W912DR18R0055_-_Attachment_G_-_DRAFT_OU-7_Optimization_Memo_-_FINAL.docx

MEMO

To:

Stephen Wood, USACE Vincent Grassi, USACE Copies:

Courtney Ingersoll, Arcadis Kathleen Hamrick, Arcadis Scott Potter, Arcadis

From:

Edidia Nefso, Arcadis

Christopher Spooner, Arcadis

Subject:

DRAFT DSCR OU-7 Remedy Optimization

Arcadis U.S., Inc.

2101 L Street NW Suite 200 Washington

DC 20037

Tel 202 912 8100 Fax 202 887 5168

MEMO

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1/6 Document5 Introduction and Purpose This memorandum presents a recommended remedial approach for the Operable Unit (OU)-7 groundwater impacts at Defense Supply Center Richmond (DSCR). The recommended remedial approach presented is based on current site conditions and a review and evaluation of historic site data and previously implemented remedial actions.

OU-7 consists of impacted groundwater beneath and downgradient of the former Fire Training Areas (FTA) and is in the southern portion of DSCR. The contaminants, consisting of chlorinated volatile organic compounds (CVOCs) are found in the Lower Water Bearing Unit (LWBU) and the Saprolite. Multiple remedial actions have been implemented to remediate CVOCs in soil and groundwater at OU-7 including soil vapor and dual-phase extraction (SVE/DPE), density driven convection (DDC), enhanced in-situ biodegradation with permeable reactive barriers (PRBs) or biowalls, and in-situ bioremediation with injections of emulsified vegetable oil (EVO). Ongoing remedial actions focus on arresting migration of the groundwater impacts rather than source treatment. The effectiveness of these remedies was reviewed to determine a more optimal remedial technology for OU-7 to address residual source mass as well as the groundwater plumes.

Background Geology The geological cross-sections at the locations of the three former FTA Pits presented in the 2013 AECOM Remedial Action Work Plan, the source areas of the CVOC plumes, were reviewed as part of this optimization for OU-7. Based on review of these cross sections and available boring logs, the stratigraphy of the former FTA Pit areas was modified in some areas. Arcadis’ interpretation of the stratigraphy is as follows:

Former FTA Pit 1 (eastern most plume area)

· The top layer below ground surface generally consists of clay and silty clay, some sand, sandy clay and sandy silt. This layer is approximately 4 to 25 feet thick and is overlain by a thin layer of organic silt in parts of the area.

· The top layer is underlain by a layer of sand and some silt and gravel approximately 10 feet thick.

· The next underlying layer consists of coarse grained poorly graded sand with gravel of approximately 3 to 11 feet thick.

· Followed by a layer of approximately 5 to 9 feet of lower clay.

· Followed by a layer of approximately 9 to 20 feet of glauconitic sand and gravel.

· And a layer of sand and Saprolite.

Former FTA Pit 2 (middle plume area)

· The top layer below the ground surface consists of silt, silty clay and silty sand with trace gravel and fill in some areas. This layer is approximately 4 to 19 feet thick.

· The top layer is underlain by a layer of silty sand, trace gravel and clay. Gravel increasing with depth at the base of the layer. This layer is approximately 10 to 20 feet thick.

· The next underlying layer consists of approximately 8 to 10 feet of lean clay with increasing sand at base of the layer.

· Followed by a layer of silty clay, silty to clayey sand, and minor gravel grading into micaceous saprolite at the bottom.

Former FTA Pit 3 (western most plume area)

· The top layer below the ground surface consists of clayey sand, silty clay and sandy clay with trace gravel and fill in some areas. This layer is approximately 4 to 9 feet thick.

· The top layer is underlain by a layer of silty sand that is generally poorly sorted with a thickness of approximately 5 to 21 feet. This silty sand layer is overlain and underlain by a discontinuous gravel layer.

· The next underlying layer is a lower clay layer that is approximately 5 feet thick.

· Followed by a layer of glauconitic sand and gravel.

Previous Remedial Approaches, Pilot Tests and Treatability Studies The remedial approaches, pilot tests and treatability studies previously conducted at OU-7 are summarized below and shown on Figure 1.

i) Soil Vapor/Dual Phase Extraction (SVE/DPE) Pilot Test (1995) – the pilot test was conducted in August and September 1995 (LAW, 1997). The western most plume associated with the former FTA Pit 3 was the subject of the dual phase pilot test. The objective of the pilot testing was to develop site-specific parameters needed to further evaluate the feasibility of and obtain design parameters for a potential DPE remedial system. Based on the results of the pilot test, the DPE technology was recommended for consideration as a part of an overall remediation plan addressing the CVOC contamination of the LWBU. However, the full-scale technology was never implemented.

ii) Density Driven Convection (DDC) Pilot Test (1999) - a pilot system consisting of one DDC well, nine nested piezometers and an above ground pressurized air supply system were used to test the effectiveness of DDC in reducing concentrations of COCs in groundwater and to evaluate the feasibility of using DDC as a full scale remedial technology (LAW, 1999). The system was operated from December 17, 1998 to January 18, 1999. It was conducted at the downgradient end of the eastern most plume associated with the former FTA Pit 1. Results of the test suggested that DDC would be a feasible remedial option for the site. However, the full-scale technology was never implemented.

iii) Time Critical Removal Action (2004) – was conducted in November, 2004, where approximately 484 cubic yards (yd3) of soil were removed from the former FTA Pit 1, 5 yd3 of soil from Pit 2 and 498 yd3 of soil from Pit 3 (MACTEC, 2005). This was done to reduce, to the extent practicable, contaminant mass from the vadose zone at OU-4 that could contribute to further degradation of OU-7 groundwater.

iv) Enhanced In-Situ Biodegradation Treatability Tests (2007 & 2009) – the treatability tests were implemented in the former FTA Pits 1 and 2 areas of the LWBU (AECOM, 2010). The tests consisted of placement of two PRBs or treatment walls (biowalls) containing organic mulch and sand downgradient of the former FTA Pit 1 and the former FTA Pit 2 source areas (Figure 2). A separate in-situ bioremediation treatability test was conducted in the former FTA Pit 3 area in April 2009. Approximately 2,125 gallons of approximately 6 percent (volume/volume) Emulsified Vegetable Oil (EVO) was injected in an area of elevated chlorinated solvent concentration within the LWBU. Results showed substantial degradation of the CVOC contamination in the LWBU; and therefore, in-situ bioremediation was chosen as an effective technology in the treatment of CVOCs for the OU-7 groundwater.

v) In-Situ Bioremediation (ISB) and Institutional Controls (ICs) (2013–present) – the record of decision (ROD) for OU-7 was signed in 2012. The final remedy for OU-7 consisted of in-situ bioremediation consisting of EVO injections in source areas and downgradient high concentration areas, monitored natural attenuation (MNA) and Long Term Monitoring and Institutional Controls (DSCR, 2012). As part of the final remedy, in-situ bioremediation that included injections of EVO was initiated in 2013 to treat CVOCs in the lower WBU at the source area and downgradient plumes (Figure 2). Institutional controls were also established to prevent exposure to OU-7 groundwater.

Based on a review of the historic CVOC concentration data and trend analysis results at wells downgradient of the in-situ bioremediation areas (MW-41, MW-48, MW-53, MW-59 and MW-83), concentrations of tetrachloroethene (PCE) and trichloroethene (TCE) are generally decreasing or remaining unchanged. Concentrations of cis-1,2-dichloroethene (cis-1,2-DCE) and vinyl chloride (VC) are generally increasing or remaining unchanged. Although, this indicates the occurrence of some degradation of the parent CVOCs, the lack of significant production of successive de-chlorination daughter products (cis-1,2-DCE, VC and ethene) suggests that complete degradation is either not occurring or is occurring locally at specific wells. Additionally, the total organic carbon (TOC) level at all these downgradient wells have not maintained a concentration of at least 10 milligrams per liter (mg/L) or higher, which is needed to sustain active anaerobic bioremediation. Because biodegradation, even in the wells directly downgradient of the in-situ bioremediation area, has not been sustained, the in-situ bioremediation system in place (EVO injections) would not achieve MNA endpoints within a reasonable timeframe.

The likely reason of the limited success of the bioremediation system in place is due to low injection volumes resulting in both poor distribution of carbon (gaps in the barriers) and a barrier insufficiently thick to biodegrade COCs before passing through each bioremediation area. The result is either no treatment as dissolved impacts move through the gaps in the barrier, or incomplete dechlorination resulting in cis-DCE and/or VC plume development. The EVO injected in 2013 is either completely depleted or no longer emulsified which provides inadequate carbon to the aquifer and therefore active bioremediation is not ongoing.

Remedial Strategy US Army Corp of Engineers Injection Network Expansion Prior to implementation of the optimized remedial strategy, The US Army Corp of Engineers (USACE) will seek to expand the existing injection well network to focus remediation in locations with relatively high groundwater concentrations. The expanded injection well network will consist of 12 new injection wells as shown on Figure 1. Following installation of these wells, the USACE will complete an ISB injection into 31 injection wells (12 new and 19 existing wells). The injection well network and recommended injection volume per wells is as follows:

· Existing Shallow Injection Wells – INJ-132, INJ-133, INJ-134, INJ-135, INJ-136, INJ-137

· Existing Intermediate Injection Wells – INJ-151 through INJ-159 and INJ-163 through INJ-166

· New Intermediate Injection Wells – INJ-200 through INJ-211

· Recommended Target Injection Volumes (per well)

· Shallow Injection Wells (6) – 900 gallons

· Existing Intermediate Injection Wells (13)– 2,350 gallons

· New Intermediate Wells (12) – 9,400 gallons

· Total Injection Volume – 148,750 gallons

· Total Lactoil (2% by volume) – 2,975 gallons.

The recommended carbon donor is Lactoil (or equivalent), which is a blended carbon donor consisting of 35% EVO and 35% sodium lactate. Because Lactoil provides both a soluble (sodium lactate) and insoluble (EVO) component, distribution and migration of Lactoil will be greater than what can be achieved with only EVO. Additionally, the soluble carbon donor provides an easily utilized carbon source to condition the aquifer and rapidly create the appropriate geochemical conditions to achieve complete dechlorination. As the sodium lactate is utilized, the EVO will provide a long-term carbon source to sustain the active bioremediation for up to 2 years. Performance groundwater monitoring should be used to determine the need for future injections, Additional injections will be necessary either through an optimized remedy or through additional injections into the existing network where appropriate. This approach targets primary transport pathways and adds source treatment as well as injections into existing injection barriers. It has been assumed that existing well infrastructure has not been biofouled or has been re-developed prior to implementation and injectability is not an issue. These new injection wells will be incorporated into an optimized remediation strategy in the future. If only a portion of the above scope is completed, any remaining scope will be rolled into an optimized remedy.

Water for the injections will either be trucked in or obtained from a local hydrant and stored in a frac tank The carbon donor will be stored on-site in 275 gallon totes. A metering pump will be used to introduce the appropriate amount of carbon. An in-line static mixer will be used to mix the injected solution.

To remediate the source area and maintain active bioremediation, multiple injections will be necessary. Injections through the proposed injection well network or as part of an optimized remedy. Injections should be completed approximately every 18 months. Up to 4 injection events are likely required to achieve COC concentrations that will support transition to an MNA remedy with a reasonable timeframe.

Well Construction and Performance Monitoring Injection Wells

· Will be constructed of 4-inch ID schedule 80 PVC casing and 4-inch ID stainless steel screen.

· The screens will consist of 20-slot stainless steel screen with an appropriate filter pack for the screen slot size.

· Will be screened from the water table down to the lower clay layer (approximately 30 feet below ground surface [ft bgs]) with an approximate screen interval of 10 to 20 feet.

Performance Monitoring

· Performance monitoring of existing wells near the applied treatment areas will be done periodically to monitor the effectiveness of the active bioremediation areas and to monitor changes in the geochemical and COC concentrations over time.

· Select performance monitoring wells at the three source areas (Table 1) will be sampled for some or all of the following parameters:

· Field parameters, including dissolved oxygen (DO), pH, oxidation/reduction potential (ORP) and turbidity;

· CVOCs using USEPA Method 8260B;

· Dissolved gases (methane, ethene, and ethane); and

· TOC

· Anions (sulfate and nitrate)

· Performance monitoring will be completed quarterly for field parameters, CVOCs, TOC, and dissolved gases. Anions are included in the approved annual monitoring program and will be sampled annually. As performance monitoring confirms complete degradation of CVOCs, development of the appropriate geochemical conditions, and utilization rates of the carbon donor, the sampling frequency may be reduced to semi-annual.

· During the injection event, select wells will be sampled for TOC (both visually and analytically) to confirm distribution. Following initial injection of the carbon source, the performance monitoring will be focused on evaluating TOC distribution and ensuring TOC is sustained above 10 mg/L in the select performance monitoring wells (Table 1).

· Based on the TOC sample results, the injection frequency of the carbon source may be adjusted (either up or down) or the TOC loading increased to allow for a longer injection frequency.

· Following confirmation of TOC distribution and evaluating utilization, additional parameters, such as dissolved gases may be monitored to demonstrate active bioremediation is occurring.

· The CVOC concentrations monitored within the treatment area will be compared to background trends. The minimum performance criteria will be a trending decrease in CVOC concentrations within the study area, at a rate of at least five percent greater than the background trend.

· As the active bioremediation system entails enhanced biodegradation, it is anticipated that shifts in daughter products (e.g., TCE to cis-1,2-DCE and VC) will be observed over the course of the program. Therefore, the overall trending decrease in CVOC concentrations will also consider shifts in daughter products (which may increase initially) as part of holistic VOC declines throughout the plume.

· Based on the results of the data, ongoing refinement of the remediation program will be conducted and may include modification of the carbon substrate, pH modification, bioaugmentation, or carbon dosing and frequency of delivery events.

Table 1. Performance Monitoring Wells

MW-431

MW-1501

Notes:

All wells will be sampled for general field parameters (DO, pH, ORP and turbidity) and CVOCs 1 Wells that will be sampled for TOC and dissolved gasses (methane, ethene and ethane)

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