Modeling Methodology for Determining Pollutant Concentrations and Loadings for Combined Sewer Overflows: A Simplified CSO Model
The Detroit Metropolitan area occupies much of the basin of the Detroit River - the basin as a whole is home to about four million people. The Detroit River provides important habitat for fish and birds. Atmospheric transport and deposition of chemical contaminants over the entire basin, leakage from sites contaminated according to Michigan’s Act 307, and industrial effluents to the system provide sources for a variety of toxic chemicals to the Detroit River through the combined sewer overflow (CSO) process (Arimoto, 1989; Michigan Department of Natural Resources, 1993; Michigan Department of Natural Resources and Ontario Ministry of Environment, 1995). The Detroit River receives treated and untreated waste water from the City of Detroit and suburban districts, industries, runoff from urban and agricultural lands, and effluent from combined sewer overflows (Lin, 1994; Roginski, 198 1; USEPA and EC, 1988; Rhee, 1995). The city is entirely served by combined sewers. As might be expected, CSOs have been demonstrated to be a major source of conventional and toxic contamination to the Detroit River, for example by the 1979-1980 major monitoring and modelling work on the section 201 Final Facilities Plan conducted by the Joint Venture (Giffels et al., 1980), to characterize the Detroit River CSO loadings and the Detroit River Remedial Action Plans (Stage I & II) (Michigan Department of Natural Resources and the Ontario Ministry of the Environment, 1991; Michigan Department of Natural Resources and Ontario Ministry of Environment, 1995).
A simplified CSO model has been developed as a rapid and inexpensive way to estimate and assess potential environmental hazards, as opposed to standard cause-and-effect modelling which typically costs $2,000,000 to $15,000,000 per study for Detroit. This model is a large-scale multiple-input multiple-output (MIMO) system. The modelling task can be eased by applying the decomposition-coordination strategy of the large-scale system theory (Huang and Fan, 1993). The goal of the present work is to use the field data generated by the current Southeast Michigan Council of Governments, U. S. Geologic, Survey, and Detroit Water and Sewerage Department (SEMCOG/USGS/DWSD CSO; Perry, 1996) study, and other resources to build a simplified CSO model capable of correlating and predicting the CSO concentrations and loadings to the Detroit River. Three groups of input variables to the model have been considered: the runoff concentrations of pollutants by land use; the runoff and CSO volumes; and the dry weather flow rates and concentrations of pollutants. This study focuses on three constituents, cadmium, copper, and lead.
The first input data, the runoff concentrations of cadmium, copper an d lead as a function of land use, were obtained from the stormwater pollutant loading factors presented in the Rouge River National Wet Weather Demonstration Project Report (Cave, et al., 1994). These runoff concentrations by land use already incorporate whatever leakage from Michigan’s Act 307 contaminated sites and atmospheric deposition occur through runoff during the antecedent period. The second input data, the runoff and CSO volume, were obtained from SEMCOG’s database system (daily precipitation) and the CDM report (Camp Dresser and McKee, 1993). The third input data, the dry weather flow rats and concentrations of cadmium, copper, and lead, have been obtained from the DWSD Industrial Waste Control (IWC) database (1993-1994) and residential sources (Salley and Kummler, 1986; Garakani et al., 1991).
Because actual case studies involved complicated geometry and source functions, the modelling was performed in two model scenarios. First, daily average based estimation of CSOs concentrations was conducted using 1980 cadmium data from the City of Detroit’s Section 201 Final Facilities Plan report for whole Detroit River basin (Kummler, 1983; Salley and Kummler, 1983). Second, a storm event based estimation of CSO concentrations was performed for two sampling sites (Conner Creek and Fischer sites). Four CSOs discharging to the Detroit River were monitored in 1994-1995 to characterize storm-related water quantity and quality to calculate their respective pollutant loads.
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