CHARACTERIZATION OF METALS IN RUNOFF FROM RESIDENTIAL AND HIGHWAY STORM SEWERS

Stormwater runoff was sampled from six storm sewer outfalls in residential and highway settings in Monmouth County, NJ to determine the colloidal and dissolved concentrations of metals. Heavy metals, common pollutants in natural waters and stormwater,are known to associate with particles, affecting the metals' transport and fate. The outfalls were instrumented with automatic samplers which collected flow-weighted samples for at least 4 storms. Samples were sequentially filtered through 5 µm and 0.45 µm filters and a 10K nominal molecular weight pore size ultrafilter (approx. 0.01 µm). The ultrafiltrate was operationally defined as dissolved. Filtrates and unfiltered aliquots were analyzed for metals, including Cu, Cd, Cr, Ni, Pb, and Zn, and total organic carbon (TOC).

Cd and Ni were not detected. Unfiltered Pb and Cr concentrations ranged from 5-40 µg/L. Unfiltered Cu and Zn concentrations ranged from 3-65 µg/L and 5-322 µg/L respectively. Pb and Cr were not detectable after 5 µm filtration. Cu and Zn partitioning varied widely. In some samples they were almost entirely dissolved (up to 90%). When particulate Cu or Zn was present it was mostly > 5 µm in size, as indicated by reductions in concentrations up to 75% after 5 µm filtration. Smaller amounts of Cu and Zn (up to 20%) were present as colloids (0.010-05 µm), contrary to expectations that colloids would carry most of these metals.

Thirty to 90% of Al, Fe, and Si was > 5 µm in size. Smaller fractions (up to 20%) were colloidal. The Fe, Al, and Si are most likely from mineral phases such as Fe oxides, silica, and clays, and are probably associated with the particulate fractions of Cu, Zn, Pb, and Cr. Organic carbon (4- >100 mg/L) was mostly dissolved (40-90%) and in 0.01-0.45 µm sized colloids (up to 40%). There was no correlation between colloidal Cu and Zn and colloidal organic carbon. Up to 30% of TOC was > 5 µm, part of which may be coatings on Fe-, Al-, and Si-bearing particles.

The results provide needed data on the characteristics of runoff, which impacts receiving waters such as streams and estuaries. Stormwater treatment strategies that rely on settling can achieve partial Cu and Zn removal by settling particles greater than 5 µm in size, but dissolved fractions can continue to move through the environment.