In 1968, Peabody Western Coal Company commenced operations of a massive 54,000 acre coal mine on Black Mesa, Arizona, an arid and semi-arid region inhabited by the Hopi Tribe and Diné Nation. The mine fuels the Navajo Generating Station, which was developed to power the Central Arizona Project canal, which annually pumps 1.6 million acre-feet of Colorado River water nearly 3,000 feet uphill during its 333 mile journey to Phoenix and Tucson. Water for mine operations is pumped from a non-renewable aquifer beneath the Hopi and Diné lands.

After more than forty years of development, conflict characterizes the history of industrial groundwater exploitation on Black Mesa, and there is little understanding of the relationship between industrial withdrawals and its impact upon the region’s hydrological and social-ecological systems.

This case study performs a postaudit of groundwater model predictions used in the mine’s impact assessment studies, and it evaluates the efficacy of regulatory oversight. The study demonstrates that groundwater models consistently underestimated water-level decline caused by industrial withdrawals, overestimated declines caused by tribal community withdrawals; failed to capture the linear relationship between declining water levels and spring discharge; and predicted water-level recoveries that have not occurred. Further, at least two of the Regulatory Authority’s four threshold criteria for material damage have been crossed, and two have never been evaluated as intended.

Peabody’s groundwater model was purportedly “validated” and subsequently implemented for regulatory purposes; it demonstrated that declining groundwater trends at Moenkopi and Tuba City (60 miles from the mine) are the result of community withdrawals and recent drought conditions. However, this postaudit demonstrates that declining discharge from Moenkopi School Spring has a strong, statistically significant relationship with the rate of Peabody’s groundwater withdrawals (r = -0.84; R² = 0.71; p < 0.0001), while neither community withdrawals nor local precipitation have a statistically significant relationship with this spring.

In 2008, the Regulatory Authority revised its material damage criteria: all prior criteria expressing negative trends were removed from regulatory purview, and remaining criteria acquired insurmountable damage thresholds and will be evaluated using Peabody model simulations rather than USGS monitoring data.