GRAND CANYON GROUNDWATER RECHARGE SOURCES AND FLOW PATHS: A MULTI-TRACER ASSESSMENT OF A CONCEPTUAL FLOW MODEL

In an effort to better understand the resilience of groundwater on the South Rim of the Grand Canyon to contamination and extraction, environmental and isotopic tracer data was compiled and collected from 33 area springs/wells. Stable isotope end-member mixing analysis and noble gas solubility models indicate recharge is largely from winter precipitation although there a measurable component from summer monsoonal runoff was identified. Gas solubility model results clearly differentiate between low elevation rapid infiltration, consistent with monsoonal runoff, from cool temperature high elevation recharge, consistent with seasonal snowmelt. Helium isotope interpretation of excess helium is consistent with radiogenic crustal sources and suggests a more limited connection between shallow meteoric and deeper endogenic flow systems as an alternative to previous results. Interpretation of age tracers (tritium, carbon-14) showed both relatively younger local and older regional flow paths which in combination with noble gas and dissolved ion chemistry are shown to be chemically distinct. In contrast to the North Rim, so-called fast flow along karst conduits does not appear to be a major mechanism of South Rim groundwater flow. From east to west the component of younger local recharge decreases and contribution from summer recharge sources increases in South Rim springs consistent with an eastern plateau winter recharge source. The wide range of interpreted groundwater ages supports mixing of the local and regional groundwater, likely facilitated by structural features based on the spatial pattern relative to faults and fractures. The multi-tracer approach used here revealed additional complexity in a previously well studied groundwater flow system suggested revision of the conceptual model of groundwater recharge source and flow paths was needed.