Well to mitigate salinity intrusion


North Marin Water District has struggled with salinity issues in its wells for decades because of their proximity to Tomales Bay. This month, it finally secured a $1.5 million grant to help remedy the problem by incorporating a new well into the system. The agency has observed salinity problems at its two wells, which draw groundwater influenced by the bay during high tides, since the 1970s; it copes by timing pumping to avoid high tides, but dry weather in recent years has aggravated the problem since rainfalls help flush unwelcome saltwater. Salinity itself is considered a secondary drinking water standard, meaning that it only affects cosmetic and aesthetic components like taste. But particulates from saltwater can also contribute to the creation of disinfection byproducts; these byproducts, which can form when the chlorine used to treat drinking water latches onto organic material, are regulated by the Environmental Protection Agency because in high concentrations they are considered a health hazard. Although the district has never formally exceeded those limits, because compliance is measured with a yearly average there have been occasional short-term spikes. Pablo Ramudo, the district’s water quality supervisor, said sodium levels have exceeded limits at some point every year for the past few years, typically during the summer. The grant will fund the construction of a pipe to connect the system to an existing well that sits out of the bay’s salty reach, two miles up Point Reyes-Petaluma Road on the Gallagher ranch. The Gallagher well, which has sat idle for years, doesn’t have the capacity to supply all of the district’s demand, so it will be mixed with water from the other two wells. The grant will cover the entire cost of the project, so none of the roughly 700 customers in West Marin will be asked to contribute. The district is currently soliciting bids for the work, and construction should begin in late spring or early summer and finish up by fall.