A high groundwater table across much of the Big Mesa in Bolinas presents a barrier for homeowners wishing to upgrade their septic systems—a necessary and an expensive step in creating an accessory dwelling unit.
Now, the Bolinas Community Land Trust has permission from the county to launch a pilot program designed to help 22 homeowners make septic upgrades, enabling the creation of accessory dwelling units deed-restricted for affordable housing.
“This is a very exciting project,” said Tom Lai, the acting director of the Community Development Agency. “Multi-years in the making, this is really a partnership between the county, the land trust and the public utilities district. To make it work, we just have to keep in mind that our ultimate goal here is to promote the development of accessory dwelling units in the Bolinas pilot area.”
The program will employ a lesser-used system that has both a pre-treatment tank and a peat moss biofilter. Together, those components purify the waste before discharging it to a bed of gravel and sand below. The trust will provide low-interest loans to participating homeowners and install the systems at once, lowering costs.
The San Francisco Water Quality Control Board has signed a waiver for the project, allowing the systems to be installed two feet—as opposed to the standard three feet—above groundwater. The Bolinas Community Public Utility District board, which plans to ration water next month, supports the project and is expected to sign off on it.
Although brand-new septic systems typically must meet the criteria of a Class I system, the water board has allowed for the installation of the peat moss systems, which are categorized as Class II. Both Class I and II systems are above ground and employ pre-treatment, but the first class typically uses more conventional infrastructure and needs a higher clearance.
The waiver will also allow for the systems to serve an additional 800 square feet; typically, the county only permits homeowners to add 500 square feet with a Class II system.
The new systems cost an estimated $45,000 each, around half the price of installing a brand-new Class I system. The trust plans to provide low-interest loans to the participating homeowners, and will manage the systems, providing any funds needed for upkeep. Property owners will pay an annual fee of approximately $650, which includes an annual inspection fee and all repairs.
Gwen Baert, a senior environmental specialist for Marin, said that most Bolinas homes built in ‘70s and ‘80s likely have Class III or Class IV systems, meaning they are built below ground level and have no pre-treatment element. She said local homeowners were likely dealing with the area’s high groundwater by pumping their systems more regularly to prevent seasonal failures. Although there are several ways for a system to fail, Ms. Baert said the most common way is for septic content to breach the surface.
“We will be bringing these systems up from a malfunctioning status to a functioning status, where we are protecting the groundwater quality also,” she said. “And then there’s the need for affordable housing. It’s a win-win.”
While the county has not conducted a comprehensive study of the seasonal groundwater table on the Bolinas Mesa, Ms. Baert said contamination from septic was certain. (The seasonal groundwater table is separate from the deeper groundwater table that would affect drinking water.)
Recent samples taken through Marin’s water quality monitoring program drawn from a site on Bolinas Beach meet state standards. Heal the Bay, a nonprofit that issues annual report cards, gave Bolinas Beach an A for water quality in the summer of 2020 and an A+ after wet weather.
Not everyone who participates in the pilot program has to install an accessory dwelling unit, though they will be encouraged. Across West Marin, where multi-family zoning is scarce, land trusts are focused on the development of second units—called ADUs—and junior accessory dwelling units, or JADUs. Without counting toward density, these units create opportunities for three homes on one lot.
“There isn’t more land in Bolinas—everything is maxed out,” said Arianne Dar, the executive director of the Bolinas land trust. “The ADUs will allow for someone to live on site full time, even if the property is not used full time.”
Ms. Dar said she is applying for funding to support the program and hopes to begin installation as early as this spring. The land trust put out a call for interested homeowners this week, and will conduct site visits to find properties that are a good fit.
Ms. Dar could not estimate how many second units exist in Bolinas, given the fact that many were built without permits. In 2018, the trust projected that it would take 50 new units—including housing options aside from new ADUs—to stabilize the housing crisis; the development of 34 units is underway.