HYDROGEOLOGY OF A PERCHED AND SEMI-CONFINED KARST AQUIFER, KAIBAB PLATEAU, GRAND CANYON

In a stacked karst aquifer setting, understanding the intermediate effects of a perched aquifer is crucial when evaluating surficial and climatological impacts to a deep, semi-confined aquifer. This study focuses on the perched and semi-confined karst aquifers of the Kaibab Plateau, north of Grand Canyon National Park. The shallow Coconino (C) aquifer is situated 300 m above the deep Redwall-Muav (R) aquifer and many R aquifer springs along the North Rim of Grand Canyon are theorized to be sourced from leakage in the C aquifer. The Kaibab Plateau has bimodal precipitation distribution in the form of winter snowmelt and summer monsoon rain, both of which are isotopically unique and invoke a hydraulic response in the Kaibab Plateau aquifers. As a result, many Grand Canyon spring’s recharge seasonality can be evaluated based on geochemistry, hydrograph interpretation and climograph data. Recent regional qualitative fluorescence dye tracer studies indicate the Kaibab Plateau aquifers have a significant horizontal flow component, but the heterogeneity of these horizontal pathways is still unknown. In order to better understand the hydrogeology of the C aquifer as it moves water from the surface to the underlying R aquifer, we sporadically sampled 30 C aquifer springs for one year and instrumented three C aquifer springs with In-Situ water level data loggers. In addition to these springs on the Kaibab Plateau, we also evaluated 10 years of continuous data from a C aquifer spring in a similar geologic setting on the Mogollon Plateau. C aquifer hydrograph analysis show quick response to large recharge events with a hydraulic response occurring in 4-6 days. C aquifer water isotopes have very similar and concentric clustering along the global meteoric waterline when compared to R aquifer water isotopes. These data suggest seasonal intermixing occurs within the shallow C aquifer before moving down to the R aquifer. Based on hydrograph analysis and stratigraphic position, many perched C aquifer springs could simply represent overflow during large recharge events. Although R aquifer springs are far more numerous and have much higher annual discharge, our results indicate that Kaibab Plateau groundwater storage primarily occurs within the perched C aquifer before being transferred down via conduits and fractures to the R aquifer.