ACHIEVING RESIDENTIAL CLOSURE AT A CHLORINATED SOLVENT SITE

Transcription

1 ACHIEVING RESIDENTIAL CLOSURE AT A CHLORINATED SOLVENT SITE Chlorinated hydrocarbons are among the most persistent and difficult groundwater contaminants to remediate, particularly to residential standards. Choosing the most effective remediation strategy for a site involves evaluating technical and business considerations. Cumulatively, these factors pose a complex web of interactions, resulting in a consensus within the industry that remediation of chlorinated solvent sites to residential standards is often impractical.

2 In some cases, these challenges spur development of novel methods and technologies to overcome site specific challenges. Remediation of the Commerce Center site in Dallas, Texas, is an example of an integrated site strategy that combines many of the recommendations offered by the National Research Council (2005) and Interstate Technology & Regulatory Council (2011): 1. Clearly define remedial objectives before beginning remediation, with specific, measurable, attainable, relevant and time-bound (SMART) attributes. 2. Integrate multiple remedial technologies in time and space. 3. Monitor and evaluate performance to determine if objectives are being met, if remedies can be optimized and when technologies should be upgraded as site conditions evolve. Remediation technologies used at the site were soil excavation, in situ chemical oxidation (ISCO), in situ chemical reduction (ISCR) and enhanced anaerobic bioremediation. Site conditions and access restrictions necessitated development of innovative application methods, and contractual requirements stipulated by the site owner required an expedited cleanup to residential standards. SITE CHARACTERIZATION Several chemical distributors have occupied the Commerce Center site since A Phase 2 investigation in 2000 identified chlorinated hydrocarbons, including dichlorobenzene, tetrachloroethene (PCE) and other volatile organic compounds (VOCs) in soil and groundwater. The origin of the VOCs was not known, although surface discharges related to on-site chemical storage were suspected. Site investigations identified an area of contaminated soil adjacent to the building and a groundwater plume extending downgradient under two adjacent properties (Figure 1). Access to the plume area is physically restricted. The downgradient margin of the site is bordered by a stormwater channel that drains to the Trinity River. Groundwater is intercepted by the stormwater channel, which terminates the groundwater VOC plume. GEOLOGIC AND HYDROLOGIC SETTING A surficial layer of approximately 2 feet of gravel fill exists near the building. In this area, the vadose and saturated zone soil is silty and sandy clay with intermittent lenses of fine sand to approximately 35 feet below grade, transitioning to predominantly sand to a depth of approximately 60 feet below grade, all deposited as fluvial sediments on the Trinity River floodplain. Due to the steep slope down to the channel, the fluvial deposits extend to a depth of approximately 35 feet at the base of the slope. The fluvial deposits are underlain by the Eagle Ford Shale. Groundwater is encountered at approximately 15 to 25 feet below grade in the upgradient source area and as shallow as 1 foot below grade at the base of the slope adjacent to the stormwater channel. Hydraulic conductivity is relatively low, with slug test estimates ranging from 2 centimeters by 10.5 centimeters to 1.4 centimeters by 10.3 centimeters per second. The aquifer is unconfined. The piezometric surface follows the ground surface topography, dipping south toward the stormwater channel with a sharp increase in gradient at the break in surface slope. The stormwater channel intercepts the water table, and groundwater discharges to the surface water. CONTAMINANTS AND DISTRIBUTION The zone of impacted, vadose-zone soil measures approximately 20 feet by 40 feet, directly adjacent to the building (Figure 1). The VOCs detected in soil exceeding the PCLs were 1,2-dichlorobenzene, 1,4-dichlorobenzene, PCE, cis-1,2-dichloroethylene (cis-dce), and trichloroethylene (TCE) (Table 1). The predominant VOCs detected in groundwater were PCE, TCE, cis-dce, and vinyl chloride (VC) (Table 1). Other VOCs were detected only intermittently or did not exceed their protective cleanup levels (PCLs). The groundwater plume measured approximately 225 feet by 230 feet and extended to approximately 35 feet below grade in the source area. A monitoring well installed downgradient of the stormwater channel did not yield detectable VOCs, indicating the plume was intercepted by and discharging to the stormwater channel. 2 BURNS & McDONNELL

3 FIGURE 1: Site Location Map, Layout BURNS & McDONNELL 3

4 Soil CONTAMINANT MAXIMUM CONCENTRATION PROTECTIVE CLEANUP LEVELS (PCLS) Tier 1 Residential Critical PCL 1,2-dichlorobenzene 120 mg/kg 18 mg/kg 1,4-dichlorobenzene 14 mg/kg 2.1 mg/kg Tetrachloroethylene (PCE) 85 mg/kg 0.05 mg/kg Trichloroethylene (TCE) 0.13 mg/kg mg/kg cis-1,2-dichloroethylene (cis-dce) 1.5 mg/kg 0.25 mg/kg Groundwater Tetrachloroethylene (PCE) Tier 1 Residential Critical PCL SW GW PCL* 920 µg/l 5 µg/l 790 µg/l Trichloroethylene (TCE) 1,200 µg/l 5 µg/l 257 µg/l cis-1,2-dichloroethylene 514 µg/l 70 µg/l 2,500 µg/l (cis-dce) 1,1-Dichloroethylene (1,1-63 µg/l 7 µg/l 58.4 µg/l DCE) Vinyl chloride (VC) 18 µg/l 2 µg/l 4,150 µg/l TABLE 1: Soil and Groundwater VOCs Exceeding Texas Risk Reduction Program Critical Protective Cleanup Levels *Groundwater-to-surface water PCL REMEDIATION APPROACH The site was entered into the Texas Voluntary Cleanup Program (TVCP), with the Texas Commission on Environmental Quality (TCEQ) as the regulatory agency. Under the TVCP, nonresponsible parties are protected from state liability and site cleanups are streamlined. The absolute remedial objective was to receive a Certificate of Completion from the TVCP for residential standards. The underlying functional objectives, required to be met in order to achieve the absolute objective, are the site-specific Texas Risk Reduction Program Tier 1 Residential Critical PCLs (Table 1). The overall remediation approach developed to meet the residential cleanup objectives consisted of three types of technologies: 1. An initial source area soil removal 2. ISCO using three emplacement and injection methods in the source and primary plume area 3. ISCR and anaerobic bioremediation at the downgradient edge of the plume and adjacent to the stormwater channel Source area excavation and ISCO injection system construction were completed from June through mid-august 2004, and the initial injection activities continued until October Based on performance monitoring results, additional optimization and treatment at focused locations were conducted in 2007, and long-term monitoring continued until October SOURCE AREA EXCAVATION The shallow depth of contamination, low permeability of the clay soil, accessibility, relatively moderate level of contamination, and desire to eliminate the source contributing to the groundwater plume as quickly and cost-effectively as possible were important factors considered for selection of a soil removal remedy. A total of 520 tons of excavated soil was disposed off-site. After excavation, sidewall samples and floor samples continued to exceed PCLs for PCE and/or TCE. Further excavation was not possible without undercutting the building or encroaching off-site property; therefore an alternative plan was developed to address the remaining soil using ISCO. 4 BURNS & McDONNELL

5 INNOVATIVE REAGENT DELIVERY ISCO was an attractive option for staying within the remediation timetable. However, the low permeability of the aquifer presented a challenge for delivering ISCO reagents. Traditional methods were anticipated to require very close horizontal and vertical spacing of injection points and long-term, on-site presence of a field crew with associated labor, mobilization and equipment, increasing costs. VOCs remained at concentrations exceeding the cleanup goals in unsaturated soil above the water table, which also posed a challenge for remediation by injection of liquid reagents due to inhomogeneous reagent distribution. Two innovative approaches for ISCO reagent delivery were developed to overcome these challenges (Figure 2). The first method was used in the excavated source area and combined placement of a blend of sand and solid potassium permanganate in deep borings and in the base of the excavation. Water was then delivered to the borings, injection wells and to the backfilled excavation in order to create increased hydraulic head, resulting in infiltration of water through the reagent and into the underlying soil and aquifer. The second method was emplacement of a slurry of permanganate via hydraulic fracturing. This approach was pioneered by Siegrist et al. (1999), and was improved by tying the emplaced solids to a network of water injection wells in order to increase groundwater flow through the emplaced solids. The overall system provided continuous, remotely controlled ISCO injection for 15 months. After initial construction was completed, field oversight was limited to one day per month for routine system checks. FIGURE 2: Schematic of the Three-Pronged ISCO Approach BURNS & McDONNELL 5

6 ISCR AT DOWNGRADIENT PLUME MARGIN The TCEQ required a more immediate response to groundwater at the plume margin because VOC concentrations in groundwater immediately upgradient of the stormwater channel exceeded the surface water to groundwater PCLs (SWGW PCLs). A primary concern during remedial design was the potential for discharge of any remedial additive to the stormwater channel. A permeable reactive barrier (PRB) comprised of granular zero valent iron (ZVI) was selected because the ZVI is not mobile after emplacement, and reaction-intermediate products are either not hazardous or have much higher SWGW PCL criteria (e.g., cis-dce), thus mitigating the potential for discharge of reagents or hazardous reaction products to surface water after injection FINAL TREATMENT Based on quarterly groundwater monitoring events between October 2005 (when water injection was discontinued) and April 2007, six discrete areas at the site (Figure 1) required additional injection in order to achieve the PCLs in a timely fashion. Five of the areas (areas one through five) were treated with additional permanganate injection. Area six was treated with additional ISCR and bioremediation. GROUNDWATER MONITORING RESULTS Detailed groundwater sampling is central to determining whether the remedies are operating as designed and whether optimization or transition of remedies is warranted. Data was actively analyzed during the course of the project, and adjustments to the treatment areas and methods were implemented as previously discussed. After remediation, quarterly groundwater monitoring events began in January 2005 and continued through October The routine monitoring program was reduced to semiannual frequency beginning in April 2007 and continued until October Figures 3, 4 and 5 show the VOC reductions in key areas of the plume after the various treatments; MW- 09 and MW-40 are located in the immediate source area, MWs 34, 39, 57, and 58 are located downgradient of the source area, and MW-61 and MW-62 are located at the downgradient edge of the plume (see Figure 1). Liquid sodium permanganate was injected at discrete locations in Areas 1 through 5. Oxidant requirements were based on the size of the treatment area, site geology, VOC concentrations, and an estimated natural oxygen demand for the soil. A sodium permanganate solution of 40% was diluted on-site to a 3% concentration for injection. Injection was accomplished using standard direct-push equipment through temporary borings. PCE TCE cis-dce VC FIGURE 3: Source Area Groundwater VOC Concentrations (See Figure 1 for a Site Map) For Area 6, the ZVI barrier upgradient of MW-61 (Area 6 in Figure 1) was augmented with the injection of EHC. EHC is a mixture of ZVI with a controlled-release carbon substrate to enhance anaerobic biodegradation. Like ZVI, EHC is an immobile, granular product that poses no concerns with potential discharge into the stormwater channel. The EHCTM slurry was prepared as a 30% solids mixture and injected using standard direct-push equipment in 2-foot vertical intervals throughout the target treatment zone. PCE TCE cis-dce VC FIGURE 4: Groundwater VOC Concentrations for Monitoring Wells MW-61 and MW-62 at the Distal Plume Margin (See Figure 1 for a Site Map) 6 BURNS & McDONNELL

7 additional treatment, and active discussions with the regulatory agency can result in site closure to residential standards within a reasonable time frame. REFERENCES Interstate Technology & Regulatory Council (ITRC) Integrated DNAPL Site Strategy. Washington, D.C.: Interstate Technology & Regulatory Council. National Research Council (NRC) Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, D.C.: The National Academies Press. SITE CLOSURE PCE TCE cis-dce VC FIGURE 5: Groundwater VOC Concentrations in Monitoring Wells Downgradient and Crossgradient of the Source Area (See Figure 1 for a Site Map) Active remediation took just more than three years (June 2004 to October 2007) and was followed by two years of confirmatory sampling through October Active discussions with TCEQ during the course of the project resulted in site closure in a stepwise process: TCEQ agreed in May 2009 that the soil-togroundwater pathway was eliminated and, thus, the residential soil objectives had been met. TCEQ agreed in March 2010 that residential groundwater objectives had been met on-site. Discussions with the city of Dallas resulted in a deed restriction in October 2011 on a small (0.15 acre) piece of the adjacent property with marginal exceedances (<10 parts per billion) of the groundwater PCLs. TCEQ issued a final certificate of completion for the entire site in November The entire remedy, from submittal of the first plan to final regulatory closure, required less than eight years. This case study demonstrates how coupling multiple remediation technologies with innovative reagent delivery methods, clearly defined remedial objectives established before remediation, performance monitoring to optimize and focus Siegel, D.I On the effectiveness of remediating groundwater contamination: Waiting for the Black Swan. Groundwater, in press. Siegrist, R.L, K.S. Lowe, L.C. Murdoch, T.L. Case, and D.A. Pickering In situ oxidation by fracture emplaced reactive solids. Journal of Environmental Engineering 125, no. 5: Stroo, H.F., A. Leeson, J.A. Marqusee, P.C. Johnson, C.H. Ward, M.C. Kavanaugh, T.C. Sale, C.J. Newell, K.D. Pennell, C.A. Lebron, and M. Unger Chlorinated ethene source remediation: Lessons learned. Environmental Science & Technology 46, no. 12: BIOGRAPHIES JOHN R. HESEMANN, PE, is a regional manager at Burns & McDonnell. He earned his bachelor s and master s degrees in geological engineering from the Missouri University of Science and Technology. CHERYL M. TOPHINKE, RG, is a senior geologist at Burns & McDonnell. She earned her bachelor s degree in geology from Missouri State University. TOM ZYCHINSKI, RG, is a technical leader and vice president at Burns & McDonnell. He earned his Bachelor of Science in geology from the University of Missouri and his master s degree in geology from the University of Illinois. BURNS & McDONNELL 7

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