SPATIAL AND TEMPORAL VARIATIONS IN TOXICITY IN AN URBAN-RUNOFF TREATMENT MARSH

摘要

Toxicity tests have not been widely used to assess the performance of urban-runoff treatment facilities. In the present study, Ceriodaphnia dubia toxicity tests were used to quantify toxicity of urban runoff at Crandall Creek and the downstream Demonstration Urban Stormwater Treatment (DUST) March in Fremont, California. Acute toxicity, expressed as the median time to lethality (LT50) for C. dubia, was used to compare the relative intensities of toxicity in the system. During or shortly after storm events, horizontal and vertical gradients in LT50 and electrical conductivity were observed, with high correlation between the two parameters. Toxicity diminished as time passed after the storm. The performance of the DUST Marsh as a treatment facility was evaluated for three aspects: detention, dilution, and toxicity removal. We found that toxic storm water generated by small- to medium-sized storms (5 to 25 mm rain) was contained in the marsh. Toxicity was greatly reduced upon dilution of storm water with pre-existing marsh water, and mixing of the water column increased the rate of toxicity diminution. Toxicity reduction, above and beyond that attributable to dilution, was evident in the marsh. Results of this study demonstrate the potential use of toxicity assessments as an integral component of marsh design and management.