Hargis + Associates Inc

Extraction well design for DNAPL recovery.

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Courtesy of Hargis + Associates Inc

The potential for downward migration of dense nonaqueous phase liquid (DNAPL) into deeper portions of a saturated hydrogeologic unit has important implications for successful groundwater remediation. Little information has been published regarding the design of extraction wells that allow DNAPL recovery without simultaneously creating a conduit for cross contamination by vertical migration through the wellbore. An extraction well design for DNAPL recovery has been developed and successfully implemented at a Superfund site located in the West Coast Groundwater Basin near Los Angeles, California.

The mud rotary drilling method was selected for well installation because the positive hydrostatic head created by the mud in the interval above the water table prevents the migration of DNAPL into the wellbore during construction. The bottom of the well screen was fitted with a section of blank casing to act as a sump for the accumulation of DNAPL. A cone-shaped cement basket was attached to the bottom of the screen to funnel DNAPL into the sump. Prior to setting the casing, a calculated volume of neat cement was tremied to the bottom of the borehole. The casing assembly described above was then quickly lowered into the borehole, displacing the cement and sealing the annulus around the sump to the base of the cement basket. The annulus above the open cement basket was completed with filter pack opposite the well screen and conventionally sealed to the ground surface.

Following well development, monitoring of the well indicated an average DNAPL accumulation rate of less than 0.1 gallon per day (gpd) under static conditions. The average DNAPL recovery rate during pumping of groundwater increased to approximately 10 gpd, which was several orders of magnitude greater than the static accumulation rate. A 28-day pilot test of the well resulted in the recovery of approximately 300 gallons of DNAPL.

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