Abstract

Combined Sewer Overflows (CSOs) are a major source of conventional and toxic contamination for the Detroit River (DR). In 1979-1980, a major monitoring and modelling effort was conducted to characterize the DR CSO loadings. The Detroit River Stage I Remedial Action Plan (RAP) used the 1979 CSO concentrations to estimate the CSO loadings to the Detroit River. However, the population, industrial productivity and control measures, and the Detroit Sewerage System Configuration have all changed substantially since that era. In order to more correctly assess the current loadings for the RAP Stage 11 process, a new estimate in the absence of new measurements had to be made. The authors served as the data analysis team for the DR RAP to provide this estimation.

First, the raw influent data of daily average concentrations provided by DWSD (Detroit Water and Sewerage Department) WWTP (Waste Water Treatment Plant) were used to analyze the trend of each parameter's annual average influent conventional and toxic concentrations. The first fiscal year (1983) and the last fiscal year (1992) concentrations were also calculated using a t-distribution and the confidence intervals (95% confidence limits) determined. Secondly, the ratio between 1983 and 1992 annual average influent toxicant concentrations for parameter was calculated. Then, this fraction was applied to the 1979 CSO concentrations to estimate current CSO concentrations.

Because of the City of Detroit's Industrial Pretreatment Program, the general decline of industrial activity in the area, the conscientious efforts of industries to reduce discharges through pollution prevention, and other factors, it was concluded that the current (1992) DR CSO toxicant concentrations are substantially less than the 1979 CSO toxicant concentrations.

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