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Project: Assessment of Best Management Practice (BMP) Effectiveness for Reducing Toxicity in Urban Runoff

Background and Objectives

Best management practices (BMPs) are used extensively in southern California to reduce contaminants in urban runoff. Most BMPs that treat urban runoff are designed to reduce or remove trash, nutrients, or toxic constituents associated with particulates. Though BMPs have been shown to reduce the larger particulates found in runoff, it cannot be assumed that they also reduce toxicity.  Previous work suggests that many structural BMPs are not capable of treating the most toxic fraction of runoff (the dissolved phase). However, information regarding the effectiveness of BMPs at reducing toxicity is comparatively limited. Toxicity assessments are useful because they incorporate the additive and antagonistic interactions of chemicals as a direct measure of biological effect. They can improve the evaluation of BMP effectiveness by accounting for the effects of a range of toxic contaminants whose chemical concentrations may or may not be routinely monitored.

To assess the effectiveness of BMPs for improving water quality impacts related to toxicity in southern California, five BMP technologies were evaluated with respect to their ability to reduce contaminant concentrations and toxicity in runoff samples.

Wetland BMPs used to remove trash and filter urban runoff in southern California.


This project was completed in 2005.


Seven BMP sites representing five different technologies were assessed for their effectiveness at reducing contaminant concentrations and toxicity. The five BMP technologies studied were: wetlands, hydrodynamic devices (e.g., continuous deflection separation (CDS) units), microfiltration, UV treatment, and screening/settlement. Samples were collected both before and after the BMP treatment processes in order to evaluate the effectiveness of each BMP system. Dry-weather and wet-weather samples were tested for toxicity using the Ceriodaphnia dubia survival and reproduction test, as well as the echinoderm fertilization test. All samples were analyzed for total and dissolved metals, while most were also analyzed for organophosphate (OP) pesticides, pyrethroid pesticides, and glyphosate. Variation in the constituents among sites reflects contributed data from multiple monitoring programs.

Seven BMP sampling locations in Los Angeles and Orange Counties.


Significant findings of this study were as follows:

• The two wetland BMPs (Wet CAT and OCWD) were effective in reducing toxicity, and had the best overall combination of toxicity and contaminant reduction. The toxicity was consistently reduced, although not completely removed in all cases, by both wetland systems. Dissolved salt concentrations at the Wet CAT site inhibited the C. dubia survival and reproduction test.

• In general, the hydrodynamic devices (CDS units) had no effect on toxicity. This was not surprising, as CDS units were designed to remove solids from runoff and exhibited little effect on the dissolved metals in this study (Dissolved metals are the forms most likely to cause water column toxicity.)

• At the L.A. metal recycling yard, outflow samples from the screening/settlement device were usually quite toxic, although toxicity was often slightly decreased following treatment. This slight decrease was not consistent enough to indicate that the BMP apparatus was able to affect toxicity. The screening/settlement sump was also inconsistent in reducing most metals and total suspended solids (TSS).

• The toxicity data for the samples from the SMURRF (screening/hydrodynamic device/microfiltration/UV treatment) site could not be used to evaluate toxicity removal effectiveness due to the influence of chlorinated water used to clean the screens. The SMURFF site did consistently reduce concentrations of most total metals and TSS.


Los Angeles and San Gabriel Rivers Watershed Council
Los Angeles Department of Public Works
Orange County Water District
Santa Monica Baykeeper
City of Santa Monica

For more information on Assessment of Best Management Practice (BMP) Effectiveness for Reducing Toxicity in Urban Runoff, contact Jeff Brown at (714) 755-3226.
This page was last updated on: 8/30/2011