Innovative NAPL Migration Barrier and Large Scale SSDS in Challenging Urban Setting


Innovative NAPL Migration Barrier and Large Scale SSDS in Challenging Urban Setting

Yelena Skorobogatov (Consolidated Edison Company of New York, Inc.)
Kathleen Slimon, Laurence Welch, Jr., P.J. Snyder and John Ripp (GEI Consultants, Inc.)



Yelena Skorobogatov                         Kathleen Slimon                 Laurence Welch, Jr.

Consolidated Edison Company
of New York, Inc.
                              GEI Consultants, Inc.             GEI Consultants, Inc.

Speaker Bios:

Yelena Skorobogatov        

Yelena Skorobogatov is a Technical Specialist with Consolidated Edison Company of  New York, Inc. specializing in management of former Manufactured Gas Plant (MGP) sites. Ms. Skorobogatov holds a Master of Science Degree in Mining Engineering – Hydrogeology from Moscow Institute of Exploration Geology, Russia.

Kathleen Slimon       

Ms. Slimon, a senior engineer/project manager with GEI, is a licensed professional engineer in the states of New York and Connecticut.  She has over 25 years’ experience in environmental engineering, working primarily in former manufactured gas plant (MGP) site investigations, feasibility studies, and remediation design and implementation management. Her experience in remedial design and implementation includes NAPL containment and recovery systems, surfactant enhanced in-situ chemical oxidation applications, and large scale complex excavation projects. In her role as a project manager Ms. Slimon has worked closely with clients in negotiations with state regulators and third party property owners to develop investigation and remediation programs acceptable to all parties.

Laurence Welch, Jr.

Mr. Welch joined GEI Consultants in 1999 and currently resides near Portland, Maine.  He is an Environmental Engineer with 18 years’ experience performing and managing remediation activities; developing remediation strategies; and evaluating, designing, and managing the construction and operation of multi-disciplinary remedial solutions.  Mr. Welch received a Master of Science degree in Environmental Engineering from Rensselaer Polytechnic Institute in 2003.

Mr. Welch has experience with numerous state regulatory programs and has participated in remediation work at former Manufactured Gas Plant sites, educational facilities, commercial and manufacturing facilities, and municipal properties. Specific remedial engineering experience includes soil vapor extraction systems, residential and commercial soil vapor intrusion mitigation systems (including soil vapor barriers and sub-slab depressurization systems), groundwater extraction and treatment systems, in-situ solidification, sediment dredging and capping, and large-scale soil excavation and remediation waste management.


The former manufactured gas plant (MGP) site is located in New York City (NYC) and was dismantled a long time ago, however, certain MGP structures, including the foundations of gas holders, tar wells and purifier houses, were left in place.  These remaining structures and associated impacts are potential sources of non-aqueous phase liquid (NAPL).

The approved remedy for the site specified installation of approximately 500 linear feet of a low-permeability, subsurface barrier wall extending down into bedrock, to prevent off-site movement of coal tar.  Another component of the approved remedy included installation of a sub-slab depressurization system (SSDS) beneath the currently occupied building to prevent potential soil vapor intrusion.  Designing and installing these remedial components presented a wide range of challenges.

The initially specified low-permeability type of subsurface barrier wall had to be re-evaluated due several constructability challenges, including site access restrictions and potential for groundwater mounding behind the wall.  After further evaluation of site conditions, a semi-permeable, NAPL migration barrier system consisting of large diameter (8- and 24-inch diameter) NAPL recovery wells and overlapping grout columns was selected. Recovery wells were chosen over conventional containment technologies due to site constraints, flexibility in construction schedule, and to prevent groundwater mounding.  The use of overlapping NAPL recovery wells for the containment of MGP NAPL is unique and may be the first of its kind implemented in the country.

A significant challenge for the SSDS design was to identify a viable pipe route above interior ceilings – areas already occupied with existing piping, ductwork, and asbestos containing material. With the site being a candidate Landmark site, all exterior system components had to be architecturally consistent with the rest of the building.  Construction could be conducted only during building users’ off-hours. The SSDS design and implementation required a multi-disciplinary technical and project management team. The result is a system consisting of 42 sub-slab vapor extraction points, 1,800 feet of interior collection piping, exterior piping to a new rooftop control and exhaust building, and two blowers with a design flow rate of 1,500 standard cubic feet per minute (SCFM).

Construction of the NAPL migration barrier was completed in the spring of 2017.  Construction of the SSDS began in 2015 and is expected to be completed in 2017.


To view the full Agenda for MGP 2017 view this link:

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