Abstract

Research on the effects of acidic deposition in the United States has focused largely on the role of sulfur deposition in the acidification of surface waters. Results from both long‐term (up to 70 years) and recent monitoring of stream chemistry in the Catskill Mountains of New York indicate, however, that nitric acid has a significant and increasing role in surface water acidification that, during high‐flow periods, rivals the role of sulfuric acid. Nitrate increases with increased streamflow throughout the year except during the late summer, when biological activity and its attendant nitrogen uptake are greatest; peak concentrations as high as 128 μeq/L have been recorded during spring snowmelt. In contrast, SO42− concentrations decrease with increased flow. The ratio of NO3−:(NO3− + SO42−) in the seven U.S. Geological Survey/U.S. Environmental Protection Agency long‐term monitoring (LTM) streams ranged from 0.36 to 0.54 from August 1983 through February 1990. Seasonal Kendall tau tests for trends suggest a general decrease in SO42− concentrations and increase in NO3− concentrations in these streams during that period. Trend analysis of 19 large streams with periods of record from 23 to 70 years indicates that NO3− concentrations have been increasing in Catskill streams since 1970. The increasing trend appears to be primarily a result of increasing peak flow concentrations; base flow concentrations in the same large streams show only small changes over the period of record. Increases in base flow NO3− concentrations in headwater LTM streams have been larger, however. Trend information and seasonal patterns suggest that Catskill watersheds may be approaching nitrogen saturation.