As a former frequent kayaker of the San Francisco Bay, I was surprised to read that maybe agriculture from the Delta isn't primarily to blame for elevated levels of mercury and other pollutants in those waters. But as someone who's been paying attention to low impact development efforts in southern California lately, the fact that a decade's worth of data points to urban runoff as a culprit as large or larger than ag made me nod my head.
I'm talking about work from the San Francisco Estuary Institute: SFEI released its Pulse of the Estuary report a few days ago. It shows the depth of knowledge for the SF bay's runoff problem has grown in nearly-staggering fashion.
Ten years ago scientists thought about a quarter of pollution in the bay came from sediments and urban runoff. Now they believe that it's double that - more than 50 percent. Correspondingly, farms now are getting blamed for far less of the pollution in the bay - even though the Central Valley itself is an enormous watershed, and almost all the freshwater coming into the bay comes from there.
(All kinds of things could be going on, btw. Ag could be managing its pollution runoff more responsibly either because water is so expensive or because rules exist. Or the scientists studying this could simply have been wrong a decade ago about relative responsibility. One thing the report asserts has happened is that sediment speeding into the bay has slowed dramatically over the last 10 years.)
Runoff rules in the San Francisco Bay Area are regional - and in fact last year water regulators there consolidated six municipal permits into one great big permit - this report suggests that's good because it integrates the region into one problem and set of solutions. It's interesting to see the Bay - whose population may be nearly 8 million in 10 years - reaping the results of rapid growth too.
Lastly, the report provides a specific example of measured results from low impact development - in Daly City, of all places.
Four rain gardens (bioretention areas) and one bioswale were installed to treat runoff from the parking lot, as well as tennis and basketball courts and a popular community area with an average of over 20,000 visitors per month. The rain gardens and bioswale, with a total surface area of approximately 4% of the entire four-acre runoff area, effectively trapped contaminants during the first year after installation.
Monitoring prior to rain garden installation showed contaminant concentrations in runoff that were between five (total mercury) and 88 times (PAH) higher than the average concentrations in Central San Francisco Bay. During 2009/10, con- sidered to be a wet year, runoff from most storms (88%) was successfully filtered through the rain gardens and bioswale before it was discharged into a tributary of Colma Creek. Preliminary results suggest that contaminant loads were reduced between 50% and 80% for mercury, copper, cadmium, nickel, lead, zinc, PCBs, and PAHs when water was captured by the LID treatment measures. Twelve percent of the storms exceeded the capacity of the rain gardens and bioswale and were bypassed into the storm drain to avoid flooding. (emphases mine)
Check the entire report out here.