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5th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 26 Apr 1992 - 29 Apr 1992
- Location: Oakbrook, Illinois, USA
- Published: 26 April 1992
21 - 40 of 40 results
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Improved Gpr Interpretation From Analysis Of Buried Target Polarization Properties
Authors Roger Roberts, Jeffrey J. Daniels and Leon PetersGround penetrating radar (GPR) investigations are often focused on the detection
of subsurface targets such as barrels, pipes, and isolated patches of
contaminants. Unfortunately many study areas also contain undesirable signals
from other objects such as logs, concrete, and disrupted soil. Knowledge of the
polarization properties of targets with different degrees of symmetry coupled with
GPR data containing polarization information enhances the interpretation of GPR
data over complex subsurface zones.
An ongoing study at The Ohio State University is investigating GPR polarization
with an array of antennas pulled by hand or towed behind an all-terrain vehicle.
The two-channel GPR system contains a switch that alternates radiation from one
antenna to a transmit antenna orthogonal to the first antenna. The receive
antennas are in a crossed-dipole configuration located at the center of the array.
Parallel, perpendicular, and cross polarization data. are received from subsurface
targets from the two orthogonal transmit antennas.
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Geophysics And The Solvents-In-Groundwater Program
By J.D. RedmanThe University of Waterloo has an active research program studying the application of geophysical methods
to subsurface contaminant detection and delineation. As part of the Solvents-in-Groundwater Program, the
focus of our recent research efforts has been on the class of contaminants referred to as dense nonaqueous
liquids (DNAPLs).
During the past two years, we have perfomed geophysical monitoring of controlled DNAPL spills in a natural
sandy aquifer. The aquifer was isolated from the surrounding environment by steel sheet-pile walls driven
into an underlying aquitard.
The geophysical monitoring of the first injection of 231 litres of DNAPL into a 3m x 3m x 3.5m deep cell
consisted of in situ dielectric permittivity monitoring, ground penetrating radar(GPR) and in situ resistivity
measurements. All of these techniques detected changes in the measured physical properties during and
following the injection. The GPR monitoring was limited in scope and the data quality, as expected, was
affected adversely by the small size of the cell and the numerous metallic monitoring devices installed in the cell.
The experience gained during this first injection experiment was applied to the methodology used for
extensive geophysical monitoring in the much larger 9m x 9m x 3.3m cell. In this cell, 770 litres of DNAPL
were injected, and monitoring was performed using neutron, den&y, and induction logging, borehole radar,
surface GPR, and in situ resistivity and dielectric permittivii devices. Most of the geophysical methods
successfully monitored the migration of the DNAPL during and following the injection.
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Effects Of Acquisition System Parameters On Refraction Survey Data
Authors T. Kanemori, Finn B. Michelsen and John H. MimsThe seismic refraction method is ideal for a variety of applications where large
features that are characterized by an abrupt increase of the seismic velocity need to be
delineated, such as the depth to the water table or the depth to basement rock. Over the
years, the techniques used to acquire and process seismic refraction data have changed
slowly. Most of the current techniques used for refraction processing were developed from
the late 1930’s to the early 1960’s (e.g., Hagiwara and Omote, 1939; Hagedoorn, 1959;
Hawkins, 1961). The most recent advance in seismic refraction processing, the Generalized
Reciprocal Method, was first published in 1980 (Palmer, 1980). Since the early 1970’s,
computer programs have been developed to facilitate processing of seismic refraction data
(e.g., Scott and Markiewicz, 1990). Most of these programs have merely changed the
processing platform from drafting boards to computer monitors with little modification to
the processing procedures.
The most recent developments for the refraction method have involved the data
acquisition systems. Features such as analog and digital filtering, IFP, and automatic gain
setting have become standard on many seismic data acquisition systems. These features
have made recording systems easier to operate and the recorded seismic traces appear to
be more robust. The results of theoretical calculations, numerical model studies, and field
tests, however, demonstrate the need to fully understand how changes to the acquisition
system settings affect the first break signal.
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Geophysical Investigation And Characterization With Usrads
Authors C.R. Flynn, M.S. Blair and J.E. NyquistThis paper describes two recent case histories in which commercially available geophysical
instruments were used with an innovative tracking and mapping system called USRADS
(UltraSonic Ranging And Data System) that automates data location and collection. USRADS uses
ultrasonics to provide real-time surveyor positioning and radio links to transmit the surveyor data
to an on-site computer for storage and real-time display. USRADS uses a standard 386 computer
for data collection and includes real-time color display of the findings. It also includes numerous
analysis and display formats for on-site, as well as utilities to facilitate post-process analysis of the
findings.
The objective of one project was to locate several suspect waste disposal trenches and to map their
boundaries. The second was to locate and map the presence of subsurface unexploded ordinance
(UXO) at a suspect artillery impact area. A Geonics EM31 terrain conductivity meter interfaced to
USRADS was used to map the suspect trenches. A Schonstedt GA-52C magnetometer interfaced
to USRADS was used to map the subsurface UXO. Correlation of findings to known site features
and additional knowledge about the sites indicates that these efforts did locate and map the
geophysical features including the suspect waste trenches and the subsurface UXO. Images of the
findings generated on-site and during post-processing are included.
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Geophysical Techniques For Characterizing Shallow Velocity-Attenuation Models
Authors G.I. Al-Eqabi and R.B. HerrmannKnowledge of seismic shear wave velocity and Q is required for both seismic
hazard analysis and the siting of critical structures since these parameters govern
the transmission of the seismic signal from the earthquake to the site and also
control the site response itself. These parameters vary spatially and may not be
readily available. The purpose of this paper is to show how these can be
estimated using surface-wave analysis techniques.
Data from USGS refraction surveys in Maine were selected which show dispersed
short-period Rg waves (fundamental mode Rayleigh waves). Processing techniques
applied to the data include filtering, waveform inversion of selected trace,
phase velocity stacking, interactive amplitude processing, inversion of dispersion
parameters for a layered shear-wave velocity and Q model, and finally verifying
the derived velocity-attenuation models by comparing both shape, absoluteamplitude
and arrival times of synthetic and observed time series. The resultant
velocity models show that the shear-wave velocity varies between 2.43-2.81
km/set for surface layer and Qp is less than 50 in the upper kilometer of the
metamorphic terrain of Maine. The models correlate with the type of geologic
rocks encountered. While the study focused on upper crustal structure, the techniques
and experimental procedure can be used for shallow site characterization
by appropriate scaling of the data acquisition.
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Shallow Reflection Seismics Using Firecrackers As The Source
Authors Chien-Ying Wang, Deng-Bo Chen and Dao-Tze TsaiDue to the high resolution requirement of shallow reflection seismics, a
firecracker source was invented to generate high frequency signals used in
studying shall ow structures. Except for a slightly lower energy level, the
firecracker source has proved to be efficient, portable, cheap and safe. Its
performance was satisfactory. In this paper, a field test in a good
reflection area was conducted to exhibit high frequency signals among the
massive noises of groundroll when delineating the underground layers. By
setting the geophones at the near as well as the far offset distances, we
obtain different but compatible seismic profiles which could provide us with
different structural details. Careful and properly adjusted field procedure
has always been a key factor for successful shallow seismic reflection studies.
The other purpose of this paper is to use the firecracker source for
investigation of near-surface faults whose locations are only grossly known.
Three seismic lines, one used in detecting the Hsincheng fault and the other
two the Shihtan fault, were shot. They all resul,t in good reflection images
illustrating the structural variation across the fault. This kind of
near-surface fault mapping provides very detailed information about the
fault and the structure, which can be used to understand the faulting
processes during the earthquake. It is believed that the developed technique
could work equally well for other engineering applications. On account of
the outstanding achievements of firecracker source, we may expect a wider
acceptance of this powerful method for shallow structure studies in the future.
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Surface And Borehole Radar Monitoring Of A Dnapl Spill In 3D Versus Frequency, Look Angle And Time
Authors K.A. Sander, G.R. Olhoeft and J.E. LuciusAs part of the University of Waterloo continuing experiments at Canadian Forces
Base Borden (CFB), a controlled spill of perchloroethylene, a dense nonaqueous phase
liquid (DNAPL), was monitored by a variety of geophysical methods. Surface ground
penetrating radar (GPR) at 300,500, and 900 MHz and hole-to-hole GPR at 160 MHz were
periodically measured at the site for 340 hours. This report presents the results of the first
66 hours of 500 MHz surface data and an example of the 900 MHz data at 14 hours. The
surface data were acquired on a one meter grid in both directions across a nine meter by
nine meter experimental cell. The borehole radar data were acquired between 8 wells
circling the spill point on a 3 meter radius, using all 28 combinations of non-repeating holeto-
hole pairs, each at 14 different depths in the cell at 25 cm intervals. The resulting multidimensional
(x, y, z, time, frequency, look angle) dataset clearly outlines the movement of
the DNAPL horizontally and vertically, and the interactions of the DNAPL with the
heterogeneous sand matrix. Subtle changes in grain size distribution and the resultant
capillary forces in the sand caused the DNAPL to spend more time and distance in
horizontal travel than vertical travel.
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A Summary Of Methods For Locating And Mapping Fractures And Cavities With Emphasis On Geophysical Methods
Authors Richard C. Benson and Lynn YuhrAssessing the existence, location and mapping of geologic anomalies, such as fractures, cavities within soil
and rock are common to both engineering and hydrologic project needs. Spatial considerations of such a
site characterization must address the detection probabilities involved by using boreholes alone. Spatial
considerations must also include the characteristic dimensions of the fracture/cavity system, including
size, depth and perkxlicity and an appreciation of methods other than drilling to improve the probability of
detection.
A summary of the common methods available for fracture and cavity detection mapping (non-geophysical
as well as geophysical) are presented along with their spatial sampling and limitations. The remote sensing
and geophysical methods are but a small part of this list. However, they include some of the most effective
methods for fracture and cavity detection and mapping. They cover the complete range of spatial
sampling, from regional to no more than the localized borehole wall. The airborne, remote sensing,
surface, and downhole geophysical methods along with methods that can be used on water covered areas
are identified along with their spatial sampling capabilities. An understanding of the tools available and
their spatial sampling limitations are a necessary part of planning and carrying out an effective
fracture/cavity investigation.
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Fracture Detection With Sellsmlc Crosshole Tomography For Solution Control In A Stope
Authors James A. Jessop, Michael J. Friedel, Michael J. Jackson and Daryl R. TweetonAs part an environmental assessment for the application of in situ leaching in an
underground mine, the U.S. Bureau of Mines conducted a seismic crosshole tomographic
investigation at the Colorado School of Mines experimental mine near Idaho Springs, CO.
Seismic tomography was used to detect fractured regions as part of an investigation aimed at
providing information that will help mining companies control solution during stope leaching.
Whole waveform data provided both direct P-wave and S-wave traveltimes for input to the
Bureau’s curved ray tomography program MIGRATOM, based on Huygens’ principle of
wavefront propagation.
The heterogeneous velocity distribution observed is attributed to the presence of both
natural and blast induced fractures. The effectiveness of the tomographic method was
corroborated by noting that low velocity zones occurred where water was lost in boreholes.
The pre- and post-blast P-wave tomograms showed similar velocity distributions. This
similarity in velocity tomograms suggests that the blast energy was contained resulting in
minimal extension of existing fractures. Where changes in velocity were observed, they
correlated to either natural fractures or blast induced damage. Tomograms also were
produced showing the variation in dynamic elastic properties, including Poisson’s ratio,
Young’s, shear, and bulk moduli. Such dynamic elastic properties assist in solving
engineering problems associated with deformation, stability and blast design.
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The Relationship Between Lineaments And Fractures In Chad Basin
Authors Solomon A. Islorho, Karen S. Taylor-When and Tom. O. NkereuwemOne of the problems facing drought stricken parts of the world is the location of potable
sources of water. The apparent increase in global temperature is also of great concern in
those areas that are thirsty and as a result any available source of water needs to be
monitored and understood for management purposes. Recent studies in the drought striken
Sahel region (especially Chad Basin) indicate the presence of fractures within and near
Lake Chad. This paper looks at the use of geophysics to determine a fracture near the lake
shore and how this might be related to geochemical water quality of the region.
In the summer of 1991, a transect twenty kilometers in length was chosen perpendicular
to a lineament that was inferred from Landsat images. This was mapped using electrical
resistivity method (using Strata Scout and ABEM Terrameter) that utilized both the
Wenner and Schlumberger arrays for both sounding and profiling. The profile shows a
number of minor anomalies with a major anomaly that corresponds to the fracture that
was mapped from the Landsat images. There is no topographic feature that corresponds to
the anomaly or lineament in the field. Water samples were also collected in the field to see
the influence of the fracture on the water quality of the region.
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The Application Of Shallow Geothermal Exploration Methods To Detect High Permeability Features In Groundwater Flow Systems
By John JansenAn exploration method using shallow soil temperature
measurements as an exploration tool to find permeable bodies at
depth in an impermeable matrix is described. The depth of seasonal
soil temperature variation is discussed. A method of measuring
soil temperatures below the depth of seasonal variation is
described. Simple finite difference modeling techniques to predict
thermal anomaly characteristics are presented. Case histories are
presented which demonstrate the application of the method for
groundwater.
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Integrated Geophysical Studies Over Buried Valleys
Authors Paul J. Wolfe and Benjamin H. RichardIntegrated geophysical studies of four buried valleys have been completed in
Southwestern Ohio’. All of the areas have been overridden by two or more Pleistocene
glaciers and each valley contains sequences of till, outwash, and lacustrine deposits. This
paper summarizes the results of those studies and proposes a strategy for shallow geophysical
investigations in glaciated areas. The techniques used in the integrated studies were gravity,
magnetics, resistivity, ground penetrating radar, seismic refraction, and seismic reflection.
Certain combinations of geophysical techniques can be most cost effective but it is
important to clearly define the goals of the study and to understand the existing field
conditions before designing the geophysical survey. If only general depth to water table and
bedrock are needed and there is essentially no cultural development in the area, electrical
methods are inexpensive and fast. If there is no clay and the groundwater is low in mineral
content, then ground penetrating radar can be quick and inexpensive.
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Cokriging Surface Elevation And Seismic Refraction Data For Bedrock Topography
Authors J.E. Nyquist, W.E. Doll, R.K. Davis and R.A. HopkinsAnalysis of seismic refraction data collected at a proposed site of the Advanced Neutron Source
(ANS) Facility showed a strong correlation between surface and bedrock topography. By combining
seismically determined bedrock elevation data with surface elevation data using cokriging, we were
able to significantly improve our map of bedrock topography without collecting additional seismic data.
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Seismic Reflectlon/F?Efractlon Survey To Characterize The Subsurface At An Npl Site In The Mojave Desert
More LessIn July and August 1991, a seismic reflection and refraction survey was performed at the Marine Corps
Logistic Base (MCLB) in Barstow, California. The purpose of the survey was to determine the
configuration of: 1) bedrock, 2) the water table, 3) confining layers, and to precisely locate faults; mostly
the Waterman, that may affect contaminant pathways.
The site is on the National Priority List (NPL) with known contamination from solvents and petroleum
hydrocarbons. The MCLB is located within an alluvial basin in the Mojave block, a large wedge shaped
fault block bounded by the San Andreas and Garlock Faults.
The seismic survey was performed in support of the Remedial Investigation/Feasibility Study being
conducted by the Navy. it was designed to be innovative in response to the depth and nature of various
geologic formations, especially in light of difficufties experienced in collecting data in this desert region.
The Bison Model 9048 seismograph was used to record data generated by the Bison Elastic Wave
Generator Ill. The data was processed by using Seistrix 2386 for reflection and Firstpix and Gremix for
refraction. A split-spread array allowed for the extraction of both CDP reflection data and forward and
reverse refraction data. A total of ten miles of data were shot within a ten day period. Vertical seismic
profiling (VSP) was performed in four wells to assist in interpreting the surface information.
Combining data from the two techniques suggest that there is a mounding of water at the en echelon
faults which cut across the highlands into the alluvium. Faults not only displace the bedrocks but,
extend into sediments near the surface. The seismic survey provided a picture of the subsurface which
will assist in planning future work.
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Geophysical Detection Of Hydrocarewn And Organic Chemical Contamination
More LessUnambiguous detection of hydrocarbon and organic contamination is the most
difficult task for noninvasive geophysical methods at hazardous waste sites. The difficulty is
two-fold: 1) the low level of geophysical contrast that these contaminants provide against the
background soil and rock, and 2) the low level of contaminant concentration considered to
be of regulatory concern. Yet, electrical and electromagnetic methods can sometimes
detect some hydrocarbon and organic contaminants. The important questions are what
levels of contamination can be reliably detected and where? The most important
consideration in answering these questions is the level of interaction that occurs between the
contaminating chemical and the host matrix materials. The most obvious of contaminants
are those which either are insoluble in water or which chemically react with clay minerals.
The best understood examples are tetrachlorethene (common dry cleaning solvent) and
toluene (common industrial solvent and principle component in gasoline). Both have low
solubility in water, high electrical resistivity and low dielectric permittivity. Thus, both are
visible to ground penetrating radar (though toluene floats on the water table while
tetrachlorethene sinks). Toluene is also catalyzed on the surface of montmorillonite to
polymerize into bibenzyl, and the electrochemistry of the polymerization reaction is
observable with the complex resistivity technique. Some organic contaminants such as
alcohols, which dissolve in water, are essentially without geophysical trace. In between are
interactions such as insoluble lenses floating on the water table, suppression of the capillary
fringe, change in soil wetting, interference with cation exchange, and others, such as a
change in the background geological noise statistics, that sometimes produce a detectable
geophysical signature. Few geophysical signatures are unique indicators of hydrocarbon or
organic chemical contaminants, but repeated measurements with time can often detect the
movement of contaminant fluids, more uniquely indicating their presence. No hydrocarbon
or organic chemical contaminant produces a geophysical contrast that is observable at the
parts-per-billion concentration level of regulatory concern. Even when the contaminant
itself produces no detectable direct geophysical signature, sometimes it may be found by
indirect means through the use of geophysical mapping and characterization of
geohydrological heterogeneity as a guide to drilling likely traps of the contaminant.
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Site Studies Of Ground Penetrating Radar For Monitoring Petroleum Product Contaminants
Authors Jeffrey J. Daniels, Roger Roberts and Mark VendlGround penetrating radar (GPR) provides a possible means of mapping
hydrocarbons in the vadose zone. Results of controlled surveys in a sand test pit
at The Ohio State University demonstrate conclusively that there is a clear high
amplitude GPR anomaly over plastic containers containing diesel fuel and
containers containing the host sand material saturated with diesel fuel.
Results of surveys at a site in Northern Indiana show a correlation between the
decrease of the GPR signal amplitude in the vicinity of the gasoline concentration
and the presence of gasoline. A decreased amplitude of the GPR signal is
present in the capillary fringe region above the water table at this site, suggesting
that vapor-phase hydrocarbons may affect the propagation of the
electromagnetic wave. Possible explanations for the observed high amplitude
reflections over the confined hydrocarbons in the test pit and the low amplitudes
in the field include: 1) the hydrocarbon product may be contained in small
dispersed concentrations in the vadose zone, or 2) the hydrocarbon product may
have a high loss tangent. Small dispersed concentrations of hydrocarbon
product above the water table cotild cause reflecting and diffracting boundaries,
resulting in many small isolated anomalies on a GPR record. A high loss tangent
for the hydrocarbon product could help to explain both the observed decrease in
amplitude for the electromagnetic wave propagating through the capillary fringe,
and the high amplitude reflections from the saturated hydrocarbons.
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Monltorlng A Controlled Injectlon Of Perchloroethylene In A Sandy Aquifer With Ground Penetrating Radar And Time Domain Reflectometry
Authors M.L. Brewster, J.D. Redman and A.P. AnnanA controlled injection of perchloroethylene (PCE) into a sandy aquifer was undertaken to investigate the
effectiveness of ground penetrating radar (GPR) and time domain reflectometry (TDR) for monitoring and
detecting the subsurface migration of organic solvents.
Surveys were performed at regular intervals during the 70 hour injection and for 38 days following the
injection. Background (pre-injection) surveys were conducted to assist in the discrimination between features
caused by PCE and those due to natural geologic variability.
PCE was chosen because it is a typical member of a class of groundwater contaminants known as dense
non-aqueous phase liquids or DNAPLs. Similar to other DNAPLs, PCE has a low dielectric permittivity relative
to pore water, therefore PCE saturated soils can be expected to have higher radar velocities.
Time domain reflectometry probes were used to measure vertical profiles of in situ dielectric permittiiity.
Decreases in dielectric permittiiity of up to 50% were observed where PCE had pooled on low permeability
lenses.
Initially, GPR reflection events were detected from pooling of the DNAPL on low permeability horizons at 1 m
depth. At later times pools were observed on successively deeper horizons. Velocity pullup of the underlying
aquitard event was also observed.
GPR and TDR provide effective methods for monitoring the progress of PCE migration in a saturated sandy
aquifer. Reflections and dielectric contrasts from PCE pools are of such a high magnitude that they may
often be distinguishable from geology without the benefii of background data. Wiih background data, zones
of residual level PCE saturations are resolvable with GPR and TDR.
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Geophysical Detection Of Perchloroethylene In A Sandy Aquifer Using Resistivity And Nuclear Logging Techniques
Authors George W. Schneider and John P. GreenhouseAs part of a larger programme to understand the behaviour of dense non-aqueous phase
liquids (DNAPLs) in the subsurface (Redman, 1992), a controlled release of 770 litres of
perchloroethylene (PCE) into a 9 by 9 metre cell within a sandy aquifer was monitored
geophysically during the summer of 1991. Surface and in situ resistivity, terrain conductivity
and neutron, induction, and density logging data were collected over a period of several weeks
to document the movement of the PCE away from the point of injection towards its final resting
place in pools on confining layers within the cell.
The very high resistivity of the PCE in its liquid state makes it a target for resistivity,
induction logging and terrain conductivity methods. Its high density, low hydrogen and high
chlorine content make it a good target, in principle, for density and neutron logs. For resistivity
measurements, two specifically designed in situ probes were installed one metre on either side
of the injection point. Two surface arrays were used to monitor apparent resistivity across the
diagonals of the cell. A resistivity scanning system was developed to ensure rapid acquisition
of surface and probe resistivity data. The density, neutron and induction logs were performed
on a regular basis in nine PVC-cased boreholes arranged on a 3 metre radius circle around the
injection point. In all cases, extensive background measurements were made.
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Use Of Resistivity Monitoring Systems To Detect Leaks From Storage Ponds
Authors Gregory P. Van, Stephen K. Park and Patrick HamiltonA resistivity monitoring system was installed beneath and around a fifty-five acre
evaporation pond at the Mohave Generating Station in Laughlin, Nevada. The system was
installed to detect leakage of brine from the pond. The system consists of sixty-three
electrodes which are placed in a 7 by 9 grid using a go-meter spacing.
The feasibility of using this monitoring system was demonstrated by a scaled-down field test.
This field test determined that distinct measurable changes in resistivity occur in response
to the influx of water into the vadose zone. A direct correlation of increases in measured
voltages relative to the amount of water put into the ground was observed. Threedimensional
numerical modeling of the leak, using a finite-difference code, confirmed that
the changes observed in the field test were due to the influx of water.
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Risk Assessment Of A Landfill Using Soil Gas Survey
More LessThe soil gas survey technique was applied at a codisposal facility to define the direction
and extent of a contaminated soil gas plume. A typical soil gas probe assembly was used to
collect soil gas samples from approximately 2.5 feet below the ground surface. Photovac lOS50
portable gas chromatographs (GC) were used for field analysis of approximately 120 samples
during three sampling events. In addition, air samples were taken from the landfill surface. Each
sample was analyzed for five target volatile organic compounds (VOCs): vinyl chloride 0/C),
1,l -dichloroethene (1,l -DCE), trichloroethene (TCE), tetrachloroethene (PCE), and benzene. The
main criterion for selecting these compounds was their level of toxicity.
The similarity of the soil gas contaminants to those measured at the landfill surface (flux
box) indicates that landfill air emissions may be a primary source for the compounds identified in
the soil gas. Potentially, the presence of a polyvinyl chloride (PVC) cap over the municipal/
industrial landfill could be forcing landfill gas out of the perimeter of the landfill into the adjacent
soil. Large amounts of landfill-generated methane may create a significant pressure gradient in the
landfill and cause a vapor phase plume to migrate in the direction of lower pressure off the landfill.
Soil gas is further considered to be a source of contamination for ambient air above ground.
An advective transport model was used to predict the concentrations of contaminants in air.
Ambient air monitoring results showed a good correlation with the values predicted by the models.
Traditionally, the soil gas survey technique is used to define the direction and extent of
contamination in soil and groundwater. In the present study, this technique was used innovatively
to define the soil gas plume and then for risk assessment purposes via the air-vapor pathway.
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