The ocean and Bolinas Lagoon are the receiving surface water bodies for groundwater in Stinson
Groundwater flow from the oceanside portions of Stinson Beach, Old Town, and Highlands areas
generally is toward the ocean.
Groundwater flow from the remaining portions of Seadrift, Calles, and Patios is toward Bolinas
Lagoon. Easkoot Creek intercepts groundwater and conveys it to Bolinas Lagoon.
Groundwater in the Seadrift, Calles, and Patios areas is not a viable drinking water supply because
of the influence of seawater.
Wastewater disposal is influencing groundwater quality in the Seadrift, Calles, and Patios areas.
The concentration of wastewater indicators, such as MBAS and nitrogen, generally decreases with
increasing distance along the groundwater flow path.
Detections of fecal coliform were confined to two wells, suggesting that fecal contamination of
groundwater is limited and localized.
Sampling and analysis of Seadrift Lagoon, Bolinas Lagoon, and Easkoot Creek water during this
study indicated that the most significant impacts of wastewater on surface water occur in Easkoot
Creek, which showed consistent high levels of fecal contamination.
Nitrate loading calculations for the Seadrift, Calles, and Patios areas predicts that current
wastewater disposal practices would result in nitrate concentrations in groundwater that are
generally within the drinking water standard of 10 mg/L.
According to nitrate loading calculations under buildout conditions, nitrate loading would increase,
but could remain within drinking water standards if the current practice of utilizing of sand filter
systems is continued.
Nitrate loading calculations demonstrate that the preponderance of nitrate in groundwater is the
result of wastewater disposal, and that landscape irrigation is a minor factor.
A policy regulating fertilizer use for landscaping is not warranted.
Wastewater disposal effects on groundwater in the Seadrift, Calles, and Patios areas do not pose a
threat to human health, and appear to have only a small influence on beneficial uses in Bolinas
Lagoon. Accordingly, no technical basis exists to support the building moratorium.
The impact on Bolinas Lagoon of Easkoot Creek, reflecting drainage from the Highlands and Old
Town, appears to be more significant.
The focus of future water quality monitoring and protection should be the Highlands, Old Town,
and Stinson Beach Park areas and Easkoot Creek.
The District's sand filter monitoring study has shown good to excellent performance for systems
installed under the current sand filter design criteria.
Review of the sand filter monitoring data reveals no compelling reason to change the current
design criteria; nonetheless, innovative technologies and practices exist that can improve onsite
wastewater system performance.
Onsite wastewater system monitoring can be improved through use of data recording control
panels and expansion of reporting requirements for septic tank maintenance work.
A community STEP system may be warranted to serve the Old Town area, as well as portions of
the Highlands, Calles, and Patios. The most promising site exists in the vicinity of the Highlands
Tank on federal park land.