U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Detection and Site Characterization

Geophysical Methods in DNAPL Investigations

The following discussion is taken from Site Characterization Technologies for DNAPL Investigations, EPA 542-R-04-017Adobe PDF Logo.

Geophysical methods provide qualitative and quantitative information on subsurface conditions. They can be deployed in single borehole, borehole to borehole, surface to borehole, and purely surface surveys. The various methods are used to measure the physical properties of the subsurface materials, such as conductivity/resistivity, dielectric constant, and density. Changes in these measurements are interpreted to indicate changes in subsurface physical/chemical properties. Because geophysical methods do not directly measure the matrix, it is almost always necessary to perform intrusive sampling to confirm the interpretation. For many surface survey methods, an accurate interpretation requires that the geophysical data be matched or calibrated with nearby borehole data. Once calibrated, survey methods can be used for accurate description of large subsurface areas in which direct exploration would be prohibitively expensive.

Depending upon the method and the deployment, results obtained with surface geophysical methods can be presented as graphs, plan-view contour maps, 2-D cross-sections of the study area, or in some cases displayed with 3-D imaging software. Their resolution and accuracy depend on several factors. One factor is the degree of interpolation between measured points used to construct the image. Some geophysical methods, such as ground penetrating radar (GPR), produce effectively continuous measurements over a single transect. Other methods, such as seismic reflection, measure data with geophones placed at specific points along a transect that require some interpolation. A second factor is depth. For all survey methods, there is a direct trade-off between the required depth of the measurement and the resolution obtained; the deeper the requirement, the poorer the resolution. Another factor that has a significant influence on the resolution of a geophysical technique is the degree of physical property contrast between geological structures or targets in the subsurface. Most geophysical methods measure changes in some physical characteristic of the geology; if these changes are not distinct, the ability of the instrument to detect them is lessened.

Borehole geophysical methods, such as electrical resistivity or natural gamma logs, provide continuous stratigraphic column information relevant to a specific location or station. Station measurements can be interpreted into a matrix diagram and then interpolated to produce a continuous generalized cross section of the study area. The accuracy and level of detail in the cross section can be enhanced only by increasing the number of stations or by employing different instrumentation that can be deployed in surface-to-borehole or borehole-to-borehole configurations.

Surface-to-borehole and borehole-to-borehole tomography are finding increasing use in environmental geophysical applications. These methods can provide 3-D images with depth. The spacing of generators and receivers generally determines the degree of resolution and accuracy obtained. Geophysical methods can be used for a variety of purposes in DNAPL investigations and remediation:

  • Geologic characterization, including lithology and thicknesses of strata and the topography of the bedrock surface below unconsolidated material, to assess preferential flow pathways;
  • Aquifer characterization, including depth to water table, general water quality, and water-bearing fractures;
  • Contaminant plume distribution when a dissolved DNAPL chemical is mixed with other contaminants with properties that can be distinguished by the geophysical method (e.g., conductive landfill leachate);
  • DNAPL mass location when the mass is sufficiently large to cause a resolvable change in the physical characteristics of the host matrix;
  • DNAPL mass remediation by steam where the geophysical technique is used to track the movement of the steam front to ensure the area thought to contain the DNAPL is completely immersed; and
  • Buried leaking drums or tank locations.

Several techniques that show promise in detecting DNAPL masses are seismic reflection with amplitude versus offset and GPR with amplitude versus offset. Complex electrical resistivity tomography (ERT) has been used to track the movement of a DNAPL mass through the subsurface but not to identify the mass if it is not moving, nor is it effective if a pre-contamination survey was not conducted. ERT has found some use in tracking in situ cleanups of DNAPLs, most notably with steam.

For Further Information

Adobe PDF LogoGeophysical Techniques to Locate DNAPLs: Profiles of Federally Funded Projects
Federal Remediation Technologies Roundtable (FRTR).
EPA 542-R-98-020, 31 pp, 1998

This report provides the results of federally funded projects to investigate the ability of innovative techniques to detect the presence of DNAPLs in the subsurface. Most of the techniques are geophysical in nature, but negative ion sensors and partitioning tracers are also covered.

Adobe PDF LogoHigh Resolution Seismic Reflection to Locate DNAPL Source Zones at Hazardous Waste Sites
Environmental Security Technology Certification Program (ESTCP).
NFESC Technical Report TR-2115-ENV, 140 pp, 2000

This report investigates the feasibility of predicting the location of DNAPLs in the subsurface using seismic reflection. It concludes that the technique is efficient at locating stratigraphic changes that may be used to predict potential flow paths; it is not effective in identifying DNAPL masses.

Adobe PDF LogoIntegrated Geophysical Detection of DNAPL Source Zones
Blackhawk Geoservices, Inc.
Strategic Environmental Research and Development Program, Project CU-1090, 68 pp, 2001

This report describes the difficulties in combining seismic reflection with electrical resistivity tomography to identify chlorinated ethene DNAPLs.

Interpreting DNAPL Saturations in a Laboratory-Scale Injection with GPR Data and Direct Core Measurements
R.H. Johnson and E.P. Poeter.
U.S. Geological Survey, Open-File Report 03-349, 40 pp, 2003

GPR was used to track a DNAPL injection in a laboratory sand tank. Before data reduction, GPR data provide a qualitative measure of DNAPL saturation and movement. One-dimensional GPR modeling provides a quantitative interpretation of DNAPL volume within a given thickness during and after the injection. With geologic conditions that are suitable for GPR surveys (i.e., shallow depths and low electrical conductivities), the procedures in this laboratory study can be adapted to a field site to identify DNAPL source zones after a release has occurred. Note that the key word in this experiment is "track"; stationary NAPLs are harder to identify.

Adobe PDF LogoGround Penetrating Radar for Environmental Applications
R. Knight.
Annual Review of Earth and Planetary Sciences, Vol 29, p 229-255, 2001

This paper contains discussions of the use of GPR in environmental investigations. Many liquid organic contaminants have dielectric properties distinctly different from those of the other solid and fluid components in the subsurface.

Abstracts of Journal Articles

Time-Lapse Electrical Resistivity Tomography Mapping of DNAPL Remediation at a Star Field Site

Adobe PDF LogoFocused Literature Search: Geophysical Methods for Locating and/or Monitoring DNAPLS

This 24-page compilation contains abstracts of literature found in a search conducted in 2004. Some of the abstracts are accompanied by an Internet link to the full text of the report or paper.

Amplitude Variation with Offset (AVO) Analysis of Ground Penetrating Radar Data for Direct Detection and Delineation of NAPL Contamination

Borehole Geophysics for Investigations of Ground-Water Contamination in Fractured Bedrock

A Case Study of Traditional and Alternative Monitoring Techniques for Solvent Contamination within Fractured Bedrock

Changes in Geoelectrical Properties Accompanying Microbial Degradation of LNAPL

Note: this abstract is of interest because it discusses the change in geoelectric properties associated with the aerobic degradation that can occur with some DNAPLs.

A Conceptual Model for the Detection of NAPL Using Amplitude and Phase Variation with Offset (APVO) Analysis of Ground Penetrating Radar Data

Cross-Hole Complex Resistivity Survey for PCE at the SRS A-014 Outfall

DNAPL Site Characterization for Waste Management at Manufactured Gas Plant (MGP) Sites

Detection of DNAPLs using Ultra High-Resolution Seismic Data and AVO Analysis at Charleston Naval Weapons Station, South Carolina

Difference Inversion of ERT Data: a Fast Inversion Method for 3-D In Situ Monitoring

Direct Detection of Dense Nonaqueous Phase Liquids (DNAPL) Using High Resolution Reflection Seismic Techniques at the SWMU-12 Site Charleston, Naval Weapons Station, Charleston, South Carolina

Direct Push Optical-Based Sensor Systems for Characterizing the Subsurface Soil Environment

Effect of Immiscible Liquid Contaminants on P-Wave Transmission Through Natural Aquifer Samples

Effect of Microbial Metabolic Byproducts on Electrical Properties of Unconsolidated Sediments

An Effective Electrode Configuration for the Detection of DNAPLs with Electrical Resistivity Tomography

Electrical Impedance Tomography for Detection of DNAPL Contamination

Electromagnetic Induction and GPR Measurements for Creosote Contaminant Investigation

Electromagnetic Surveys for 3-D Imaging of Subsurface Contaminants. Cost and Performance Report

Evaluation of Geophysical Methods for the Detection of Subsurface Tetrachloroethyene in Controlled Spill Experiments

Geoelectrical Signatures of NAPL Impacted Soils: Implications for the Monitoring of Natural Attenuation

A Geophysical Method for Detection and Quantification of Dense Non-Aqueous Phase Liquids (DNAPL) in the Subsurface

Mapping of TCE and PCE Contaminant Plumes Using a 3-D Induced Polarization Borehole Data

Nonlinear Complex-Resistivity Survey for DNAPL at the Savannah River Site A-014 Outfall

Using Amplitude Variation with Offset and Normalized Residual Polarization Analysis of Ground Penetrating Radar Data to Differentiate an NAPL Release from Stratigraphic Changes

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