STRATIGRAPHIC AND STATISTICAL ANALYSES OF GROUNDWATER FLOW WITHIN A PERCHED, KARST-SILICICLASTIC AQUIFER, KAIBAB PLATEAU, GRAND CANYON

The hydrogeologic effects of overlying perched aquifers are essential to understand when evaluating the hydrogeology of underlying, semi-confined aquifers. This study focuses on the stacked karst aquifer connections of one such stacked karst aquifer system on the Kaibab Plateau, north of Grand Canyon, Arizona. The shallow epikarst Coconino (C) aquifer is situated 300 m above the deeper karst Redwall-Muav (R) aquifer. To better understand complicated karst groundwater flow within the overlying C aquifer, stratigraphic data were collected from 9 locations and 22 springs were sampled for 18 months. C aquifer stratigraphic analyses show that the primary water bearing unit thins dramatically from south to north as evaporite lithofacies of the overlying unit become thicker and more prevalent. Stratigraphic analyses also indicate that there is significant karst development within carbonate lithologies in a sandwiched groundwater flow setting in western discharge areas. Principal Component Analyses (PCA) corroborate these findings and show that the hydrogeochemistry of northern groundwater discharge areas have elevated SO₄^2-, Cl^-, and Na ^– while western discharge areas have elevated alkalinity as CaCO₃ and Ca⁺. Stable isotope end-member mixing and elevation-isotope analyses indicate similar groundwater storage areas and recharge elevation for all C aquifer springs. The overall geochemistry of the C aquifer of the Kaibab Plateau is altered as groundwater flow moves laterally from primary recharge areas and interacts with unique carbonate and evaporite lithofacies of the C aquifer. The variability of C aquifer groundwater can be divided into three geochemically similar discharge areas based on PCA results: 1) elevated chloride-sulphate type water to the north, 2) minimal dissolved ion abundance water in the east, central, southern areas and 3) higher calcium-alkalinity as CaCO₃ water to the west. Results from this study helps reduce the complexity of karst groundwater flow within perched karst aquifers and enables researchers to better understand the hydraulic connectivity between shallow and deep aquifers in a stacked karst aquifer setting.