Hargis + Associates Inc

Field Screening Methods for the Presence of DNAPL, Superfund Site, southern California

Characterizing the lateral and vertical extent at a Dense Non Aqueous Phase Liquid (DNAPL) impacted site can prove to be a challenging task. Field methods employed to verify the presence of DNAPL have included the use of down-hole Ultraviolet Induced Florescence technologies, the application of down-hole detection materials such as dye impregnated fabrics, and the installation of monitor wells with DNAPL trap sumps. All of these methods can be effective, however each has its accuracy limitations and cost considerations.

A DNAPL reconnaissance program has been successfully completed at a Superfund Site located in southern California. The depth of investigation, greater than 90 feet below land surface, did not allow the use of field screening techniques with CPT technology. Rather, the program consisted of the drilling of soil borings using continuous core drilling and screening the core sample at the surface. Screening methods included visual inspection of the core, the application of hydrophobic dye-impregnated fabric to the core sample, the collection of soil gas data using a field organic vapor analyzer, and the collection of confirmatory soil samples for laboratory analysis. The application of a hydrophobic dye impregnated fabric proved to be the most valuable of the field screening methods. When the fabric is applied to the soil core, the fabric will almost immediately react by staining, providing clear and obvious evidence of the presence of DNAPL at that particular sample depth. To confirm the presence of DNAPL, soil vapor measurements were made and laboratory samples were collected and analyzed for the DNAPL constituents.

Laboratory samples later confirmed the presence of DNAPL in soil cores where DNAPL was suspected based on reactions with the hydrophobic dye-impregnated fabric. The use of the fabric to undisturbed core proved to be an invaluable tool to the success of this field investigation.

Data obtained during the field program were used to refine the lateral and vertical extent of DNAPL. These data combined with the detailed lithologic information allowed a better understanding of DNAPL behavior and migration pathways at the site.

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