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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

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
Resistivity Profile
 

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
Resistivity Profiles
 
 

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.

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J R Associates, 1886 Emory Street, San Jose, CA  95126

Ph: (408) 293-7390 Email: jra@Greatgeophysics.com