Size fractionation of metals in runoff from residential and highway storm sewers

Stormwater sampling for particulate, colloidal, and dissolved metals was conducted for several storms at six outfalls in Monmouth County, NJ. Samples were initially sequentially filtered through 5 μm, 0.45 μm, and 10 kDa filters. Of the heavy metals, Cu and Zn were mostly either dissolved (< 10 kDa) (20–100%) or in the particulate size fractions > 5 μm (0–> 70%). Pb and Cr were associated exclusively with particles > 5 μm in size. Fe, Al, and Si were found mostly in larger size fractions (> 70%), with smaller amounts 0.45–5 μm in size. Preliminary data from a small set of samples passed through coarser filters suggested that metals may actually be largely associated with particles larger than 20 μm. Variable and sometimes large dissolved fractions of Cu and Zn can contribute to erratic metals removal by structural best management practices (e.g., wet ponds, detention basins). The size fractionation of stormwater constituents has implications for the design and performance of stormwater control structures and the aquatic toxicity risks posed by the metals. The results demonstrate the importance of obtaining particle size data when planning stormwater treatment.