The National Park Service says the source of the water in Montezuma's Well, a flowing sinkhole in Arizona's Montezuma Castle National Monument, is poorly known.
A new report from the USGS notes:
Water emerges from the middle limestone facies of the lacustrine Verde Formation, but the precise origin of the water and its travel path are largely unknown. Some have proposed artesian flow to Montezuma Well through the Supai Formation, which is exposed along the eastern margin of the Verde Valley and underlies the Verde Formation. The groundwater recharge zone likely lies above the floor of the Verde Valley somewhere to the north or east of Montezuma Well, where precipitation is more abundant.But the report concludes that more info is needed to test these conceptual ideas.
Ref: Johnson, R.H., DeWitt, Ed, Wirt, Laurie, Arnold, L.R., and Horton, J.D., 2011, Water and rock geochemistry, geologic cross sections, geochemical modeling, and groundwater flow modeling for identifying the source of groundwater to Montezuma Well, a natural spring in central Arizona: U.S. Geological Survey Open-File Report 2011–1063, 62 p.