Abstract
Bioremediation of a Large
Chlorinated Solvent Plume, Dover AFB, DE
Aleisa Bloom, (Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA)
Robert Lyon (bob.lyon@aecom.com), Laurie Stenberg, and Holly Brown
(AECOM, Germantown, Maryland, USA)
ABSTRACT: Past disposal practices at Dover Air Force Base (AFB), Delaware, created a large solvent plume called Area 6 (about 1 mile long, 2,000 feet wide, and 345 acres). The main contaminants are PCE, TCE, and their degradation products. The remedy is in-situ accelerated anaerobic bioremediation (AAB). AAB started in 2006 and is focusing on source areas and downgradient plume cores. Direct-push injections occurred in source areas where contamination is typically between 5 and 20 feet below ground surface. Lower concentration dissolved-phased contamination is present downgradient at 35 and 50 feet below ground surface. Here, permanent injection/extraction wells installed in transects perpendicular to the flow of groundwater are used to apply AAB. The AAB substrate is a mix of sodium lactate, emulsified vegetable oil, and nutrients.
After eight years, dissolved contaminant mass within the main 80-acre treatment area has been reduced by over 98 percent. This successful application of AAB has stopped the flux of contaminants to the more distal portions of the plume. While more time is needed for effects to be seen in the distal plume, AAB injections will soon cease, and the remedy will transition to natural attenuation.
INTRODUCTION
Oak Ridge National Laboratory Environmental Science Division (ORNL) and AECOM (formerly URS Corporation) have successfully implemented in situ accelerated anaerobic bioremediation (AAB) to remediate chlorinated solvent contamination in a large, multi-sourced groundwater plume at Dover Air Force Base (AFB). AAB has resulted in significant reductions of dissolved phase chlorinated solvent concentrations. This plume, called Area 6, was originally over 1 mile in length and over 2,000 feet wide (Figure 1). It originated from at least four separate source areas that comingled in the subsurface to form the large plume. The major contaminants of concern (COCs) are tetrachloroethene (PCE), trichloroethene (TCE), and 1,1,1-trichloroethane (1,1,1-TCA), which were historically used for degreasing operations in the maintenance of aircraft and support vehicles.
Relatively small areas of elevated PCE, TCE, and 1,1,1-TCA were delineated in the shallow portion of the water table aquifer by direct-push groundwater sampling. Focused direct-push AAB treatment occurred in March 2006 at these source areas (Figure 1). Downgradient of the these areas and deeper in the aquifer, AAB treatment was implemented using rows of extraction/injection wells oriented perpendicular to groundwater flow to create multiple reductive zones across the plume cores, defined as areas where more than 1,000 micrograms per liter (ug/L) total solvent concentrations were present. Initial indications of successful degradation were observed within 6 months of starting injections.
FIGURE 1. Dover AFB Area 6 plume.
This paper describes the AAB implementation and progress of remediation after 8 years of treatment and periodic groundwater monitoring.
SITE LITHOLOGY
Contamination at the site is limited to the surficial aquifer, which consists of 35 to 50 feet (ft) (11 to 15 meters [m]) of unconsolidated Pleistocene deposits of the Columbia Formation. The Columbia Formation consists of fine to coarse sand with silt and clay lenses and less common gravel lenses. Silts and silty sands are generally encountered to a depth of 10 to 12 ft (3.05 to 3.65 m) below ground surface (bgs) and grade to medium- and coarse-grained sands to a depth of 35 to 50 ft (11 to 15 m) bgs. There is a clay and silt unit (part of the Calvert Formation) below the surficial aquifer that acts as an aquitard to the downward migration of contaminants. The depth to the water table varies across the site but usually ranges from 8 to 15 ft (2.4 to 4.5 m) bgs in the treatment area.
REMEDIAL APPROACH AND OPERATIONS
Because the Columbia Aquifer is used as a source of potable water off base, the remedial goal is to restore the aquifer to usable condition, i.e., reduce all chemicals of concern to below drinking water maximum contaminant levels (MCLs). To achieve this goal, AAB was selected as the best remedial alternative to reduce the solvent contamination. Source areas with high solvent concentrations were present in the shallow portion of the aquifer. From the source areas, dissolved solvents migrated downgradient and deeper in the aquifer with the flow of groundwater. In the deeper portion of the aquifer, the individual plumes comingle to form the larger Area 6 Plume, which covers approximately 345 acres.
Types of Substrate. Previous AAB pilot tests at Dover AFB used either sodium lactate or emulsified vegetable oil (EVO) as substrates to stimulate microbial growth. The best results were obtained with sodium lactate, which is a soluble substrate and easily utilized by microorganisms such as Dehalococcoides. EVO had limited success because it could not quickly create and maintain the robust reducing environment necessary for reductive dechlorination. This was especially apparent in the downgradient portions of the plume. Thus, for the downgradient Area 6 plume core treatments, a combination of sodium lactate and EVO is being used, each supplying 50 percent of the total organic carbon (TOC). A relatively low TOC concentration ranging between 2,000 and 4,000 milligrams per liter (mg/L) was injected. The sodium lactate rapidly creates a robust reducing environment, and the EVO provides TOC longevity. However, in the source areas where high concentrations of solvents were present (including daughter products), EVO was typically used and injected at concentrations up to 17,000 mg/L.
Delivery Methods. For AAB to work, TOC substrates must be distributed throughout the targeted zone so that microorganisms utilize the substrate while in contact with the contamination. Source areas typically contained the highest solvent concentrations and are located in the shallow portion of the aquifer. Substrate was injected using direct-push technology on 10 ft by 10 ft grids to distribute TOC throughout the source areas. High TOC concentrations (up to 17,000 mg/L TOC) were injected to reduce the number of reinjection events. In the downgradient and deeper portion of the plume, AAB treatment was applied by creating reducing zones perpendicular to groundwater flow, with rows of injection/extraction wells installed across the plume cores at a spacing of about 50 feet. Nine transects were installed with a total of 112 wells (Figure 2).
FIGURE 2. Area 6 Plume transects.
Substrate is delivered by extracting water from every other well, adding the substrate via a mobile process trailer, and injecting the amended water back into every other well; this creates a push-pull effect between the wells in a given transect. This push-pull delivery method is shown conceptually in Figure 3. Transects were installed to accommodate Base infrastructure, and substrate injections using towed trailers is flexible and scheduled to avoid disrupting Base activities. Substrate concentrations range from 2,000 to 4,000 mg/L TOC, and reinjection is required about every 2 years.
Operations. The primary operations are the distribution of TOC using the Area 6 transects, and monitoring groundwater geochemistry and contaminant concentrations. Process trailers designed for the AAB treatment can extract water out of the four wells, add substrate and nutrients, and inject into four or five wells at one time. Originally, each set up required 200 to 250 hours to distribute TOC across the 50-foot well spacing and it would take over a year to complete one injection event.
The push-pull method was initially effective for distributing substrate across the plume cores perpendicular to groundwater flow. However, later injection events were more difficult because biofouling or oil clogging of the aquifer decreased the hydraulic conductivity. After the fourth injection event, flow rates were reduced to the extent that the push-pull circulation could not be sustained and modification were made to improve substrate distribution. Modifications included: 1) extracting water from only one or two wells along a transect and injecting into all remaining wells; 2) replacing wells that were severely fouled; and 3) pulsing substrate injections. Currently, injections are conducted until at least 35,000 gallons have been injected into each injection well to minimize TOC gaps between the wells. Additionally, prior to an injection event, all wells are flushed with hot water to remove any remaining solidified oil from the previous injection and open up the well screen. At the conclusion of an injection event, water only is circulated for a minimum of 24 hours to flush the TOC from the well screen and gravel pack.
FIGURE 3. Conceptual push-pull method.
Monitoring wells are sampled semi-annually and analyzed for volatile organic compounds, dissolved gasses (ethane, ethane, and methane), TOC, total and dissolved iron, sulfate, dissolved oxygen, pH, conductivity, oxidation reduction potential, and temperature. Modifications to the injection solution are made annually based on monitoring data. In areas where indications of active degradation (i.e., generation of daughter products and ethene) are poor, attempts are made to inject water from an area where degradation is occurring and contains Dehalococcoides and Dehalobacter microorganisms. Supplementing water with the necessary micrograms has been very successful in improving degradation results in poorer performing areas.
RESULTS
After 2006, the source areas received a second direct-push injection in 2010 and the downgradient transects have received a minimum of four injection with several of them up to six injections to maintain sufficient TOC concentrations. After the initial 2006 injections, positive signs of degradation were observed within the first 6 months in the source areas and in several transects. After the first year, degradation indicators continued to improve at most of the transects, specifically, daughter products were being generated. For this paper, the areal extent and contaminant concentrations are used to illustrate the effectiveness of the AAB of the chlorinated ethenes. (Similar results were observed for chlorinated ethanes and are not presented here.) Figure 4 shows the areal extent and concentrations of PCE, TCE, cis-1,2-DCE, and vinyl chloride in 2006 prior to the initial AAB treatment and in January 2015 (9 years later). As shown on this figure, the areal extent of each contaminant has been significantly reduced. The high concentrations represented by the red and yellow in 2006 are gone in 2015 and replaced by the blue and green of relatively low concentrations. The parent material, PCE and TCE, has shrunk to a few localized areas. Cis-1,2-DCE and vinyl chloride are being generated from the degradation of the PCE and TCE, and even their overall concentrations are significantly lower in 2015 compared to 2006.
Another measure of AAB progress is to estimate the change in total dissolved solvent mass within the treatment area. Both chlorinated ethenes and ethanes are included in this analysis. Figure 5 shows the areal extent and total dissolved solvent concentrations and mass in 2006 prior to AAB treatment and in January 2015 after eight years of AAB. In 2006, the estimated total COC dissolved mass was 93.06 pounds (42,222 grams). As of January 2015, the estimated total COC dissolved mass was 1.87 pounds (848 grams), which is more than a 98 percent reduction in mass in the area of AAB application. This large change in dissolved mass indicates that AAB has been very effective to date.
FIGURE 4. Aerial extent of total COC concentration >100 ug/L
and dissolved COC mass.
CONCLUSIONS
The following conclusions can be drawn based on 9 years of AAB system operations and data collection:
• AAB is an effective remedial treatment alternative for large multi-sourced solvent plumes.
• AAB, as originally designed and then modified as conditions changed, is resulting in impressive degradation, as shown on Figures 4 and 5, with 98 percent of the total dissolved mass degraded in 9 years.
• Multiple substrate delivery methods should be used depending on site conditions with consideration for multiple injection events. These methods will likely need to be modified as geochemical and hydraulic conditions change in reaction to remediation. In Area 6, the push-pull injection method worked well for the first few years, but had to be modified due to reduced hydraulic conductivity.
• Aggressive source area treatment is a necessary component of the overall treatment strategy, as residual source mass can be many times the dissolved mass.
• Bioaugmentation with local water containing Dehalococcoides microorganism can jump start areas showing poor degradation.
• Bioremediation of large plumes using passive reductive barriers requires time for contaminated water to migrate through the reactive zones. Stakeholders must be aware that the remedy is very effective but requires time to complete.
FIGURE 5. Aerial extent and chlorinated ethenes concentrations prior to AAB treatment in 2006 and current extent and concentration in 2015.
