HR: 1330h
AN: H42B-1081    [PDF]
TI: Optimizing a Hydrogeologic Investigation of a Fractured Bedrock Aquifer With a Seismic Reflection Survey
AU: * Truskowski, M
EM: melinda.truskowski@erm.com
AF: Environmental Resources Management, 5950 South Willow Drive, Suite 200, Greenwood Village, CO 80111 United States
AU: Warner, J
EM: jim.warner@erm.com
AF: Environmental Resources Management, 5950 South Willow Drive, Suite 200, Greenwood Village, CO 80111 United States
AU: Tisoncik, D
EM: dan.tisoncik@ual.com
AF: United Airlines, 1200 E. Algonquin Road, Elk Grove Township, IL 60007 United States
AB: This project involves extensive investigation and focused remediation of chlorinated solvents in fractured bedrock and overlying alluvium in Northern California. Primary contaminants include 1,1,1-TCA, 1,1-DCE, and 1,1- DCA. The source area includes recoverable DNAPL, with a maximum accumulated thickness of 29 feet in a well, and dissolved concentrations in excess of 500 mg/L. The dissolved plume extends away from the source through an extensive fracture network to a depth of 450 feet. A bedrock high corresponds to the source area, with the absence of overlying sediments providing a migration pathway into the bedrock for DNAPL. The depth to bedrock and location/depth of fracture zones are highly variable across the site, with both factors significantly influencing drilling costs. The investigation involved drilling wells to depths up to 700 feet, borehole geophysics, a seismic reflection survey, and aquifer testing. The seismic reflection survey was performed to identify zones of increased fracturing, and to map the topography of the upper bedrock interface. To address the objectives of the survey, both p-wave and shear wave methods were tested in an effort to identify fracture orientation and density. A split-spread configuration with station spacing of five feet and 144 recording channels was used to collect the data for both methods. A microvibrator was used as the energy source, varying its orientation to generate p- and shear waves. Preliminary review of the test sections revealed abundant faulting, which would complicate p-/shear rotational analysis and is the likely cause of areas of increased fracturing. Based on the data quality and survey objectives, the remainder of the survey, totaling linear 6,570 feet, was collected with a p-wave source and receivers, using a 50 to 300 Hertz linear sweep. Unique aspects of the survey include a target depth of up to 1,000 feet and operational constraints associated with working in the aircraft movement area of a major maintenance facility. To address the challenges of abundant noise due to vehicular activity, both aircraft and ground equipment, and the need to quickly clear an area for aircraft movement, surficial adhesive was used to couple the geophones to the concrete surface. The seismic reflection survey identified en-echelon, southeast dipping normal faults and antithetic, southwest dipping reverse faults that are generally consistent with the orientation of features in the nearby San Andreas fault zone. The results have been used to create a hydrogeologic model of the site that includes three major fault blocks and predicted areas of increased fracturing, ground water flow, and contaminant transport. The seismic interpretation was developed by integrating extensive borehole data from the site related to lithology and fracture orientation, and is consistent with ground water flow and contaminant occurrence. The results demonstrate the effectiveness of seismic reflection surveys to support investigation and remediation in structurally complex areas.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2003 Fall Meeting