Treatment of RDX and/or HMX Using Mulch Biowalls

Site Name:

Solid Waste Management Unit (SWMU)-17, Pueblo Chemical Depot (PCD)

Location:

Pueblo, Colorado

Period of
Operation:

November 2005 to July 2007

Cleanup
Type:

Pilot Scale

Technology:
In situ organic mulch-pea gravel Permeable Reactive Barrier (PRB)

  • Biological reduction of RDX- and HMX-contaminated groundwater is stimulated by passing groundwater through an in situ mulch PRB filled with a mulch/gravel mixture.
  • A soil-bentonite funnel served as a hydraulic control to limit, or even eliminate, groundwater flow bypassing the PRB.
  • Both the PRB and the soil-bentonite wall were 2- feet (ft) thick. The completed length of the mulch/gravel PRB was approximately 105 ft from the unsaturated zone to the west.
  • A 33%:67% (volume:volume) pea gravel:mulch fill mixture was used for the mulch PRB.
  • Groundwater monitoring wells were installed and sampled prior to the PRB installation to establish a baseline. Additional monitoring wells were installed at the PRB and downgradient from the barrier.

Cleanup Authority:
Environmental Security Technology Certification Program (ESTCP)

Contacts:

Charles J. Newell
GSI Principal Investigator
GSI Environmental, Inc.
2211 Norfolk
Suite 1000
Houston, TX 77098-4054
Telephone: 713-522-6300
Fax: 713-522-8010
cjnewell@gsi-net.com

Farrukh Ahmad
GSI Co-Principal Investigator
GSI Environmental Inc.
Houston, TX 77098-4054
Telephone: 713-522-6300
Fax: 713-522-8010
fahmad@gsi-net.com

David T. Adamson
GSI Co-Principal Investigator
GSI Environmental Inc.
Houston, TX 77098-4054
Telephone: 713-522-6300
Fax: 713-522-8010
dtadamson@gsi-net.com

Christopher Pulskamp
PCD Contact
Pueblo Chemical Depot (PCD)
Pueblo, CO 81006
Telephone: 719-549-4252
Fax: 719-549-4318
Christopher.Pulscamp@us.army.mil

Contaminants:
hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-
1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)

Waste Source:
Ordnance production and ordnance demilitarization activities.

Type/Quantity of Media Treated:
Groundwater (36,479.5 cubic feet (ft3))

Purpose/Significance of Application:
This is the first application of a mulch biowall for the treatment of an RDX or HMX plume. The overall purpose of this project was to demonstrate and validate mulch PRB technology in the field at the pilot scale for the treatment of explosives contamination in groundwater. Data collection focuses on technology effectiveness, longevity, bioactivity, and health and safety concerns.

Regulatory Requirements/Cleanup Goals:
The state-mandated, site-specific cleanup criteria of 0.55 ppb RDX and 602 ppb HMX were used as the cleanup goals for the demonstration project. The primary performance criteria for the site include: (1) Achieve greater than a 90% reduction of RDX across the treatment zone. (2) Reduce the RDX concentration to below 0.55 ppb at the edge of the treatment zone. (3) Reduce the cumulative concentration of RDX transformation intermediates to less than 20% of the RDX molar concentration immediately upgradient of the PRB.

Results:
Data from the demonstration site indicated the following:

  • A 90% reduction in RDX and HMX was measured in the treatment zone once a pseudo-steady-state was established.
  • RDX concentrations of below 0.55 ppb were consistently recorded in all wells downgradient of the mulch PRB.
  • No RDX intermediates were found in the wells downgradient of the mulch PRB

Cost Factors:
The unit costs of implementation were $10.26/ft3 or $1.37/gallon. The total cost for this demonstration was $374,389.

Description:
The purpose of this project was to demonstrate and validate mulch PRB technology in the field for treating explosives contamination in groundwater. The organic mulch used in the PRB acts as a slow-release source of electron donor that stimulates the biological reduction of RDX and HMX contamination. For this field demonstration, a groundwater explosives plume was selected in the Solid Waste Management Unit (SWMU)-17 area located at the Pueblo Chemical Depot (PCD) in Pueblo, Colorado. The facility has a history or ordnance production demilitarization activities which contributed to the munitions contamination in the groundwater at the site. The main munitions found in the groundwater at the site include RDX and HMX.

The technology design consisted of a 100-ft long and 2-ft thick in situ mulch-pea gravel PRB along with hydraulic controls used to prevent the groundwater from bypassing the PRB. The biological reduction of RDX- and HMX- contaminated groundwater occurred by passing the groundwater through the mulch/gravel barrier. The mulch PRB was installed in November 2005 and ran until July 2007. During the course of this 22-month demonstration, groundwater samples were collected to determine the effectiveness of the mulch biowall to reduce RDX and HMX concentrations in groundwater.

The groundwater monitoring wells were installed and sampled prior to PRB installation to establish baseline conditions. Samples were also taken at, and downgradient of, the PRB. The results indicated that all of the performance objectives had been met by June 2006. By then, a sustained treatment zone had been created down gradient of the mulch PRB that achieved >90% RDX and HMX removal, RDX concentrations <0.55 ppb except for one detection, and no accumulation of toxic intermediates in the any of the treatment zone wells.