Influence of Hydrology on Rainfall-Runoff Metal Element Speciation

Examination of speciation for metal elements transported in urban rainfall-runoff events is critical when evaluating the potential fate, bioavailability, and effective control of such constituents. In many urban areas anthropogenic activities result in rainfall pH levels that are acidic and low in alkalinity. As a result, finely abraded metallic components and exposed metal infrastructure can be leached into rainfall-runoff. This study examines the influence of hydrology on storm water metal element speciation at the upper end of a Portland cement concrete small urban watershed. This study focused on Pb, Cd, Cu, and Zn; metal elements commonly found in urban and transportation land uses. For this site partitioning results demonstrated that Cd and Cu partitioned nearly equally between particulate and dissolved phases while Zn was generally particulate-bound and Pb was highly particulate-bound. Utilizing water quality analyses, measured ion balances and speciation modeling, results for Cd and Zn indicated that divalent ionic forms of these metals dominated the dissolved species for all events, while Pb was predominately associated with dissolved organic matter (DOM), and Cu was predominately associated with carbonate species or DOM. Of the three events examined, only the mass-limited events demonstrated a change in speciation during the passage of the hydrograph. Results from this study indicate that effective control of storm water metal elements at the upper end of the urban watershed requires unit operations and processes that account for the ionic, complexed and particulate-bound species and account for the hydrology at the upper end of the urban watershed.