EFFECTIVE LARGE-SCALE PERMEABILITY OF MULTIPLE GEOLOGIC UNITS

As typically defined, aquifers are geologic units that can store and transmit water at rates fast enough to supply reasonable amounts to wells (e.g., Fetter, Applied Hydrogeology, 2001). While both an aquifer's water storage and transfer capabilities are important to evaluate groundwater sustainability, this presentation will focus on effective large-scale permeability determinations of multiple geologic units. In particular, I will present a method to calculate mean anisotropic permeabilities and their applicable depth ranges in approximately horizontal geologic units where permeability may be assumed to decrease exponentially or logarithmically with depth. When combined with hydrogeologic models (e.g., Saar and Manga, JGR, 2004), this approach can provide estimates of the vertical permeability distribution at the crustal scale where near-surface pumping or tracer tests are unlikely to reflect deeper crustal material properties. Therefore, the presented approach should not be viewed as a substitute for more lithologically specific measurements of permeability for a given (shallow) region of interest but as a method to approximate the depth-dependence of permeability across multiple geologic units. The latter can be useful when direct permeability measurements at greater depths are technically or economically not feasible but overall water budgets and fluid fluxes are considered from the surface to several kilometers depth.