Conceptual models of groundwater flow in the Grand Canyon region, Arizona
The conceptual models of groundwater flow outlined herein synthesize what is known and hypothesized about the groundwater-flow systems that discharge to the Grand Canyon of Arizona. These models interpret the hydrogeologic characteristics and hydrologic dynamics of the physical systems into a framework for understanding key aspects of the physical systems as they relate to groundwater flow and contaminant transport. This report describes five individual groundwater-flow systems draining to the Grand Canyon: Kaibab, Uinkaret-Kanab, Marble-Shinumo, Cataract, and Blue Spring. These systems are present in the saturated parts of the lower Paleozoic carbonate section exposed on the walls of the Grand Canyon; specifically, the Mississippian Redwall Limestone down through the Cambrian Muav Limestone of Tonto Group. Together, the systems described in this report compose the regional groundwater-flow system. Local to subregional flow systems in the sedimentary units of the overlying Permian section could provide transport pathways from the land surface to the regional flow system. Despite the potential importance of the local systems, the focus of this report is on the systems present in the lower Paleozoic section because all major springs in the Grand Canyon discharge from those units. The most important hydrogeologic characteristics include system boundaries imposed by major tectonic structures, and the degree to which karstification influences the magnitude and direction of flow in each system. Important hydrologic dynamics include locations and rates of potential groundwater recharge, vertical pathways to the regional aquifer, and the locations, magnitude, geochemical signature, and hydrostratigraphic setting of groundwater discharge from springs. Unknown properties or conditions that represent the greatest uncertainties in our current understanding of the regional groundwater-flow system are identified for additional consideration. Groundwater data are sparse owing to geographic remoteness and extreme depth to water throughout much of the study area. This paucity of information was diminished with the development of a structural contour map of the top and bottom surfaces of the regional aquifer, and a Soil-Water-Balance model that produces spatial distributions of rates of potential recharge. Investigation of the five groundwater-flow systems reveals important, though mostly qualitative, characteristics controlling the rates and directions of groundwater flow. Karstification has produced dissolution-enhanced conduit flow pathways to various degrees in each of the systems. Parts of each system exhibit relative structural uplift or downdropping of the hydrostratigraphic units of the regional aquifer, with some uplifted sections dipping inward toward the Grand Canyon and others dipping outward. The Kaibab groundwater system is archetypical of an uplifted, inward-dipping karst system, whereas the Blue Spring groundwater system and most of the Cataract groundwater system are representative instances of a downdropped or basin karst system. The Uinkaret-Kanab groundwater-flow system is structurally similar to the basin karst systems but karstification has not progressed to nearly the same degree. The Marble-Shinumo groundwater system does not fall cleanly into either category and its boundaries are the most uncertain of all the groundwater systems.