September 1997

SD-5 South Plume Response Alternatives
A fact sheet providing information on potential cleanup alternatives for one of the plumes emanating from the MMR.

The purpose of this fact sheet is to describe the alternatives proposed by remedial project managers for the Air Force Center for Environmental Excellence (AFCEE), the U.S. Environmental Protection Agency (EPA), and the Massachusetts Department of Environmental Protection (DEP) to clean up the Storm Drain 5 (SD-5) South groundwater plume emanating from the Massachusetts Military Reservation (MMR).

Project managers from AFCEE, EPA, and MassDEP will be evaluating alternatives to recommend the most appropriate cleanup solutions for the SD-5 South plume. To date, seven alternatives have been proposed, based on available data. Other suggestions for alternatives are welcome from the public.

This fact sheet describes the alternatives currently under consideration. It also describes alternatives that were considered but have not been carried forward through the decision-making process as a result of analysis by the Technical Review and Evaluation Team, the remedial project managers, and other technical staff.

Public Involvement in the Decision Process

Public involvement during the decision-making period is critical to selecting an alternative that not only cleans up the plume and meets regulatory requirements, but also is most acceptable to the affected communities. The project managers from AFCEE, EPA, and MassDEP invite you to participate in the following public involvement opportunities:

  • A public comment period will run from September 16 to November 21.
  • A posterboard session describing the alternatives will be held on October 8.

A posterboard information session describing the proposed alternatives will be held on Wednesday, October 8, from 3:30 p.m. to 9:00 p.m. The posterboard session will provide you with an opportunity to talk to project staff one-on-one. It will be held at the Barnstable County Fairgrounds Administration Building on Route 151 in Falmouth. Call the Community Involvement Office at (508) 968-4678 for more information or directions.

  • Another posterboard session and public meeting, describing the advantages and disadvantages of the alternatives, will be held on November 12.
  • Fact sheets, news releases, and other information will be distributed.
  • Citizen team meetings, which are open to the public and advertised in local newspapers, will continue as scheduled. These alternatives will be discussed at the citizen team meetings.
  • Neighborhood availability sessions will be held and may be requested by contacting the Community Involvement Office.

More information about the alternatives and the advantages and disadvantages associated with them will be presented in a decision criteria matrix document and fact sheets before the public meeting in November. The matrix document will include more information about human health risk, ecological impacts and risks, implementation issues such as access to property, relative costs, and other considerations. Detailed designs will be developed after the decision is made on which alternative to implement.

A response will be prepared to all written comments received during the public comment period. The responses will be documented in a responsiveness summary and distributed to citizen team members and all who commented, and will be placed on the website and in local libraries.

The written comments and responses will be considered by the AFCEE, EPA, and MassDEP project managers in recommending the most appropriate alternative. The alternative selected by AFCEE will be announced on December 15th, followed by a draft execution plan on February 2nd.

Additional documents can be found in the main libraries in Falmouth, Bourne, Sandwich, and Mashpee, and in the Coast Guard Library on base. Information can also be obtained by visiting the website at http://www.mmr.org, or by calling the Community Involvement Office at (508) 968-4678.

SD-5 SOUTH PLUME RESPONSE ALTERNATIVE A

Alternative A would use both extraction, treatment, and reinjection (ETR) and recirculating well technologies. This alternative has extraction wells northwest of Johns Pond along Hooppole Road and an extraction well fence in the interior of the plume along Highland Avenue. Treated groundwater would be reinjected along Hooppole Road using either the same well casing used for extraction, but at a different depth, or in a close-coupled reinjection well (the reinjection well would be approximately 10 feet from the extraction well). There would also be three recirculating wells near the Johns Pond shoreline.

The following figure illustrates the conceptual layout of wells for Alternative A, described below.

Components of the Alternative

Extraction Fence 1

  • Captures the portion of the plume northwest of Hooppole Road, thereby reducing the mass of contamination that flows into and under Johns Pond.

Extraction Fence 2

  • Reduces contaminant mass in the central portion of the plume.
  • Would need to be operated for approximately 10 years or less because the SD-5 North ETR system will prevent further plume migration from the base.

Reinjection Fence 1

  • Returns treated water to the aquifer, balancing the effects of drawdown (lowering of the water table) due to Extraction Fences 1 and 2.
  • To reduce dispersion of the contaminants and to reduce hydraulic impacts, treated water would be returned to the aquifer below the deepest part of the contaminant plume. This water would be reinjected through the same well casing used for extraction or through an additional well installed within approximately 10 feet of the extraction well.

Recirculating Wells

  • Reduce contaminant mass between Hooppole Road and the Johns Pond shoreline.
  • The wells would be located at the state boat ramp, a vacant lot, and community-owned property, and on any other available property.
  • These wells would be operated for no more than 1 year after the Hooppole Road Extraction Fence 1 is installed because clean groundwater would be reinjected upgradient at Hooppole Road. The travel time for that clean groundwater from Hooppole Road to the Johns Pond shoreline would be less than 1 year.

Considerations

  • Extraction Fence 2 allows for the removal of contaminants from within the plume.
  • Potential contaminant discharge to Johns Pond would be reduced by including the recirculating wells near the Johns Pond shoreline.
  • There are potential ecological impacts with the construction of the recirculating wells along the Johns Pond shoreline, requiring mitigation.
  • Drawdown (lowering the level of the water), mounding (raising the level of the water), and movement of treated water through nearby ponds are within the hydrological thresholds.
  • Natural attenuation would be part of this alternative. A portion of the plume east of Extraction Fence 1 at Hooppole Road would not be captured. However, the recirculating wells would reduce the contaminant mass. Continued monitoring would be required to track the migration and degradation of the uncaptured portion of the plume.
  • Property access for the systems may be difficult to obtain.

SD-5 SOUTH PLUME RESPONSE ALTERNATIVE B

Alternative B would use ETR technology. This alternative has extraction wells northwest of Johns Pond along Hooppole Road. Treated groundwater would be reinjected along Hooppole Road using either the same well casing used for extraction, but at a different depth, or in a close-coupled reinjection well (the reinjection well would be within approximately 10 feet of the extraction well).

The following figure illustrates the conceptual layout of wells for Alternative B, described below.

Components of the Alternative

Extraction Fence 1

  • Captures the portion of the plume to the northwest of Hooppole Road, thereby reducing the mass of contamination that flows into and under Johns Pond.

Reinjection Fence 1

  • Returns treated water to the aquifer, balancing the effects of drawdown due to Extraction Fence 1.
  • To reduce dispersion of the contaminants and to reduce hydraulic impacts, treated water would be returned to the aquifer below the deepest part of the contaminant plume. This water would be reinjected through the same well casing used for extraction or through an additional well installed within approximately 10 feet of the extraction well.

Considerations

  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.
  • Natural attenuation would be part of this alternative. A portion of the plume east of Extraction Fence 1 at Hooppole Road would not be captured. Continued monitoring would be required to track the migration and degradation of the uncaptured portion of the plume.
  • Property access for the system may be difficult to obtain.

SD-5 SOUTH PLUME RESPONSE ALTERNATIVE C

Alternative C would use recirculating well technology. This alternative uses a line of recirculating wells along Hooppole Road, on the northwest side of Johns Pond.

The figure on page 6 illustrates the conceptual layout of wells for Alternative C, described below.

Components of the Alternative

Recirculating Well Fence 1

  • Captures most of the plume northwest of Hooppole Road, thereby reducing the mass of contamination that flows into and under Johns Pond.

Considerations

  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.
  • Natural attenuation would be part of this alternative. A portion of the plume east of Recirculating Well Fence 1 at Hooppole Road would not be captured. Continued monitoring would be required to track the migration and degradation of the uncaptured portion of the plume.
  • Property access difficulties may be reduced because piping may not be necessary between all the wells. However, multiple small treatment facilities would be required.

SD-5 SOUTH PLUME RESPONSE ALTERNATIVE D

Alternative D would use ETR technology. This alternative uses an extraction well fence aligned with the direction of groundwater flow (referred to as "axial") in the body of the plume. Two additional extraction wells would be installed along Hooppole Road to provide additional plume capture. Treated groundwater would be reinjected northeast and southwest of the plume.

The following figure illustrates the conceptual layout of wells for Alternative D, described below.

Components of the Alternative

Axial Extraction Wells

  • Reduce contaminant mass in the central portion of the plume and help to hydraulically "steer" the plume

Hooppole Road Extraction Wells

  • Provide additional capture for the portion of the plume not captured by the axial extraction wells.
  • Reduce the mass of contamination that flows into and under Johns Pond.

Reinjection Wells

  • Return treated water to the aquifer, balancing the effects of drawdown due to the extraction wells.
  • Treated water would be returned to the aquifer at approximately the same depth as the extraction wells to reduce hydrologic impacts due to system operation.

Considerations

  • Axial extraction wells allow for the removal of contaminants from within the plume.
  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.
  • Natural attenuation would be part of this alternative because a portion of the plume would not be captured. Continued monitoring would be required to track the migration and degradation of the uncaptured portion of the plume.
  • Property access would be complicated by extensive piping necessary to connect the extraction and reinjection wells to a central treatment facility.
  • Potential contaminant discharge to Johns Pond would be reduced by including Hooppole Road extraction wells.

SD-5 SOUTH PLUME RESPONSE ALTERNATIVE E

Alternative E would use recirculating well technology. This alternative uses recirculating wells aligned with the direction of groundwater flow (referred to as "axial") in the body of the plume. Two additional recirculating wells would be installed along Hooppole Road to provide additional plume capture.

The figure on page 9 illustrates the conceptual layout of wells for Alternative E, described below.

Components of the Alternative

Axial Recirculating Wells

  • Reduce contaminant mass in the central portion of the plume.

Hooppole Road Recirculating Wells

  • Provide additional capture for the portion of the plume not captured by the axial recirculating wells.
  • Reduce the mass of contamination that flows into and under Johns Pond.

Considerations

  • Axial recirculating wells allow for the removal of contaminants from within the plume.
  • Potential contaminant discharge to Johns Pond would be reduced by including Hooppole Road recirculating wells.
  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.
  • Natural attenuation would be part of this alternative because a portion of the plume would not be captured. Continued monitoring would be required to track the migration and degradation of the uncaptured portion of the plume.
  • Property access may be simplified since piping to a central treatment facility would not be necessary for this alternative. However, multiple small treatment facilities would be required.

SD-5 SOUTH PLUME RESPONSE ALTERNATIVE Y

Alternative Y relies on the process of monitored natural attenuation. Natural subsurface processes that contribute to natural attenuation include dilution, volatilization, biodegradation, adsorption, and chemical reactions with subsurface materials. These processes may reduce contamination to acceptable levels over time. Natural attenuation is not a technology, but rather a strategy that uses natural processes to reduce contaminant concentrations. Monitoring and institutional controls are used to manage a known condition of contamination.

Components of the Alternative

Alternative Y consists of the following components:

  • Groundwater monitoring activities.
  • Modeling of particle flow paths toward potential receptors
  • Evaluation of contaminant degradation rates over time.
  • Implementation of institutional controls.

Monitored natural attenuation is not the same as "no action", although it often is perceived as such. The EPA Superfund program includes monitored natural attenuation, but only on a case-by-case basis. Under EPA groundwater standards, monitored natural attenuation may be used under the following conditions:

  • If compliance with standards, when compared to remediation time frames, are reasonable,
  • If adequate monitoring and institutional controls are established and maintained throughout the period of natural attenuation,
  • If institutional controls would result in conditions that were protective of human health and the environment, and
  • If the groundwater is not currently nor is projected to be a source for a public drinking water system.

Considerations

  • Natural attenuation can be combined with other alternatives to address low level plume contamination.
  • Natural attenuation has a low cost as compared to other alternatives under consideration.
  • There would be potential human health or ecological risks from migration of contaminants to areas where groundwater is being used. Institutional controls, such as restricting use of private wells, would limit that potential risk. No unacceptable risks would be allowed.
  • Long-term management and sampling is required to maintain community safety.
  • There is uncertainty associated with contaminant degradation.
  • There is uncertainty associated with institutional controls.

ALTERNATIVE Z: NO-ACTION ALTERNATIVE

This alternative means leaving the site in its present condition without any remediation system installed. The no-action alternative is developed for each Superfund site to assess impacts on human health and the environment if no measures are taken to clean up current site conditions. It also serves as a "benchmark" for comparison to other cleanup alternatives considered, so that comparisons can be made of the magnitude of environmental effects of the action alternatives.

Under the no action alternative, no further cleanup activities would be conducted to comply with the EPA or MassDEP groundwater standards at the site. AFCEE would not collect samples to continue characterization of groundwater and no institutional controls would be used.

Limited monitoring would take place to support a five-year review, which would be conducted to determine that there is no impact on public health or the environment. A five-year review is required by regulation.

The no action alternative would be selected only at sites that posed little or no risk to human health and the environment. For SD-5 South, the no action alternative would NOT provide adequate protection of human health and the environment. Therefore, it is under consideration only for purposes of comparison.

PLUME RESPONSE ALTERNATIVES NOT CARRIED FORWARD

Alternative F

Alternative F uses ETR technology. This alternative uses extraction wells along the northwest shoreline of Johns Pond and there would be an extraction well fence aligned with the direction of groundwater flow (referred to as "axial") in the body of the plume north and east of Ashumet Pond. Treated groundwater would be reinjected along Hooppole Road north and south of the plume.

Considerations

  • There are potential ecological impacts with the construction of the extraction wells along the Johns Pond shoreline, requiring mitigation.
  • Property access, especially along the Johns Pond shoreline, would be extremely difficult to obtain.
  • The axial extraction fence north and east of Ashumet Pond would capture less than 5 percent of the total plume mass.
  • Compared to an extraction fence along Hooppole Road, the extraction fence along the shoreline of Johns Pond captures less than 5 percent more of the total plume mass.
  • Alternatives F and G have the highest predicted mass capture of the alternatives that were evaluated.
  • Alternative F has the highest pumping rate of each of the alternatives considered.
  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.

Alternative G

Alternative G uses ETR technology. This alternative uses extraction wells along the northwest edge of Johns Pond. Treated groundwater would be reinjected along Hooppole Road north and south of the plume.

Considerations

  • There are potential ecological impacts with the construction of the extraction wells along the Johns Pond shoreline, requiring mitigation.
  • Property access, especially along the Johns Pond shoreline, would be extremely difficult to obtain.
  • Compared to an extraction fence along Hooppole Road, the extraction fence along the shoreline of Johns Pond captures less than 5 percent more of the total plume mass.
  • Alternatives F and G have the highest predicted mass capture of the alternatives evaluated.
  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.

Alternative H

Alternative H uses ETR and recirculating well technologies. This alternative uses an extraction well fence along the northeast edge of Ashumet Pond in addition to a line of recirculating wells along Hooppole Road on the northwest side of Johns Pond. For the ETR component, treated groundwater would be reinjected either from the same well casing used for extraction, but at a different depth, or in a close-coupled reinjection well.

Considerations

  • The ETR fence along the Ashumet Pond shoreline captures only the 1 percent of the total plume mass that is predicted to enter Ashumet Pond.
  • There are potential ecological impacts with the construction of the ETR fence along the Ashumet Pond shoreline, requiring mitigation.
  • Drawdown, mounding, and movement of treated water through nearby ponds are within the hydrological thresholds.
  • Property access, especially along the Ashumet Pond shoreline, would be extremely difficult to obtain.
  • This alternative was modified by deleting the ETR fence along Ashumet Pond. The modifications are presented in Alternative C.
  • Natural attenuation would be part of this alternative. Continued monitoring would be required to track the migration and degradation of the uncaptured portion of the plume.