THE YELLOWSTONE CONTROLLED GROUNDWATER AREA-HISTORY, PURPOSE, AND LONG-TERM MONITORING

The Yellowstone Controlled Groundwater Area (YCGA) was created in 1994 as part of a Reserved Water Right Compact (Compact) between Montana and the National Park Service. It encompasses 1,170 mi² of land in Montana adjacent to Yellowstone National Park (YNP), and was established to protect hydrothermal resources in YNP from potential impacts from groundwater development adjacent to YNP. The impetus for the YCGA was a private well drilled 9-mi north of Mammoth Hot Springs, in a known geothermal resource area, that intercepted warm (57°C) groundwater. Test pumping the 490-ft deep well reduced the discharge from LaDuke Hot Springs, located 650 ft away, by 92%. This prompted a comprehensive hydrogeologic investigation by the U.S. Geological Survey to evaluate the potential impacts from geothermal groundwater development in the La Duke Hot Springs area on hydrothermal features in YNP. The investigation concluded that there were possible hydrogeologic connections between the La Duke Hot Springs area, located outside of YNP, and Mammoth Hot Springs.

The YCGA, established following the USGS investigation, includes permitting requirements for new wells that include documentation of well location, groundwater temperature, proposed pumping rate, and installation of water meters. Creation of the YCGA also established a long-term groundwater monitoring program overseen by the Montana Bureau of Mines and Geology (MBMG) under the auspices of a Technical Oversite Committee. The long-term monitoring program was initiated in 2006 and MBMG currently monitors 34 sites comprised of 21 wells and 13 springs. The sites include hot springs and warm-water wells (> 26°C), and cold-water wells and springs. The primary monitoring goal is to establish baseline conditions for spring discharge and temperature, and groundwater level and temperature for sites adjacent to YNP. To date, the data show seasonal and short-term climatic variability but no significant changes in groundwater levels, spring discharges, or water temperatures that are attributable to groundwater development within the YCGA.