Chemical constituents in storm flow vs. dry weather discharges in California storm water conveyances

This research evaluated concentration data for selected water quality param eters in selected California urban separate storm sewer systems during stor m event discharges and during dry weather conditions. We used existing moni toring data from multiple regulatory agencies and municipalities originally collected for compliance or local characterization, which allowed systemat ic assessment of seasonal patterns over a wide region. Long term mean conce ntration for most parameters in most streams was higher during storm discha rges than during dry weather flows to at least 95 percent confidence in 20 of 45 comparative evaluations, and lower statistical confidence in 22 other comparisons. Some regional differences emerged: in four evaluated streams in the San Francisco Bay Area, total concentration of lead, copper and zinc were lower during dry weather than during storm flows to at least 99.9 per cent confidence, with only one exception; while the other four evaluated Ca lifornia streams showed the same tendency, but to much lower statistical co nfidence.