Abstract A conceptual model of anisotropic and dynamic permeability is developed from hydrogeologic and hydromechanical characterization of a foliated, complexly fractured, crystalline rock aquifer at Gates Pond, Berlin, Massachusetts. Methods of investigation include aquifer‐pumping tests, long‐term hydrologic monitoring, fracture characterization, downhole heat‐pulse flow meter measurements, in situ extensometer testing, and earth tide analysis. A static conceptual model is developed from observations of depth‐dependent and anisotropic permeability that effectively compartmentalizes the aquifer as a function of foliation intensity. Superimposed on the static model is dynamic permeability as a function of hydraulic head in which transient bulk aquifer transmissivity is proportional to changes in hydraulic head due to hydromechanical coupling. The dynamic permeability concept is built on observations that fracture aperture changes as a function of hydraulic head, as measured during in situ extensometer testing of individual fractures, and observed changes in bulk aquifer transmissivity as determined from earth tides during seasonal changes in hydraulic head, with higher transmissivity during periods of high hydraulic head, and lower transmissivity during periods of relatively lower hydraulic head. A final conceptual model is presented that captures both the static and dynamic properties of the aquifer. The workflow presented here demonstrates development of a conceptual framework for building numerical models of complexly fractured, foliated, crystalline rock aquifers that includes both a static model to describe the spatial distribution of permeability as a function of fracture type and foliation intensity and a dynamic model that describes how hydromechanical coupling impacts permeability magnitude as a function of hydraulic head fluctuation. This model captures important geologic controls on permeability magnitude, anisotropy, and transience and therefor offers potentially more reliable history matching and forecasts of different water management strategies, such as resource evaluation, well placement, permeability prediction, and evaluating remediation strategies.
Earth and Space Science Open Archive posterOpen AccessYou are viewing the latest version by default [v1]Designs and Results from Three New Borehole Optical Fiber Tensor StrainmetersAuthors Scott DeWolf iD Larry Murdoch Leonid Germanovich Robert Moak Micheal Furgeson iDSee all authors Scott DeWolfiDCorresponding AuthorClemson UniversityiDhttps://orcid.org/0000-0002-7440-6973view email addressThe email was not providedcopy email addressLarry MurdochClemson Universityview email addressThe email was not providedcopy email addressLeonid GermanovichGeorgia Institute of Technology Main Campusview email addressThe email was not providedcopy email addressRobert MoakClemson Universityview email addressThe email was not providedcopy email addressMicheal FurgesoniDClemson UniversityiDhttps://orcid.org/0000-0001-7802-8001view email addressThe email was not providedcopy email address
A unique configuration of horizontal sheet-like electrodes was used in the field at a site in Ohio that was underlain by silty clay glacial drift to induce electroosmotic flow and to characterize the effects of electroosmosis on soil properties (e.g., electrical conductivity and pH). The lower electrode was created at a depth of 2.2 m by filling a flat-lying hydraulic fracture with granular graphite, and the upper one was a metallic mesh placed at a depth of 0.4 m and covered with sand. The electrodes were attached to a DC power supply, creating an electrical gradient of 20–31 V/m and a current of 42–57 A within approximately 20 m3 of soil. Total energy applied was 5,500 kW⋅h during approximate 4 months of operation. Electroosmotic flow rates of 0.6–0.8 L/h were observed during tests lasting several weeks, although total flow rate (electroosmotic plus hydraulic) was strongly influenced by fluctuations of the ground-water table. The ratio of applied current to voltage decreased from 0.9 to 0.6 A/V and was mainly due to a decrease in electrical conductivity of the soil. A low pH front developed at the anode and migrated toward the cathode. The velocity of the pH front per unit voltage gradient was 0.014 (cm/day)/(V/m). This was 40 times slower than what has been reported from laboratory experiments using kaolinite as a medium. These results confirm the feasibility of using horizontal electrodes at shallow depths, but they also underscore some important differences between the geochemical effects observed during field tests in natural soils and those seen in laboratory tests using ideal materials.
