title: Imaging Pathways in Fractured Rock Using Three-Dimensional Electrical Resistivity Tomography.
abstract: Major challenges exist in delineating bedrock fracture zones because these cause abrupt changes in geological and hydrogeological properties over small distances. Borehole observations cannot sufficiently capture heterogeneity in these systems. Geophysical techniques offer the potential to image properties and processes in between boreholes. We used three-dimensional cross borehole electrical resistivity tomography (ERT) in a 9 m (diameter) × 15 m well field to capture high-resolution flow and transport processes in a fractured mudstone contaminated by chlorinated solvents, primarily trichloroethylene. Conductive (sodium bromide) and resistive (deionized water) injections were monitored in seven boreholes. Electrode arrays with isolation packers and fluid sampling ports were designed to enable acquisition of ERT measurements during pulsed tracer injections. Fracture zone locations and hydraulic pathways inferred from hydraulic head drawdown data were compared with electrical conductivity distributions from ERT measurements. Static ERT imaging has limited resolution to decipher individual fractures; however, these images showed alternating conductive and resistive zones, consistent with alternating laminated and massive mudstone units at the site. Tracer evolution and migration was clearly revealed in time-lapse ERT images and supported by in situ borehole vertical apparent conductivity profiles collected during the pulsed tracer test. While water samples provided important local information at the extraction borehole, ERT delineated tracer migration over spatial scales capturing the primary hydrogeological heterogeneity controlling flow and transport. The fate of these tracer injections at this scale could not have been quantified using borehole logging and/or borehole sampling methods alone.
authors.forename: Judith
authors.initials: J
authors.lastname: Robinson
authors.affiliation: Department of Earth and Environmental Sciences, Rutgers University, 101 Warren Street, Smith Hall Rm 135, Newark, NJ, 07102. lslater@andromeda.rutgers.edu.
authors.forename: Lee
authors.initials: L
authors.lastname: Slater
authors.affiliation: Pacific Northwest National Laboratory, Richland, WA, USA. TJ@pnnl.gov.
authors.forename: Timothy
authors.initials: T
authors.lastname: Johnson
authors.affiliation: United States Geological Survey, Reston, VA, USA. ashapiro@usgs.gov.
authors.forename: Allen
authors.initials: A
authors.lastname: Shapiro
authors.affiliation: United States Geological Survey, Menlo Park, CA, USA. tiedeman@usgs.gov.
authors.forename: Claire
authors.initials: C
authors.lastname: Tiedeman
authors.affiliation: Department of Earth and Environmental Sciences, Rutgers University, 101 Warren Street, Smith Hall Rm 135, Newark, NJ, 07102. dimntar@rutgers.edu.
authors.forename: Dimitrios
authors.initials: D
authors.lastname: Ntarlagiannis
authors.affiliation: United States Geological Survey, Storrs, CT, USA. cjohnson@usgs.gov.
authors.forename: Carole
authors.initials: C
authors.lastname: Johnson
authors.affiliation: United States Geological Survey, Storrs, CT, USA. daylewis@usgs.gov.
authors.forename: Frederick
authors.initials: F
authors.lastname: Day-Lewis
authors.affiliation: United States Geological Survey, Lawrenceville, NJ, USA. placombe@usgs.gov.
authors.forename: Pierre
authors.initials: P
authors.lastname: Lacombe
authors.affiliation: United States Geological Survey, Lawrenceville, NJ, USA. timbrig@usgs.gov.
authors.forename: Thomas
authors.initials: T
authors.lastname: Imbrigiotta
authors.affiliation: United States Geological Survey, Storrs, CT, USA. jwlane@usgs.gov.
authors.forename: John
authors.initials: J
authors.lastname: Lane
mesh_terms: D017097:Electric Impedance; D004784:Environmental Monitoring / Q000295:instrumentation / Q000379:methods*; D060587:Groundwater / Q000032:analysis*; D014054:Tomography / Q000295:instrumentation / Q000379:methods*; D014241:Trichloroethylene / Q000032:analysis; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D014241:Trichloroethylene