Abstract

The Illinois Environmental Protection Agency has designated portions of the Illinois River in Peoria, Woodford, and Tazewell Counties, Illinois, as impaired owing to the presence of fecal coliform bacteria. The U.S. Geological Survey, in cooperation with the Tri-County Regional Planning Commission, examined the water quality in the Illinois River and major tributaries within a 47-mile reach between Peoria and Hennepin, Ill., during water year 2008 (October 2007–September 2008). Investigations included synoptic (snapshot) sampling at multiple locations in a 1-day period: once in October 2007 during lower streamflow conditions, and again in June 2008 during higher streamflow conditions. Five locations in the study area were monitored for the entire year at monthly or more frequent intervals. Two indicator bacteria were analyzed in each water sample: fecal coliform and Escherichia coli (E. coli). Streamflow information from previously established monitoring locations in the study area was used in the analysis. Correlation analyses were used to characterize the relation between the two fecal-indicator bacteria and the relation of either indicator to streamflow. Concentrations of the two measured fecal-indicator bacteria correlated well for all samples analyzed (r = 0.94, p <0.001), indicating a strong linear correlation. Presence of one fecal-indicator bacteria generally indicates the presence of another at a similar magnitude and may support substitution of generalized data gaps for other analyses. Hydrologic conditions during the study period can be characterized as wetter than normal, with the mean annual flow in the Illinois River about 37-percent above the long-term average. However, for the Illinois River below Peoria Lake at Peoria, a statistically significant negative correlation coefficient indicates a weak inverse relation between values of streamflow and fecal-indicator bacteria (fecal coliform rho = −0.44, p = 0.0129; E. coli: rho = −0.43, p = 0.0157). The correlation between fecal indicators and streamflow in tributaries or in the Illinois River at Hennepin was found to be statistically significant, yet moderate in strength with coefficient values ranging from r = 0.4 to 0.6. Indirect observations from the June 2008 higher flow synoptic event may indicate continued effects from combined storm and sanitary sewers in the vicinity of the Illinois River near Peoria, Ill., contributing to observed single-sample exceedance of the State criterion for fecal coliform.

First posted May 8, 2012