Determining Water Quality Impacts of Combined Sewer Overflow on the Mississippi River Using Full Factorial Analysis

The City of Rock Island, Illinois is completing a Combined Sewer Overflow Long Term Control Plan as part of a Consent Decree with the U.S. EPA. The Consent Decree includes a requirement that Rock Island document existing water quality so that CSO impacts could be identified and potential improvements with various CSO could be assessed. Because of complex river morphology, large flow rate disparity between CSO and river flows, and multiple sources of pollution in the metropolitan area, Rock Island chose to characterize CSO impacts on water quality by monitoring.

The monitoring program used Full Factorial Analysis, a subset of the broader technique known as Design of Experiments, to guide the development of the monitoring program and analyze the data. Full Factorial Analysis is particularly useful for maximizing the information derived from a water quality monitoring program. In Rock Island’s case, a conventional One Factor at a Time analysis technique would have required five times the number of samples as the Full Factorial Analysis to gather similar information. This paper describes the Full Factorial Analysis technique, its potential benefits, and its application to water quality studies.

The City of Rock Island, Illinois is located in western Illinois at the confluence of the Rock River with the Mississippi. Rock Island is one of ten contiguous communities comprising the “Quad Cities,” a metropolitan area of about 350,000 people. This is the largest metropolitan area between the Minneapolis/St. Paul, Minnesota area and St. Louis, Missouri. Rock Island is the most downstream community on the Illinois side of the river and has about 13% of the population.

The City is served by combined sewers, partially separated sewers, and separate sanitary and storm sewers. The City of Rock Island is completing Combined Sewer Overflow (CSO) Long Term Control Plan (LTCP), prepared to conform to requirements of a Consent Decree with the United States Environmental Protection Agency (USEPA). The USEPA brought suit against the City of Rock Island (the City) on August 18, 2000, for alleged violations of the Clean Water Act (CWA). As a result of this suit, the City is required by the USEPA to prepare a Combined Sewer Overflow (CSO) Long Term Control Plan (LTCP) consistent with the requirements of the Consent Decree entered into the United States District Court for the Central District of Illinois in the matter of U.S. v. City of Rock Island, Illinois, et al., Civil Action Number 4:00-CV-04076.

The Consent Decree referenced the CSO policy with respect to the level of control to be achieved by the plan. Two options are available to communities to establish the level of control: the Presumptive Approach and the Demonstrative Approach. The intent of either approach is to complete collection and treatment system improvements that eliminate to the greatest degree practicable discharges that contribute to failure to achieve water quality standards. The City chose the Presumptive Approach allowing an annual average of four and maximum of six uncontrolled CSO events to prepare its plan, rather than attempt to construct and validate a water quality model to assess CSO impacts to the Mississippi River with different levels of control. An element of the Consent Decree required documentation of existing water quality for assessment of CSO impacts to the receiving waters. Accurate definition of CSO impacts to the receiving waters was found to be difficult, given the difference between river flow and CSO discharge volume, the fact that available monitoring data was not specific to wet weather impacts, and sample locations were not established to evaluate CSO impacts. A monitoring program was designed by Symbiont to fill data gaps with respect to CSO discharge under different river flow conditions. This monitoring program was based upon the requirements of Design of Experiments Full factorial analysis to allow better discernment of the potential effect of CSO discharge on observed water quality.

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