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WATERSHED MONITORING <br /> FOR THE <br /> LAKE DECATUR WATERSHED <br /> by <br /> Illinois State Water Survev <br /> BACKGROUND <br /> The Illinois State Water Survey has been conducting the Watershed Moniroring and Land <br /> Use Evaluation for Lake Decatur Watershed study since spring of 1993. This study is a result <br /> of the signing of a Letter of Commitment (LOC) between the City of Decatur and the Illinois <br /> Environmental Protection Agency. The LOC was initiated because of high nitrate concentrations � <br /> in the city's public drinking water supply since 1979. The study's main goal has been to assist <br /> the City of Decatur in complying with the LOC by following three specific objectives: 1) <br /> Establishment of a watershed monitoring network that will quantify the amount of nitrate being <br /> contributed by tributaries upstream of Lake Decatur; 2) Apply the AGNPS model for the <br /> watershed to analyze nitrate yields from different land use practices, which can be used to <br /> evaluate the impacts of BMPs on water quality; and 3) Analyze land uses in the watershed and <br /> review altemative land management practices and mitigation proj ects that could reduce the nitrate <br /> concentrations in Lake Decatur to acceptable drinking water levels. <br /> A major technical report was published in January 1996 (Demissie, et al., 1996 - ISWS <br /> Misc. Pub. lb9). The report provides the background information on the watershed by reporting <br /> on the historical water quality problems in Lake Decatur, watershed physical characteristics, and <br /> land uses. A discussion of the sources and transformations of nitrogen, as well as a detailed <br /> hydrologic analysis of the watershed are included. The hydrologic and water quality monitoring <br /> data (May 1993 through April 1995) were presented and the mathemarical modeling effort <br /> discusses the effects of best management practices on the nitrate loading into Lake Decatur. <br /> Subsequent one-year progress reports have been published in ISWS Contract Reports 602 <br /> (Keefer and Demissie, 1996), 620 (Keefer, Demissie, Shaw, and Howard, 1997), and 637 (Keefer <br /> and Demissie, 1999). The latest publication (ISWS Contract Report 637) reports annual <br /> watershed monitoring data from May 1993 through April 1998 with monthly data presented for <br /> May 1997 through April 1998. <br /> The watershed monitoring component of the study was established in May of 1993 and <br /> will be concluding in April 1999. Figure 1 shows the location of the stations that are being <br /> monitored throughout the watershed. In order to best evaluate any impending land management <br /> 1 <br />