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Journal of Applied Geophysics 44 2000 167–180 www.elsevier.nlrlocaterjappgeo
Investigations of geoelectrical signatures at a hydrocarbon contaminated site
Estella A. Atekwana, William A. Sauck, Douglas D. Werkema Jr.
Department of Geosciences, Western Michigan UniÕersity, Kalamazoo, MI 49008, USA Received 24 March 1998; accepted 5 November 1998
Abstract
This study provides an evaluation of the utility and resolution of different geoelectrical methods in mapping contaminant distribution in the subsurface, and provides a window into the processes that may control their response at a site in Central
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Michigan. In situ and 2D surface resistivity, ground penetrating radar GPR , and electromagnetic methods EM constrained Ž
. by soil boring data were used to investigate the electrical properties of a light nonaqueous phase liquid LNAPL
contaminant plume that resulted from 50 years of leakage into a glacio-fluvial geologic setting. Overall, the electrical signature from the in situ resistivity measurements were best able to image the subsurface stratigraphy and the associated
contamination zone. GPR also mapped the subsurface stratigraphy. In particular, the GPR recorded a reflector that is subparallel to the water table, and occurs a few meters above the current free product level, which is coincident with the top
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of an oil-stained, light-gray sand layer. Further, regions of attenuated GPR reflections shadow zones due to enhanced conductivities were found to be coincident with low apparent resistivities. 2D geoelectrical measurements successfully
imaged the top of the saturated zone and the underlying clay layer, but was unable to resolve any anomalous region that could be attributed to the hydrocarbon contamination. Likewise, the EM results provided no evidence of the presence of the
free product plume at depth. Throughout this investigation, geoelectric measurements consistently recorded low resistivities Ž
. high apparent conductivities associated with zones containing the freerresidual product plume instead of high resistivities
as has been suggested by the simple intuitive model. From this, it is inferred that substantial modification of the geochemical characteristics of the plume, surrounding media, and associated groundwater has occurred as a result of biogeochemical
reactions. It is evident from this study that in situ resistivity measurements combined with surface geoelectrical measure- ments can characterize the distribution of conductive zones that may be associated with the biodegradation of LNAPL in the
subsurface. Thus, the application of these techniques to hydrogeologic, contaminant monitoring, and remediation studies are far reaching. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Biogeochemical; Degradation; Conductivity; Contamination; LNAPL; Geoelectrical
1. Introduction
Ž Geophysical surveys
particularly, ground Ž
. penetrating radar
GPR , electrical resistivity,
Corresponding author. E-mail: d.werkemawmich.edu
Ž ..
and electromagnetic methods EM are increas-
ingly being used at sites contaminated by non- Ž
. aqueous phase liquids NAPL , as an aid in the
characterization and monitoring of these sites. Often, geophysicists have based the interpreta-
tion of the results from these surveys on the
0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 9 2 6 - 9 8 5 1 9 8 0 0 0 3 3 - 0
Ž results from simple intuitive models e.g., Mazac
´
. et al., 1990 . This insulating layer model as-
sumes that the presence of NAPLs in the sub- surface can be inferred because the contami-
nated groundwater and soils would have a lower electrical conductivity and lower relative per-
mittivity than the surrounding uncontaminated media. Such a model has received wide support
from controlled spill and laboratory experiments
Ž using mostly fresh hydrocarbon products King
and Olhoeft, 1989; Schneider and Greenhouse, 1992; Daniels et al., 1992; Endres and Redman,
1993; de Ryck et al., 1993; Redman et al., 1994; Monier-Williams, 1995;; Grumman and
Daniels, 1995; Endres and Greenhouse, 1996;
. Campbell et al., 1996 .
However, none of these geophysical experi- ments replicates the dynamic field conditions
usually present at contaminated sites. Thus, it is not surprising that the geophysical investiga-
tions at hydrocarbon spill sites have yielded
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remarkably variable results. Benson 1995 and Ž
. Benson and Mutsoe 1996 show enhanced GPR
reflections at hydrocarbon-contaminated sites. Other investigators report attenuation of GPR
signal amplitude resulting in a ‘shadowing’, ‘fuzzy’, or ‘muted’ appearance in the vicinity of
gasoline contamination in the capillary fringe ŽDaniels et al., 1992; Grumman and Daniels,
. 1995; Maxwell and Schmok, 1995 . Further-
Ž more, high apparent conductivities
Monier- .
Williams, 1995 or low apparent resistivities
ŽBenson and Stubben, 1995; Gajdos and Kral, 1995; Benson and Mutsoe, 1996; Sauck et al.,
. Ž
1998 and high interpreted resistivities Benson .
et al., 1997 associated with areas of known hydrocarbon plumes have been reported. In some
cases, no noticeable change in the geoelectric signature was observed at a site even though
hydrocarbon contaminants were detected in
Ž groundwater monitoring wells Grumman and
. Daniels, 1995 .
The above investigations illustrate that the geoelectrical properties of hydrocarbon contam-
inant plumes and their host media vary and suggest that the widely-acceptable insulating
layer model does not adequately describe the geoelectrical signatures observed at many NAPL
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contaminated sites. Recently, Sauck et al. 1998 have proposed that the volume impacted by
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hydrocarbon spills in this case LNAPLs in the natural environment changes from electrically
resistive to a more conductive behavior with time due to a variety of biogeochemical pro-
cesses. This temporal change in the resistive behavior of hydrocarbon contaminated zones
has not been adequately exploited in geophysi- cal interpretations and models at contaminated
sites, although it has been suggested that many hydrocarbons and organic chemical contaminant
plumes change with time due to a variety of
Ž active processes
Olhoeft, 1992; Benson and .
Stubben, 1995; Benson et al., 1997 . Support for this hypothesis comes from geochemical
studies at hydrocarbon contaminated sites which indicate the presence of highly conductive
groundwater
below some
LNAPL plumes
ŽBaedecker et al., 1987; Cozzarelli et al., 1990; Baedecker et al., 1993; Bennett et al., 1993;
. Eganhouse et al., 1993 . Apparently, hydrocar-
bon degradation by bacterial activity in the va- dose zone and groundwater produces carbonic
and organic acids which contribute to enhanced mineral dissolution of the aquifer materials
Ž
. McMahon et al., 1995 . This leads to the pro-
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duction of a high total dissolved solids TDS leachate plume that is reflected in the increased
groundwater conductance
observed in
and Ž
around the zones of active biodegradation Be- .
nson and Stubben, 1995; Benson et al., 1997 . Consequently, at a given location the composi-
tion and physical properties of the freerresidual product plume and surrounding media will
change or evolve with time.
The apparent ambiguity in the spatial and temporal geoelectrical properties at hydrocarbon
spill sites can be explained by changes in the vadose zone, aquifer, and their associated pore
fluids. Changes that can influence the measured geoelectrical signature include, but are not lim-
Ž . ited to: 1 porosity changes in the vadose zone
as the hydrocarbon displaces and occupies pre-
Ž . viously air filled pores,
2 displacement of
water in the capillary fringe by hydrocarbons, Ž .
3 the thickness, saturation, and distribution of the residualrfree product above the water table,
Ž . 4 the thickness and distribution of the leachate
Ž . plume in the upper part of the aquifer,
5 changes in aqueous pore fluid chemistry due to
Ž . microbial degradation, and 6 reaction between
the aquifer solids and microbial degradational products. Clearly, the observed geophysical sig-
natures are in response to complex physical and biogeochemical changes occurring within a dy-
namic system.
Ž In light of the above hypothesis that hydro-
carbon impacted media will change from elec- trically resistive to conductive behavior with
. time , this study investigates the geoelectrical
signature of a hydrocarbon-impacted site with a 50-year spillage history, using multiple geoelec-
trical methods. In addition, the distribution of the hydrocarbon contaminants in the subsurface
and their relationship to the observed geoelectri- cal measurements are evaluated. It is assumed,
based on the spill history, that sufficient time has elapsed to modify the hydrochemical envi-
ronment within and around the existing plume, and presumably the geoelectrical signature.
2. Background
