Variability of Karstic Permeability Between Unconfined and Confined Aquifers, Grand Canyon Region, Arizona

摘要

Most of the ground water in the Grand Canyonregion circulates to springs in the canyon through the thick,deeply buried, karstified Cambrian-Mississippian carbonatesection. These rocks are collectively called the lower Paleozoiccarbonates and comprise the Redwall-Muav aquifer where saturated.The morphologies of the caves in the Grand Canyon areprimarily a function of whether the carbonates are unconfined orconfined, a distinction that has broad significance for ground-water exploration and which appears to be generally transferableto other carbonate regions. Caves in unconfined high-gradientenvironments tend to be highly localized, partially saturated,simple tubes, whereas those in confined low-gradient settingsare saturated 2- or even 3-dimensional mazes. The highlyheterogeneous, widely spaced conduits in the unconfinedsettings make for difficult drilling targets, whereas the moreubiquitously distributed mazes in confined settings are far easier to target.The distinctions between the storage characteristics within thetwo classes are more important There is minimal ground-waterstorage in the unconfined systems because cave passages tend tobe more widely spaced and are partially drained. In contrast,there is maximum storage in the saturated mazes in the confinedsystems. Consequently, system responses to major stormrecharge events in the unconfined systems are characterized byflow-through hydraulics. Spring discharge from the unconfinedsystems tends to be both flashy and highly variable from seasonto season, but total dissolved solids are small. In contrast, thepulse-through hydraulics in the artesian systems causefluctuations in spring discharge to be highly moderated and, inthe larger basins, remarkably steady. Both total dissolved solidsand temperatures in the waters from the confined aquifers tendto be elevated because most of the water is derived from storage.The large artesian systems that drain to the Grand Canyonderive water from areally extensive, deep basins where the waterhas been geothermally heated somewhat above mean ambient air temperatures.Karst permeability is created by the flow system, so dissolutionpermeability develops most rapidly in those volumes ofcarbonate aquifers where flow concentrates. Predicting wherethe permeability should be best developed in a carbonate sectioninvolves determining where flow has been concentrated in thegeologic past by examining the geometry and hydraulicboundary conditions of the flow field. Karstification can beexpected to maximize in those locations provided enoughgeologic time has elapsed to allow dissolution to adjust to theimposed boundary conditions. The rate of adjustment in theGrand Canyon region appears to be related to the degree ofsaturation. The artesian systems are far better adjusted tohydraulic gradients than the unconfined systems, a finding thatprobably implies that there is greater contact between thesolvent and rock in the saturated systems.These findings are not arcane distinctions. Rather, successfulexploration for ground water and management of the resource ismaterially improved by recognition of the differences betweenthe types of karst present For example, the unsaturated conduitkarsts in the uplifts make for highly localized, high risk drillingtargets and involve aquifers with very limited storage. Theconduits have highly variable flow rates, but they carry goodquality water largely derived from seasonal flow-through fromthe surface areas drained. In contrast, the saturated basin karsts,with more ubiquitous dissolutional permeability enhancement,provide areally extensive low risk drilling targets with largeground-water storage. The ground water in these settings isgenerally of lesser quality because it is derived mostly from long term storage.