R96-51 AUTHORIZING AGREEMENT ILLINOIS STATE WATER SURVEY

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/ • • . WATERSHED MONITORING ' FOR THE LAKE DECATUR WATERSHED ' bY i Illinois State Water Survey BACKGROUND The Illinois State Water Survey has been conducting the Watershed Monitoring and Land Use Evaluation 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 Agenc . The LOC was initiated because of hi h nitrate concentrations Y g in the city's public drinking water supply since 1979. The study's main goal has been to assist the Ci of Decatur in com 1 in with the LOC b followin three s ecific o 'e t'v • ty p y g y g p b� c i es. 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 imy�cts of BMPs on water quality; and 3) Arialyze 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 final technical report is completed and will be printed in January 1996. The report provides the background information on the watershed by reporting on the historical water quality 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 are resented and the mathematical modelin effort discusses the effects of best mana ement P g g practices on the nitrate loading into Lake Decatur. A one-year progress report was published in November 1994 as an Illinois State Water Survey Miscellaneous Report 159. The watershed monitoring component of the study was established in the spring of 1993 , and will be concludin in A ril 1 g p 9 9 6. F i g u r e 1 s h o w s t h e l o c a t i o n o f t h e s t a t i o n s t h a t a r e b e i n g monitored throughout the watershed. In order to best evaluate any impending land management changes, continued monitoring in the watershed is highly recommended. A fourth year of watershed monitoring at a reduced level is proposed to accomplish this. The addition of a fourth year of monitoring will enhance the baseline 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 1