The site geology generally consists of dense clay to 5 feet below ground surface (bgs) transitioning to clay with thin sand/silty sand interbeds at 10 feet bgs with an underlying denser clay layer from 16 to 20 feet bgs. In the last 10 years, the depth to groundwater has ranged from 8 to 11 feet bgs. Major chemicals of concern are benzene, tertiary butyl alcohol (TBA), trichloroethylene and perchloroethylene.
Although the site had been investigated by six previous consultants since 1988, including installation of five groundwater monitoring wells, collection of 10 hydro-punch samples, completion of 60 soil borings and periodic groundwater monitoring, no clear understanding had yet emerged about several important issues, including:
· Identification of source areas of contamination
· Potential pathways of contaminant transport
· Application and pros and cons of remediation methods to address these issues
Thus, it was the regulatory agency’s belief that operations at the site had caused all contamination and no other sources existed.
In response, a time-driven and cost-effective membrane interface probe (MIP) investigation coupled with selected Geoprobe/hydro-punch sampling based on real-time MIP data was initiated.
Based on the MIP data, the distribution of volatile organic compounds in soil and groundwater could be determined; it was established that dense non-aqueous phase liquids (DNAPLs) were present and that the DNAPL and TBA contamination originated from other sites. The MIP data also provided an excellent basis to develop a remediation approach for the site to address only the gasoline and benzene contamination clearly associated with the site.