DECISION-MAKING FOR WET WEATHER RAINFALL-RUNOFF UNIT OPERATIONS, PROCESSES, AND CONTROLS BASED ON PARTITIONING, DISTRIBUTION, AND SPECIATION OF METALS IN THE URBAN WATERSHED

Urban infrastructure and activities can be a significant source of metal element generation and transport during wet weather rainfall-runoff events in the urban environment. In urban wet weather runoff (rain or snow) metal elements partition between the particulate and dissolved fractions. This partitioning can favor the dissolved fraction for low rainfall pH levels, low pavement residence times in terms of minutes, and low solids levels and alkalinity; and favors the particulate fraction under higher pH, alkalinity, residence time and solids levels. The dissolved fraction speciation is a function of organic and inorganic complexes including carbonates, organic matter and sulfates. The particulate fraction distributes across the gradation as a function of particle surface area, amphoteric surface charge, particle geochemistry, granulometry and size-based particulate loading. High metal element concentrations and the persistence of metal elements influence characteristics of wet weather rainfall-runoff, receiving waters, the existing soil/residual complex, and ultimately the aquatic life and food chain. A fundamental understanding of the nature and extent of storm water pollution integrated with the development of sustainable treatment and re-use of storm water is coming of age in the USA. To ensure sustained progress, questions associated with the viability and sustainability of treatments, cost/benefit alternatives, and a focus on the relative roles of source control/in-situ treatment compared to centralized storm water treatment must be addressed. Storm water quality is now a major urban environmental issue and storm water is becoming the wastewater of focus for the new century. While the hydrologic/hydraulic fundamentals of urban storm water quantity control are taught in selected engineering curricula, there is a need for integration of water quality and unit operation/process treatment control theory applied specifically for storm water. The sources, generation, unsteady delivery and diffuse nature of storm water require a unique perspective for an academic course on unit operations and process treatment design loaded by the urban anthropogenic hydrology. Development of viable stormwater unit operations and processes (UOPs) targeting dissolved, complexed and particulate fractions are dependent on knowledge of these phenomena and synthesis of such knowledge for UOP design and application. This paper will present specific field and lab studies illustrating such phenomena for rainfall-runoff. Integration of these concepts can apply for structural and non-structural controls.