Isotopic and geochemical tracers of groundwater flow in the Shivwits Plateau, Grand Canyon National Park, USA

Understanding patterns of groundwater flow are important when quantifying and mitigating threats to critical groundwater resources. Environmental tracers, determined from discrete sampling, could aid in characterizing spring systems through determining flow paths, recharge areas, and carbon cycling. This study explores the novel inclusion of δ¹³C of dissolved organic carbon (DOC), δ¹³C of dissolved inorganic carbon (DIC) and fluorescent dissolved organic matter (fDOM), along with more commonly used multi-tracer approaches that involve water isotopes, major ions, and saturation indices, to characterize springs of the Shivwits Plateau in Grand Canyon National Park, USA. Carbon isotope ratios and fDOM concentrations for all springs reflect source values associated with regional surface vegetation and heterotrophic degradation of terrestrial DOM. Principal component analyses show that springs can be grouped by geochemical variability into: (1) a shallow epikarst system, (2) a flow path through gypsiferous beds of the Toroweap Formation on the eastern side of the plateau, (3) a short canyon-slope flow path dominated by runoff, and (4) a deeper complex flow system in the Redwall Limestone characteristic of mixing of all other flow systems. As appropriations from the Colorado River already exceed its annual streamflow, characterizing groundwater resources for water supply in an increasingly arid climate will be paramount. These results demonstrate the effectiveness of geochemical techniques for groundwater flow characterization, particularly in inaccessible environments.