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August 2004 <br /> WATERSHED MONITORING <br /> FOR THE <br /> LAKE DECATUR WATERSHED <br /> by <br /> Illinois State Water Survey <br /> Champaign, IL <br /> 1NTRODUCTION 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 /> current capacity of the lake is approximately 21,500 acre-feet. The drainage area of the <br /> Sangamon River upstream of Decatur is 925 square miles and includes portions of seven counties <br /> in east-central Illinois. <br /> Lake Decatur occasionally has high concentrations of total dissolved solids and nitrate- <br /> nitrogen (nitrate-N). Nitrate-N concentrations in Lake Decatur have exceeded the Illinois <br /> Environmental Protection Agency (IEPA) drinking water standard of 10 milligrams per liter <br /> (mg/1) on occasions each year for the period between 1979 and 2004, except from 1993 to 1995. <br /> This created a serious situation for the drinking water supply of the City of Decatur and resulted <br /> in the construction of an ion exchange nitrate 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 ten years of data <br /> (May 1993-April 2003) reported by the Illinois State Water Survey (Demissie and Keefer, 1996, <br /> 1998; Keefer and Bauer, in review; Keefer and Demissie, 1996, 1999, 2000, 2002; Keefer et al., <br /> 1997), it is observed that the nitrate-N loads are relatively uniform over the entire watershed but <br /> tend to be slightly higher at the tributary streams in the upper Sangamon River watershed than at <br /> the Sangamon River stations closer to the lake. Nitrate-N loads vary from year to year with <br />