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WATERSHED MONITORING FOR THE LAKE DECATUR WATERSHED by Illinois State Water Survey BACKGROUND �i The Illinois State Water Survey (ISWS) has been conducting the Watershed Monitoring for Lake Decatur Watershed study since spring of 1993. This study is a result of a Letter of Commitment (LOC) between the City of Decatur and the Illinois Environmental Protection Agency. The LOC was initiated because of high nitrate concentrations in the city's public drinking water supply since 1979. The initial goal of the study was to assist the City of Decatur in complying with the LOC by following three specific objectives: 1) establishment of a watershed monitoring network that will quantify the amount of nitrate being contributed by tributaries upstream of Lake Decatur; 2) development of a non-point source pollution model for the watershed that will simulate nitrate yields from different land use practices, which can be used to evaluate the impacts of BMPs on water quality; and 3) analysis of current land uses in the watershed and formulate alternative land management practices and mitigation projects to reduce the nitrate concentrations in Lake Decatur to acceptable drinking water levels. A technical report covering the first two years of study was published in January 1996 (MP169). The report provides the background information on the watershed by reporting on the historical water qualiry problems in Lake Decatur, watershed physical characteristics, and land uses. A discussion of the sources and transformations of nitrogen, as well as a detailed hydrologic analysis of the watershed are included. The hydrologic and water quality monitoring data aze presented and the mathematical modeling effort discusses the effects of best management practices on the nitrate loading into Lake Decatur. Another report (ISWS CR602) on the third year of data collection was published in July 1996. The watershed monitoring component of the study was established in May of 1993 and will conclude in April 1997. Figure 1 shows the location of the stations that are being monitored throughout the watershed. In order to best evaluate any impending land management changes, continued monitoring in the watershed is highly recommended. A fifth year of watershed monitoring is proposed to accomplish this. The addition of a fifth year of monitoring will enhance the data already gathered thereby 1) developing a stronger base for attracting future funding for land management programs throughout the watershed and 2) provide the background data needed to determine the progress of any future best management practices implemented in the watershed. The continuation of the watershed monitoring has been recommended by several 1