Project Description

Following are the steps most often required by the New Hampshire Department of Environmental Services in the investigation and remediation of a site that has been impacted by wastes associated with former MGP operations:

  • Site Investigation
  • Development of Remedial Alternatives
  • Remedial Design
  • Construction of the Remedy

The Lower Liberty Hill project is now in the Construction of the Remedy phase. Below are a summary of the prior phases and a summary of the upcoming Construction phase.

Site Investigation

During the investigation, samples of groundwater, soil, sediment, surface water, soil gas and indoor air were collected. Groundwater and soil samples were taken both east and west of Liberty Hill Road. Drinking water wells were tested at the properties in the vicinity of the site. Following is a summary of the findings:

  • No MGP-related contaminants were detected in any drinking water wells.
  • Coal tar in soil and groundwater was found at some properties south of Liberty Hill Road (69, 77, 83 and 87 Liberty Hill Road). These four properties were purchased by the company from their owners. No significant current exposure pathways to coal tar were identified. No coal tar or associated contaminants were found east of Liberty Hill Road.
  • A small groundwater seep was identified at the site adjacent to Jewett Brook during site investigation activities. This area was evaluated as a potential exposure pathway. The evaluation indicated that the seep area does not represent a potentially significant exposure to human or ecological receptors. However, at NHDES's request, the seep area was filled with gravel in November 2008 to prevent potential contact with the groundwater in the seep.

NHDES requested further investigations to determine whether site contaminants could impact Jewett Brook, or the golf course on the other side of the brook. That investigation showed limited impacts to the brook and virtually no impacts on the golf course side of the brook.

Development of Remedial Alternatives

During 2006 and 2007 National Grid and its consultants developed a number of remedial alternatives for evaluation by the NHDES. Four alternatives were presented in the Remedial Action Plan (GEI, February 2007). Following discussions with NHDES a Remedial Action Plan Addendum was prepared (GEI, November 2007), which presented four alternatives:

  • RAA1 - Excavate all tar-impacted soil;
  • RAA4 - Excavate tar-impacted soil to 3 feet deep and contain tar-impacted soil below 3 feet deep with a slurry wall and impermeable cap;
  • RAA5 – Excavate all soil to 3 feet and tar-saturated soil below 3 feet, treat soil off-site by thermal desorption, contain remaining tar-impacted soil below 3 feet using a slurry wall and impermeable cap; and
  • RAA6 – In-situ thermal desorption of all tar-impacted and tar-saturated soil.

After significant discussion, further investigation, evaluation, and refinement of the RAA5 Remedy, NHDES issued a final decision selecting RAA1 as the most appropriate alternative. EnergyNorth had recommended RAA5a (a modification of RAA5) as the most appropriate Remedial Alternative and appealed NHDES' final decision to the State Waste Management Council in December 2011. Prior to a resolution of the appeal, EnergyNorth/Liberty Utilities agreed to implement RAA1 as set forth in its December 2012 Conceptual Remedial Design Report and the appeal was withdrawn.

Conceptual Remedial Design Report

The objectives of the remedial action are to remove coal tar source material that represents a potential ongoing source of contaminants to groundwater and to eliminate potential future exposure of human and environmental receptors to contaminants in soil and groundwater. The Conceptual Remedial Design Report addresses the excavation and off-site treatment of soil that meets the remedial action objectives. Residual contaminants will exist in groundwater downgradient from the excavation area following the implementation of RAA1, but will attenuate over time due to natural degradation processes. This report establishes the criteria for establishing the excavation limits, delineates the area to be excavated based on existing information, proposed a Pre-Design Investigation (PDI), and provides the conceptual design for the implementation of the work. The limits of the excavation have been established based on the results of field investigations conducted to date and the PDI conducted in 2013. Excavation limits will be modified if observed conditions during excavation are substantially different from conditions encountered during the PDI (see below). The excavation limits may be increased or decreased depending on observations during the work.

Sprayed on foam being used to control odors

The total estimated quantity of soil for off-site treatment is approximately 40,000 cubic yards. Although this quantity is less than the total estimated quantity of soil requiring excavation in the original estimate of RAA1, it does not represent a substantial decrease in the mass of coal tar that will be removed from the Site. The reduction in the volume of soil requiring excavation is due primarily to the lateral extent of excavation being smaller than the original estimate of RAA1 due to the elimination of small seams of tar-stained soil. Even though the tar-stained material does not represent a significant volume of contaminant mass, the volume of soil requiring removal at the perimeter of the excavation in order to capture the seams of stained material was substantial. By applying the refined criteria for excavation, additional soil between two areas of tar-impacted soil that did not require removal was identified and is instead available for reuse as backfill.

The excavation will be largely backfilled with thermally treated soil (contaminated soil taken from the Site, treated and returned to the Site certified as clean soil). The treated soil to be reused will be limited to soil excavated only from the Site. This approach will allow reuse of a large quantity of soil that otherwise would have required excavation and transport from another off-site source.

Drill rig like this will be used to
install excavation support along
Liberty Hill Road

Pre-Design Investigation

The PDI conducted in 2013 further defined the lateral extent of the areas of contaminated soil as well as the depth at which it is found. It also further defined the field geology, necessary to design the methodology and sequencing of the excavation project.

Remedial Design

Based on the Conceptual Remedial Design as modified by the findings of the Pre-Design Investigation, a Remedial Design for the Site has been developed and submitted to the NHDES in October 2013. In essence, the remedial plan divides the work into two phases (North and South) with each phase planned for one season (work will not be performed in winter months). Each phase consists of:

  • Excavation of Soil
  • Temporary Stockpiling of Re-Use Soil
  • Transport off-site of Treatment Soil
  • Backfill with Re-Use Soil and Treated Soil
  • Site Restoration
Truck on wash rack

The total amount of soil expected to be excavated is 93,000 cubic yards, of which 40,000 cubic yards are expected to be contaminated soil. Clean soils that are excavated (Re-Use Soil) will be stored on site, and contaminated soils (Treatment Soil) will be trucked to a thermal treatment facility. During the soil transport phase of the project, truck traffic is expected to be up to twelve to twenty trucks outgoing each day with a similar amount returning with clean soil. All of the trucks leaving the site will be tightly covered, and all soil on tires and the sides of the trucks will be removed prior to their leaving the Site. In all, it is estimated that approximately 2,000 truckloads of contaminated soil will be hauled from the Site and 2,000 truckloads of clean soil will be returned.

Example of an air monitoring system

To minimize impacts from the project to the community, a secure perimeter fence will placed around the entire Site; air monitoring stations will be placed at the fence line (with weekly reports on air quality placed on the website and any exceedances noted in the weekly reports on the Home Page); vibration and noise monitoring equipment will be in use with conservative action levels to stop work if the levels are exceeded; and truck drivers will be required to follow strict operating procedures to reduce noise and vibration with trucks transporting soil kept to specified routes and allowed travel times. A specialized groundwater treatment facility will be constructed on-site to process the water removed from the soil before it is trucked, which will restore the water to levels determined by the NHDES and the US Environmental Protection Agency (USEPA) to be safe before it is discharged to Jewett Brook.