Groundwater Remedial Optimization Work Plan - U.S. Army SSC.pdf

Final Groundwater Remediation Optimization Work Plan

For U.S. Army Soldier Systems Center

Natick, Massachusetts

Prepared for

United States Army Environmental Command

Northeast Environmental Services and Support Division 2450 Connell Road, Bldg. 2264

JBSA Ft Sam Houston, TX 78234-7664

Prepared by:

Plexus Scientific Corporation 5510 Cherokee Avenue, Suite 350

Alexandria, VA 22312

Contract W91ZLK-13-D-0010

Delivery Order 0004

Ju 2019

Groundwater Remediaiton Optimization Work Plan U. S. Army Natick Soldier System Center, Natick, Massachusetts i

TABLE OF CONTENTS

INTRODUCTION

1.1 SITE DESCRIPTION

1.2 GEOLOGY

1.3 HYDROGEOLOGY

OPTIMIZATION WORK PLAN OBJECTIVES

UPGRADE OF THE GWETS

3.1 SCADA/PLC/HARDWARE

3.2 BAG FILTER EQUIPMENT AND CONTROLS UPGRADE

3.3 REPLACE FLOW METERS

3.4 REPAIR PULL BOXES AT MW-96B AND EW-2

EXTRACTION WELL REHABILITATION

4.1 SPECIFIC CAPACITY AND DISCHARGE PRESSURE TESTING

4.2 EXTRACTION EQUIPMENT REMOVAL/INSPECTION/REHABILITATION

4.3 SURGE/BAIL/AIR-LIFT

4.4 CHEMICAL ADDITION AND IN-WELL RECIRCULATION

4.5 WELL REBUILD/SPECIFIC CAPACITY AND DISCHARGE PRESSURE TESTING

4.6 CONTINGENT WELL REHABILITATION USING PANTONITE

EXPANSION OF THE EXTRACTION WELL NETWORK

5.1 HORIZONTAL DIRECTIONAL DRILLING

5.2 EXTRACTION WELL INSTALLATION

5.3 1,4-DIOXANE PRE-TREATMENT SYSTEM

5.4 ARIEM BUILDING

SLUG VOLUME REMOVAL

1,4-DIOXANE PRE-TREATMENT SYSTEM UPGRADE

7.1 1,4-DIOXANE PRE-TREATMENT SYSTEM EQUIPMENT DESCRIPTION

7.2 BENCH SCALE TREATABILITY TESTING

7.3 1,4-DIOXANE PRE-TREATMENT SYSTEM CONTROLS UPGRADES

7.4 PUMP AND INSTRUMENTATION UPGRADES

OPTIMIZATION OF THE MONITORING WELL NETWORK

UPDATE LONG-TERM MONITORING PLAN

PROJECT SCHEDULE

REFERENCES

ii

TABLES

Table 4-1 Well Recirculation Amendments Table 7-1 Bench Test Matrix

FIGURES

Figure 1-1 Site Layout, Natick Soldier Systems Center Figure 1-2 Interpreted Deep Overburden Piezometric Surface – Event 77 Figure 3-1 Current GWETS Configuration Process Flow Diagram Figure 3-2 New Bag Filter System Process Flow Diagram Figure 4-1 Flow Data Time Trend – 2017/2018 Figure 4-2 Extraction Well Rehabilitation Process Schematic Figure 5-1 Remote Wells New Extraction System Process Flow Diagram Figure 5-2 Proposed New Extraction Wells and Pipe Routing Figure 5-3 Typical Extraction Well Vault and Trench Detail Figure 5-4 Dioxane Treatment System Shed Process Flow Diagram

APPENDICES

Appendix A Safety Data Sheets Appendix B Project Schedule iii

LIST OF ACRONYMS AND ABBREVIATIONS

% percent µg/L micrograms per liter ADL Arthur D. Little ARIEM U.S. Army Research Institute of Environmental Medicine bgs below ground surface CAD Computer-Aided Design COC Contaminant of Concern ECC Environmental Chemical Corporation, LLC.

ESD Explanation of Significant Differences FPGS Former Proposed Gymnasium Site ft foot/feet gpm gallons per minute GPR Ground Penetrating Radar GWETS Groundwater Extraction and Treatment System HDPE high-density polyethylene HMI Human Machine Interface I&C Instrumentation and Controls I/O input/output lf linear feet MassDEP Massachusetts Department of Environmental Protection MCP Massachusetts Contingency Plan mg/L milligrams per liter MSDS Safety Data Sheets NSSC Natick Soldier Systems Center O&M Operations and Maintenance OL Overload ORP oxidation reduction potential PCE tetrachloroethene PLC programmable logic controller Plexus Plexus Scientific Corporation PTS pre-treatment system PVC polyvinyl chloride RAO Remedial Action Objective ROD Record of Decision SCADA Supervisory Control and Data Acquisition TCE trichloroethene TD total depth TSDF Treatment, Storage, and Disposal Facility USEPA United States Environmental Protection Agency WSW Water Supply Well

INTRODUCTION

Plexus Scientific Corporation (Plexus) has been awarded Delivery Order 0004 under contract number W91ZLK-13-D-0010, with the Department of the Army, U.S. Army Mission and Installation Contracting Command to provide environmental remediation services for groundwater remediation at U. S. Army Natick Soldier Systems Center (NSSC) in Natick, Massachusetts.

Based on the results of an optimization investigation of the current remedy (Plexus, 2017), an Optimization Plan (Plexus, 2018) was developed in collaboration with the Army, United States Environmental Protection Agency (USEPA), and Massachusetts Department of Environmental Protection (MassDEP); it recommends specific remedy upgrades and additions at the following four groundwater treatment Areas:

T-25 Area, which requires extraction well rehabilitation;

Buildings 22 and 36 Area, which requires extraction well rehabilitation;

Buildings 63, 2, and 45 Area, which requires extraction well rehabilitation, upgrade or replacement of the 1,4-Dioxane PTS pre-treatment system (PTS), re-routing of flow through a PTS, the installation of additional extraction wells, and adjustment of design pumping rates; and

U.S. Army Research Institute of Environmental Medicine (ARIEM) Building Area, which requires the installation of additional extraction wells and setting design pumping rates.

Additionally, the Groundwater Extraction and Treatment System (GWETS), which services each of the above treatment areas, requires upgrades to and replacements of various individual components.

1.1 Site Description

NSSC is located approximately 17 miles west-southwest of Boston in the Town of Natick, Massachusetts. NSSC is a research facility that occupies approximately 74 acres on the eastern shore of the South Pond of Lake Cochituate (Figure 1-1).

NSSC, formerly called the Quartermaster Research and Engineering Command and subsequently the U.S. Army Natick Research and Development Command, has been a permanent U.S. Army installation since October 1954. NSSC’s mission includes research and development activities in food engineering, food service, clothing, equipment, materials engineering, and aero-mechanical engineering.

Previous investigations at NSSC have focused on characterizing physical conditions and contaminant distribution at several locations or sites within NSSC. These sites consist of: The T- 25 Area, the Water Supply Well (WSW) Site, the Former Proposed Gymnasium Site (FPGS); the

Boiler Plant Area; the Buildings 22 and 36 Area; the Buildings 63, 2, and 45 Area; and the ARIEM Building Area (Figure 1-1). The primary Contaminants of Concern (COCs) are tetrachloroethene (PCE), trichloroethene (TCE), and 1,4-dioxane.

The Record of Decision (ROD) for the T-25 Area (Arthur D. Little [ADL], 2001) specified the following Remedial Action Objectives (RAOs):

Prevent contamination in the groundwater, above federal and state drinking water standards, from migrating outside of the T-25 Area toward off-facility receptors;

Prevent any potential exposure to groundwater beneath the T-25 area and off-facility with contaminant concentrations in excess of federal and state drinking water standards;

Restore aquifer to drinking water standards within a reasonable time frame; and, Monitor potential future migration of groundwater contamination to verify that elevated concentrations decrease over time.

An Explanation of Significant Differences (ESD; Environmental Chemical Corporation, LLC.

[ECC], 2013) was later developed to expand application of the T-25 Area RAOs to additional areas at NSSC, including the Buildings 22 and 36 Area; the Buildings 63, 2 and 45 Area; and the ARIEM Building Area.

NSSC is located in a Zone 2 aquifer and is located approximately 2,500 feet (ft) southeast of the Town of Natick’s Springvale Municipal Water Supply Well Field.

1.2 Geology

The geology at the facility consists predominantly of fill and reworked native soils underlain by glaciolacustrine, lacustrine, and glacial till deposits. Surficial soils are comprised of primarily well-graded sands. The surficial soils are underlain by poorly graded, very-fine to fine grained sands which are embedded with thin layers of silty sand and sandy silt. A layer of grey clayey silt and silt underlies the sands (MACTEC Engineering and Consulting, Inc., 2008).

Soil boring observations have noted a transition to poorly graded sands and gravels below the grey clayey silt in the southern portion of the Buildings 22 and 36 Area that continues south into the Building 63, 2, and 45 Area. Along the eastern shoreline, adjacent to Pegan Cove, a subsurface layer of peat has been observed in soil borings and excavations. Logs of borings advanced during the 2017 optimization investigation were consistent with the known descriptions of, and the heterogeneity of, the previously described site soil formations (Plexus, 2017).

Bedrock is not exposed at NSSC but does outcrop on the eastern shore of South Pond of Lake Cochituate near Pegan Brook. Historically, w ell borings from the three study areas have noted the bedrock is composed of granodiorite/hornblende gabbro, which is consistent with the regional bedrock geology.

1.3 Hydrogeology

NSSC is located within the Lake Cochituate drainage basin, which encompasses approximately

17.7 square miles. The main hydrogeologic feature at NSSC and the surrounding area is the South Pond of Lake Cochituate. The South Pond of Lake Cochituate occupies 0.9 square miles, flows northward to the Middle Pond, and through to North Pond of Lake Cochituate. The North Pond drains via the Cochituate Brook into the Sudbury River.

Groundwater in the unconsolidated soils at the Installation is divided into two aquifer intervals:

the unconfined water table aquifer, or “A” interval, and the locally semi-confined, deeper overburden aquifer, or “B” interval. The water table ranges in depth from 0 to 28 ft below ground surface (bgs).

Figure 1-2 illustrates the interpreted groundwater flow conditions for the deep overburden aquifer (“B” interval) across the NSSC while the GWETS is in operation. Deeper overburden groundwater appears to move from east to west beneath NSSC, discharging to the South Pond of Lake Cochituate with local variations beneath the eastern bay and the central and southern portion of the installation. The influence of groundwater pumping is apparent in Figure 1-2 as cone-of-depression distortions of the piezometric contours in the vicinity of extraction wells.

Based on the current data, all impacted groundwater is found entirely within the locally semi-confined, deeper overburden aquifer, or “B” interval, and no contamination is present in the unconfined water table aquifer, or “A” interval. The average hydraulic conductivity rates in the “B” interval estimated during the 2017 optimization investigation (Plexus, 2017) ranged from 0.45 to 1.87 ft/day across the installation, which is consistent with previous capture zone model input values and the known descriptions of, and heterogeneity of, the screened soil formations.

OPTIMIZATION WORK PLAN OBJECTIVES

As documented in the Optimization Investigation Tech Memo (Plexus, 2017) and Optimization Plan (Plexus, 2018) the GWETS is starting to provide diminishing returns. The reduction of the GWETS effectiveness is due to multiple factors that are related to the decrease of contaminant concentrations in groundwater, the reliability of the system, and the ability of the GWETS to sustain yields in extraction wells. The Optimization Plan (Plexus, 2018) identified those factors and areas of the current remedy that require an updated strategy to optimize the remedial approach and minimize the time to achieve site closure, along with recommendations towards meeting those optimization goals. This Optimization Work Plan presents the approach to implement these optimization steps that will meet the recommendations presented in the Optimization Plan (Plexus, 2018).

UPGRADE OF THE GWETS

Upgrades to individual components of the existing GWETS will be completed to allow for increased flow from the additional extraction wells, to modernize the system, and to replace obsolete control software and hardware. The Current GWETS Configuration Process Flow Diagram is shown in Figure 3-1. The upgrades to the GWETS are detailed in the sections below.

3.1 SCADA/PLC/Hardware

Plexus will work directly with its instrumentation and controls (I&C) subcontractor to modernize the controls of the GWETS, as well as to replace the obsolete control software and hardware. All new I&C will be seamlessly integrated into the system to allow for increased user functionality and easier operation.

The current Allen Bradley SLC™ Series programmable logic controller (PLC) will be upgraded to a modern Allen Bradley CompactLogix Series unit, or similar alternative. This work will include all PLC program migration to the new software platform and Human-Machine Interface (HMI) front-end database modifications to read the new tag-based PLC structure. The I&C subcontractor will complete the on-site, physical replacement and re-wiring of all main control panel PLC modules and inputs/outputs. The I&C subcontractor will also be responsible for all associated testing with the new PLC and associated programming. Once complete, the I&C subcontractor will update existing main control panel Computer-Aided Design (CAD) drawings to reflect new equipment modifications for incorporation in the updated Operation and Maintenance (O&M) Manual.

In addition to upgrading the main control panel PLC, the Supervisory Control and Data Acquisition (SCADA) hardware and software will be upgraded to modern industry standards. The current SCADA desktop computer will be replaced with one running the latest Windows Operating System. Existing Wonderware SCADA Software will be installed and upgraded to the latest version. The I&C subcontractor will migrate the existing HMI application to run on the installed Wonderware version as well as install and upgrade existing WIN-911 Alarm Monitoring Software to its latest version. All associated testing will be complete to ensure full desired operational functionality.

In addition to modernizing the system control components, the PLC and HMI Wonderware SCADA programming will be modified to integrate the six new extraction wells (outlined in Section 5.2) that will be brought online and treated by the GWETS. This will include all modifications and the database additions to add communications, control, graphics, and alarming for the six new extraction wells, new bag filter housings, and differential pressure-based control valving. Six new groundwater extraction well process control panels will be installed to duplicate existing configuration of the current control panels at the GWETS. Control panels will include stainless steel enclosure, flanged disconnect switch, control power transformer, circuit breaker fuses, motor starter with Overload (OL), enclosure heater, surge protectors, 24 volts Direct Current (VDC) power supplies, PLC, ethernet switch, fiber optic patch panel, terminal blocks, and wireway. The I&C subcontractor will include UL-508A labeling and update associated CAD drawings for incorporation in the updated O&M Manual. The I&C subcontractor will also conduct all associated on-site operational readiness testing and startup.

3.2 Bag Filter Equipment and Controls Upgrade

The existing two single-bag filter housings have become degraded over time and will be replaced with new stainless steel housings. The existing bag filter configuration of the GWETS is depicted in Figure 3-1. Two additional single-bag filter housings will be added to bring the total of single bag filters up to four, as depicted on Figure 3-2. The bag filter arrangement will consist of two sets of two bag filters with each set connected in parallel so that flow can be directed through one set (side) and be split evenly across its two bag filters while the other set (side) will receive no flow until the differential pressure reaches a desired set point. Once that desired differential pressure is achieved across the operating bag filters, the PLC will signal control valving, as shown on the New Bag Filter Process Flow Diagram in Figure 3-2 to re-direct flow through the other set (side) of parallel bag filters. An operator will then be required to change out the spent bag filters before flow can be re-directed back through the original set (side) again. This will allow for reduced down time. Differential pressure readings will be displayed on the HMI via the updated Wonderware SCADA programming.

3.3 Replace Flow Meters

Flow meters for all ten currently operating extraction wells will be replaced with appropriately sized magnetic flow meter/indicators and signal converter transmitters to communicate with the PLC and allow for the display of flow data on the HMI screen. Due to different vault construction throughout the extraction well network, varying pipe sizes may exist within the well vaults, so flow meters will be sized accordingly. Flow meter/indicators and transmitters will also be installed for the six new extraction wells to be installed as part of this system optimization, as well as a flow meter/indicator and transmitter on the effluent of the 1,4-Dioxane PTS, for a total of 17 new flow meters. These flow meters will allow for flow data to be displayed both locally and remotely, so wells can be monitored, and flow data recorded. The following wells will have new flow meters and transmitters installed: EW-3, EW-4, EW-6, EW-7, EW-8, EW-9, EW-10, EW-11, EW-12, MW-165B-2/MW-165BR-4, MW-182B-4, MW-96B, MW-95B, MW-39B, MW-94B, MW-90B, and a total effluent flow meter for the 1,4-Dioxane PTS. Plexus will install the flow meters, the electrical subcontractor will wire the flow meters and ensure proper connection, and the I&C subcontractor will integrate them (with appropriate testing) into the PLC modules and Inputs/Outputs, and update the HMI Wonderware SCADA Software.

3.4 Repair Pull Boxes at MW-96B and EW-2

The electrical pull boxes associated with wells EW-2 and MW-96B will be repaired as described below:

The pull box located between wells EW-2 and EW-3 is extensively damaged and will be replaced in its entirety with a new pull box and lid. The old box will be dug out by hand and the new box installed at grade. Dig-Safe notification will be made at least 72 hours prior to excavation. Power will be shutoff at the electrical panel to de-energize the line prior to replacement of the box.

The pull box associated with well MW-96B has a damaged cover, but otherwise appears to be in usable condition. Repairs to the electric line itself were completed in 2017 and the circuit has functioned properly until late Winter 2019 when additional repairs were made. A new lid will be installed and the accumulated sediment inside of the box will be removed. The lid will be sealed to the box to make it as water-tight as possible. Power will be shutoff at the plant electrical panel to de-energize the line prior to removal of the sediment from the box.

EXTRACTION WELL REHABILITATION

Over time, the yield on the groundwater extraction wells has generally declined. In some cases (for example, at wells EW-7 and EW-8), the drop-in yield is significant with little water production currently achievable. The drop in well yield is likely due to a combination of biological fouling and inorganic precipitation within and outside the well bore. Siltation may also be contributing to the observed loss in water production. Figure 4-1 provides a graphical representation of groundwater production over the period from January 2017 through November 2018.

Plexus proposes to rehabilitate the groundwater extraction wells to improve well yields. A phased program will be conducted to assess the results of alternative rehabilitation processes. Three of the 12 currently active wells will be initially targeted for rehabilitation. Varying chemical additions and in-well distribution processes will be tested within these three wells to determine appropriate methodologies prior to implementing the site-wide program. Wells MW-90B, EW-3 and EW-7 have been selected as the pilot rehabilitation test wells. Each of these wells has experienced significant yield degradation over the last three to four years of operation. During this rehabilitation pilot test, the extraction wells will be offline no longer than 48 hours, and no significant loss of capture is anticipated.

The remaining nine existing groundwater extraction wells will be rehabilitated using the most effective processes identified during the initial testing phase of the program. The phased rehabilitation program is described in the following sections of this work plan.

4.1 Specific Capacity and Discharge Pressure Testing

Prior to initiating any well rehabilitation efforts, the current specific capacity of each extraction well will be measured. The specific capacity is calculated as the sustained groundwater production rate divided by the drawdown occurring within the well at that production rate. The drawdown is calculated as the difference between pumping water levels and static levels within the well. The initial testing will provide a baseline for comparison to post-rehabilitation groundwater production rates.

The wellhead discharge pressure at varying well flows will also be measured prior to conducting any well modifications. This measurement will be used to assess the occurrence of scale formation within the subsurface piping between the extraction well and the treatment systems.

4.2 Extraction Equipment Removal/Inspection/Rehabilitation

The in-well equipment will be removed from each extraction well and inspected for wear, corrosion and damage. The impellors will be inspected to determine if repair or replacement of pump components is warranted. The pumps will also be pressure washed to remove visible external scale.

4.3 Surge/Bail/Air-lift

Following removal of the extraction pump, the total depth (TD) of the well will be measured and compared to well completion logs to assess the presence and thickness of accumulated sediment.

A portable compressor will also be used to air lift water and sediment from the well. TD will be measured occasionally to determine when sediment has been removed. Water and sediment removed will be contained at the wellhead for later decanting and disposal at the groundwater treatment system.

4.4 Chemical Addition and In-well Recirculation

Following manual removal of accumulated sediment, a small volume of chemical dispersants will be added to the well followed by biocide and acids. Collectively, the well rehabilitation amendments are designed to address biological growth and inorganic precipitates. The specific amendments proposed for this pilot program are listed in the following Table 4-1.

Copies of the Safety Data Sheets (SDSs) for these chemicals are provided in Appendix A. The specific chemicals proposed were selected based on their low toxicity and compatibility with well materials.

A portable pump will be inserted into the base of the well and allowed to run in a recirculation mode, as depicted on Figure 4-2. The in-well recirculation will promote distribution of the rehabilitation amendment chemicals over a 24 to 48-hour period, without continuous monitoring or extraction/disposal of groundwater, as all fluids are initially returned to the well. The well rehabilitation chemicals will remain within the immediate vicinity of the well bore due to the simultaneous extraction and re-infiltration of groundwater. During groundwater circulation, samples will be collected periodically for field measurement of pH and oxidation-reduction potential (ORP) to assess the continued chemical activity of the added rehabilitation amendments.

As necessary, additional rehabilitation chemicals may be added if field measurements indicate neutralization of the amendments has occurred.

During well recirculation, a concentrated volume of rehabilitation amendments will be metered into the subsurface discharge line and isolated from the recirculation cell. This approach will attempt to reduce in-pipe scale/biological growth, if present. A biodegradable and nontoxic dye (Cole-Parmer Fluorescent Yellow/Green Dye Tracer) will be added to the solution to determine when the fluid has displaced the stored volume within the pipe. The volume required to displace the stored volume will be recorded for each well. The amendment solution will be allowed to remain in the subsurface piping for a minimum of 48 hours before it is flushed from the system.

At the conclusion of the recirculation program, approximately 100 gallons of groundwater will be removed from each extraction well (where possible) to remove rehabilitation amendments added to the subsurface. Poor-yielding wells will be pumped dry for a minimum duration of 2 hours, if the 100-gallon target volume cannot be achieved during that time period.

The extracted water will be contained in a 275-gallon tote at the wellhead and will either be treated at the GWETS or transported off-site for disposal. Pre-disposal waste characterization samples will be collected and characterized to determine the appropriate means of waste disposal. Extracted water will not be processed through the GWETS without prior approval from MassDEP.

The well rehabilitation amendments will be removed from the aquifer by two mechanisms. First, as described above, approximately 100 gallons of groundwater will be removed immediately after completion of the recirculation program. The majority of the amendments will be removed by this process. Second, the extraction well will be put back into service within two days of the well rehabilitation program. Once the well is returned to service, the extracted groundwater will be recovered on a continuous basis and transported to the GWETS through existing underground piping. The hydraulic containment achieved by the continuous extraction of groundwater will prevent migration of the amendments away from the extraction wells. While the amendments chosen for the well rehabilitation process have low toxicity, removal of the chemicals from the subsurface will prevent unexpected negative impacts to the aquifer as a result of the well rehabilitation process.

4.5 Well Rebuild/Specific Capacity and Discharge Pressure Testing

Following well recirculation and chemical treatment, the in-well equipment will be re-installed.

The specific capacity and wellhead discharge pressure will again be measured for comparison to measurements taken prior to well rehabilitation. A comparison of pre- and post-rehabilitation and relative performance within the three test wells will be used to design the site-wide rehabilitation program. A brief technical memorandum will be prepared to document the results of the pilot program. This memorandum will be maintained as an internal document for future reference;

however, a copy will be provided to the State for informational purposes.

As part of the system’s future operations, well yield and pumping level (drawdown) will be monitored to determine when a well’s specific yield is again declining. In response to an observed decline (to the degree that hydraulic containment is potentially compromised), the well rehabilitation processes described above will be repeated. The frequency of this process will be well-specific with its occurrence documented in subsequent annual monitoring reports. The decision metrics and procedures for this corrective action will be added to the O&M Manual.

4.6 Contingent Well Rehabilitation using Pantonite

Should well yield improvements using the in-well circulation program described above prove to be insufficient, a contingent approach of mechanical redevelopment and acidification will be conducted. In this contingent program, the extraction well equipment will again be removed, and a well development rig will be used. The inorganic acid solution Pantonite (MSDS also provided in Appendix A) will be added to the well and surged mechanically by the development rig to push this solution out into the formation. A minimum of 100 gallons of water will again be removed from each well at the conclusion of the redevelopment program and containerized for characterization and likely off-site disposal.

EXPANSION OF THE EXTRACTION WELL NETWORK

The Optimization Plan (Plexus, 2018) concluded that the extraction well network needed to be expanded in order to provide increased efficiency and effectiveness of groundwater capture and treatment. Six additional extraction wells will be installed and plumbed in a manner to allow them to be incorporated into the existing 1,4-Dioxane PTS if required (i.e. if 1,4-dioxane is detected).

Flow from the new wells will also be able to bypass the 1,4-Dioxane PTS treatment and flow directly to the GWETS if treatment for 1,4-dioxane is not required. The details of the expansion of the extraction well network are described in the sections below. Figure 5-1 depicts how these new wells will be incorporated into the existing 1,4-Dioxane PTS and the GWETS. Figure 5-2 depicts the location of the additional extraction wells as well as the approximate location of the proposed subsurface horizontal directional drilling boring path or trenching in order to incorporate the new wells into the existing treatment system.

Two of the additional extraction wells (EW-9 and EW-10) are to be installed where 1,4-dioxane has historically been detected. The bench testing program described in Section 7.2 is designed to confirm the existing PTS can achieve acceptable 1,4-dioxane destruction at the higher hydraulic loading rate anticipated with the addition of flow from these two wells to the extraction network.

Based on historical groundwater monitoring and extraction data, the remaining four additional groundwater extraction wells (EW-11, EW-12, MW-165BR-4, and MW-182B-4) are outside the 1,4-dioxane plume footprint. Extraction of 1,4-dioxane by these wells, therefore, is not anticipated.

As described above, the piping system will be configured to allow water from any of the additional extraction wells to be routed through the PTS, if warranted based on sustained groundwater quality data indicating the presence of dioxane above acceptable levels. The existing PTS can likely accommodate some additional flow should one or more of the remaining extraction wells require pre-treatment. The hydraulic and treatment capacity of the PTS will be evaluated by the proposed bench testing program.

In the unlikely event that 1,4-dioxane is found in a number of the additional extraction wells, additional modifications to the PTS may be warranted. Specifically, the reactor sizes can be increased to maintain sufficient residence times, in line with reaction rates determined by the bench tests. A supplemental work plan will be prepared in the event this unanticipated scenario is realized during the system’s operation.

5.1 Horizontal Directional Drilling

A Plexus subcontractor will perform the horizontal directional drilling work. The planned approach is discussed below; however, this may require modification prior to or during field implementation based on any new information that becomes available regarding utilities, buildings, or subsurface geology.

The NSSC Directorate of Public Works Engineering Chief will be notified prior to commencement of any ground penetration activities. A minimum of 48 hours prior to any excavation, saw cutting of asphalt/concrete, or advancement of drill borings, a Massachusetts Dig Safe (811) call will be placed. In addition, a private utility mark-out contractor will determine the location of all suspected utilities in the proposed work areas using Ground Penetrating Radar (GPR) or suitable alternative.

Where needed, follow-up soft digging (air lance, hydro-excavation, hand digging, etc.) will be used to further identify and expose the utilities identified with GPR. All utility locations will be logged using Global Positioning System coordinates and measured depths below grade.

Excavations will then be backfilled to original conditions.

A track-mounted directional drill rig will be used to drill two horizontal borings in which the groundwater extraction well piping, I&C cable conduit (fiber optic), and electrical power conduit will be installed. The drill rig will be positioned approximately 15 to 25 linear feet (lf) from the end of the boring location(s). A topographic survey along the proposed alignment of the boring(s) will be performed prior to drilling. This information will then be used to generate a bore path profile to be used by the drilling foreman. The bore path profile acts as a plan that tells the foreman the angle that the drill should be set and provides checkpoints every 10 lf along the entire bore length.

A support truck with two 1,000-gallon tanks containing a mixture of bentonite clay and water will be located by the drill rig, and a 2-inch hose will be extended from the drill to the support truck.

Electronic locating equipment will be used to control both the line and grade of the boring and will be recalibrated on site to confirm the accuracy of the measurement. The lead drill rod and cutting assembly is 10 lf long and contains a transmitter, which emits a signal to a locator receiver box.

The signal transmitted at the cutting head informs both the operator and locator of the depth of the drill head, the angle of inclination and the direction in which the cutter head is moving.

A pilot hole will be installed following the development of the drill path to ensure flow. Upon completely drilling the first 10 ft rod into place, a second rod is then threaded to the first and coupled to the drill rotation motor. Accumulated cuttings and fluids are removed from the pit through a vacuum excavation method. The slurry is pumped through the head softening the earth while simultaneously removing the cuttings from the face back to the mud-holding pit. This process continues by advancing the drill rods through rotation and thrusting.

Two separate boreholes will be drilled; one for the groundwater extraction line, and one for the electrical power conduit and the instrumentation and control conduit (fiber optic) high-density polyethylene (HDPE) piping will be installed for the groundwater extraction line, which will be a minimum of 12 inches vertically below the electrical and control conduits, and a minimum of 48 inches below grade. The electrical power and the control conduits will be installed in the second borehole a minimum of 24 inches below grade, and with 12 inches horizontal separation from the groundwater extraction line. The electrical power and control wiring conduits’ boreholes will re-surface approximately every 300 lf to allow the wiring to be strung through pull boxes. Detectable warning tape will be placed within these conduits. Where trenching is used for groundwater extraction piping and conduit installation (i.e. EW-9 and EW-10), detectable warning tape will be installed above the lines in the trench.

A mud holding pit will be excavated approximately 4 ft square by 3 ft deep to collect cutting and excess drill slurry. The drill rods and cutter head assembly are drawn back into the borehole to the end of the pipe position. A small pit approximately 6 ft by 6 ft is excavated for the containment of excess drill fluids and to allow the operator room to change tooling. The sloped exit trench is excavated from the end of the pipe up to the ground surface.

The HDPE pipes are strung out and fused together as needed on the exit side of the crossing. A vacuum truck will remove the slurry from around the drill head while a laborer removes the black reamer assembly. The drill operator then pulls the balance of the drill rods from the excavation back to the insertion point, pulling the HDPE piping back to the insertion point using the drill rods.

Once the pipe is installed properly, the crew will then clean up their work area using a vacuum excavator and/or a backhoe. Hard surfaces such as asphalt and/or concrete impacted by the horizontal direction drilling activities will be repaired as necessary. Waste drill cutting mud will be either hauled offsite in the vacuum truck for disposal at a suitable Treatment, Storage, and Disposal Facility (TSDF) (if the volume of mud is significant) or transferred to 55-gallon steel drums onsite for later disposal (if the volume of mud is minimal).

Figure 5-2 presents the plan view of the proposed horizontal directional drilling bore path as well as the short length of conventional trenching required for new extraction wells EW-9 and EW-10.

5.2 Extraction Well Installation

Five additional extraction wells will be installed using a track-mounted sonic drill rig, a sixth extraction well, MW-182B-4, is already installed. The approximate locations of these new wells are depicted in Figure 5-2. Figure 5-1 depicts how these new wells will be incorporated into the existing 1,4-Dioxane PTS and the GWETS. An eight-inch cased hole will be drilled while collecting soil samples. The wells will be constructed with ten to 20 feet of four-inch diameter Schedule 40 polyvinyl chloride (PVC) slotted screen and riser, as specified by well in Section 5.3 and 5.4, below. The screen will be sand packed, sealed with bentonite and then cement/bentonite grouted to the surface with cement/bentonite grout. The wells will be secured at the surface with a temporary cap.

Drilling tools will be decontaminated by hot water pressure washing as needed at a temporary constructed decontamination pad. All drill cuttings, drill water, and decontamination wastewater will be contained in drums and staged on site for disposal.

After the well installation is complete, Plexus will complete the installation of well vaults at each wellhead along with all necessary trenching, piping, and electrical work required to connect the new wells to the existing influent network.

This work will include the following:

Installation of 6 inches of pea gravel, geotextile fabric, detectable tape, and backfilling with compaction in 6-inch lifts for all trenching

Installation of 6 inches of crushed stone base for asphalt and concrete repairs

Repair and replacement of disturbed concrete, asphalt, and grass areas

Installation of concrete well vaults 48-inch by 48-inch by 36-inch with H-20 load-rated doors

Installation of Corning 12-Strand Single-mode Outdoor Fast-Access Fiber Optic Cable.

Installation of submersible pumps with ½ horse power, 460-volt, 3 phase motors

Installation of 460-volt 3-phase feed to each pump

Installation of 1-inch HDPE piping for water line

Installation of 1-inch PVC conduit with appropriately sized conductors for each pump feed with associated termination and disconnecting means

Installation of 1-inch PVC conduit for instrumentation and control fiber optic cable.

Installation of variable speed drives at selected extraction wells.

Figure 5-3 presents the extraction well vault plan and the pipe trench detail. To the degree practical, the well vaults will be constructed in a similar manner and provide a similar configuration to the existing well vaults on site. This is subject to change based on field conditions and availability of components.

5.3 1,4-Dioxane Pre-Treatment System

Based on groundwater modeling results and contaminant distribution, the Optimization Plan (Plexus, 2018) recommended that additional extraction wells be installed to re-attain capture of the Target Capture Zones. The existing process flow configuration of the 1,4-Dioxane PTS is depicted in Figure 5-4. Figure 5-1 depicts how the new extraction wells will be incorporated into the existing 1,4-Dioxane PTS Building. It is anticipated that all piping from extraction wells in the Buildings 63, 2, and 45 Area will be routed through the 1,4-Dioxane PTS shed to allow for the flow from each individual well to be pre- treated if needed.

Both proposed new extraction wells EW-9 and EW-10 will be constructed of four-inch-diameter, schedule 40 PVC casing and screened within the “B” aquifer interval. Extraction well EW-9 will be installed to approximately 30 ft bgs, with a 15-foot screened interval; EW-10 will be installed to approximately 29 ft bgs, with a 20 ft screened interval.

5.4 ARIEM Building

The preferred remedy (ADL, 2001) to address impacted groundwater at the ARIEM building is continuous groundwater extraction.

This will be achieved by installing two new extraction wells, one near MW125B-2 (identified as EW-11) and one between MW-182B-4 and MW165B-2 (identified as EW-12), as well as converting existing wells MW-182B-4 and MW165B-2 to extraction wells. Groundwater extracted from these four wells will be piped through the 1,4-Dioxane PTS shed to the existing GWETS.

Extraction wells EW-11 and EW-12 will be constructed of four-inch-diameter, schedule 40 PVC casing, installed to approximately 40 ft bgs and 37 ft bgs, respectively, both in the “B” aquifer interval, with a 15-foot screen.

Converted extraction well MW165BR-4 will be constructed of four-inch-diameter, schedule 40 PVC cased, reinstalled to approximately 32 ft bgs, in the “B” aquifer, with a 10-foot screen.

Figure 5-1 presents the proposed locations of new and converted extraction wells, as well as the conceptual, proposed routing of piping from these extraction wells to the GWETS.

SLUG VOLUME REMOVAL

Regular slug volume removal activities will be continued until MW-182B-4, MW165B-2, and EW-11 become operational as GWETS extraction wells. Periodic extraction of contaminated groundwater from the newly installed extraction well MW-182B-4 and existing well MW165B-2, both in the ARIEM Building Area and outside the current capture zones for the GWETS, will occur on a quarterly basis. If groundwater results indicate a rebound of contamination at MW- 114B-2, then this well will be added to the periodic slug volume removal scope of work.

The periodic slug volumes will be removed by “pulse pumping” via vacuum extraction. The vacuum enhanced extraction will maximize groundwater recovery. By increasing the pressure gradient at the extraction well, groundwater flow rates, radius of influence, and the overall effectiveness of the remedy will be maximized.

Extracted groundwater will be transported to Building 94, where the water will be processed in the GWETS. Subsequent removals will target the same volumes and frequency that have been previously established. Target slug volumes of contaminated groundwater to be removed from MW-182B-4 and MW165B-2 combined will be approximately 960 gallons every quarterly event (480 gallons/well).

Historically, MW-181B-2 and MW-182B-4 have provided low yields, so actual extracted volumes may be closer to 500 gallons total over four hours at each well.

Performance monitoring of the remedy will consist of sampling groundwater at MW182B-4 and MW165B-2 prior to and one week after each slug removal event. Groundwater samples will be collected in accordance with USEPA Region I Low Stress (low-flow) Purging and Sampling Procedure and analyzed for PCE and TCE by Method 8260B.

1,4-DIOXANE PRE-TREATMENT SYSTEM UPGRADE

7.1 1,4-Dioxane Pre-Treatment System Equipment Description

The existing 1,4-Dioxane PTS has been shown to consistently achieve effluent 1,4-dioxane concentrations below the Massachusetts Contingency Plan (MCP) groundwater criteria (0.3 micrograms per liter [µg/L]) when operating parameters are within acceptable limits.

Operationally, the system has experienced down time and, infrequently, effluent discharges exceeding the MCP criteria. Exceedances of the criteria can be attributed to a lack of I&C for the system rather than failures of the chemical (Fenton’s reaction) process. As a key component of the proposed site activities, upgrades will be performed to improve operational reliability and improve system monitoring/control between site visits by the system operator.

The existing 1,4-Dioxane PTS consists of a pH adjustment tank, three chemical feed pumps, a reaction tank and a transfer pump. Groundwater from EW-2 (which was taken offline in 2011), EW-3, and EW-4 may be directed to enter the pH adjustment tank, where sulfuric acid (H2SO4) is added via chemical feed pump and mixed to reduce the pH of the groundwater. The pre-treated groundwater then enters the reaction tank via gravity feed where hydrogen peroxide (H2O2) and iron sulfate (FeSO4) are added via chemical feed pumps and mixed to treat the groundwater for 1,4-dioxane. The treated groundwater is then pumped and combined with the ARIEM building well lines where it enters the GWETS. The new extraction wells (EW-9 and EW-10), which are part of this system optimization will be piped into the 1,4-Dioxane PTS for 1,4-dioxane treatment.

As part of this system optimization, the 1,4-Dioxane PTS equipment and process will be unchanged, but the chemical dosing will be adjusted based upon the results of the bench scale testing conducted, as described in Section 7.2, below.

7.2 Bench Scale Treatability Testing

Treatability testing was conducted in support of the 1,4-Dioxane PTS design process prior to the system’s implementation. The treatability study tested groundwater containing up to 30 µg/L 1,4-dioxane, representative of conditions present prior to 2010, when the 1,4-Dioxane PTS became operational. Influent 1,4-dioxane concentrations have been less than 1 µg/L since May 2015.

Additional treatability testing is proposed to reassess the system’s performance under current, low influent 1,4-dioxane concentration conditions. Re-testing under current conditions is also planned to optimize reagent dosage rates and to confirm that the reactor tank size is appropriate for the higher flows anticipated for the system with the addition of flow from two more groundwater extraction wells (EW-9 and EW-10).

The design conditions to be tested by the treatability test are as follows:

System Flow: up to 4 gallons per minute (gpm) o The estimated maximum groundwater flow into the PTS is less than 4 gpm based on recent production rates achieved at existing wells EW-3 and EW-4. The yield from proposed wells EW-9 and EW-10 is unknown prior to their installation. Flow exceeding 4 gpm is not anticipated based on historical flow data from wells of similar construction.

Reactor Residence Time: 60 minutes o The existing 1,4-Dioxane PTS reactor has a volume of 350 gallons. At the anticipated influent flow rate (less than 4 gpm), the reactor residence time will be 87.5 minutes. A shorter residence time will be simulated in the treatability test to account for higher system flows (up to approximately 6 gpm) should the influent flow exceed current estimates.

Influent 1,4-Dioxane Concentration Ranges: 1 µg/L and 10 µg/L o As stated above, influent 1,4-dioxane levels have been below 1 µg/L since May 2015.

1,4-Dioxane contribution from the two proposed wells (EW-9 and EW-10) is unknown at this time. The treatability test will simulate influent 1,4-dioxane concentrations up to 10 µg/L. Based on groundwater quality in the area of the new extraction wells, sustained influent 1,4-dioxane below 10 µg/L is anticipated.

1,4-Dioxane Analytical Method Detection Limit: 0.1 µg/L o The 1,4-dioxane effluent target for the 1,4-Dioxane PTS is 0.3 µg/L. A laboratory reporting limit of 0.1 µg/L will be used during the treatability test.

pH: 3.5 (representative of conditions typically employed for the system)

Hydrogen Peroxide Dosages: 100 milligrams per liter (mg/L), 1,000 mg/L and 10,000 mg/L o Three peroxide dosages will be used in the treatability testing program. The dosages will represent between 10:1 and 10,000:1 ratio of peroxide to 1,4-dioxane, depending on the influent 1,4-dioxane level.

Supplemental Ferrous Iron Concentration: 2.5 mg/L and 20 mg/L

While iron supplementation has not typically been required due to naturally-occurring iron levels at the site, the treatability test will add iron to maintain a minimum influent rate of 2.5 mg/L. A second, higher iron dosage (20 mg/L) will be tested to assess the influence of iron concentration on system performance. Should natural iron levels exceed 2.5 mg/L, background iron concentration will be used as the low range test condition with respect to iron. No attempt will be made to reduce natural iron content below background levels.

As in the previous bench testing, the reaction will be quenched at the conclusion of the 1-hour test using Sodium Sulfite (Na2SO3) to neutralize the Fenton’s reaction through decomposition of the hydrogen peroxide to oxygen. Quenching of the reaction before sample collection in this manner is necessary to assess 1,4-dioxane concentrations at the end of the treatment system residence time.

Without this step, further 1,4-dioxane treatment will likely occur within the sample containers prior to laboratory analysis, biasing the results of the treatability test.

Influent and effluent 1,4-dioxane analyses will be conducted at an off-site laboratory. All samples will be analyzed for 1,4-dioxane using USEPA Method 8270-sim to achieve the required low detection limits. Two primary influent samples will be collected and analyzed during the test along with 12 primary samples effluent samples. Duplicate samples will be collected on 100 percent (%) of the primary influent samples (two samples) and 20% of the primary effluent samples (two samples). In total, 18 samples will be collected and analyzed.

Table 7-1 provides a summary of the testing matrix planned for the NSSC bench testing program.

As shown on Table 7-1, 12 test conditions will be simulated in the laboratory. The results of the treatability test will be provided in a brief technical memorandum prepared after receipt and validation of bench test results. Based on influent and effluent water quality achieved under varying test conditions, recommendations for amendment dosages will be provided and form the basis of future PTS operations.

7.3 1,4-Dioxane Pre-Treatment System Controls Upgrades

As previously described for the GWETS, Plexus will work directly with its I&C subcontractor to upgrade the controls of the 1,4-Dioxane PTS. The new I&C will be integrated into the existing system to allow for increased user functionality and easier operation. The current PLC will be upgraded with additional input/output (I/O) modules to improve system operations.