Using Geophysics
to Identify Contaminate Pathways
Mapping
buried permeable stream channels is a common problem at most groundwater
remediation sites. Buried channels shift direction frequently and are
difficult to trace from boring to boring. Often many borings must be
drilled in a small area to intercept a buried channel. The geophysical
technique we describe here, resistivity profiling, is
a powerful method for tracing buried channels through a site.
The facility,
illustrated below, consisted of many buildings surrounded by asphalt driveways
and parking areas. The facility manufactured electronic components for
many years. The groundwater beneath parts of the site contained
detectable amounts of solvents. Many borings were drilled during a
preliminary groundwater investigation. The borings were drilled to determine
the extent of the impacted groundwater and to determine the geology beneath the
site. Well graded sands and gravels were found in borings A and B but not
in other borings. The sand and gravel channel was highly permeable and
was a potential contaminate pathway. It was important to the remediation
effort to define the extent of the gravel bed and whether or not it extended
off site.
Site Map
To aid the
groundwater investigation, J R Associates performed high resolution electrical
profiling to map the depth and extent of the sand and gravel channel.
This method was chosen because of its ability to distinguish gravels from clays
and its ability to collect useful data in paved areas without disrupting the
day to day operations of the facility. Also the data could be collected
along the ground surface and did not require any additional boreholes.
We planted
electrodes a few inches into the ground at 20-foot intervals along three
profile lines. Measurements of the earth's resistivity
were made between different pairs of electrodes. Well graded sands and
gravels do not conduct electricity and are considered electrically
resistive. Clay layers conduct electricity easily and are considered
electrically conductive. By taking numerous measurements of the soil's
electrical properties for different depths and locations, we developed a
three-dimensional map of the electrically resistive gravel deposit.
Electrical Resistivity Profile of Line 1
Shown above is the
electrical profile collected along Line 1. The illustration shows a
section of soil 850 feet long by 100 feet deep beneath Line 1. The color
represents the soil's electrical resistivity measured
in Ohm-feet. The dark blue indicates electrically resistive material and
corresponds to the gravel bed. The pink indicates an electrically
conductive material and corresponds to a fat
clay. The light blue, green, yellow and red indicate mixed sands, gravels
and clays. In general, the color moves from light blue to red as the clay
content of the soil increases.
Shown below are
the results of three profiles collected across the site. The profiles
clearly show the extent of the gravel deposit. We were able to determine
the gravel deposit was limited to the southern half of the site and did not
extend to the site's northern border. The data also showed an impermeable
clay layer underlies most of the site. The location and depth of the
gravel was used in planning new exploratory and extraction wells. The
information regarding the clay layer was used in designing a cut-off wall to
prevent off-site migration of the contaminates.
Electrical Resistivity Profiles of Lines 1 Through 3
This case study
illustrates how electrical resistivity profiling was
used to map a subsurface gravel deposit and impermeable clay layer. The
geophysical data gave the geologists the information needed to successfully
plan new exploratory and extraction wells and aided in designing a cut-off
wall. Resistivity is just one of many
geophysical techniques offered by J R Associates. Please contact us to discuss
how our services could benefit you.
J
R Associates,
Ph: