U.S. EPA Evaluates Direct Push Electrical Logging for Site Investigation

The U.S. EPA's Office of Solid Waste and Emergency Response has recently published a new document entitled:

Innovations in Site Characterization: Geophysical Investigation at Hazardous Waste Sites

The EPA document number for this publication is EPA-543-R-00-003. Several different geophysical techniques are evaluated in this document through "case studies" at real sites where the technology was applied. Some of the geophysical methods evaluated are: ground penetrating radar (GPR), seismic surveys, magnetometry, gamma logging, electrical conductivity and electrical resistivity. At two of the case study sites Geoprobe's electrical logging system was evaluated. At both locations the SC200 soil conductivity probe was advanced to depths of up to 60 feet with a Model 5400 Geoprobe® direct push machine.

The first case study was completed at an underground storage tank facility in Kansas. At this location the electrical logs were obtained in alluvial sediments of the Saline River at depths of up to 60 feet. The EPA's evaluation notes that the electrical logs delineated fine grained clayey to sandy silts over lying sands. It was also reported that the electrical logs defined the presence of a persistent clay layer across the site which divided the sand into two distinct layers. The report also noted that contouring on the contact between the overlying clayey to sandy silts and underlying sands located a saddle like structure which created a preferential flow path for the LNAPL contaminants almost perpendicular to the local ground water flow direction.

A former manufactured gas plant in Marshalltown, Iowa was the location of the second case study where the SC200 probe was used for subsurface investigation. The geology at this site consisted of glacial till overlying limestone bedrock. The EPA's evaluation indicates that the electrical logging system was able to accurately identify the lithology, and in fact identified some lithological features not detected by conventional methods. It was also found that the SC200 probe identified the presence of DNAPLs in the subsurface by consistently detecting low electrical conductivity values where DNAPLs were present. The review indicated that the SC200 probe was damaged by large glacial till cobbles or boulders in the subsurface. Geoprobe now has a more rugged soil conductivity probe (the SC400) which has been on the market for over three years.

In the performance review the EPA found that Geoprobe's electrical logging system was a cost effective method for delineating subsurface geology in unconsolidated formations. At the first site 10 logs were obtained to a depth of 60 feet for a cost of approximately $3600.00. At the glacial till site a two-person crew completed 27 logs in 5 working days, with a total of 700 feet of logging completed (average depth of almost 26 feet per log). All 27 electrical logs were completed at a cost of less than $8000.000 at this facility. In the Lessons Learned section of the document the evaluation notes "The probe (SC200) provides more detailed stratigraphic information than conventional auger borings".

Copies of this report are available free of charge from the National Service Center for Environmental Publications (NSCEP) at www.cluin.org/pub1.cfm (Search for Publication No: EPA 542-R-00-003).  Or contact NSCEP at PO Box 42419, Cincinnati, Ohio 45242-2419, Telephone 800-490-4198, Facsimile 513-489-8695.

Wesley McCall, R.G. 11/30/00