Explosives remediation at Pueblo Chemical Depot changes course

Treatment of soil and groundwater contaminated by energetic compounds has been underway since 1997 at area of interest (AOI) 3B of the Pueblo Chemical Depot (PCD) in Colorado. AOI 3B is part of the source of an explosives-contaminated groundwater plume. Initial work involved excavation and onsite composting of soil to address the contaminant source. In 2001, a pump and treat system using granular activated carbon and ion exchange began treating groundwater with high concentrations of RDX, 2,4-DNT, and nitrate near the PCD boundary. An electrolytic reactive barrier (e-barrier) subsequently was demonstrated at AOI 3B under the federal Environmental Security Technology Certification Program (ESTCP) to determine the technology’s efficacy in addressing remaining RDX and 2,4-DNT in source area groundwater. [For more information, see the September 2007 issue of TNT.]

Demonstration of the e-barrier concluded earlier this year. Samples collected after three years of operation indicated little reduction of RDX, TNT, and 2,4-DNT concentrations exiting the barrier and continued offsite migration of energetic compounds. Results suggested that e-barrier performance was limited due to wider distribution of the contaminant plume than originally estimated and its location directly above a thick shale aquitard. A complete cost and performance report will be available later this year from the ESTCP (www.estcp.org).

Remediation of the explosives plume is moving forward. PCD recently began implementing full-scale corrective measures involving enhanced in situ bioremediation (EISB) with a groundwater infiltration and recirculation system to stimulate microbial degradation of contaminants in the capillary fringe. EISB was selected based on successful application of the technology at similar sites and in PCD pilot studies.

Construction of the state-approved EISB system began with installation of two infiltration galleries within the floors of two previously excavated adjacent areas totaling 6,100 yd2. Each gallery consists of 3,100 linear feet of 1- and 2-inch perforated PVC pipe configured in a grid that will allow even distribution of amended water across the excavation area.

The EISB system includes seven injection and four downgradient extraction wells installed to depths of 10 to 20 feet (Figure 1). Well screens are positioned within the bottom two-thirds of the overall aquifer thickness. Flow rates from extraction wells are expected to range from 3 to 10 gpm.
Explosives remediation at Pueblo Chemical Depot changes course

Figure 1. Six of the seven wells used for sodium lactate injections at the PCD are situated directly within infiltration galleries that recirculate amended groundwater across the contamination hot spot.

Extracted groundwater will be amended with a solution of 0.5 to 2.0% sodium lactate and then reinjected into infiltration galleries or injection wells on a rotating schedule. In order to maintain reducing conditions in the aquifer, the solution will contain sufficient sodium lactate to maintain dissolved oxygen content below 1.0 mg/L. Pulsed injection will be conducted to allow flushing of the screen with recirculated groundwater between pulses, thereby minimizing biological growth on the screen and preventing associated biofouling.

The infiltration and injection recirculation system will be housed in a weatherproof enclosure and will consist of an influent tank, carbon substrate tank, pumps, sampling ports, and instrumentation with various fittings. The system will be built to recirculate groundwater via the injection pump or in a closed-loop mode with recirculation accomplished solely by the extraction well pump.

A nutrient solution containing carbon, nitrogen, and phosphorous in a 100:10:1 ratio also will be injected into the subsurface, either as part of the sodium lactate amendment or through separate events as warranted by groundwater nutrient conditions. The single set of extraction wells will maintain hydraulic control of sodium lactate solution as well as surface infiltration of carbon substrate solution.

Injection currently is scheduled to begin in July 2009 and continue for a minimum of 36 months. Recirculation and/or sodium lactate and nutrient addition may be extended or shortened following review of each quarterly monitoring event. Lactate recirculation will be considered complete and the system will be shut down when regulatory site-specific groundwater cleanup goals are met. During the post-recirculation stage, contaminants are expected to continue to degrade while groundwater moves under the regional gradient and additionally contacts accumulated explosives-degrading bacteria. Regulatory closure will be achieved once concentrations for contaminants of concern remain below specified cleanup levels at compliance points for three consecutive years after the remedy is complete. Cleanup requirements for RDX and 2,4-DNT are 0.825 µg/L and 0.1328 µg/L, respectively, based on risk-based groundwater cleanup levels specified by the State.

Read the full article online at http://www.clu-in.org/products/newsltrs/tnandt/view.cfm?issue=0709.cfm#1

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