Objective:
This ESTCP project evaluated the ability of electromagnetic
(EM) resistivity surveys to detect chlorinated organic compounds
in the form of dense non-aqueous phase liquids (DNAPLs) and
generate a 3-D model of these DNAPL source areas in soil and
groundwater. Through the EM survey, an electromagnetic field
is induced at the surface with a transmitter coil, after which,
the subsequent resistivity contrasts produced by the presence
of key subsurface heterogeneities and/or anomalies are measured
with a receiver in an instrument well or borehole. Changes
in the electromagnetic field are recorded including what is
hoped to be a set of unique resistivity contrasts representative
of the presence of subsurface DNAPL. A proprietary electromagnetic
offset log (EOL) modeling procedure is used to interpret the
data and generate a 3-D model of subsurface features and DNAPL
source areas.
Results:
The EM surveys consisted of general site research; installation
of instrumentation wells; measuring and recording the resistivity
from the source at 0.1 foot increments from the bottom to
top of the instrumentation well; data analysis and interpretation
by the EOL modeling procedure; and confirmation borehole drilling,
soil analyses (EPA Methods 8260/8270), and groundwater analyses
(EPA Method 8240). The study concluded that EM surveys could
not detect subsurface DNAPL source areas because these source
areas are too diffuse to alter the resistivity of subsurface
materials and provide a unique "fingerprint" of the subsurface
location where the DNAPL is situated.
Benefits:
EM surveys successfully detect and delineate light non-aqueous
phase liquid (LNAPL) sources areas in groundwater and in the
capillary fringe above groundwater where it forms a detectable
layer of low electrical conductivity. EM surveys are cost
efficient at sites greater than two acres. The cost for an
EM survey is estimated at $35,000 per 2-acre site. A detailed
3-D illustration of subsurface geologic features can be generated
with the EM survey data to identify optimal locations for
LNAPL and DNAPL monitoring and recovery wells.
Implications:
EM resistivity data requires interpretation by qualified
and experienced personnel with a strong understanding of the
technology. An EM survey is limited to a radius of approximately
300 feet around each instrumentation well and to a maximum
depth of 300 feet below grade. Multiple instrumentation wells
are needed with the exact number depending upon the size of
the study area. Confirmatory borings and media sample analyses
are required to interpret the results. Therefore, it is not
a stand-alone investigation method and requires subsurface
intrusive activities. Metal objects and structures at the
surface too close to the transmitter coil can result in "noise"
and affect the results of the survey. (Project Completed - 2000)
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