Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

Utilizing High Resolution Site Characterization Technology and In-Field Quantitative Data to Dynamically Delineate a Chlorinated Solvent Plume

Background/Objectives:

Eagle Synergistics conducted a high resolution site characterization (HRSC) investigation at the Dallas-Ft. Worth International Airport (DFW Airport). The location of the investigation was a former air cargo operations area where chlorinated solvents (PCE and breakdown products) were previously identified in surface and subsurface soils and groundwater at concentrations above Texas Commission on Environmental Quality Protective Concentration Levels. The objectives of the HRSC investigation were to delineate the extent of dense nonaqueous phase liquid (DNAPL) in the subsurface and update the conceptual site model for remedial action design purposes. HRSC technology and a portable gas chromatograph (gc) were utilized to complete these objectives.

A membrane interface probe (MIP) was run in conjunction with the electrical conductivity dipole (EC) and the hydraulic profiling tool (HPT). Collectively, this technology is known as the MIHPT, which indicates volatile hydrocarbon and solvent contamination in addition to soil electrical conductance and indicators of formation permeability. Typical standard MIP configurations use three gas phase detectors: a photo-ionization detector (PID), flame-ionization detector (FID) and a halogen specific detector (XSD). The PID responds to both chlorinated and non-chlorinated hydrocarbons. The FID will respond when organic compounds are present in high enough concentrations; whereas the XSD responds only to halogenated compounds. Based upon which detector or detector series a contaminant responds to, the type, halogenated or petroleum based, of contaminant can be determined. To determine contaminant concentration from MIP responses, confirmation soil and/or groundwater samples must be collected for laboratory analysis. A portable gc allowed for contaminant concentrations to be analyzed in the field on a real-time basis.

Approach/Activities:

The HRSC investigation included advancing 26 MIHPT borings and collecting five composite portable gas gc samples. The first borings were advanced near the source area as identified by previous groundwater and soil sampling data. Subsequent HRSC borings were placed at new locations based on real-time HRSC data. This soil and groundwater analytical data, lithology, and hydraulic conductivity data were instantly utilized by field personnel to determine the next boring locations.

While onsite, a portable gc sampled five different zones of interest. The lab-grade, real-time data speciated particular contaminants of concern, specifically differentiating the chlorinated solvents first identified by the MIHPT. Additionally, this data were used to identify the possibility of a separate hydrocarbon plume affecting the PID and FID detectors within the MIHPT system.

Results/Lessons Learned:

The approach of high resolution to site characterization investigations allows field personnel to “chase” the contamination to accurately delineate the plume and/or source and subsequently create a more informed conceptual site model. Obtaining an understanding of lithological and hydrogeological data, along with the contaminant information, supports the determination if contamination is near to prominent migration pathways or bound up in a storage zone. Utilizing a more thoughtful approach supported a higher quality understanding of the site, which leads to a remediation strategy to concisely target the plume and source.

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Thomas Wiberg