Fence Springs of the Grand Canyon, USA: insight into the karst aquifer system of the Colorado Plateau region

Fence Springs are the highest discharge springs of the Redwall-Muav (R-M) karst aquifer in Marble Canyon of eastern Grand Canyon, Arizona, USA. Vents on opposite banks of the Colorado River within the Fence fault system have similar chemistries indicating the springs are connected hydrologically within the confined karst aquifer below the river. Stable isotope data fingerprint the recharge area for both springs to be the Kaibab Plateau, west of the river. Chemical variation in nearby R-M springs indicates mixing between karst base flow (represented by Fence Springs) and fast-traveled meteoric waters (Vasey’s Paradise). A 7-year record (2012—2019) suggests the karst base flow has steady temperature (~20 °C) and specific conductance (~2,000 μS/cm) and no seasonality. A progressive decrease of ~1 °C in both springs and ~100 μS/cm in Fence East over 7 years reflects declining spring discharge accompanying declining meteoric recharge. Fortuitous high-flow experiments in the Colorado River during Glen Canyon Dam management operations provide data analogous to a “slug test” for the groundwater system. Rapid increase in river level from ~142 to 1,218 m 3 /s caused the springs to be inundated and mix with river water. Recovery curves showed rapid return of spring temperature from ~10 to 20 °C and specific conductance from 500 to 2,000 μS/cm once river stage fell below ~283 m 3 /s. After 2016, an increase in short-term fluctuations during recovery suggests declining spring discharge through the 7-year period. This multitracer hydrochemical dataset combined with spring monitoring helps establish a baseline for groundwater in eastern Grand Canyon.