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  • Watershed monitoring for the Lake Decatur watershed, 2000-2003.


  • Lake Decatur is the water supply reservoir for the City of Decatur. The reservoir was created in 1922 by constructing a dam to impound the flow of the Sangamon River. The dam was modified in 1956 to increase the maximum capacity of the lake to 28,000 acre-feet. The drainage area of the Sangamon River upstream of Decatur is 925 square miles and includes portions of seven counties in east-central Illinois, which are primarily in agricultural production. Lake Decatur has high concentrations of total dissolved solids and nitrates, and nitrate-nitrogen (nitrate-N) concentrations have been exceeding drinking water standards in recent years. This created a serious situation for the drinking water supply of the City of Decatur because nitrate-N cannot be removed from finished drinking water through regular water purification processes. Nitrate-N concentrations in Lake Decatur have exceeded the Illinois Environmental Protection Agency (IEPA) drinking water standard of 10 milligrams per liter (mg/L) on occasions each year between 1979 and 2002, except in 1993, 1994, 1995, and 2000. In June 2002, the City of Decatur activated a newly constructed nitrate-removal facility. Since 1993, the Illinois State Water Survey has been monitoring the Lake Decatur watershed for trends in nitrate-N concentrations and loads and to identify any significant changes in the watershed. The continued purpose of the monitoring is to collect reliable hydrologic and water quality data throughout the watershed for use by city planners to efficiently operate the nitrate removal facility and by resource managers to develop watershed management alternatives based on scientific data. This report presents annual data for 10 years of monitoring (May 1993-April 2003) and monthly data for Project Years (PYs) 8-10 of monitoring (May 2000-April 2003). Based on these data, it can be concluded that the average unit nitrate-N loads are relatively uniform over the entire watershed, but tend to be slightly higher at the tributary stations than at the Sangamon River stations. There also can be considerable differences in loads at tributary stations from year to year. Nitrate-N loads vary with concentration and streamflow. Average annual runoff has varied from 4 to 14 inches over the monitoring period. Concentrations were lowest in PY 7 and highest in PY 1 due to extremely low and high streamflows, respectively. Flow-weighted nitrate-N concentrations have been increasing at the Monticello and Big Ditch stations during the study period. The highest nitrate-N concentrations during the monitoring period were observed in PY 6 and PY 7. Area-weighted annual nitrate-N yield into Lake Decatur has varied between 10 (1999) and 38 (1998) lb/acre during the 10-year monitoring period (1993-2003).

Originally Deposited as: 999999994472

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Language(s): EN-English

Volume or Year: 2005
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Date Created: 4 19 2006
Date Last Modified: 4 19 2006

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1. Watershed monitoring for the Lake Decatur watershed, 2000-2003. (20061005195606_ISWSCR2005-09.pdf).
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