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New Jersey Turnpike Authority, Parkway Maintenance (District 6) Remediation, USA

Shortlisted for Brownfield Awards Category 10 - Best International Project


The New Jersey Turnpike and Garden State Parkway are a system of controlled-access highways in the U.S. state of New Jersey and maintained by the New Jersey Turnpike Authority (NJTA). The 173-mile Parkway, from Cape May to the New York state line, was constructed between 1952 and 1956. It is a major thoroughfare, providing access to various localities in New Jersey and New York and is one of the most heavily travelled highways in the United States. Mott MacDonald has supported the NJTA with environmental services as part of their asset management since 1988 at police barracks, toll plazas, communication tower sites, and the majority of their 21 service areas and 20 maintenance yards.


Figure 1: Parkway Maintenance site during ground investigation


The Parkway Maintenance District 6 site is located in Clark Township. It is a fuelling, salt storage, and equipment maintenance facility with a footprint of approximately 100m by 240m. The underground storage tanks (USTs) were removed in 2002 and the petroleum discharge reported to the NJ Department of Environmental Protection (NJDEP). The environmental consultants at that time thought the soil contamination had been removed and remedial investigations focused on groundwater contamination. However, this was later found to be incorrect and free phase hydrocarbons were identified beneath the site. Mott MacDonald was retained to provide Licensed Site Remediation services to move the project to case closure. To do this, we had to address chlorinated volatile organic contaminants (CVOC) and petroleum hydrocarbons in groundwater and develop a conceptual site model to explain why groundwater contamination remained an issue.

Figure 2: Site location plan

The site is underlain by the Rahway Till, an unconsolidated glacial drift deposit including pebbles, cobbles and boulders. Groundwater is resting at approximately 5m to 7m below ground level and flows to the south east towards the Rahway River. To the north of the site are residential properties and a park (Clark Field), to the east and south is the Garden State Parkway and beyond this to the south is a park and the Rahway River at a distance of 100m. This park and river are hugely important to the community for recreation purposes, especially canoeing, kayaking and fishing. To the west are residential properties.


Mott MacDonald addressed the CVOC and petroleum contaminants separately. The CVOC concentrations were highest in a deep well that had been installed previously. We installed additional deep wells and confirmed the results. We conducted a Preliminary Assessment and identified upgradient sources of CVOC at former industrial sites. With these lines of evidence, we eliminated the NJTA as a source and closed out this area of concern with the NJDEP.

The residual petroleum contamination source was more elusive as all post-excavation soil samples results were below soil remediation standards. We developed a conceptual site model based on our familiarity with contaminant migration in the terminal moraine deposits of the Rahway Till. The additional deep wells confirmed that there was a strong component of vertical groundwater flow downwards throughout the facility. When applied to petroleum discharges, it was assumed that petroleum followed the same vertical route. That being the case, the post-excavation soil samples were not collected deep enough to evaluate residual source material.

Mott MacDonald evaluated numerous remedial alternatives for the NJTA and settled on two potential approaches. The first was to re-excavate the former tank area to a greater depth. This had limitations based on a new salt dome being constructed on top of the former UST area and other nearby structures restricting access. In addition, the horizontal and vertical extent had not been determined to adequately select the area of excavation. The alternative was to perform in-situ remediation. Several in-situ treatments were considered, and calcium peroxide (CaO3) was selected as most suitable to enhance biodegradation based on the hydrogeochemistry. The injection of CaO3 within the groundwater required the application for a Discharge to Ground Water Permit-by-Rule from the NJDEP. 

The next step in the process was to determine how to apply the CaO3 as the Rahway Till has low permeability that limits injection distribution. We selected to combine the investigation of the horizontal and vertical extent of the contamination with the application of CaO3. A grid was installed over the source area, and roto-sonic drilling was used to collect continuous soil cores so that the geology and contaminant migration would be adequately interpreted. Fifty soil borings were installed in May and June 2020, at the height of the pandemic which required additional health and safety consideration and the introduction of strict protocols for working together. Some soil borings were installed within the salt dome. The CaO3 powder was mixed as a slurry and placed within the soil column where contamination was observed during drilling. The remainder of the borehole was grouted.


Figure 3: Proposed remediation treatment exploratory hole location plan

The roto-sonic drilling led to a better understanding of the site geology. It was originally thought that the till deposits extended to approximately 9m (30 feet) where bedrock was encountered. However, in reinterpretation, it was determined that the terminal moraine till deposits extended to a depth of 7.5m to 9m (25-30 feet) below grade and contained cobbles and boulders. Below the till, englacial fluvial deposits of sand and rounded gravel were present to a depth of 12m (40 feet). The fluvial deposits were saturated, explaining how the off-site and upgradient CVOCs migrated to the site. It also explained how petroleum contaminants in groundwater migrated horizontally toward the nearby Rahway River. The refined conceptual site model of contaminant migration is one of petroleum contaminants migrating vertically downward until it reaches the fluvial deposits, whereby it migrates horizontally with groundwater flow toward the east.


Figure 4: Plume map for VOCs in groundwater (2018)


We conducted groundwater compliance monitoring prior to emplacing CaO3 to establish a baseline of contaminant concentrations and geochemical parameters. Since the injection, we have continued to monitor the groundwater quality on a quarterly basis. 

Our work has meant that the contaminant concentrations have been decreasing overall for two selected indictor contaminants, as illustrated in the graphs below. Benzene concentrations within the plume (MW2, MW2D and MW3) show a range from 1.4ug/l to 8.1ug/l in March 2020 which has since reduced to a range of 0.2u/g to 7.7ug/l a year later with downward trends in each well. For the downgradient wells (MW6S and MW6D) benzene concentrations decreased from 2.2ug/l and 8.1ug/l to 1.1ug/l and 7.7ug/l, respectively. For VOCs, similar trends were seen with reductions across all boreholes. Concentrations within the plume decreased from a range of 351ug/l to 16,680ug/l in March 2020 to a range of below method detection to 6,690ug/l. Similar decreasing trends were also measured for toluene, ethylbenzene and xylene, with only minor exceptions.  However, we will continue to monitor the site an evaluate the trends.


Figure 5: Benzene and VOC concentration plots over time

Note: The red dotted line indicates the date that the CaO3 injection was completed.

New Jersey Department of Environmental Protection Groundwater Quality Criteria (NJDEP GWQC) for Class II-A Aquifers is 1ug/l for benzene and 500ug/l for VOC(TICS).

It is anticipated that there will be a levelling off in decreasing trends as the CaO3 is consumed in the degradation process. We will then evaluate whether the contaminant concentrations have reduced significantly enough to enter a monitored natural attenuation phase, or whether additional remediation is required such as more CaO3, selective soil removal, or in-situ thermal treatment.


The investigation and remediation undertaken by Mott MacDonald at the Parkway Maintenance (District 6) site demonstrates our determination to provide a well-considered selection of best practice ground investigation and remediation techniques with the potential to surpass the original requirements for the identified contamination issues. Our innovative selection of a sonic drilling technique allowed significant refinement of the ground model as the boreholes were able to advance through boulders at depth, which were previously understood to be bedrock. The result was a significant improvement in the understanding of the geology, hydrogeology and contaminant flow paths and ultimately the conceptualisation of the site. 

We understood that the site’s physical constraints and anticipated depth of contamination dictated the need for an in-situ remediation solution, and our expert knowledge of geology and geochemistry made CaO3 the preferred amendment. We deployed the CaO3 using a bespoke solution as the Rahway Till has low permeability that limits injection distribution. In order to provide a robust and cost-effective solution we combined horizontal and vertical delineation data to identify the most efficient installation of the CaO3 amendment in the contaminated zones. 

The results to date indicate clear environmental benefits through the treatment of the identified source area of groundwater contamination and the nearby receptors including the Rahway River. And importantly, social benefits have been realised in reducing potential risks to the receptors in the river and surrounding park which are used for recreation purposes. We will continue with groundwater monitoring and review the data and ensure any further works are completed as required.  In addition to the external benefits and environmental gain, this project is now being used within Mott MacDonald as part of the Land Contamination and Remediation Technical Practice knowledge sharing. The project is the subject of upcoming talks and supports the existing peer review initiatives between the US and UK.