Rethinking groundwater flow on the South Rim of the Grand Canyon, USA: Characterizing recharge sources and flow paths with environmental tracers

In the arid landscape south of the Grand Canyon, natural springs and seeps are a critical resource for endemic species and Native American tribes. Groundwater is potentially threatened by expanding populations, visitations, and mineral extraction activities. Environmental tracers including noble gases, stable isotopes of hydrogen and oxygen in water, tritium, and carbon-14 were used to characterize recharge sources and flow paths in South Rim aquifers. Results confirm the regional Redwall-Muav Aquifer as the primary groundwater source to springs. However, a second local recharge source is required to explain the detection of tritium. Two probable sources are identified as: low-elevation infiltration of surface run-off with warm noble gas recharge temperatures, high excess air, and relatively low fractions of winter recharge, and high-elevation plateau recharge with cool recharge temperatures, low excess air, and fraction of winter recharge of ~ 1. Previous investigators have linked spring occurrence with regional faults and fractures. We show such features are also the likely control chemical mixing between the regional and local groundwater sources, the transport of deeply sourced and local recharge fluids, groundwater age, and thus the relative vulnerability of groundwater to depletion and contamination. The new conceptual model of groundwater sources and flow paths suggest many South Rim springs may respond on the order of 10s to 100s of years to groundwater depletion and contamination, even though the majority of groundwater flow is along longer flow paths with longer lag times. The magnitude of response to short term changes in the flow system remains unclear.