The Aquatic Effects Research Program (AERP) within the National Acid Precipitation Assessment Program (NAPAP), the U.S. federal plan for effects research for acidic deposition, funded the EPA Bear Brook Watershed in Maine (BBWM) project. The major objectives of BBWM were to (1) identify and quantify the major processes that control the surface water acidity, (2) assess the quantitative and qualitative response, at the watershed scale, to the artificially increased levels of acidic deposition to one of the two contiguous catchments, and (3) evaluate the ability of the existing models of acidification such as MAGIC and others to predict the effects of acidification on a watershed scale. Previous quantitative studies of the response of the watershed include the application of MAGIC to the watershed behavior after one year of manipulation, and mass balance studies after two years. Randomized Intervention Analysis (RIA) can be used to determine if a non-random change has occurred. This method was applied to BBWM to ascertain if a non-random change has occurred in the chemically manipulated watershed. RIA, and other statistical methods like auto correlation analysis and cross correlation analysis also provide new support for certain geochemical arguments.