Abstract:
Most of the ground water in the Grand Canyon region circulates to springs in the canyon through the thick, deeply buried, karstified Cambrian through Mississippian carbonate sectionThese rocks are collectively called the lower Paleozoic carbonates and comprise the Redwall-Muav aquifer where saturatedThe morphologies of the caves are primarily a function of whether the carbonates are unconfined or confined, a distinction that has broad significance for groundwater exploration and which appears to be generally transferable to other carbonate regionsCaves in unconfined high-gradient environments tend to be highly localized, partially saturated, simple tubes, whereas those in confined low-gradient settings are saturated 2- or even 3-dimensional mazesThe highly heterogeneous distribution of the unconfined conduits makes for difficult drilling targets, whereas the more ubiquitously distributed confined mazes are far easier to targetThe distinctions between the storage characteristics within the two classes is probably even more importantThere is minimal groundwater storage in the unconfined systems because they are well drainedIn contrast, the saturated mazes exhibit maximal storageConsequently, system responses to major storm recharge events in the unconfined systems is often dominated by flow-through rather than the pulse-through hydraulics as found in the confined systemsSpring discharges from the unconfined systems tends to be both flashy and highly variable from season to season, but total dissolved solids are smallIn contrast, the pulse-through hydraulics in the artesian systems causes spring discharge responses to be highly moderated and, in the larger basins, remarkably steadyBoth total dissolved solids and temperatures in the waters from the confined aquifers tend to be elevated because most of the water is derived from storageKarst permeability is created by the flow system, consequently predicting where the permeability is best developed in a carbonate section involves determining how circulation should be ideally organized through an examination of the geometry of the flow systemThe areas where flow concentrates are the areas where karstification will maximize, provided enough time has elapsed to allow dissolution to adjust to the imposed boundary conditionsThe rate of adjustment in the Grand Canyon region appears to be related to the degree of saturationThe artesian systems are far better adjusted to hydraulic gradients than the unconfined systems, a finding that probably implies that there is greater contact between the solvent and rock in the saturated confined systems