WaterSubject

Stormwater runoff from urbanized watersheds has been receiving increasing attention from the public and scientific community in recent years because it is perceived to be a large source of pollutants to coastal waterbodies around the nation. Despite separate stormwater and sanitary sewer systems in southern California, the stormwater runoff from urbanized watersheds has contributed substantial loadings of a variety of constituents to receiving water environments. For example, the cumulative loads of lead and zinc from all of the urbanized watersheds in the Southern California Bight to the coastal oceans were estimated to be 39 and 316 metric tons during the 1994/95 water year, respectively. These inputs represent over half of the combined mass emissions from all sources, which include traditional point sources such as publicly owned treatment works, industrial facilities, and power generating stations.

Although mass emissions from urban runoff in southern California have been determined to be large, little is known about the extent that stormwater inputs affect biota. Some studies have analyzed the effect of stormwater discharges on freshwater organisms including algae, invertebrates, and fish. However, studies that have examined the effects of stormwater discharges on marine species are rare. Southern California has very little freshwater habitat because of its semi-arid climate; therefore, the ocean environment has generated the most concern from the public. The coastal environment of the SCB has been estimated to generate as much as $9 billion per year in revenues from recreational uses.

This study addressed two questions of environmental concern to further assess the impacts of stormwater discharges on aquatic organisms. The first question was: “How do the toxic responses to stormwater runoff differ between freshwater and marine organisms?” The goal of this study objective was to evaluate potential impacts to either the freshwater or marine communities that receive runoff discharges. The second question was: “Which constituents are responsible for toxicity in freshwater and marine organisms?” The goal of this study objective was to identify the constituent(s) of concern in stormwater discharges and to determine whether the constituents are similar between the different species.

The toxic responses differed between freshwater and marine species after exposure to stormwater. Strongylocentrotus, a marine species, was extremely sensitive to stormwater, exhibiting responses during every storm at concentrations as low as 6–12% stormwater. In contrast, another marine species, Mysidopsis, exhibited no response to stormwater for any of the storms sampled. Ceriodaphnia, the freshwater species, exhibited intermediate toxic responses; two of three samples were toxic at relatively high concentrations of 50–100% stormwater. Moreover, the pattern of toxicity among storms was not consistent. No single storm was the most toxic to both the marine and freshwater species.

Organophosphate pesticides in stormwater runoff from Chollas Creek are responsible for toxicity observed in the freshwater species C. dubia. Diazinon is the most likely constituent. The TIEs characterized toxicity during each storm as organophosphate pesticides. The TIE manipulations that remove hydrophobic organic compounds (C-18 column) or neutralized organophosphate pesticides (PBO) both effectively removed toxicity. Moreover, concentrations of diazinon and chlorpyrifos, both organophosphate pesticides, were high enough in the stormwater samples to induce toxicity. Confirmation of diazinon as the likely constituent was accomplished through the use of pH manipulations that degrade diazinon but not chlorpyrifos. The predicted toxicity of diazinon based upon measured concentrations in our samples and responses of Ceriodaphnia from the peer-reviewed literature was sufficient to account for 95% of the observed toxicity in each of the storms measured. Chlorpyrifos was further discounted because Mysidopsis, a species that is known to be even more sensitive than Ceriodaphnia to this pesticide, exhibited no toxic response to the same stormwater sample.

Trace metals in stormwater runoff from Chollas Creek are responsible for toxicity observed in the marine species S. purpuratus. Zinc, and to a lesser extent copper, were the most likely constituents. The TIEs characterized toxicity during each storm as trace metals. The TIE manipulations that sequestered heavy metals (EDTA) effectively removed toxicity. Moreover, concentrations of zinc, and to a lesser extent copper, were high enough in the stormwater samples to induce toxicity. Confirmation of zinc as the likely constituent was accomplished through the use of cation exchange columns that can be used to reintroduce the sequestered metals. The predicted toxicity of zinc and copper based upon measured concentrations in our samples and responses of S. purpuratus from laboratory-spiked seawater experiments was sufficient to account for between 55% and 95% of the observed toxicity, depending upon which storm was measured.

Impacts of urban stormwater discharges have the potential to impact both freshwater and marine habitats. The differences in sensitivity and stormwater constituents among various organisms justifies the use of multiple species in monitoring and assessment programs. However, a link between discharge monitoring and impacts in the receiving water environment also needs to be established. Measurable impacts in receiving waters not only provides the rationale and justification for management action, but it provides an assessment of the magnitude and extent of beneficial use impairment necessary to set reduction goals.