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  • Watershed monitoring for the Lake Decatur Watershed, 1999-2000.


  • 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. Lake Decatur has high concentrations of total dissolved solids and nitrates, and nitrate-N concentrations have been exceeding drinking water standards in recent years. This has created a serious situation for the drinking water supply of the City of Decatur, since nitrate-nitrogen (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 for the period between 1970 and 2000, except from 1993 to 1995. 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 purpose of the monitoring is to collect reliable hydrologic and water quality data throughout the watershed for use by city planners and resource managers to develop watershed management alternatives based on scientific data. This report presents the annual data for all seven years of monitoring (May 1993-April 2000) and monthly data for Year 7 of monitoring (May 1999-April 2000). Based on the seven years of data, it can be concluded that the unit of nitrate-N loads are relatively uniform over the entire watershed but tend to be slightly higher at the tributary streams in the upper Sangamon River watershed than at the Sangamon River stations closer to the lake. Nitrate-N loads vary with concentrations and streamflow and were the lowest in Year 7 because of the low streamflows during that year. Flow-weighted nitrate-N concentrations have been increasing during the study period at the Monticello station. The highest nitrate-N concentrations during the monitoring period were observed in years 6 and 7.

Originally Deposited as: 999999994336

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Language(s): EN-English

Volume or Year: 2002
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Date Created: 9 24 2004
Date Last Modified: 6 20 2003

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1. Watershed monitoring for the Lake Decatur Watershed, 1999-2000. (20061002184454_ISWSCR2002-01.pdf).
Document Size:963331 Software: Adobe Acrobat Version: 7.0