Critical evaluation of stable isotope mixing end-members for estimating groundwater recharge sources: case study from the South Rim of the Grand Canyon, Arizona, USA

Springs and groundwater seeps along the South Rim of the Grand Canyon (Arizona, USA) are important for the region’s ecosystems, residents (human and animal), and economy. However, these springs and seeps are potentially vulnerable to contamination, increased groundwater extraction, or reduced recharge due to climate change. In this study, statistical methods are used to investigate δ 2 H and δ 18 O in precipitation, surface water, and groundwater to determine groundwater source. A mixing model for δ 18 O is developed using statistically distinct seasonal end-members represented by modeled winter (Nov–Apr) precipitation and summer (May–Oct) surface-water run-off. The calculated fraction of winter recharge ( F win ) indicates that South Rim groundwater is primarily sourced from snow-melt and winter rains with an average F win of 0.97 ± 0.09. Groundwater sourced from the highest elevations of the study area are more depleted than the winter end-member, suggesting values of F win are overestimated or a meaningful portion of winter recharge occurs at lower elevations. Lower-elevation recharge from the Coconino Plateau is supported by consistent spatial trends in δ 2 H and δ 18 O with respect to longitude, F win values