Hollister Water Hardness & Quality Report (2026)

Sunnyslope County Water District and the City of Hollister provide drinking water to Hollister, California, in San Benito County, serving a population of approximately 36,318 residents. The supply blends groundwater drawn from the San Benito groundwater basin with purchased surface water imported via the Central Valley Project. Treatment occurs at the West Hills Water Treatment Plant and Lessalt Water Treatment Plant, where well water is mixed with imported surface water before distribution. Sunnyslope reports overall hardness at 120–180 mg/L, rising to about 250 mg/L in summer, while the City of Hollister ranges from 79 to 563 ppm.

The primary aquifer is the San Benito groundwater basin, a closed-loop system in San Benito County whose configuration limits outflow except during major storm events when the San Benito River connects to the Pajaro River. Local rock formations and soil contribute high mineral content to groundwater, historically resulting in a hard supply. Imported surface water from Central Valley reservoirs carries a lower mineral content typical of California rivers and lakes; blending these sources has moderated the overall hardness profile compared to periods when pure groundwater dominated, yielding a moderately hard to hard overall character.

At hard to moderately hard levels, scale builds on pipes, faucets, and appliances, potentially clogging plumbing and leaving spots on glassware; water heaters, dishwashers, and washing machines are most impacted over time. Regular vinegar descaling helps mitigate buildup. Notably, new salt- or potassium-based water softener installations are banned in both the Sunnyslope and City of Hollister service areas to protect the groundwater basin from added salinity. Surface water blending has improved overall water quality, and treatment at local plants includes processing imported surface water mixed with well water to aid aquifer protection.