Theme: 05. Fractured Rock and Complex Geology

Topic Area: 5c. Remediation Approaches in Fractured Rock and Karst Aquifers

Formatted Title:

Dual-Phase Extraction of TCE from Fractured Cretaceous Sandstone: F.E. Warren Atlas Missile Site, Greeley, Colorado

Background/Objectives:

Background/Objectives. This Formerly Used Defense Site (FUDS) operated as F.E. Warren Atlas “E” Missile Site 12 (FEW12) from October 1960 to March 1965. The site is situated in Weld County, Colorado near the city of Greeley, and covers 18.5 acres. It is currently a recreational park owned by Weld County. Former FEW12 facilities included an underground Launch Operations Building, Launch and Service Building, flame pit, discharge headwalls, and a retention pond. Wastewater and residual cleaning solvent containing trichloroethene (TCE) and residual rocket fuel were discharged to the ground after fueling and defueling of the missile. These discharges created an artificial, perched water zone within the Cretaceous Fox Hills Sandstone. The saturated thickness was approximately 30 feet near the missile complex and gradually decreased to zero (0) feet approximately 700 feet downgradient. The Decision Document for FEW12 identified enhanced reductive dechlorination (ERD) as the selected remedy. EHC^® amendment, to stimulate biotic degradation and/or zero valent iron (ZVI) for abiotic chemical reduction, was injected using hydraulic fracture or high-pressure injection techniques over four events between 2009 and 2019.

Approach/Activities:

Approach/Activities. Bhate was contracted by USACE to optimize the remedy and advance the site to closure. To accomplish this, Bhate performed additional data gap investigation (DGI) activities to improve the conceptual site model (CSM). Bhate determined that there was very little sandstone pore space saturation with perched water limited to bedding planes and fractures. Furthermore, the DGI identified dissolved TCE concentrations an order of magnitude higher than the historical well network defined. Past ERD injections failed to generate site-wide TCE concentration reductions, but rather appeared to redistribute the contaminant mass across the site. Bhate proposed a different remedial approach based on the re-evaluation of the CSM, which would also account for the higher TCE concentrations. The new approach recommended by Bhate was dual-phase-extraction (DPE) in the artificial, perched water zone within the fractures.

Following a 48-hour DPE pilot test, a porosity of 0.17% was calculated, water extraction rates of less than 0.1 gallons per minute (gpm) were recorded, and drawdown of up to 0.9 feet was observed in neighboring monitoring wells. Using the pilot test data and CSM, an expanded pilot test DPE design was developed and installed within the 2.6-acre target treatment zone at the site. The system included 11 DPE near facility wells and 11 downgradient DPE wells, 22 self-actuating pneumatic pumps, two separate vapor extraction blowers, three granular activated carbon (GAC) vessels for water treatment, a compressor to supply air to the pumps, and three injection wells for treated water discharge.

Results/Lessons Learned:

Results/Lessons Learned. One year after the pilot test, three monitoring wells within the test zone of influence exhibited sustained TCE reductions of 56 to 92%. During startup of the expanded DPE system, DPE wells dewatered within minutes of startup. After 30 days of system operation, the water extraction rate from all 22 DPE wells combined was approximately 1 gpm, vacuums of 40-50 inches of water were applied to each DPE well, and a maximum drawdown of over 10 feet was observed in adjacent monitoring wells. GAC units were performing as designed and air emissions were within limits for direct discharge. Only one injection well was required to handle the discharge flow.

The ERD amendments in the extracted water coupled with the high dissolved oxygen created by the DPE system generated microbial growth in the DPE well sumps, piping, bag filter, and first GAC unit. Slow-release chlorine was added to the system to minimize the growth to prevent fouling of the first GAC unit. Influent samples are collected monthly, and concentrations are trending down; effluent concentrations from the first GAC vessel are non-detect. Bhate will have three quarters of groundwater sample results to share by the time of the conference.

Anthony Sedlacek

Affiliation

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(Group 1, Poster Board #4) Dual-Phase Extraction of TCE from Fractured Cretaceous Sandstone: F.E. Warren Atlas Missile Site, Greeley, Colorado