HYDROCHEMISTRY AT GRAND CANYON: WHO KNEW GROUNDWATER HYDROLOGY COULD BE SO COMPLICATED?

Springs and associated riparian environments provide critical habitats for both aquatic and terrestrial wildlife in the Grand Canyon region. Springs also provide drinking water for Grand Canyon National Park (GCNP). Grand Canyon springs are fed by world-class karst aquifer systems (both shallow and deep) on the Colorado Plateau, but increasing pressure on groundwater resources from climate change, mining and other development activities pose major challenges to resource managers. The shallow and deep karst systems of the region interact in ways that are revealed by recent studies. General hydrologic models for the Colorado Plateau aquifers highlight the importance of recharge areas (‘springsheds’) for water supply. Ongoing work by several groups is helping to understand these complex relationships using multiple tracer methods and modelling. A robust monitoring and geochemical sampling program can provide data for understanding the sustainability of spring-fed water supplies for anthropogenic use. Our ongoing geochemical studies of spring waters (including dissolved gases) have identified the importance of mantle-derived volatiles and CO2 that contribute dissolved salts potentially impairing water quality. Faults are important conduits for fluid transport and an essential component of the hydrogeologic framework for understanding fluid flow pathways and groundwater transit times. Grand Canyon hosts a multi-porosity groundwater system resulting from variable ages and mixing of meteoric recharge, karst system transport, matrix sandstone transport, fault connectivity, and endogenic inputs. Quantitative forecasting of the effects of climate change on water quality depends on our understanding of deep inputs as well as aquifer recharge flowpaths and quantities. Results from Grand Canyon and other spring-supported stream systems in the western U.S. indicate the need for development of hydrologic baselines that recognize these complexities. This can be accomplished through use of both natural and artificial tracers to unravel mixing and environmental sensors to monitor real time changes. These investments are needed to inform water management decisions that address societal and ecosystem needs, and at Grand Canyon National Park a new infrastructure for water delivery.