Origin of Groundwater in the Mogollon Highlands, Arizona; Stable O and H Isotopes and Tritium as Key Components for Regional Hydrogeological Conceptual Model Development

Groundwater from the fractured Proterozoic “Payson Granite” aquifer, regionally known as the “X” aquifer, is the sole source of potable water for Payson, AZ and surrounding communities.  Recharge is found to occur both locally (from precipitation falling on the central Mogollon Highlands), and from the nearby Mogollon Rim (where recharge supplies water to the Coconino, or “C” aquifer and subjacent aquifers in the Paleozoic and Proterozoic strata of the Colorado Plateau).   Stable O and H isotope data are listed below as (δ¹⁸O‰, δD‰).  For X-aquifer groundwater the data define a linear trend of slope near 5, with extreme values corresponding to C aquifer groundwater (<–11.0, <–76) and evaporated water from artificial lakes in Payson (>–9.4, >–69).   Average recharge at the elevation of Payson (AREP) plots near   (–10.4, –75), in the middle of the trend.   The linear trend is best explained by mixing of AREP with C aquifer water and lake water, the C aquifer water predominating closer to the Mogollon Rim.   Tritium is present in most X aquifer groundwater, but is lowest where C aquifer water predominates.   Winter precipitation dominates recharge in both X and C aquifers.   These data were then considered along with regional geological mapping, general water chemistry analyses, and hydrostatic elevations.   A complex picture of the region’s hydrogeological framework was identified. Structural networks of faults and fracture systems crossing through the bedrock are found to ultimately convey and interconnect recharge from atop the Mogollon Rim through the “C” regional aquifer (northern) and into the regional “X” aquifer system (southern) where mixing with local recharge occurs.  From the hydrogeological and stable isotope and Tritium data, a regional-scale conceptual model of the Mogollon Highland’s hydrogeological system has been developed for the Bureau of Reclamation’s Mogollon Rim Water Resources Management Study “MRWRMS” (BOR final draft pending).