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Urban stormwater runoff is traditionally defined as that portion of precipitation which drains from city surfaces exposed to precipitation and flows via natural or man-made drainage systems into receiving waters. But, urban stormwater runoff also includes discharges from many other anthropogenic activities/sources, which find their way into storm drainage systems. The importance of inappropriate discharges into storm drain systems stems from their significant impacts on receiving water quality.

Initial studies by Pitt, et al.(1993) and Lalor (1994) reviewed various methodologies to investigate illicit discharges into storm drain systems. The Center for Watershed Protection (CWP) and the University of Alabama are currently conducting a technical assessment of techniques and methods for identifying and correcting illicit and inappropriate discharges geared towards NPDES Phase II Communities, with support provided by Section 104(b)3 funding from the US Environmental Protection Agency (Bryan Rittenhouse is the project officer).

Investigation of non-stormwater discharges into storm drainage proceeds along a hierarchy of procedures ranging from exploratory techniques to verification procedures. Exploratory techniques involve an extensive mapping effort to identify the locations of all outfalls for sampling and to outline and characterize the drainage areas contributing to all outfalls. This is followed by the screening analyses at the outfalls which include several visual observations and sampling at repeated intervals at the outfalls in order to measure chemical tracers which would help to identify the general categories of non-stormwater flows. Bacterial concentrations of stormwater flows are more problematic indicators of specific contamination. The use of the flow chart method for identifying most significant flow component would result in identifying the most likely source of contamination based on the concentrations of chemical tracers. Based on recent field studies at the University of Alabama, Tuscaloosa, the flowchart method has been significantly improved to include fewer chemical tracers while giving better results. The flowchart helps differentiate between the major two sub-groups of sources- clean water sources: tap water, spring water and irrigation runoff and the dirty water sources: carwash, laundry, sewage and industrial sources. The use of detergents to differentiate between the clean and dirty water sources can be replaced by boron as a suitable tracer. This is a significant improvement considering the potentially carcinogenic chemicals used in the detergent tests (benzene or chloroform).

Karri (2004) at the University of Alabama developed a computer simulation model based on chemical mass balance equations and Monte Carlo simulation to identify the most likely source of contamination in dry-weather flow samples. The model compares the tracer concentrations of outfall samples against local chemical tracer concentrations of source area samples and calculates the most probable source of contamination.

By using both the flowchart and the modeling methods, the most probable source of inappropriate discharge into the storm drain system can be identified. The watershed survey includes manhole sampling at successive intervals and identifies the likely source from the chemical tracer analysis. Flow measurements at successive manholes can give a clear indication of the location of the candidate pipes responsible for the inappropriate discharges.

The above methodology is being employed at Tuscaloosa, Alabama, to study the sources of inappropriate discharges into the Cribbs Mill Creek. Initially, a local library of source area samples was collected and analyzed for characteristic tracer concentrations. The Cribbs Mill Creek was surveyed and the outfalls mapped and sampled to obtain tracer concentrations of the dry weather flows. Using the flow chart method, the most likely sources of contamination have been identified. The simulation model predicted the same major contamination sources when the tracer concentrations were modeled. The field verification phase is currently being completed.
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Document Type: Research Article

Publication date: 01 January 2004

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