The City of Peoria is preparing a Combined Sewer Overflow (CSO) Long Term Control Plan. A monitoring program was implemented to characterize CSO discharges to the Illinois River and their impacts to water quality (Peoria, 2007). Elevated background levels of bacteria make discernment
of CSO impacts to the receiving waters difficult. The primary goal of the monitoring program was to characterize the specific bacterial impacts of CSO discharge to the Illinois River such that the benefits and water quality improvements from CSO abatement with respect to alternate levels of
CSO control could be identified. The monitoring program used both fecal coliforms and E. coli to characterize bacterial contamination. Fecal coliform was used because it is the water quality standard used by Illinois to indicate conformance to recreational water use. E. coli was used because
it is considered by USEPA to be a better indicator of human sources of bacteria. In both cases, statistical methods were employed to discern the impacts from CSO apart from elevated background levels, which for fecal coliforms often exceeded regulatory limits by 2 to 4 orders of magnitude. Monitoring
took place at 5 transects of 4 sampling stations each (20 locations total) along the Illinois River. One transect was located immediately upstream of the segment of the river subject to CSO discharge, three transects were located within the area of the river subject to CSO discharge, and one
transect was located downstream from all CSO discharges and the treated discharge of the Greater Peoria Sanitary District. The sampling grid allowed characterization of both longitudinal and lateral variation in bacterial contamination. Sampling rounds took place during both dry (no CSO) and
wet (CSO) weather for both low and high river stage conditions. Wet weather sampling rounds were sampled before, during, and after CSO discharge. This collection of sampling stations and rounds allowed characterization of background concentration, longitudinal and lateral variation in water
quality, variation of water quality during a CSO event, and impacts of river stage on water quality. Statistically valid conclusions require characterization of natural variation in bacteria concentration. Variance was characterized by replicating the low river stage, dry weather sampling
round, and collecting samples at nominally equivalent conditions. Sample variance was further characterized by collecting water quality samples at 3-hour intervals over a 24-hour period at each sampling station at Transects 1 and 5. Discerning CSO impacts to the Illinois River used several
graphical and statistical techniques ranging from simple to complex. Pie charts depicting the percent of samples meeting water quality samples compared dry weather, pre-CSO, during-CSO, and post-CSO conditions. Bar charts depicting geometric mean concentration compared longitudinal, lateral,
and temporal variation in water quality. Exceedance probability plots compared the distribution of bacteria concentrations during dry weather, pre-CSO, during-CSO, and post-CSO conditions. The graphical techniques provided useful visual indications of water quality trends, but could not be
used to declare if the observed differences were statistically valid. For this purpose, the monitoring program used full factorial regression analysis. The full factorial regression analysis, documented by Gonwa, et.al. (2008), exploited the orthogonal sampling design of the monitoring program
to determine which observed changes in bacterial water quality were statically significant and which were not. The study found that fecal coliform levels were consistently elevated above water quality standards at all locations, with increased concentrations observed during CSO events downstream
from discharges. E. coli concentrations were lower than fecal coliform concentrations and were found to be a better indicator of CSO impacts. Upstream sources of fecal coliform contribute to elevated levels of fecal coliform in the Illinois River at Peoria beyond the time when impacts of CSO
are present. Full factorial regression analysis indicated that statistically different concentrations of both E. coli and fecal coliform are observed when CSO is present. The analysis also found a depression of dissolved oxygen concentrations, although not below the water quality standard.
In general, higher variability in nominally replicate samples made it more difficult to discern variation from elevated background concentrations in fecal coliform samples than for E. coli samples. The study has been completed and a draft technical memo submitted to U.S.EPA and Illinois
EPA for their review and comment in advance of CSO control alternative development. The findings of the study have advanced the understanding of the use of fecal coliform and E. coli as indicators of human sources of contamination. The results are being used to support cost-beneficial levels
of control, and potentially a Use Attainability Analysis, if required, for full Long Term Control Plan conformance with the water quality standards.
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