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

摘要

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, tourism, 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 is the primary groundwater source to springs. However, a second local recharge source is required to explain the detection of tritium. Probable sources are: (1) low-elevation infiltration of surface run-off with warm recharge temperatures and high excess air determined from noble gas models, and relatively low fractions of winter recharge, and (2) high-elevation plateau recharge with cool recharge temperatures, low excess air, and fraction of winter recharge of approximately 1. Previous investigators have linked spring occurrence with regional faults and fractures. Such features also likely control the 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 suggests that many South Rim springs may respond on the order of tens to hundreds 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.