ABSTRACTHexavalent chromium (Cr+6) of natural origin has been found in the ground water in Paradise Valley, Maricopa County, Arizona. The concentrations exceed 0.05 milligrams per liter, the U.S. Public Health Service's upper limit for Cr+6.Paradise Valley is a northwest‐trending, down‐faulted basin of about 225 square miles that is located in the semi‐arid Basin and Range physiographic province. The accumulation of fine‐grained sediments (in part gypsiferous) near the center of the basin indicates that the material was deposited in a playa‐type sedimentary environment.The Cr+6is present in the ground water in a 75‐square mile area and forms an elongate northwest‐trending pattern in the southern part of the basin. The largest Cr+6concentrations are located in the approximate center of this pattern and decrease toward the margins. The Cr+6distribution in the ground water corresponds to the location of the fine‐grained part of the alluvial deposits; the highest Cr+6concentrations are associated with the finest‐grained sediments. Evidence indicates that the Cr+6is fairly evenly distributed vertically in the ground water, at least throughout the finer‐grained part of the aquifer system.Thermodynamic calculations and Eh and pH measurements of the ground water indicate that the water containing Cr+6is oxidizing and alkaline enough to account for the observed concentrations. An alkaline environment, largely a result of primary silicate hydrolysis in the fine‐grained sediments, allows chemical oxidation of the trivalent chromi