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

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... or 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 <0.9 for 9 of the 50 samples, and age tracer data indicating young groundwater discharging from springs which is distinct from old groundwater observed in the regional flow system. These results suggest a new conceptual model is needed to account for recharge sources from low elevation and summer precipitation. Results imply resource managers may need to reconsider current land-use and water management practices on the South Rim to protect future water quantity and quality.