Saltwater intrusion at North Marin wells reaches historic high

08/26/2020

Salt levels are unprecedented in the drinking water pulled from North Marin Water District’s two wells in Point Reyes Station, following a year of little rainfall. The district has struggled for decades with periodic and seasonal salinity intrusion resulting from the wells’ proximity to Tomales Bay, but the problem is especially dire this summer as freshwater becomes scarce. There is little health concern, but the district warned of a change in the water’s taste. “We’ve always had some salinity intrusion at these wells,” said Pablo Ramudo, the district’s water quality supervisor. “There’s a strong tidal influence in nearby Lagunitas Creek, and when the tide is high, the water that is charging the aquifer can be from the seawater.” The problem worsens during the dry months when there’s less freshwater in the creek to supply the aquifer, and it is becoming more problematic over time. Mr. Ramudo says that trend could be a combination of changed hydraulics that resulted from the Giacomini Wetlands restoration over 10 years ago and rising sea levels. Four years ago, the salinity started increasing about 50 percent a year. This year, several components of salinity showed even greater increases: the sodium rose by 70 percent from last year, chloride by 50 percent, and bromide by 62 percent. Those percentages will likely keep rising as the dry season wears on, though Mr. Ramudo said the water would remain drinkable for the foreseeable future; salinity itself is considered a secondary drinking water standard, meaning that it only affects cosmetic and aesthetic components like taste. To ameliorate the issue, North Marin is maximizing the use of its third well, which sits two miles up the Petaluma-Point Reyes Road on the Gallagher Ranch, far from the bay’s reach. The well, which the district began using in 2015, is able to provide all of the water for the West Marin areas served by the district during the winter months. But in July, it met only 38 percent of demand. (Users have brought usage down 15 percent since the district asked for voluntary reductions this summer.) The district’s long-term strategy is to dig a second well on the Gallagher Ranch, where a site investigation is underway. In a recent notice to its customers, the district explained the implications of higher salinity for those drinking its water in the meantime: “While there is no direct health concern from the salt for most people at this concentration, it does affect the taste. Customers that may be on sodium restricted diets should consult their physicians to see if the additional sodium is a concern for them.” Bruce Fox, a Point Reyes Station resident, noticed the change. “It’s a little bitter, a little off, in a way I can’t pin down,” he said. “I wouldn’t say it’s bothering me, but it’s something I noticed. It’s not keeping me from drinking the water, but it’s tasting a little different, and I hadn’t thought of why.” For now, the district is managing the water pulled from the two wells differently. The primary concern is the rising levels of bromide, which can contribute to the formation of disinfection byproducts. The byproducts, regulated by the Environmental Protection Agency because they are considered a health hazard in high concentrations, form when the chlorine used to treat drinking water reacts with organic material. To keep the byproducts at safe levels, North Marin is closely monitoring for the presence of bacteria in the water, which takes additional time and resources, so that it can add the minimum amount of chlorine necessary and thereby keep disinfection byproducts low. It is also hoping to use the water pulled from the wells more quickly, minimizing the time spent in storage tanks and the amount of chlorine that must be added. If disinfection byproducts rise to an unsafe level, the district said it would alert customers and consult with the California Division of Drinking Water. “We have to think of it in the terms of short-term strategies and long-term strategies,” Mr. Ramudo said. “We’re managing the amount of chlorine and reducing the water age, but in the long-term, we are unsure what the final concentration of salinity is going to be in that aquifer. There could be a point where that water will no longer be usable.”