The need to identify polluted soils and groundwater has increased significantly as the amount of regulations and the knowledge of risks has increased. The private sector as property owners, and firms selling and purchasing properties are both concerned about subsurface contamination. This vast new need to assess potentially polluted sites accurately and economically, poses new demands on assessment techniques. The application of geophysical methods such as ground penetrating radar (GPR) and infrared thermography to assess soil contamination has become possible as data handling has become more and more accurate. These issues have been considered with case examples by Weil et al. (1994).
The assessment process has three general phases. The first phase is to define all potentially contaminated sites by using old information, maps and conducting interviews with site knowledgable personnel. The second step of the process is confirmation of the contamination by means of soil and groundwater analysis. In many cases, the actual contaminated area is only a small part of the site. Often there is no information available on the groundwater hydrology of the site either. In this case the ground penetrating radar works well. It produces data from large areas for a reasonable price with reasonable accuracy to facilitate the detection of the contamination. The final phase in the process is further analysis and possible remediation of the plume.
The above mentioned processes have been carried out worldwide,
including in Denmark and Finland both in the public and the
private sector. The conclusion of Johanna Peltola's and Jesper
Christensen's (VT graduate students) discussion of these
experiences is available here. 
(450 Kb)
GPR techniques are good for defining the overall existence of contaminants in groundwater and soil. Such substances as salts (e.g. chloride), petroleum products and chlorinated hydrocarbons are well detected by GPR. The detection depth is dependent on soil type and moisture conditions. However, GPR works best when detecting in shallow soils and aquifers than into depth.
The GPR technique is similar in principle to seismic reflection and sonar techniques. GPR is like taking an x-ray of the ground. Pulse mode GPR systems radiate short pulses of high frequency electromagnetic energy into the ground from a transmitting antenna. The propagation of the radar signal depends on the frequency-dependent electrical properties of the ground. When the radiated energy encounters an inhomogeneity in the electrical properties of the subsurface, part of the incident energy is reflected back to the radar antenna. Reflected signals are amplified, transformed to the audio-frequency range, recorded, processed, and displayed. From the recorded display, subsurface features such as soil/soil, soil/rock, and unsaturated/saturated interfaces can be identified. In addition, the presence of floating hydrocarbons on the water table, the geometry of contaminant plumes, and the location of buried cables, pipes, drums, and tanks can be detected.
Electrical conductivity of the soil or rock materials along the propagation paths introduces significant absorptive losses which limit the depth of penetration. The radar frequency selected for a particular study is chosen to provide an acceptable compromise between deeper penetration and higher resolution. High-frequency radar signals produce greater resolution, but are more limited in depth of penetration. To get accurate information on the depth of signal response, GPR has to be calibrated with a core sample.
To view an animation of GPR principles click 
(950 Kb)
The process of GPR data collection is relatively simple. The radar unit is towed along a grid outlined on the site. The unit sends the data to a computer where it is stored and analyzed.
The primary output information from radar is in two
dimensional form (vertical profile). See typical GPR field data
plot in form of cross
sectional picture. This type of data can be immediately
interpreted for large subsurface objects, but for more detailed
information it is necessary to use post processing techniques. By
scanning the vertical profiles over the site, a three dimensional
picture can be constructed to better illustrate the overall
conditions. See 3D
picture of GPR survey results .
Both light and heavy non aqueous phase liquids (LNAPL's and DNAPL's) pose additional problems in detecting and defining the size of the plume. Dissolved contaminants can be detected from groundwater samples, however, a more effective method for detecting NAPLs is the use of GPR.
A study by Daniels et al. (1995) suggests that GPR can be used to map hydrocarbons in the vadose zone. The electromagnetic permittivity contrast between liquid hydrocarbon and water (or air) is high in clean sands, gravel, and also in clayey or loamy soils (around 2:1). The single greatest factor influencing GPR data repeatability was the moisture content of the soil and so the results are dependent on climatic conditions. The data of highest quality was gained when the subsurface was least saturated. The study also indicates that the GPR technique was good for detecting hydrocarbon pools floating on the water table and also in monitoring their migration.
Several studies (Sander et al. 1992; Brewster et al. 1995) on the use of GPR for observing DNAPL migration have been carried out since the late 1980's. An investigation by Brewster et al. (1995) was made in a completely saturated sandy aquifer. Tetrachloroethylene (TCE) was released to the aquifer and its migration was monitored with GPR and other geophysical methods. Pooling and lateral spreading of TCE on low permeability horizons was successfully observed. Mass of the plume was calculated from radar measurements with accuracy varying from 48% to 100%.
Click here to view an animation of a comparison of sampling
methods. (250 Kb)
The animation shows a plume in a large area. Wells are drilled to attempt to locate the area of contamination, then the GPR method is done. It is easy to see how the GPR is much more efficient and effective in locating the plume. The use of wells should compliment the GPR. The GPR is used to locate and map the plume, then the wells are used to determine exact concentrations of the contaminant, the groundwater flow, and other parameters.
A GPR with two men can detect an area of more than 2 hectares per day with good resolution depending on the site characteristics. In Finland, a private survey of 2 hectares an old gas station lot was carried out with GPR to detect possible petroleum leaks. The approximate costs of both GPR method and conventional method are represented in next tables.
| GPR and subsequent sampling | ||
|---|---|---|
| Two men and the radar | 2000 $ | |
| Subsequent sampling (2 points) | 1500 $ | |
| Total cost | 3500 $ |
| Conventional methods (20 sampling wells) | ||
|---|---|---|
| Installing the wells | 2000 $ | |
| Chemical Analysis | 20*200 $=4000 $ | |
| Total cost | 6000 $ |
The GPR is able to detect any kind of contamination. In addition to the lower price, the GPR data provides valuable information on hydrological conditions (water table and soil moisture) and other characteristics of the whole area, e.g. underground pipelines.
The biggest shortage of GPR is that the observed anomalies have to be interpreted according to available information on the subsurface conditions. When characterizing the contaminants and estimating the mass of the plume, conventional methods have to be applied along GPR measurements once when the plume is located.
This is not really a shortcoming, it merely states that for a remediation to be carried out most efficiently and effectively, GPR and conventional methods should be used together. These are not competing measures, they are complementary.
The repeatability of results is highly dependent on seasonal conditions in terms of soil moisture (Daniels et al. 1995). On some soil types GPR techniques cannot be applied as the radar signal is attenuated by those soils (e.g. clay particles).
Significant amounts of plume contaminants may be in the volatilized form. These compounds are more difficult to observe than adsorbed or dissolved contaminants. Daniels et al (1995) states that volatilized compounds strongly change the permittivity of radar signals. There has not been further investigations, but the result suggests that GPR could also be good at detecting these volatilized compounds.
There are some other uses of GPR. For example, a GPR unit can be attached to a truck and driven over bridges to inspect the concrete structure. Also, GPR was used to inspect the foundation of the Capitol building, and used to detect oil plumes under ice. Click here to see pictures of these other techniques.
Brewster, M.L., Annan, A.P., Greenhouse, J.P., Kueper, B.H., Olhoeft, G.R., Redman, J.D. and Sander, K.A. 1995: Observed migration of a Controlled DNAPL Release by Geophysical Methods. Ground Water, Vol. 33, No. 6, Nov.-Dec.1995, pp. 977-987.
Daniels, J.J., Roberts, R. and Vendl, M. 1995: Ground penetrating radar for detection of liquid contaminants. Applied Geophysics, Vol. 33 (1995), pp. 195-207.
Weil, G.J., Graf, R.J. and Leann, M.F. 1994: Investigations of Hazardous Waste Sites Using Thermal IR and Ground Penetrating Radar. Photogrammetric Engineering & Remote Sensing, Vol. 60, No. 8, Aug. 1994, pp. 999-1005.
Send comments or suggestions to:
Student Authors: Josh Anderson joander8@vt.edu
and Johanna Peltola jpeltola@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
