Application of Chemical Oxidation Followed by Anaerobic Degradation Remedial Technologies for Trichloroethene in a Multi-System Aquifer

Remedial enhancements to an existing pump-and-treat (P&T) system were selected for a pilot study to address trichloroethene (TCE)-impacted groundwater within a fractured bedrock system extending into a residential neighborhood. The focus for implementing the remedial enhancements was the farthest downgradient area of the dissolved plume not fully responding to P&T system operations due to pumping well distance as predicted by groundwater modeling. This area is hydrogeologically complex due to an upward vertical gradient where TCE impacted bedrock groundwater moves into overlying unconsolidated sediment. To enhance treatment of this area, an aggressive in situ treatment train was implemented to quickly reduce groundwater impact using RegenOx and then establish a long-term remedy using NewmanZone® to stimulate microbial growth as part of an anaerobic reductive dechlorination (ARD) bioremediation process. Application of RegenOx and NewmanZone® was accomplished via permanent injection points into the bedrock and unconsolidated sediments (alternating points screened within glacial till and fluvial deposits/fill). INTRODUCTION An area located approximately 1,250 feet north from a Facility with an operating pump-and-treat (P&T) remedial system addressing volatile organic compounds (VOCs), specifically trichloroethene (TCE) in bedrock groundwater, was selected for a pilot study to evaluate remedial enhancements. The pilot study area was selected based on groundwater data indicating the existing P&T system would not effectively treat groundwater within a reasonable time frame (as predicted by groundwater modeling). This area is located at the northern end of a residential neighborhood and was the most downgradient edge of TCE concentrations in the bedrock aquifer that was being addressed under a Consent Order. Based on the Remedial Investigation/Feasibility Study (RI/FS) work, project characterization data, and a review of the most applicable remedial technologies the following remedial technologies were selected for combined application to evaluate remediation of TCE in groundwater with continued Pump and Treat remediation system operation. • In situ chemical oxidation (ISCO) technologies (specifically using RegenOxTM) and • In situ anaerobic bioremediation via anaerobic reductive dechlorination (ARD) process technologies (specifically using Newman Zone®). The use of the above technologies was envisioned to be applied as a treatment train with initial passive injection of RegenOxTM for ISCO to peak-shave TCE concentrations followed by active injection/recirculation of Newman Zone® for ARD to continue TCE reductions. Ultimately, the use of these remedial technologies was projected to address residual TCE concentrations at the northern portion of the residential neighborhood (pilot study area) while enhancing the long term effectiveness of the existing P&T system to address current (pre-pilot study) TCE concentrations ranging from non-detect to 9,100 μg/L. The RegenOx chemical oxidant formulation consists of sodium percarbonate and sodium carbonate monohydrate activated by ferrous sulfate (comparable with Fenton’s Reagent chemistry, but is not as strongly reactive and is less exothermic). RegenOx reacts with chlorinated solvents such as TCE as well as other carbon based (organic) materials and breaks these constituents down into harmless byproducts (CO2 and H2O). RegenOx has been shown to react with VOCs over a relatively short time frame (up to 30 days). The oxidizing conditions initialized by the RegenOx formulation have been shown to subside with groundwater conditions returning to pre-injection conditions over time. This characteristic allows for use of this product as an initial treatment to quickly reduce VOC concentrations and subsequent application of in situ bioremediation treatment for long-term VOC reductions (Regenesis: http://www.regenesis.com/products/ chemOx/regenOx/). NewmanZone® is an electron donor formulation consisting of sodium lactate, soybean oil, food-grade additives, and proprietary surfactants and stabilizing agents that stimulates the ARD process. ARD is an electron donor/carbon source (substrate) rate driven, microbial mediated, oxidation-reduction reaction capable of degrading chlorinated solvents (i.e. TCE) to the final end products, chloride, ethene and ethane. The use of NewmanZone® for ARD process stimulation was selected based on the formulations ability to be long lasting and readily distributable in a groundwater system (Remediation & Natural Attenuation Services, Inc.: http://www.rnasinc.com/newman-zone/). PILOT STUDY AREA SETTING The pilot study area is situated within the north end of a small residential community which is bordered to the south by the Facility. The geology and hydrogeology of the area consists of unconsolidated sediments that are underlain by bedrock with an interconnected aquifer system. The unconsolidated glacial till sediments (clay and silt with unsorted variable grained sand, coarse gravel and boulders) has a thickness of 15 to 20 feet and has been reworked by surface water leaving alluvial deposits. Portions are underlain by fill material observed within ten feet of grade. The above material is underlain by compacted glacial till (lodgment till) 10 to 15 feet thick. The lodgment till is underlain by fractured sedimentary bedrock consisting of shale inter-bedded with sandstone and siltstone and/or mudstone with high clay content at about 30 to 37 feet below grade (bg). Groundwater flow direction in the unconsolidated sediments (overburden) is northward, and in bedrock is southward in the pilot study area. A vertical groundwater flow component for the groundwater system was observed to upward at wells MW-12A and MW-12B at the north end of the pilot study area, which becomes downward at cluster well MW-24 (S,I,D) on the southern end of the area. Bedrock groundwater quality in the pilot study area for TCE historically ranged from non-detect to 8,400 μg/L. Overburden groundwater quality in the pilot study area for TCE historically ranged from 16 μg/L (estimated) to 19,000 μg/L. The presence of TCE degradation product cis-1,2-dichloroethene (cis-1,2-DCE) has been typically observed at one to two orders of magnitude below TCE concentration with an occasional exception. The presence of vinyl chloride (VC) was typically not observed in the pilot study area with exceptions of wells MW-12A, MW-12B, MW-19A, and MW-20A. PILOT STUDY OVERVIEW The pilot study activities were conducted in two phases to accommodate the batch injection of RegenOxTM followed by the active injection of NewmanZone®. The pilot study work included the following activities. • Installation of twenty overburden (OIW-1 through OIW-20) injection wells in two off-set parallel rows of ten with alternating well depths to top of lodgment till and top of bedrock. • Installation of three bedrock (IW-1 through IW-3) injection wells; IW-2 and IW-3 to 80 feet and IW-1 to 100 feet. • Conduct baseline monitoring for RegenOx injection. • Initiate batch injections of RegenOx solution into the twenty three injection wells and monitoring pilot study area. • Construction of active recirculation/injection system to use groundwater pumped from former potable well 41 that was treated via activated carbon, amended with Newman Zone®, and injected into the injection wells via subsurface plumbing. • Following stabilization and return of groundwater conditions to pre-injection levels, conduct baseline monitoring event for NewmanZone® injection. • Initiate active injection of NewmanZone® via recirculation system start-up with operation & maintenance (O&M) and monitoring pilot study area. RegenOxTM Pilot Study. Baseline monitoring of groundwater geochemical parameters (via field analysis) and groundwater quality (via laboratory analysis) was conducted at select monitoring wells to evaluate pre NewmanZone® injection groundwater. Field geochemical parameters collected were pH, temperature, conductivity, Oxidation-Reduction Potential (ORP), and Dissolved Oxygen (DO). Groundwater was also field screened via colorimetric test strip for peroxide (indicates presence of RegenOx). Groundwater samples were laboratory analyzed for VOCs (EPA 8260). Additionally, groundwater samples from MW-21AS and MW-22AI were laboratory analyzed for dissolved gases— methane, ethane, ethane (EPA 5021A), anions chloride, sulfate, nitrate, formate, acetate, lactate/propionate, and butyrate (EPA 6500), ammonia (Hach 8155), phosphate (Hach 8048), and pH. RegenOxTM batch injection activities were conducted over a four week period with a one week break (third week) in conducting injections to allow relaxation of subsurface back-pressures encountered during the second week of injections. The injection process consisted of mixing and injecting the Part B activator complex (5% solution), flushing with 1.5-3 well volumes of clean water, mixing and injecting the Part A oxidant complex (5% solution), and flushing with 1.5-3 well volumes clean water. A total of 8,561 gallons of RegenOxTM was injected to the pilot study area with 3,535 gallons injected to the overburden and 4,981 gallons injected to the bedrock. Monitoring during and after RegenOx injection activities were implemented. An overall increase in groundwater geochemical parameters from baseline data and detection of peroxide was expected to show RegenOxTM was reacting with carbon sources (i.e. VOCs) in the pilot study treatment zone. The following field geochemical and peroxide data was observed (see Figure 1 for well locations). FIGURE 1, Well location map. • DO values fluctuated from event to event with only MW-19A, MW-20A, and MW-22AI showing increased values. • ORP values increased at MW-20A, MW-23AS, and MW-23AI. • Conductivity values increased at MW-12B, MW-19A, MW-20A, MW-23AS, MW-23AI, MW-23AD, MW-26A, and MW-27A. • The pH values increased at MW-12A, MW-12B, MW-21AS, MW-22AI, MW23AI, and MW-23AD. • Temperature values increased at MW-12B and MW-19A. • Peroxide monitoring showed the presence of peroxide at MW-12B and MW23AD. Based on increases of three or more field geochemical parameters and detection of peroxide over 12 weeks of monitoring at wells MW-12B, MW-19A, MW-20A, MW23AI, and MW-23AD, the occurrence of an oxidation reaction/presence of RegenOxTM is apparent in the pilot study treatment zone. VOC analysis for TCE from baseline to post injection sampling for wells in the pilot study treatment zone showed seven wells to decrease, three wells to increase and two wells to continue showing non-detectable concentrations (see Table 1). For the most part, the presence of cis-1,2-DCE concentrations, followed TCE concentrations with cis-1,2DCE concentrations an order of magnitude lower than TCE concentrations which is similar to pre-injection conditions. TABLE. RegenOxTM pilot study TCE concentrations. Wells Baseline RegenOxTM TCE Data* Post-RegenOxTM TCE Data* (Eight Weeks) TCE Percent Change MW-12A 9,100 4,700 -48% MW-12B 5,500 3,900 -29% MW-19A 2,800 3,700 +32%