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.
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.
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.
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)