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WATERSHED MOriITORING <br /> FOR THE <br /> LAKE DECATUR WATERSHED <br /> by <br /> Illinois State Water Survey <br /> Champaign, IL <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 maYimum 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 nitrate-nitrogen <br /> (nitrate-N), where nitrate-N concentrations have been exceeding drinking water standards in <br /> recent years. Nitrate-N concentrations in Lake Decatur have exceeded the Illinois Environmental <br /> Protection Agency (IEPA) drinking water standazd of 10 milligrams per liter(mg/1) on occasions <br /> each yeaz for the period between 1979 and 2002, except from 1993 to 1995. This has created a <br /> serious situation for the drinking water supply of the City of Decatur and resulted in the <br /> construction of an ion exchange nitra.te removal facility in 2002. <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 /> lake and watershed management alternatives based on scientific data. Based on seven years of <br /> data (May 1993-April 2000) reported by the Illinois State Water Survey (Demissie, M. and <br /> Keefer, L.L. 1996; Keefer, L.L. and Demissie, M. 1996; Keefer, L.L., Demissie, M. et al. 1997; <br /> Keefer, L.L. and Demissie, M. 1999; Keefer, L.L. and Demissie, M. 2000; Keefer, L.L. and <br /> Demissie, M. 2(3fl2), it is observed that the nitrate-N loads are relatively uniform over the entire <br /> watershed but tend to be slightly higher at the tributary streams in the upper Sangamon River <br /> watershed than at the Sangamon River stations closer to the lake. Nitra.te-N loads vary from yeaz <br />