Partitioning Analog for Metal Elements in Urban Rainfall-Runoff Overland Flow Using the Soil Conservation Service Curve Number Concept
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Employing the basic proportionality concept of the popular Soil Conservation Service curve number (SCS-CN) methodology, 12 metal elements, Zn, Cd, Pb, Ni, Mn, Fe, Cr, Mg, Al, Ca, Cu, and Na, are partitioned between dissolved and particulate bound forms. Study results are based on observed runoff-water quality data for a 300 m2 instrumented paved urban site in Cincinnati, Ohio. In the SCS-CN method, the parameter CN represents the degree of saturation (%) for a watershed loaded by a 10 in. (254 mm) base precipitation amount and leads to the derivation of (a) the seldom explored potential maximum retention (S)-curve number (CN) mapping relation, and (b) a relationship between the antecedent 5 day precipitation amount and the antecedent moisture conditions (AMC). In order to apply this metal partitioning analog, two parameters, the potential maximum desorption and the partitioning curve number (PCN), are postulated as analogous to the SCS-CN parameters S and CN, respectively. These parameters are introduced, along with -PCN and -ADP, where ADP is the antecedent dry period similar to the AMC. Based on these parameters analogous relationships are developed. During a rainfall-runoff event at the upper end of the urban watershed (residence times of less than 1 h), was found to depend on the alkalinity and pH of the rainfall runoff for a given set of physical and chemical particulate characteristics and residence time. The PCN-based ranking of metals is found to be in general agreement with that available in the literature.