RANDOMIZED INTERVENTION ANALYSIS OF THE RESPONSE OF THE WEST-BEAR-BROOK WATERSHED, MAINE TO CHEMICAL MANIPULATION

A paired watershed manipulation study was conducted to study the quali tative and quantitative impacts of elevated acidic precipitation on th e chemistry of soils, soil water, and stream water. The Bear Brook Wat ershed, Maine (BBWM) is drained by two first order streams, East Bear Brook and West Bear Brook. The streams were chemically and hydrologica lly monitored for two years (1987-1989) and exhibited similar behavior . The West Bear watershed was then chemically manipulated with the bim onthly addition of (NH4)(2)SO4 (150 mol ha(-1) per application). To as sess whether changes in stream water chemistry occurred following the chemical manipulation, and if so when, Randomized Intervention Analysi s (RIA) was performed using time-paired data from the two watersheds. RIA, along with autocorrelation analysis, statistically evaluates the behavior of the various analytes under the influence of artificial aci dification and therefore provided an objective basis for determining w hether changes in the geochemical behavior of West Bear Brook were tem porally associated with the chemical manipulation, RIA analysis using weekly data yielded higher probabilities of stream water chemistry eff ects being temporally linked with the manipulation than RIA analysis u sing monthly data. Using monthly data, there is a lower probability th at short-term excursions in water chemistry related to hydrology can b e detected. According to RIA analysis of weekly data for three years o f manipulation (1989-1992), the statistically-determined order of impa ct on water chemistry was (K+, Mg2+, Na+, Ca2+, total Al, pH, SO42-, N O3-, DOC) > Si > Cl-. Autocorrelation analysis indicated that several analytes exhibited increasingly deterministic behavior, including SO42 -, base cations, and DOG. Both RIA and autocorrelation analysis indica ted no temporal relationship between the manipulation and hydrology.