Electromagnetic Resistivity Surveys
Three-dimensional (3-D) electromagnetic resistivity (i.e., resistance) surveys are used to develop high-resolution images of subsurface contamination zones and local geologic features. In a 3-D resistivity survey, an above-ground electromagnetic coil transmits a magnetic signal through the ground to a receiver located in a well. This receiver records the signal strength at various points, from near the surface to the bottom of the well (often deeper than 300 feet). Differences in signal strength, caused by different materials’ varying inherent electrical resistance, indicate the presence of different materials beneath the surface. For example, free hydrocarbons can be detected because they are highly resistive compared to subsurface waters. Large pockets of contaminants, such as dense non-aqueous phase liquids (DNAPL), can also be detected. The above-ground coil is moved to a succession of surface locations on a predetermined grid, and new measurements are taken until the entire site is mapped
Accurate subsurface images provide a more thorough understanding of the subsurface environment, so that monitoring wells can be located and screened at the most effective intervals for evaluating DNAPL presence. In turn, recovery wells can be located and screened for optimum product removal.
Limitations and Concerns
It is not clear that contaminants can be identified with the degree of assurance that is required to meet regulatory requirements.
It appears that this technique is appropriate for large concentrations of contaminants. However, it is not clear which contaminants can be successfully identified, or if there are situations where false readings are likely to occur.
It is not clear whether the technology can discriminate between naturally occurring metals and metals that are present due to human activity.
The 3-D resistivity method is not a stand-alone means of effective site characterization. The technique is interpretative, and it requires confirmatory (validation) sampling and chemical analyses to verify that subsurface contamination is present.
The survey area is limited to a radius of approximately 300 feet around each instrumentation well, so each survey encompasses a circular area about 1.6 acres in size. Hence, the general location of a suspected surface source is needed to focus the survey. Also, depths imaged by the survey only go as deep as the instrumentation well, which typically has a maximum depth of approximately 300 feet.
There has been some concern regarding the technology’s capability to detect small amounts of DNAPL. This is believed to be a significant source of error when delineating areas with low levels of DNAPL saturation.
The transmitter coil must be situated so that large metal objects do not interfere with the signal. Aside from above ground objects such as dumpsters or railroad tracks, the locations of underground pipes and tanks must be known ahead of time.
Personnel that design the survey and collect resistivity data in the field must have a strong understanding of the technology to ensure that high quality data is obtained.
3-D imagery is a tool used to characterize the geology of soils and landfills. It may be especially useful in identifying pockets of DNAPLs and buried items in landfills.
Technology Development Status
The 3-D resistivity survey has been used in exploration for oil and gas, as well as subsurface fresh water, since the 1950s. It is still being field tested for use at contaminated sites.
Other Resources and Demonstrations
The Department of Defense (DoD) Environmental Security Technology Certification Program (ESTCP) evaluated this system for DNAPL at two DoD installations: the former Alameda Naval Air Station, renamed Alameda Point, and Tinker Air Force Base.
See Innovations in Site Characterization: Geophysical Investigation at Hazardous Waste Sites, EPA, for a description of all current geophysical investigation techniques, http://www.clu-in.org/download/char/geophys_innovate_a.pdf.
See Two-Dimensional Resistivity Investigation of the North Cavalcade Street Site, Houston, Texas, August 2003, http://www.clu-in.org/download/contaminantfocus/dnapl/Detection_and_Site_Characterization/north_calvacade_creosote_geophysicssi_2005-5205.pdf and http://pubs.usgs.gov/sir/2005/5205/ .
See Using Electrical Resistivity Imaging to Evaluate Permanganate Performance During In Situ Treatment of an RDX-Contaminated Aquifer (0635), http://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200635/ER-200635, for description of project to assess whether permanganate is viable for RDX remediation.