THE UNIQUE GEOLOGY AND GEOCHEMISTRY OF MONTEZUMA WELL, A NATURAL SPRING AT MONTEZUMA CASTLE NATIONAL MONUMENT, CENTRAL ARIZONA

The National Park Service requested a geologic and geochemical study by the U.S. Geological Survey to better understand the source(s) of groundwater to Montezuma Well (a natural spring) in Montezuma Castle National Monument, central Arizona. The results of this study indicate how a unique combination of geology and geochemistry creates the features found at Montezuma Well. The majority of groundwater recharge occurs in the topographically high area of the Mogollon Rim with elevations greater than 7,000 feet; whereas, the elevation of the water surface at Montezuma Well is 3,560 feet. Recharge through the Mogollon Rim follows fractures in the cover basalt and the underlying Permian sandstones and then flows rapidly through the karstic Redwall Limestone at depth. Geologic inferences indicate the presence of a basalt dike underneath Montezuma Well that affects groundwater flow and geochemistry. This basalt dike appears to be a barrier for regional groundwater flow and a locus for a component of deep-seated groundwater flowing upward along bedrock fractures (hence the formation and unique geochemistry of Montezuma Well). The presence of this basalt dike and deep fracture system forces the groundwater flowing at depth (< 750 feet) to the surface, resulting in groundwater discharge at Montezuma Well. This fracture system appears to contribute a small amount of brine related to volcanic degassing (confirmed using helium isotopes). This brine contains carbon dioxide, salts, and trace elements such as arsenic, which mixes with water in the main groundwater system (depths < 750 feet) during discharge. The proportion of brine waters is probably quite small. The increased carbon dioxide dissolves limestone at depth, which subsequently helps to maintain open fractures and probably created the cavity that forms Montezuma Well. The water within Montezuma Well has high concentrations of carbon dioxide, calcium, and alkalinity. Subsequent degassing of the carbon dioxide at the surface causes travertine (calcium carbonate) deposition. This deposition currently occurs in prehistoric and historic irrigation ditches transporting water away from Montezuma Well. Similar processes in the past have created extensive travertine deposits in the surrounding area.