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 lake receives water from the 925-square-mile watershed of the Upper Sangamon River that includes portions of seven counties in east-central Illinois. Lake Decatur has high nitrate-N concentrations which have been consistently exceeding the Illinois Environmental Protection Agency (IEPA) drinking water standard of 10 milligrams per liter (mg/L) since 1980. 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. Since 1993, the Illinois State Water Survey (ISWS) has been monitoring the Lake Decatur watershed for trends in discharge, nitrate-N concentration, and nitrate-N yield and to identify any significant changes in the watershed. The purpose of the monitoring was to collect reliable scientific data throughout the watershed for use by city planners and resource managers to develop watershed management alternatives based on scientific data. The ISWS originally established eight monitoring stations in the Upper Sangamon River watershed. Three of those stations have been in continuous operation from May 1993 to April 2008. A companion study funded by the Agricultural Watershed Institute allowed for continued monitoring at those three stations from May 2008 through September 2008. The purpose of this report is to document and present the discharge, nitrate-N concentration, and nitrate-N yield data for the 15 water years of monitoring (October 1993–September 2008), as well as to determine any annual trends in the Lake Decatur watershed. The intended purpose of the monitoring data and trends investigation results is to assist the City of Decatur planners and resource managers in developing watershed management alternatives to manage the current and future city water supply.
In summary, eight of the 15 water years experienced above or extremely above normal precipitation. Based on the 100-year streamflow record at the Monticello station, 4 of the top 11 total annual discharges occurred during the monitoring period (Water Year (WY) 1993 - 2nd, WY2008 - 3rd, WY1994 - 10th, and WY1998 - 11th), whereas WY2000 had the fourth lowest annual discharge. The 15-year mean annual nitrate-N yield delivered to Lake Decatur from the Upper Sangamon River watershed was 23 lb/acre and varied from 6 lb/acre (WY2000) to 42 lb/acre (WY2008). During the 15-year monitoring period, there were no significant trends in discharge and nitrate-N yields for the Long Creek (101), Friends Creek (102), and Monticello (111) stations in the Lake Decatur watershed. Annual average nitrate-N concentration for the Monticello (111) station had a statistically significant increasing trend with an increase of 0.087 mg/L per year based on the statistical fit of the annual average data. This dataset was augmented by a longer data record (WY1975–2006) from the downstream gaging station near Oakford to investigate longer-term trends in the Sangamon River watershed. There were no trends detected for nitrate-N concentration or yield within the 32-year record from the Oakford gage.
Keefer, L.L., Bauer, E., and Markus, M. (2010): Hydrologic and nutrient monitoring of the Lake Decatur Watershed: Final Report 1993–2008. Illinois State Water Survey, Contract Report in Review 2010.