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Dansville manufactured gas plant project description

Project Description

Following are the steps most often required by the New York State Department of Environmental Conservation (NYSDEC or DEC) in the investigation and remediation of a former MGP site.  These steps are generally completed in the order listed.

  • Remedial Investigation/Feasibility Study
  • Interim Remedial Measures
  • Record of Decision
  • Design and Construction
  • Operation, Maintenance and Monitoring

The Dansville Site has been divided into two operable units in order to expedite remediation of MGP-impacted site soils.  A comprehensive plan to address all of the MGP-impacts for this site and surrounding area is complicated by the presence of unrelated chlorinated solvent impacts from an adjacent former dry cleaning facility.  The DEC is developing a plan to clean up the former dry cleaner impacts under a separate investigation and feasibility study.


Operable Unit 1 (OU1) includes two on-site soil excavation areas largely coincident with the location of former plant structures.  Operable unit 2 (OU2) consists of all the remaining on-site soil, groundwater for the entire site, and soil and groundwater in the areas of off-site migration.  Off-site migration of MGP impacts have been identified within the mixed commercial properties and a residential neighborhood located generally northwest of the site.


Dansville MGP OU1 is currently in the Design Phase.   Dansville OU2 is currently in the Remedial Investigation/Feasibility Study Phase.  Implementation of a remedy for OU1 and the feasibility study for OU2 are both on hold pending DEC’s development of a plan to address the adjacent dry cleaner site.


2006 OU1 Remedial Investigation Summary:

  • Contaminants detected in soil and groundwater are consistent with those expected for a former MGP site Chief among these are the BTEX compounds – Benzene, Toluene, Ethylbenzene and Xylenes, which are volatile organic compounds (VOCs); and PAH’s – polycyclic aromatic hydrocarbons, which are semivolatile organic compounds (SVOCs).  The primary byproduct responsible for the impacts is coal tar. In addition, a number of chlorinated VOCs believed to originate from an upgradient source (a former dry cleaning facility unrelated to the former MGP) were also found to be present in soil and groundwater.
  • Coal tar observed in the subsurface was generally found in the shallow water bearing zone found between approximately 12 to 16 feet below grade.  Some coal tar sheens and staining were noted at shallower depths in the vicinity of the subsurface MGP structures.
  • Shallow groundwater at the site was found to be impacted by VOCs and SVOCs in the saturated zone present in gravel and sand layer above the silty clay confining unit.
  • The greatest potential for exposure to MGP residuals is via direct contact with subsurface soils encountered during excavation work.  This potential exposure could be mitigated by using properly trained personnel and personal protective equipment.  Site data suggest that vapor intrusion into the service center building is a potentially complete pathway.  A vapor intrusion pathway investigation will be conducted.
  • There is no wildlife present at the site except for some transient species that visit the site occasionally. In short, the site does not provide habitat for wildlife resources.
  • The soil vapor intrusion evaluation concluded that a vapor intrusion pathway between the subsurface and the indoor air for the service center building is complete.   The primary compounds identified are chlorinated VOCs which are not typically related to MGP operations.  The source of these compounds appears to be the former dry cleaning facility located immediately south and east of OU1.  Lower concentrations of BTEX compounds are also present in the subsurface and indoor air.  While BTEX are associated with MGP operations, the relative concentrations of BTEX in indoor air and sub-slab vapor suggests that additional indoor air sources (such as gasoline) may account for the presence of BTEX in indoor air.

2006 OU2 Remedial Investigation Summary:

  • Contaminants detected in soil and groundwater are consistent with those expected for a former MGP site Chief among these are the BTEX compounds – Benzene, Toluene, Ethylbenzene and Xylenes, which are volatile organic compounds (VOCs); and PAH’s – polycyclic aromatic hydrocarbons, which are semivolatile organic compounds (SVOCs).  The primary byproduct responsible for the impacts is coal tar. In addition, a number of chlorinated VOCs believed to originate from an upgradient source (a former dry cleaning facility unrelated to the former MGP) were also found to be present in soil and groundwater.
  • Coal tar was observed in the subsurface soil to the west and northwest of the Dansville former MGP site, as well as at one location east of the site.  Coal tar was intermixed with water within the pore spaces generally 2 feet or less in thickness of a loose sandy gravel shallow aquifer, and at depths ranging from approximately 11 to 17 feet below grade surface.
  • Groundwater at OU2 has been impacted by VOCs and SVOCs related to MGP coal tar in the shallow saturated sandy gravel layer, above the silty clay confining unit.  The impacts are consistent with observed soil impacts and groundwater flow direction, which is generally to the northwest.
  • Soil gas samples collected in OU2 found levels of BTEX and chlorinated VOCs above the water table and at various shallower depth intervals.  Attenuation of VOCs was noted in some of the shallower samples but was not consistent.
  • Construction and utility workers involved in excavations could potentially be exposed to MGP impacted soil and groundwater, however, this is unlikely considering that the soil impacts are generally greater than 10 feet below grade and depth to groundwater ranges from 7 to 14 feet below grade.
  • The SRI concluded that there is a potential for exposure to MGP residuals via vapor intrusion into the residences within the groundwater plume of VOCs.  However, the presence of unrelated chlorinated VOCs collocated with MGP-related VOCs prompted the DEC to conduct a separate soil vapor intrusion investigation of OU2 as part of the investigation for the dry cleaner site.  Accordingly, NYSEG was not required to conduct a soil vapor intrusion investigation for OU2.
  • OU2 does contain fish and wildlife habitat and populations.  However, no pathways exist between the impacts found and fish and wildlife resources.  As a result, no further assessment of fish and wildlife resources is needed for OU2.

Interim Remedial Measures:

The site has undergone the following remediation through the implementation of interim remedial measures (IRMs), including:

  • A limited site excavation occurred in 1988 when approximately 1,500 yards of surface and shallow subsurface soil were removed from the site for a paving project.  Little information was recorded regarding the soil removal.  However, it is known that the excavation may have advanced into the contents and/or surrounding soil of the large subsurface gas holder foundation.
  • Soil Vapor Intrusion IRM (2006) – A sub-slab depressurization system was installed in the service center building to mitigate soil vapor intrusion of chlorinated VOCs into the building.  Results of post-installation soil vapor intrusion sampling indicate that concentrations of chlorinated VOCs in the indoor air and sub-slab samples were reduced by nearly 100% as compared to pre-installation sampling results.  The chlorinated VOCs are believed to originate from the former dry cleaning facility located adjacent to the former MGP.

In 2008, the NYSDEC issued a Record of Decision (ROD) for Dansville OU1 which describes the remedial goals and specific remedial requirements to achieve those goals.


The remediation goals for this OU1 of the site are to eliminate or reduce to the extent practicable:

  • exposures of persons at or around the site to VOCs, SVOCs, PAHs, and NAPL in soil;
  • the release of contaminants from soil into groundwater that may create exceedances of groundwater quality standards;
  • the release of contaminants from the soil and groundwater into indoor air, outside air, off-site soil and groundwater through soil vapors.

Further, the remediation goals for the site include attaining to the extent practicable:

  • soil cleanup objectives;
  • ambient water quality standards.

The remedial requirements for the Dansville OU1, as listed in the NYSDEC ROD are the following:

  1. A remedial design program will be implemented to provide the details necessary for the construction, operation, maintenance, and monitoring of the remedial program.  Any uncertainty identified during the RI/FS would be resolved, including a more precise delineation of the lateral and vertical extent of the proposed excavation.
  2. Installation of a construction barrier and hydraulic control system to ensure a stable excavation and provide groundwater management required to perform the excavation below the water table.  The self hardening slurry wall, or other viable construction barrier, may be left in place following the completion of the soil removal activities, or may be placed temporarily during the excavation work. The groundwater management system will be developed based on site-specific information and will be adequate to manage all dewatering handling, treatment, or disposal needs of the site.
  3. Demolition of the southern portion of the on-site building as necessary to enable the excavation of contaminated soils.  The northern portion of the current site building will remain in place.
  4. Excavation of MGP waste, NAPL and contaminated soils meeting one or more of the following criteria:  visible tar or oil; the presence of sheens or odors with total PAHs over 1,000 ppm; or total BTEX concentration above 10 ppm.   It is estimated that this will result in the excavation of contaminated soils to a depth of 16 feet below the ground surface; however soil excavation would proceed deeper if soils exceed one or more of the above criteria.  Treatment and/or disposal of excavated materials meeting the above criteria will occur at an off-site facility.
  5. Excavated materials which are below the criteria would be stockpiled and evaluated for reuse on-site.  The excavation will be backfilled with stockpiled soils and clean soil which is soil that meets the Division of Environmental Remediation=s criteria for backfill or local site background, and the ground surface would be prepared to meet future land use requirements.
  6. A soil cover will be constructed over all vegetated areas to prevent exposure to contaminated soils.  The minimum one-foot thick cover will consist of clean soil underlain by an indicator such as orange plastic snow fence to demarcate the cover soil from the subsurface soil.  The top six inches of soil will be of sufficient quality to support vegetation.  Clean soil will constitute soil that meets the Division of Environmental Remediation=s criteria for backfill or local site background.  Non-vegetated areas (buildings, roadways, parking lots, etc.) will be covered by a paving system or concrete at least 6 inches thick.
  7. Imposition of an institutional control in the form of an environmental easement that will require: (a) commercial use, which will also permit industrial use, (b) compliance with the approved site management plan, and; (c) restricting the use of groundwater as a source of potable or process water, without necessary water quality treatment as determined by NYSDOH.
  8. Development of a site management plan which will include the following institutional and engineering controls: (a) management of the final cover system to restrict excavation below the soil cover=s demarcation layer, pavement, or buildings.  Excavated soil will be tested, properly handled to protect the health and safety of workers and the nearby community, and will be properly managed in a manner acceptable to the Department; (b) continued evaluation of the potential for vapor intrusion for any buildings developed on the site, including provision for mitigation of any impacts identified; (c) identification of any use restrictions on the site; and a monitoring plan to monitor the effectiveness of the remedy.
  9. The property owner will provide a periodic certification of institutional and engineering controls, prepared and submitted by a professional engineer or such other expert acceptable to the Department, until the Department notifies the property owner in writing that this certification is no longer needed. This submittal will: (a) contain certification that the institutional controls and engineering controls put in place are still in place and are either unchanged from the previous certification or are compliant with Department-approved modifications; (b) allow the Department access to the site; and (c) state that nothing has occurred that would impair the ability of the control to protect public health or the environment, or constitute a violation or failure to comply with the site management plan unless otherwise approved by the Department.

 

OU1 Remedial Design

  • A pre-design investigation (PDI) sampling event was conducted in November and December of 2008.  The PDI report was reviewed and approved by DEC in July 2009.
  • A 50% remedial design was submitted to DEC on October 1, 2009.  Final design to be submitted for DEC review by April 30, 2010.
  • Construction activities for OU1 will be contingent upon a DEC plan to address the upgradient dry cleaner site.