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. �� . <br /> WATERSHED MONTTORING I <br /> FOR THE <br /> LAKE DECATUR WATERSHED <br /> by <br /> Illinois State Water Survey <br /> Champaign, TL <br /> INTRODUCTION AND BACKGROUND <br /> Lake Decatur is the water supply reservoir for the City of Decatur. The reservoir was <br /> created in 1922 by constructing a dam to impound the flow of the Sangamon River. The dam <br /> was modified in 1956 to increase the maximum capacity of the lake to 28,000 acre-feet. The <br /> drainage area of the Sangamon River upstream of Decatur is 925 square miles and includes <br /> portions of seven counties in east-central Illinois. <br /> Lake Decatur has high concentrations of total dissolved solids and nitrates, and nitrate-N <br /> concentrations have been exceeding drinking water standards in recent years. This has created a <br /> serious situation for the drinking water supply of the City of Decatur, since nitrate-nitrogen (N) <br /> cannot be removed from finished drinking water through regular water purification processes. <br /> Nitrate-N concentrations in Lake Decatur have exceeded the Illinois Environmental Protection <br /> Agency (IEPA) drinking water standard of 10 milligrams per liter (mg/1) on occasions each year <br /> for the period between 1979 and 2000, except from 1993 to 1995. <br /> Since 1993, the Illinois State Water Survey has been monitoring the Lake Decatur <br /> watershed for trends in nitrate-N concentrations and loads and to identify any significant changes <br /> in the watershed. The purpose of the monitoring is to collect reliable hydrologic and water <br /> quality data throughout the watershed for use by city planners and resource managers to develop <br /> watershed management alternatives based on scientific data. Keefer and Demissie (2002) report <br /> that based on seven years of data-(May 1993-April 2000), it can be concluded that the nitrate-N <br /> loads are relatively uniform over the entire watershed but tend to be slightly higher at the <br /> tributary streams in the upper Sangamon River watershed than at the Sangamon River stations <br /> cioser to the lake. Nitrate-N loads vary with concentration and streamflow and were the lowest <br /> between the sampling period of May 1999-April 2000, because of the low streamflows during <br /> that year. Flow-weighted nitrate-N concentrations have been increasing during the study period at <br />