Manteca Water Hardness & Quality Report (2026)

Manteca, California, in San Joaquin County in the northern San Joaquin Valley — a rapidly growing Central Valley community on I-5 near Stockton — draws its municipal water supply from the South San Joaquin Irrigation District (SSJID) water deliveries (Stanislaus River–New Melones Reservoir source) and local San Joaquin Valley groundwater wells via the City of Manteca Water Division. The SSJID delivers Stanislaus River water to a large portion of the northern San Joaquin Valley. Water hardness in Manteca measures 103.5 mg/L — classified as moderately hard.

Manteca's moderate hardness reflects the northern San Joaquin Valley's water blend from the Stanislaus River and local alluvial groundwater. The Stanislaus River at New Melones Reservoir (Calaveras and Tuolumne Counties) originates in the Sierra Nevada (Cretaceous granodiorite of the Summit Ranger District and the Mother Lode metamorphic belt — Jurassic phyllite and slate, Calaveras Complex). The Stanislaus River source is moderately soft. However, delivery through SSJID irrigation canals into the flat San Joaquin Valley floor introduces concentration through evapotranspiration and canal contact with calcareous alluvial soils. Local San Joaquin Valley alluvial groundwater wells contribute additional dissolved minerals from the Quaternary alluvial fan deposits. The blend produces the 103.5 mg/L moderate hardness in Manteca.

At 103.5 mg/L, Manteca residents encounter moderate scale accumulation. Faucet aerators and showerheads develop deposits after several months — monthly cleaning with citric acid solution is practical maintenance. City of Manteca Water Division and SSJID consistently deliver water meeting all California SWRCB and EPA Safe Drinking Water Act requirements.