WATER SUPPLY IN GRAND CANYON NATIONAL PARK: AN ONGOING RECHARGE EXPERIMENT FROM SOUTH RIM WATER TREATMENT, DOWN THE BRIGHT ANGEL FAULT, TO INDIAN GARDENS SPRINGS

Grand Canyon National Park receives over 6 million visitors per year with the Park’s water needs supplied by an ingenious pipeline system that conveys groundwater from Roaring Springs, a karst spring flowing from the regional world-class Redwall-Muav aquifer. Roaring Springs is of excellent water quality, and is sourced from an outlet (elevation 5,200') on the North Rim, and is used to provide water to both N Rim and S Rim locations. For S Rim usage, water flows down the pipeline across the Colorado river (~2,400') and up to to Indian Gardens (3,767'), then is pumped at about ~ 500,000 gallons/day to South Rim water tanks (7,000'). After public use it is reclaimed at the Water Reclamation Plant (WRP) and discharged at the surface at the edge of Grand Canyon Village, near the trace of the Bright Angel fault where it rapidly reinfiltrates. This has been going on since ~ 1966. The first part of this study examines the water budget of pipeline transport, water delivery to South Rim tanks, water delivery to the WRP after use in Grand Canyon Village, and discharge from the WRP.

A second part of this study is to geochemically characterize N Rim and South Rim groundwater variability towards the practical application of using tracers to evaluate water supply, water pathways, and water quality at South Rim. North Rim water has a distinctive stable isotope fingerprint relative to South Rim groundwater. Roaring Spring water from the North Rim has δ¹⁸O= -13.5 permil (‰) and very low total dissolved solids (TDS). In contrast, S Rim groundwater has δ¹⁸O = -11 ‰ and higher TDS. Preliminary data indicates that water emerging from South Rim WRP, has δ¹⁸O= -13 ‰, retaining its N Rim 'fingerprint'. Water emerging at Indian Gardens Spring directly below the WRP along the Bright Angel fault has δ¹⁸O = -12.5 ‰, and hence is interpreted to be ~ 60% North Rim water and 40% South Rim water. This agrees with calculations based on chloride concentrations, and will be further investigated using ⁸⁷Sr/⁸⁶Sr and pharmaceutical tracers.

The Bright Angel fault is one of a regional set of reactivated Precambrian structures that creates an orthogonal grid of northwest and northeast trending faults. This fault network provides multiple flow pathways. The overall goal of this study is to evaluate an ongoing anthropogenic hydrologic “experiment” that the Park has been conducting over the past 60 years in order to help develop a present baseline that can be used to inform resource management and future water challenges, even as the Park undertakes massive restructuring of the North Rim water intake.