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.