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Last Updated: Thursday, January 5, 2012 1:08 PM

First Flush Study

The Caltrans First Flush Characterization Study is measuring the pollutants that could get flushed into the storm drain system during the initial part of a rainstorm, usually within the first hour of rainfall.

First flush refers to the first significant rainstorm of a season, as well as the first portion of each rainstorm, when enough rainfall accumulates to collect samples. Relatively high concentrations of pollutants may be flushed into storm drains during a first flush. This study evaluates the amount and concentration of litter, bacteria, and chemical constituents that may be transported from Caltrans highway storm drain facilities during the first flush of a storm event.

Caltrans can use data from this study to assist in designing Best Management Practices (BMPs) to reduce pollutants and improve the quality of stormwater runoff. Proper capture of major first flush pollutants may reduce the size of structural BMPs.

Elements of the First Flush Study


Weather Forecasting:
Accurate weather forecasting is an important aspect in preparing for the intensive sampling needed for this study. Researchers work with meteorologists to predict when a storm will begin so they will be ready with sampling equipment to catch the first flush runoff as a storm begins.




Stormwater Sampling:
Up to eight storms may be sampled each rainy season at three coastal and three inland monitoring sites in the Los Angeles region. Researchers collect runoff throughout a storm, sampling every 15 minutes the first hour, then once every hour for the remainder of the storm event. More than 12 grab samples are collected during each storm following Caltrans sampling protocols. Samples are also collected by automatic samplers for comparison.

Water Quality Analysis: Runoff samples are analyzed at an independent laboratory to determine what chemicals are present. Key water quality constituents evaluated for this study include metals (total recoverable and dissolved), nutrients (phosphorus and nitrogen), conventional water quality parameters, oil and grease, and coliform bacteria.
Early Results: Initial results have confirmed a first flush phenomenon for most water quality constituents. High concentrations were generally observed in the first set of stormwater grab samples collected, followed by a significant drop in concentrations after two hours. This effect was especially evident for constituents with the highest relative concentrations (lead and zinc).
First flush pollutant concentrations are relatively high at the beginning of the storm and drop off over time, as shown in this pollutograph.

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FOR MORE INFORMATION CONTACT

Division of Environmental Analysis, Stormwater Unit
California Department of Transportation
P.O. Box 942874, MS-27
Sacramento, CA 94274-0001
Email: Division of Environmental Analysis, Stormwater Unit