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Marsh Cleanses Runoff
Meters, models, fleas and storms were the all-star cast of a recent study designed to test a Fremont marsh's ability to clean up urban runoff before it reaches the Bay. The study, conducted by Lawrence Berkeley Laboratory under the direction of Dr. Susan Anderson (see Now in Print), confirmed that toxicity was reduced in samples collected several days after storms - and not by dilution alone.
The 55-acre marsh - named the Demonstration Urban Stormwater Treatment (DUST) Marsh - includes ponds, wetlands and debris basins engineered to trap and filter stormwater from a 4.5-square-mile area of Fremont.
"When you stand in the marsh, you can see immediately when the stormwater arrives and where it goes just by dipping the conductivity meter in the water," says Dr. Revital Katznelson, Visiting Researcher at the lab. Stormwater is less saline than marsh water and thus exhibits lower electrical conductivity.
Katznelson was able to separate evaluation of the marsh's treatment processes (such as contaminant degradation and sedimentation) from other performance aspects (such as containment and dilution) through a combination of electrical conductivity measurements, dilution modelling and bioassays on Ceriodaphnia (water fleas).
The study found, among other things, that toxicity diminished as time passed after the storm, and that stormwater lingered in the upper strata of the marsh before mixing.
More importantly, the results helped evaluate how well the DUST marsh performs as a treatment facility. In terms of detention, the marsh did succeed in containing most of the toxic stormwater generated by those storms with 0.2-1.0 inches of rainfall, but some flowed out through the marsh's exit culverts after larger storms of 1 inch or more. In terms of dilution, the mixing of storm and marsh water did greatly reduce toxicity. Total mixing, and the disappearance of vertical stratification, occurred within several days. In terms of toxicity, some of the observed reduction could not be explained by dilution, indicating that toxic substances were indeed being removed, broken down or sequestered by marsh processes. The study also served as a model for cooperation among scientists (LBL), regulators (the S.F. Regional Board), and county agencies (Alameda County Urban Runoff Clean Water Program). In fact, ecologists and engineers from the latter are already using the study's findings to improve the marsh. They've installed a log boom across a main channel to break the previously uninterrupted flow of surface waters and to force bottom water - instead of surface water - to move to the marsh exit. And they hope to add another basin, increasing the marsh's holding capacity by 40%, as well as to create more meanders and islands, enhancing turbulence. Indeed, the effort to increase mixing is something of an innovation in treatment marsh design, which has traditionally worked to minimize mixing and promote sedimentation.
Contact: Dr. Susan Anderson (510)486- 4654
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