Arcata Water Hardness & Quality Report (2026)

Arcata, California, in Humboldt County — a Humboldt County city adjacent to Eureka and McKinleyville on Humboldt Bay in the Redwood Coast region of northern California, home of Cal Poly Humboldt — receives its municipal water from the Arcata Community Services District, drawing from the Mad River (Humboldt County) and supplemented by local groundwater wells in the Arcata Bottom.

The moderately hard 127.5 mg/L hardness and TDS of 350.4 mg/L are harder than expected for a Franciscan Complex-draining Coast Range supply (typically very soft), suggesting the Arcata mixed supply includes a harder groundwater component. The Mad River supply drains the Northern Coast Ranges — Jurassic Franciscan Complex (highly deformed, insoluble mafic-siliceous melange), producing soft river water. However, the Arcata Bottom aquifer (Pleistocene alluvial fan-terrace) can be moderately hard due to calcareous marine terrace deposits, explaining the elevated mixed-supply hardness.

At 127.5 mg/L, Arcata's water is moderately hard — scale builds in kettles and appliances over months, dishwashers benefit from rinse aid, and faucet aerators need periodic cleaning. Quarterly descaling is appropriate. The PFAS level of 5.8 ppt warrants a certified drinking water filter — the Humboldt Bay National Wildlife Refuge area, Arcata Airport (AFFF), and the north California coastal military and industrial PFAS background contribute to Arcata's readings.