ADEQ's newly released study on Ambient Groundwater Quality of the Agua Fria Basin found that 30% of the 46 sites sampled "had concentrations of at least one constituent that exceeded a health-based, federal, or State water quality standard."
A sample from a well near Black Canyon City had an arsenic concentration of 2.25 mg/L, one of the highest arsenic concentrations ever found in groundwater in Arizona. [right]
The report said the cause of the elevated arsenic concentrations is uncertain, but noted that in Arizona "such conditions are often associated with clay-ric sediments, volcanic rocks, geothermal environments, and/or areas with gold deposits." All of those rock types are present in the area, so the natural presence of arsenic is not a surprise.