Groundwater Study Helps Protect Montezuma Well

Water plays a central role in human development and resource protection in America's arid west. The National Park Service (NPS) and its local communities need accurate scientific information to know how to balance the requirements of humans and nature. A recent study by scientists from the U.S. Geological Survey (USGS) contributes knowledge that can help achieve that balance.

Montezuma Castle National Monument in the Verde Valley of central Arizona contains a prominent water feature known as Montezuma Well. This natural spring is a collapsed travertine dome partially filled with water that rises from deep in the ground and exits through a swallet (a dissolved channel in travertine). Even in times of drought, about 1.6 million gallons (6 million l) of water flow through four vents at the well's bottom each day, remaining at a constant temperature of 75.2°F (24°C). High levels of dissolved carbon dioxide, in addition to high levels of arsenic, calcium, and other chemicals, distinguish the groundwater in the well from shallow groundwater in the surrounding area. The chemical makeup prohibits fish from living in the deep well, but other life thrives. This oasis in the desert has significant natural value as a home to endemic species as well as a rich cultural history with centuries of human use. Until now, however, no one was sure where the water originated.

The National Park Service needs to understand the sources and flow paths of groundwater in order to protect Montezuma Well. To achieve this goal, the NPS requested a geologic and geochemical study by the USGS. The research involved two stages. The first stage identified the flow paths for groundwater that supplies Montezuma Well and the surrounding region. Chemical and isotope analyses of groundwater samples and rock samples provided the clues needed to map the flow of water. Scientists then constructed geologic cross-sections and prepared a conceptual model of groundwater flow to the well. The second stage of the research determined the source of excess carbon dioxide, confirmed the geochemical reactions along the groundwater flow path, and compared the unique geochemistry of water in and around the well to existing data about the watershed.

The USGS study found that the majority of groundwater recharge for Montezuma Well occurs in the topographically high area of the Mongollon Rim with elevations greater than 7,000 feet (2,134 m). The elevation of Montezuma Well is 3,560 feet (1,085 m). Water flowing through the Mongollon Rim follows fractures in the cover basalt and the underlying Permian sandstones, and then flows quickly through the karstic Redwall Limestone, a deep underground aquifer. The presence of an impermeable vertical basalt dike forces the deep water to travel to the surface.

In April 2011, USGS hydrogeologist Ray Johnson and geologist Ed DeWitt discussed their findings at public meetings and hosted public field trips to view Montezuma Well. Using all available data, Johnson and DeWitt believe the groundwater flowing to the well is at risk from deep groundwater development in the vicinity of the well and up gradient from the well within the Redwall Limestone. While the shallow groundwater in the surrounding Verde Formation is highly developed for community use, this water appears to be relatively disconnected from the well. By sharing their research and the results, Johnson and DeWitt are educating people about the role of science in land management decisions and providing practical water management information for local communities. Park managers now have a better understanding of how continued development in the Verde Valley can affect groundwater flowing to Montezuma Well, allowing them to make management decisions that protect this important natural feature today and into the future.

Read more about arsenic around Montezuma Well.

Read Johnson and DeWitt's Presentation Abstract.