Remediation of a Former MGP Site - Central New Jersey
Candace Baker, LSRP, Senior Project Manager, LANGAN
Co-Author: Brian Blum, CPG, LSRP, LANGAN
An approximately one acre Manufactured Gas Plant (MGP) in central New Jersey (the “site”) operated from 1872 through 1922. The above-ground MGP structures were demolished, MGP residuals beneath the ground were left in place, and the site was used mainly for storage until the property was sold to a church in 1964. The church, which occupied a corner lot across the street from the former MGP property, constructed an auditorium building and parking lot on the property in 1978. The property remained relatively unchanged from this state until remediation activities began. MGP residuals in soil at the site were remediated as per NJDEP regulations under the direction of a Licensed Site Remediation Professional (LSRP). The church has plans to redevelop the site with a senior housing complex and community center, requiring an unrestricted use remedial action as opposed to restricted use with a deed restriction and associated engineering controls. An excavation and off-site disposal remedy was chosen to accomplish this goal. Soil remediation was conducted from July 2017 through September 2018. The site is in a residential neighborhood bordered by residences to the north and west, the church across the street to the west; and two popular municipal parks to the east and south. Due to the proximity of these sensitive receptors, our client opted to complete the bulk of the remedial excavation beneath a temporary enclosure with an air treatment system. Due to the complexity of the project and the sensitive nature of the surrounding setting, a public relations firm was engaged to interface with the community before, during, and after the remediation. Multiple state and local permits were required to complete the remediation. In addition to residual MGP product, soil contamination consisted of BTEX compounds, PAHs, metals, and cyanide. The church’s auditorium building was demolished prior to excavation. Sheet piling was used to stabilize each excavation area and minimize ground water infiltration. The temporary structure was constructed on the north end of the site and moved three times to the south as excavation was completed in each area. Negative air pressure was maintained inside the structure and carbon units were used to capture vapors. Post-excavation samples were collected from the bottom and sidewalls of each excavation area to demonstrate compliance with the NJDEP Soil Remediation Standards. Construction dewatering was conducted as necessary and extracted ground water was treated in an on-site treatment system and then discharged to the local combined sewer system. Remnant subterranean MGP structures including gas holders, tar wells, piping, and building foundations were removed as they were encountered in the excavation. In addition, four underground storage tanks were encountered and closed during the excavation. An unforeseen pocket of tarry material was discovered in the central portion of the site at the bottom of one of the excavation areas, prompting the need to dig deeper than the designed extent of the excavation supports. Excavation drilling using a large diameter (53-inch) solid stem auger with overlapping boreholes was implemented to remove the remaining impacted MPG residuals in soil instead of constructing a more expensive coffer dam. The boreholes could not be traditionally backfilled due to the depth, so flowable fill was grouted into the hole from the bottom by tremie pipe. The site was backfilled with quarry fines and finished at grade with approximately six inches of crushed stone. The site is construction ready from an environmental perspective, and has been returned to the church for redevelopment.
Candace Baker has worked in the environmental industry for over 20 years. She has spent the last 6 years as a Sr. Project Manager for LANGAN and is a New Jersey Licensed Site Remediation Professional. She holds a bachelor's degree in environmental science from Simon's Rock College and a master's degree in civil and environmental engineering from Carnegie Mellon University.