First Flush of Urban Runoff

NERD ALERT: The vast majority of San Diego’s inland water pollution problems are caused by non-point source sources. As opposed to a factory which discharges pollutants from a single pipe (point source), non-point sources come from many areas at once and are generally more diffuse. Our stormwater system is one example of non-point source pollution. Water from a large area is collected when we overwater our lawns, or clean our sidewalks with a hose, or from just about everywhere during a rain event.

Urban runoff is the single biggest threat to healthy waters in San Diego.

During the long dry periods in San Diego, pollutants collect on the ground. Nutrients from wayward fertilizer applications, oil from leaky cars and copper from brake pads are examples. These pollutants are spread very thinly over a very large area and accumulate over time. When it rains, all this pollution get flushed down the stormwater system and into our rivers and eventually out into the ocean. We call the peak we see after a relatively long dry period a “first flush” event.

Take a look at this graph created by Weston Solutions, a local consultant group, made of copper concentrations in a creek during a storm event:StormCopper

Copper concentrations rise along with the rise in water flow, until it reaches a peak. We then see a steep decline. What’s happening here? The first bit of rain that falls flushes copper and moves it quickly to the creek. Levels quickly drop as the streets are washed clean and the pollution flows to the ocean.

You can read the whole report here.

Almost every single pollutant will show this trend. Look at the picture below taken in the San Luis Rey watershed after a rain event. You can see the pollution heading into the ocean. Dissolved metals, bacteria, sediment and even trash follow this same pattern. Be sure to read Alicia’s blog post to learn how this phenomenon affects our marine debris problem.

girlschasesglobedotcomSome of this pollution has serious health risks associated with them. This is why all of our beaches have advisories issued after a rain event. Since the county does not have the resources to post warnings after every rain event, be sure to check our beach status page before heading out into the ocean. We update this information as soon as the county releases updates. I check every time I go in.

How can we minimize problems associated with urban runoff?

Do you have any other clever ideas? Leave them in the comments.