Theme: 10. Characterization, Fate and Transport

Topic Area: 10c. Advanced Geophysics and Remote/Direct Sensing Tools and Techniques

Formatted Title:

Real-Time Contaminant Analysis to Delineate Unique Plumes on Site

Background/Objectives:

This site was originally developed as a retail fuel facility and was later used as an auto body shop and auto sales facility. Two underground storage tanks (USTs) were installed at the site around 1958 and included one 1,000-gallon and one 500-gallon UST. The USTs were taken out of service in May 1985. In May 2004, the 1,000-gallon UST was removed, and the 500-gallon UST was closed in place. Based on the analytical results from the tank closure, a release of gasoline was reported. A site investigation conducted in May 2010 also detected diesel constituents; however, they were believed to be from an off-site source since the USTs at the site did not contain diesel fuel. Further site investigation activities were conducted throughout 2011 and 2012, which included the installation of 15 monitoring wells to further delineate the plume. No sampling was conducted between January 2012 and September 2020. A mobile vacuum event was conducted in January 2012 to recover free product from two monitoring wells. Four dual phase mobile vacuum extraction (DPMVE) events were conducted at the site between March 2021 and May 2021. However, post DPVME events, free product was still encountered in several monitoring wells.

Approach/Activities:

Based on the presence of free product at the site, high resolution site characterization (HRSC) was proposed for the site to delineate the extent of free product. Twenty-eight borings were advanced using the Optical Image Profiler (OIP) to identify free product in the subsurface. Four membrane interface probe (MIP) borings were advanced to identify dissolved and adsorbed phase hydrocarbons. The OIP and MIP data indicated hydrocarbon contaminants at two different depths. A portable gas chromatograph (gc) was used during field operations to investigate and analyze contaminants of concern in real time. Doing so, produced lab grade data to support the delineation of two separate plumes and effectively ascertain the precise plume locations.

Results/Lessons Learned:

As portrayed in the former remediation strategies at this site, many remediation efforts underperform or fail due to lack of understanding of the site conditions and processes. Therefore, without an accurate conceptual site model (CSM), a site cannot be accurately and adequately defined. Two technologies, HSRC and the portable gc, contributed to the success of establishing two unique plumes in real-time. Initially, the OIP and MIP produced free phase and volatile phase detector data at depths where hydrocarbon contamination was identified. From these detector responses, the portable gc, distinguished the two plumes from each other based on lab-grade contaminant analysis in the field. With now greater understanding of the subsurface characteristics and contaminant distribution, an updated, comprehensive CSM can better inform remedial action by precisely targeting contamination.

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Barry Hebert