San Anselmo Water Hardness & Quality Report (2026)

San Anselmo receives its water from the Marin Municipal Water District (MMWD), which supplies communities throughout Marin County. The primary water sources are surface water drawn from seven local reservoirs: Alpine, Bon Tempe, Kent, Lagunitas, Nicasio, Phoenix, and Soulajule Lakes, all situated within the Mt. Tamalpais watershed. An additional supply comes from Lake Henshaw in San Diego County, alongside treated imported water arriving from the Sacramento-San Joaquin Delta. Water treatment is handled by two facilities: the Fairfax Treatment Plant and MMWD's Stafford Treatment Plant. These plants serve the approximately 12,700 residents within San Anselmo's 2.6 square mile service area.

The area's geology is largely defined by the Franciscan Complex melange, a collection of highly sheared sedimentary and volcanic rocks dating back to the Mesozoic era. While there are some limited areas where serpentinite and radiolarian chert appear, the region lacks significant carbonate aquifers. Instead, any groundwater present is typically found in fractured bedrock aquifers. This geological makeup, characterized by non-calcareous rocks, means the water flowing from the reservoirs is naturally very soft. The minimal mineral pickup occurs as water infiltrates through the acidic soils and granitic terrains found upstream in the watershed.

Because San Anselmo's water is remarkably soft, homeowners rarely encounter significant scale buildup on their plumbing fixtures, pipes, or appliances. You'll notice faucets remain cleaner, free from tell-tale white deposits, and appliances like dishwashers and water heaters face a reduced risk of mineral encrustation. A water softener isn't necessary and isn't recommended, as over-softening could unnecessarily introduce sodium into your supply. Basic cleaning is usually sufficient for maintenance, and the water's soft nature even means you can use less detergent for laundry and bathing. While the water meets federal standards, recent analyses show two contaminants exceeding EPA health guidelines, prompting advice for vulnerable groups to consider a certified filter.