McKinleyville Water Hardness & Quality Report (2026)

McKinleyville, California, in Humboldt County — an unincorporated Humboldt County community adjacent to Arcata and Eureka on the north California coast — receives its water from the McKinleyville Community Services District, drawing from the Mad River (Humboldt County) through the north California coast water treatment system.

The very soft 31.5 mg/L hardness and low TDS of 64.5 mg/L reflect the north California coast Mad River supply's nearly insoluble Franciscan melange character — among the softest northern California coast communities. The Mad River at Humboldt County drains the North Coast Ranges — Jurassic Franciscan Complex melange (graywacke, chert, and serpentinite — predominantly insoluble), Mesozoic Yager Formation (slightly calcareous turbidite — minor hardness contributor), and Pleistocene Mad River alluvial terraces (slightly calcareous). The predominantly insoluble Franciscan Complex and high coastal rainfall dilution produce the very soft McKinleyville supply.

At 31.5 mg/L, McKinleyville's water is very soft — essentially no scale forms, soap lathers well, and appliances maintain excellent efficiency. Annual descaling is sufficient. The PFAS level of 3.0 ppt warrants a certified drinking water filter — the Humboldt County north California coast corridor has limited major AFFF military PFAS sources, and the Mad River watershed is relatively pristine.