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 nitratenitrogen (nitrate-N) concentrations have exceeded drinking water standards in recent years. This has created a serious situation for the City of Decatur drinking water supply 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 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 this initiative is to collect reliable hydrologic and water quality data throughout the watershed to assist city planners and resource managers in developing watershed management alternatives based on scientific data. This report presents the annual data for 13 years of monitoring (May 1993–April 2006) and monthly data for project years (PY) 11, 12, and 13 (May 2003–April 2006). Based on the 13 years of data, it can be concluded that nitrate-N loads are relatively uniform throughout 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 concentration and streamflow and were the lowest in PY 7 and 13 due to low streamflows during those drought years. Flow-weighted nitrate-N concentrations at the Sangamon River station near Monticello (111) increased from 6.12 to 10.54 mg/L for PY 1 through PY 7 and fluctuated around 9.12 mg/L from PY 7 through PY 13. The highest nitrate-N concentrations during the entire monitoring period were observed at stations located at Long Creek (101) at Twin Bridge Road, Friends Creek (102) near Argenta, and Sangamon River near Monticello (111) in PY 7, 9, and 12.
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