San Luis Water Hardness & Quality Report (2026)

San Luis, Arizona, in Yuma County — a major southwest Arizona US-Mexico border city (San Luis is one of the most important US-Mexico border communities in Arizona — located directly on the Arizona-Sonora border across from San Luis Río Colorado, Mexico, in the Colorado River Delta region; San Luis is the largest US city in the lower Colorado River Delta area and has grown rapidly driven by cross-border commerce and immigration), a diverse Yuma County community with a significant Mexican-American majority population (San Luis is one of the most heavily Hispanic cities in the United States — over 98% Hispanic or Latino, reflecting its close cultural ties with San Luis Río Colorado, Mexico), adjacent to the Yuma agricultural region (one of the most productive agricultural areas in the United States — the Yuma area produces the majority of US winter lettuce and other winter vegetables), and a community with deep ties to both Arizona and Sonora — draws its municipal water supply from the Colorado River via the City of San Luis Water Division. Water hardness in San Luis measures 226 mg/L — classified as very hard.

San Luis's very hard supply reflects the Colorado River's highly calcareous character in the lower Arizona basin. The Colorado River at San Luis–Yuma County carries mineral-rich water from the entire Colorado Plateau and Rocky Mountains, including the Tertiary evaporite and calcareous alluvium of the lower river. Limited softening produces the very hard 226 mg/L.

At 226 mg/L, San Luis residents face significant hard water challenges. Regular water softeners and descaling are strongly recommended. City of San Luis Water Division consistently delivers water meeting all Arizona ADEQ and EPA Safe Drinking Water Act requirements.