Five Additional Technologies Evaluated After ERH Application at Camp Lejeune
ERH application used an array of 91 electrodes to deliver three-phase electricity in a 15,900-ft2, volatile organic compound (VOC) contaminated source area [see February 2006 TNT]. Six months of active subsurface heating at 100°C achieved a 97-99% reduction of VOC concentrations in groundwater and soil at the test area. The VOC bulk removal rate was as high as 440 lbs/day during peak performance, and a total of 48,000 pounds of VOCs was removed. At a unit cost of $137/yd3, ERH project costs totaled nearly $2.1 million for the total treatment volume of 15,311 yd3. ERH implementation at this site involved management of roughly 2,000,000 gallons of water from the interior of the treatment area, which was boiled off during the heating and collected for treatment and disposal.
Subsequent treatability studies evaluated performance and design criteria of four additional technologies for addressing dissolved-phase contaminants: enhanced reductive dechlorination (ERD) by injecting emulsified vegetable oil (EVO) and sodium lactate, chemical reduction by way of ZVI injection using pneumatic fracturing, air sparging through a horizontal directional drilled (HDD) well, and a permeable reactive barrier (PRB) with mulch as a reactive medium. Each technology was evaluated on the basis of contaminant reductions and radius of influence (ROI). Study area selection was based on site layout, plume location, and avoidance of interaction among the four subareas. For example, the horizontal well was placed in an isolated elongated plume, and the PRB was constructed in the downgradient portion of the plume.
Field activities began with installing 11 new monitoring wells. Baseline samples were collected from new and existing wells in each subarea, and groundwater was monitored throughout the study. Highest baseline concentrations of predominant contaminants of concern in the study area were 21,000 µg/L for trichloroethene (TCE) and 14,000 µg/L for 1,1,2,2-tetrachloroethane (PCA). Results of the treatability studies for the dissolved-phase plume were as follows:
ERD through EVO/Lactate Injection: Over one week, approximately 3,050 pounds of EVO and 3,300 pounds of sodium lactate were injected into the subsurface through four borings and chased with water (Figure 2). The injection depth interval was 10 to 25 ft below ground surface (bgs). Groundwater samples were collected from five monitoring wells one, three, and six months after the injection. Monitoring indicated a maximum 35-foot ROI surrounding each ERD injection location and an average TCE reduction from 333 to 8.7 µg/L. Analysis of field parameters, daughter products, natural attenuation indicator parameters, and microorganisms also suggested that reductive dechlorination had occurred.
Figure 2. EVO and sodium lactate were injected into the subsurface and chased with water in one treatability study area at Site 89.
Chemical Reduction through ZVI Injection and Pneumatic Fracturing: Over one week, approximately 11,600 pounds of ZVI carried by nitrogen gas were injected into the subsurface through six borings with the aid of pneumatic fracturing. The injection interval was 12.5 feet to 25 feet bgs. Due to a high water table, pulsing was conducted to avoid daylighting of nitrogen gas to the surface near the injection point, which would significantly reduce the ROI. This approach inadequately fractured the formation and resulted in poor ZVI distribution across the target area. Sampling of five monitoring wells one, three, and six months after the injection showed no reduction in contaminant concentrations. Oxidation-reduction potential (ORP) declined over the monitoring period, indicating that subsurface conditions were slowly becoming favorable for reductive dechlorination.
Air Sparging in HDD: Air was delivered through a 600-foot HDD sparge well with a 240-foot-long screen approximately 40 feet bgs near an existing building. The system was activated in December 2006 and operated continuously for approximately six months with an air compressor `up time` of 89%. After three months of air sparging, pneumatic fracturing was conducted in four borings spaced 50 feet apart along the axis of the horizontal well screen, at 3-foot intervals from 12.5 to 25 feet bgs. Monthly monitoring was conducted at eight groundwater monitoring wells and three soil vapor wells. Results indicated a 60-foot ROI from the sparge well, with little ROI enhancement from pneumatic fracturing. Average TCE concentrations decreased from 875 to 58 µg/L. Soil vapor concentrations increased during active sparging, but site-specific screening criteria (based on industrial exposure) were not exceeded.
PRB with Mulch: A one-pass continuous trencher was used to construct a 210- by 2-foot trench to a depth of 25 feet, which intercepted groundwater flowing to an onsite creek at a rate of 0.14 ft/day. Approximately 200 yd3 of mulch from the base`s recycling area and 480 yd3 of sand were placed in the trench to achieve a ratio of 40% mulch as a reactive medium and 60% sand as an aggregate. Groundwater monitoring included sampling at 11 monitoring wells one, three, and six months after PRB construction. Average TCE groundwater concentrations in the wall decreased from 13,573 to 442 µg/L. Study results indicated favorable conditions for contaminant degradation (low ORP, high total organic carbon, and low dissolved oxygen) but determination of PRB effectiveness over the six-month study was limited by slow groundwater flow.
Overall effectiveness of each technology was evaluated during the treatability study in terms of reducing concentrations of chlorinated VOCs within the surficial aquifer while balancing the technology`s cost and implementation ease (Figure 3).
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.
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