Analysis of colloidal phases in urban stormwater runoff

The composition and morphology of colloidal materials entering an urban waterway (Brays Bayou, Houston, U.S.A.) during a storm event was investigated. Analyses of organic carbon, Si, Al, Fe, Cr, Cu, Mn, Zn, Ca, Mg, and Ca were performed on the fraction of materials passing through a 0.45 mu m filter. This fraction, traditionally defined as "dissolved", was further fractionated by ultracentrifugation into colloidal and dissolved fractions. Colloids, operationally defined by this procedure, accounted for 17% of the carbon, 32% of the silica, 79% of the Al, 85% of the Fe, 52% of the Cr, 43% of the Mn, and 29% of the Zn present in filtrates when averaged over the storm event. However, the composition of colloidal material was observed to change over time. For example, colloids were predominantly composed of silica during periods of dry weather flow and at the maximum of the stormwater flow, while carbon dominated the colloidal fraction at the beginning and declining stages of the storm event. These changes in colloidal composition were accompanied by changes in colloidal morphologies, varying from organic aggregates to diffuse gel-like structures rich in Si, Al, and Fe. The colloidal phase largely determined the variability of elements in the 0.45 mu m filtrate.