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 in a Remedial Action Plan
  • Remedial Design
  • Design and Construction

Sonic drilling during investigation
field work

The Lower Liberty Hill project is now in the Remedial Action Plan phase, although National Grid has also begun developing the design for some of the remediation requirements.


During the investigation, samples of groundwater, soil, sediment, surface water, soil gas and indoor air were collected. Groundwater and soil samples were taken both north and south 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 contaminants were detected in any drinking water wells.
  • Coal tar in soil and groundwater was found adjacent to and potentially beneath some homes south of Liberty Hill Road (69, 77, 83 and 87 Liberty Hill Road).  These four properties were purchased by National Grid. No significant current exposure pathways to coal tar were identified. No coal tar or associated contaminants were found north 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, National Grid filled in the seep area with gravel in November 2008 to prevent potential contact with the groundwater in the seep.
  • NHDES requested that National Grid conduct 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 a possibility of very 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 KeySpan (now National Grid) and its consultants developed a number of remedial alternatives for evaluation by the NHDES. These include a “No Action” alternative, which was developed and analyzed as required by New Hampshire law, but which was immediately rejected from further consideration. 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 the following 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.

These four Remedial Alternatives were compared to each other and ranked based on nine criteria, including effectiveness, ease of implementation, reliability, timeliness, public impacts, short-term risks, long-term risks, long-term benefits, and cost effectiveness. The costs for these 4 alternatives ranged from approximately $5 million to approximately $16 million.

Proposed Remedial Action Alternative (RAA5)

In the Remedial Action Plan Addendum, National Grid and its consultants recommended RAA5 as the preferred alternative to the NHDES.  In February 2008, NHDES issued a review letter for the November 2007 Remedial Action Plan Addendum and indicated that RAA5 “appears to meet the criteria for approval established in the Contaminated Site Management Rules”. In a subsequent letter dated June 26, 2008, NHDES deferred their final decision on RAA5 until additional work was completed by National Grid. 

In July and August 2008, National Grid and their consultant met with representatives of NHDES and consultants to the Town of Gilford and a town resident to discuss additional work to be performed. National Grid conducted additional investigation in the Fall 2008 and Winter 2008/2009, including installing additional wells, conducting a pumping test and preparing a groundwater model for the Site. Based on the results of the additional work, a RAP Addendum No. 2 report was prepared (GEI, August 2009) that recommended a revised remedy for the Site (RAA5a).
The key elements of RAA 5a are:

  • Excavate Tar-impacted Soil to 3 Feet. Tar impacted soil is material with any degree of tar impacts (e.g. staining, blebs, saturation).
  • Excavate Tar-saturated Soil below 3 feet to a currently estimated depth of 45 feet. Tar-saturated soil is material where all the pore spaces between the grains are filled with tar, rather than water or air.
  • Contain Remaining Tar-impacted Soil below 3 Feet Using a Slurry Wall, with concrete portions in some areas.
  • Construction of an impermeable cap over the area surrounded by the Slurry Wall.
  • Perform limited groundwater pumping from within the contained area and treatment of the pumped water.
  • Treat Soil Off-site with Thermal Desorption.

    Soil sample showing dark stained
    area impacted by coal tar

This Remedial Action Plan would result in removing approximately 80 percent of the tar material from the site, and contain the remaining 20 percent within the slurry wall and under the impermeable cap which would prevent the infiltration of precipitation into the contained area. An Activity and Use Restriction (AUR) will prohibit residential use and other activities that may disturb the cap or slurry wall. The estimated cost of this Remedial Action Plan is $10.9 million.

This proposed remedy was presented by National Grid in a public meeting held in Gilford, NH in September 2009. NHDES is evaluating the Remedial Action Plan Addendum No. 2 prior to issuing a decision.