Winding westward along Marin County’s northern border, San Antonio Creek encompasses about 20 percent of the Petaluma River watershed. While the state has continuously designated the main stem of the Petaluma River a contaminated water body due to excessive levels of bacteria tied to fecal matter since 1975, San Antonio Creek, a tributary to the river, has gone unaffected by the river’s bacteria problem. Until now.
A state water oversight board may pass a plan laying out the steps to lower the levels of bacteria in the river and its watershed, including the San Antonio Creek. The federal Clean Water Act for contaminated water bodies requires the state to create a plan, known as a Total Maximum Daily Load [TMDL].
At a Wednesday, Nov. 13 meeting in Oakland, the San Francisco Bay Regional Water Quality Control Board will consider approving an amendment to the board’s water quality control plan for the region, a document known as a basin plan. The proposed amendment will enact the TMDL—a limit for how much fecal indicator bacteria can be found in a waterbody—and identify actions required to reach that goal.
Scientists working for the water board, one of nine similar regional bodies tasked with setting water quality rules in California, have been assembling the plan for several years, according to Farhad Ghodrati, an environmental scientist with the San Francisco Bay board.
Bacteria has impared the main stem of the Petaluma River since 1975, but to date, the San Antonio Creek has escaped the regrettable distinction. If the water board passes the proposed amendment next week, they will add San Antonio Creek to the state’s list of water bodies burdened with excessive levels of fecal bacteria.
“The testing we did as part of this TMDL development showed that the bacteria levels in all tributaries, including San Antonio Creek, were well above the impairment threshold level,” Ghodrati, the state scientist, told the Pacific Sun.
Although there are over 100 potentially dangerous bacteria related to fecal matter, water scientists generally only test for a few varieties. These “fecal indicator bacteria,” including E. Coli, are a sign that animal waste has contaminated the water body. If those levels are above the bar set by the water quality control board, they will add the water body to a list of “impaired” waterways, as required by the federal Clean Water Act.
“High FIB levels indicate the presence of pathogenic organisms that are found in warm-blooded animal (e.g., human, cow, horse, dog, etc.) waste and pose potential health risks to people who recreate in contaminated waters,” a report prepared by water board staff states.
Multiple tests for traces of E. Coli between winter 2015 and summer 2016 across 16 testing stations in the Petaluma River watershed revealed levels far in excess of water board requirements.
Water board rules allow for the discovery of excessive levels of E. Coli in less than 10 percent of samples, but tests in the Petaluma River watershed showed excessive levels in 65 to 100 percent of samples in a series of six tests conducted over 18 months.
“This result shows that the magnitude of impairment in the river is pretty significant, and some of the highest concentrations we have seen in the region,” Ghodrati said of the E. Coli results.
While many strains of E. Coli are harmless, others can cause health problems, including diarrhea and vomiting, according to the Centers for Disease Control and Prevention.
There are many potential sources for the excessive fecal matter throughout the Petaluma River Watershed. The two largest are agricultural uses and various human sources, including city and county sewer systems, private septic tanks, boats on the river and homeless encampments. For each of the sources, the water board recommends actions to reduce bacteria levels.
Five years after instituting the TMDL, water board officials will test the waters within the Petaluma River Watershed again and consider their options.
The culprits in San Antonio Creek watershed may include owners of on-site sewage management systems—such as septic tanks—and agricultural uses, including cows and horses.
Three of the San Antonio Creek testing sites “are located downstream of several horse facilities in the rural areas of the watershed and showed the highest concentrations of horse markers,” the report notes.
Marin County Supervisor Judy Arnold was on vacation and unavailable to comment on the possible new status of the creek in her district.