Long-Term Changes in the Acid-Base Status of Western Maryland Streams

Recovery of streamwater acid neutralizing capacity (ANC) resulting from declines in regional acid deposition was examined using data from two long-term monitoring stations located on the Appalachian Plateau in western Maryland. The monitoring stations are located at the outlets of two small, predominantly forested watersheds (Big Run and Black Lick) in Savage River State Forest. Runoff from both watersheds has been continuously gaged since 1995; water chemistry records (including data on ANC, pH, sulfate, nitrate, base cations) date to 1989 for Big Run and to 1996 for Black Lick. A stream gage operated by USGS on Savage River was used to extend the hydrologic record of mean daily runoff from the sites through the entire period of water chemistry monitoring. The watersheds are surrounded by regional atmospheric deposition monitoring stations that have been collecting data since the late 1970's or early 1980's. Two computational methods were used to estimate daily, monthly, and annual loadings and discharge- weighted concentrations of ANC, sulfate, nitrate and base cations over the period of record: (1) WATFLOW, which uses a linear interpolation scheme to estimate daily concentrations; and (2) LOADEST, which employs a 7-parameter multiple regression model of loading for each constituent. Two statistical methods were used to evaluate long-term trends in both the estimated constituent loadings and concentrations: (1) linear regression analysis; and (2) a seasonal Kendall tau test. Our presentation includes a detailed comparison of the two estimation techniques, as well as a discussion of the statistical significance and scientific relevance of the results.