Consolidated Edison Company of New York, Inc. NYSDEC Brownfield Cleanup Program
Posted on February 12, 2006
Client: Consolidated Edision Company of New York, Inc.
Location: West 42nd Street Former Manufactured Gas Plant Site
Project Type: NYSDEC Brownfield Cleanup Program Site No. C231024, Remedial Action and Remedial Construction Oversight
Project Period: June 2002 - February 2006
Project Cost: $710,000
Working under the New York State Department of Environmental Conservation (NYSDEC) Brownfield Cleanup Program, the firm was retained by Consolidated Edison Company of New York, Inc. (Con Edison) to oversee the remediation of a former manufactured gas plant (MGP) located on West 42nd Street, New York. While Con Edison did not own the site, it was responsible for the investigation and remediation of the former MGP operations that historically operated at the site. The current site owner was in the process of redeveloping the site as a high-rise apartment complex. The remediation activities were performed by the site owner in accordance with a Remedial Work Plan (RWP) developed by D&B Engineers and Architects, P.C. and approved by the NYSDEC. A primary goal of D&B’s oversight activities was to ensure that the site remediation was conducted in accordance with all elements of the RWP. In addition, due to the highly urbanized nature of the site, D&B worked closely with the site owner’s remediation contractor to ensure that all health and safety and air monitoring protocols were strictly followed.
The former MGP site is located on West 42nd Street, between 11th and 12th Avenues, in the borough of Manhattan, New York. The construction of the Metropolitan Gas Light Company’s West 42nd Street plant began in 1860. The plant operated as a coal gasification plant from 1863 into the early 1920s.
After generation, the gas was pumped to four gas holders located at the eastern end of the block for storage before being distributed to customers. Each of the gas holders was constructed of brick and had a capacity of 250,000 cubic feet.
The MGP was likely demolished in 1925. In 1932 the New York Central Railroad Company acquired the former MGP site which, in turn, constructed a railroad yard on the site. By the 1980s, all rail yard structures were demolished and the site was utilized as a parking lot.
Background/Remedial Work Plan Development
Working closely with Con Edison and the site owner, D&B developed a remedial strategy for the site that was integrated into the planned site redevelopment consisting of a high-rise apartment complex. The remediation plan was based on findings of an extensive site characterization and human health exposure assessment completed by D&B.
The site investigation program included the characterization of on-site soil and groundwater quality, the assessment of MGP tar and nonaqueous phase liquid (NAPL) and test pit excavations to determine the approximate locations of former MGP structures.
A human health exposure assessment was also conducted to determine how and when receptors might be exposed to contaminants of potential concern at the site based on existing and future site conditions. The elements of the exposure assessment included determining migration pathways and potential receptors, assessing exposure pathways for the various site media and identifying the need for corrective measures.
As part of the RWP, D&B performed an evaluation of several remedial options with regard to their effectiveness at mitigating potential exposure pathways and overall reduction of contamination. Based on this evaluation it was determined that the most effective remedial alternative was to combine site remediation with the future development of the site. This remedial alternative included excavation of impacted soil and former MGP structures, long-term environmental monitoring and placement of institutional controls.
Scope of Services
The scope of services for the remediation and construction of the former MGP site included the following:
Remedial Design Support
To ensure that the site owner’s contract documents included all elements of the approved RWP, D&B reviewed the following:
- The site owner’s Site Operation Plan;
- Contractor specifications for sheeting and bracing, excavation, storage, handling, transport and disposal of contaminated material, site dewatering and disposal of contaminated groundwater;
- Contractor Health and Safety Plan and Emergency Response: Air monitoring decision prompt diagrams;
- Traffic control plans;
- Facility disposal certifications; and
- Design drawings addressing excavation depths and backfill, logistics planning, and excavation and construction sequence.
Remediation and Construction Oversight
D&B performed full-time remediation oversight for a total of 9 months. Specific elements of the site remediation and construction included the following:
- The abandonment of groundwater monitoring wells to NYSDEC specifications for existing wells with the potential to serve as preferential contaminant flow pathways during and after the site remediation activities;
- The excavation, loading and off-site transport of approximately 8,500 tons and 80,000 tons of urban fill and MPG-impacted soil, respectively, to a depth of approximately 20 feet below grade;
- The demolition and removal of foundations associated with the four former gas holder walls and foundations, Purifying House, Office Building and other structures;
- The remediation of six discovered underground storage tanks (USTs) and subsequent closure of the associated NYSDEC spill cases;
- The collection of 32 post excavation samples to document the quality of soil left in place; and
- The emplacement of a geotextile membrane and approximately 14,000 tons of approved backfill.
Health and Safety Compliance
An important component of the site remediation involved odor, dust and air quality monitoring, and odor suppression. Odor suppression was especially critical, given the low odor thresholds associated with MGP-related compounds. These key project components included:
- Monitoring of odors and volatile organic compound (VOC) vapors at the excavation fronts, other work areas and at the perimeter at the site. These activities were conducted in accordance with the requirements of the site Health and Safety Plan and a Community Air Monitoring Program designed to address the site contamination; and
- The odor and VOC vapor suppression techniques included, beyond standard mitigation measures, the application of a suppression foaming agent and the use of an odor neutralizing misting system at the perimeter of the site.
Site Remediation Execution
The site remediation included the installation of steel sheeting at the site perimeter, removal of obstructions for sheeting, dewatering of site surface and groundwater, rendering and stabilization of excavated soil, and management of soil loading and truck inspection and decontamination. The implementation of the site remediation program included the following:
- Installation of steel sheeting at the property boundaries down to either a substantial clay layer or bedrock and the installation of toe-pins and tiebacks. The sheeting connections were treated with a waterproof sealant prior to installation, effectively creating a “cut-off” wall which limited the lateral movement of groundwater.
- During the installation of the steel sheeting, obstructions were encountered in the northeast corner of the site, preventing the advancement of the piles. The obstructions could not be excavated due to the potential of undermining West 42nd Street. Therefore, a borehole air rotary rig was used to remove the obstructions by drilling through the competent material and replacing it with sand.
- Dewatering of site groundwater was accomplished using extraction wells and portable pumps to lower the water table and to facilitate the removal of soil and former MGP structures. The pumped water was managed by settling, separation, filtering and liquid phase carbon, and the effluent was discharged to a combined sewer system compliant with a NYCDEP permit.
- Due to varying degrees of impacted soil determined by pre-excavation site characterization, zones of impacted soil were delineated in the field prior to excavation and tracked during loading for special handling and rendering at the selected disposal facility.
- Soil was excavated in a systematic approach to avoid commingling of waste streams (urban fill, MGP-impacted or C&D) and to avoid the generation of dust and/or odors, and that could affect the surrounding community. The excavation was conducted to the design depth of approximately 20 feet below grade using tracked excavators. The excavation front was continuously monitored and the soil was dynamically loaded for direct transport off the site.
- To mitigate odors during excavation of the low odor threshold MGP-related constituents, odor suppressant foam was applied during excavation. The odor suppressant foam application system was on standby during excavation activities and the foam was “power sprayed” onto excavated materials when elevated volatile organic vapors or sustained odors were detected at the excavation front.
- The moisture content of the soil was monitored during excavation and loading and, if the moisture content was too high per facility requirements, a stabilizing agent was added to the soil.
- A truck management system was implemented and involved establishing truck traffic patterns to and from the site, queuing of trucks in the vicinity of the site, and strictly controlling ingress and egress at the site. This system also included an inspection of all trucks entering the site and the review of required permits.
- To ensure that MGP materials were not “tracked off” of the site, the truck loading operations included the cleaning of all trucks after loading and prior to leaving the site using an automated power water spray system. After cleaning, each truck was inspected to ensure all soil was removed from the truck chassis and tires prior to leaving the site.
All remedial activities were documented by D&B through the maintenance of daily logs in a field book, preparing Daily Activity Reports and maintaining a site digital photograph log. Additionally, to ensure that the remedial activities were appropriately conducted and managed, weekly progress meetings were conducted and attended by representatives from Con Edison, the site owner, the NYSDEC and D&B.
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