R95-55 AUTHORIZING AGREEMENT ILLINOIS STATE WATER SURVEY

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. , • f. WATERSHED MONITORING FOR THE LAKE DECATUR WATERSHED by Illinois State Water Survey BACKGftOUND The Illinois State Water Survey has been conducting the Watershed Monitoring and Land Use Eualuation for Lake Decatur Watershed study since spring of 1993. This study is a result of the signing of a Letter of Commitment (LOC) between the City of Decatur and the Illinois Environmental Protection Agency. Th LOC was initiated because of high nitrate concentrations in the city's public drinking water supply since 1979. The study's main goal has been 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) Develop 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) Analyze 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 progress report highlighting the first year of data collection, a detailed hydrologic analysis, the development of the non-point source pollution model, and establishment of the land use database was published in November 1994 as an Illinois State Water Survey Miscellaneous fteport Number 159. The report also includes expanded discussions on the historical water quality problems in Lake Decatur, the land uses and physical characteristics of the watershed, and a preliminary review of the scientific literature on sources of nitrate and nitrogen transformation in the environment. The watershed monitoring component of the study was established in the spring of 1993 and will be concluding in April 1995. 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 third year of watershed monitoring at a modified level is proposed to accomplish this. The addition of a third year of monitoring will enhance the baseline data already gathered thereby developing a stronger base for attracting future funding for land management programs throughout the watershed. The continuation of the watershed monitoring has been recommended by several federal, state, and county agencies(USEPA, IEPA, county Soil&Water Conservation Districts(SWCD))as well as local watershed planning committees such as the Lake Decatur Watershed Planning Committee and Champaign County Soil&Water Conservation District Water Quality Committee. 1 �