In the previous chapter, the national-scale evaluation of long-term trends in water quality conditions identified numerous waterways that were characterized by substantial improvements in worst-case DO after the CWA (from 1961-1965 to 19861990). The signals of worst-case DO improvements that have been detected from the "noise" of the STORET database document the tremendous progress that has been achieved as a result of implementation of the effluent control regulations of the CWA in 1972. Having identified numerous watersheds and RF1 reaches, however, the inquisitive reader could easily list a number of questions to fill in the information needed to tell a more complete history about environmental management and water pollution control decisions in these watersheds.
Typical questions might include the following: What are the population trends? Are point or nonpoint sources the largest component of pollutant loading? What have been the long-term trends in effluent loading from municipal and industrial sources over the past 25-50 years? Has industrial wastewater loading declined because obsolete manufacturing facilities have been abandoned? What have been the long-term trends in key water quality parameters over the past 25-50 years? Have reductions in wastewater loads had any impact on biological resources or recreational activities?
This third leg of the three-legged stool approach focuses on answering these types of questions. The uniqueness of each watershed requires an investigator to go beyond STORET and other centralized databases to identify, obtain, and compile sufficient historical data to answer these questions and others. By necessity, the selection of specific waterways based on case studies has often been used as an appropriate technique for policy evaluations of the environmental effectiveness of water pollution control decisions. That technique is used in Chapters 5 through 13 of this study.
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