Topic Area: 2a. In Situ Bioremediation Applications

Formatted Title:

Polluted to Potable: Key ISB Lessons Learned from a Hard Earned (Groundwater) Par

Background/Objectives:

A historic tree farm turned residential property adjoining a scenic private country club in Far Hills, New Jersey, was impacted by releases from a 1,000-gallon heating oil underground storage tank (UST) and a 290-gallon gasoline UST. AKRF was hired to complete the Remedial Investigation (RI), develop a complete conceptual site model (CSM), and devise and implement a successful remedy to address residual vadose-zone impacted soils and groundwater quality. The areas of concern occurred in close proximity to Moggy Brook, a protected stream with storied significance in relation to glacial geology in the Watchung Mountain region. Dissolved concentrations of primarily methyl tert-butyl ether (MTBE – maximum concentrations of 1,700 µg/L), and benzene (maximum concentrations of 140 µg/L) occurred in an overburden plume of approximately 5,000 square feet which ranged in depths from approximately 5 feet below ground surface (fbgs) to approximately 40 fbgs (top of bedrock). Overburden groundwater flowed northwesterly from the former gasoline tank excavation area toward and under an existing garage detached from the residence and continued toward and beneath the former fuel oil tank excavation area creating a comingled and diving plume condition. Groundwater contamination (including soils in the saturated zone) required horizontal and vertical delineation which was completed in concert with an evaluation of the potential for an in situ bioremediation program to achieve remedial objectives.

Remedial Goal/Objective: The objective of the project summarized in this case study was to achieve unrestricted residential use and return groundwater quality to its condition prior to any release from the historic USTs, in this case the NJDEP’s Class IIA Groundwater Quality Standards (GWQS) which assume groundwater as a potable drinking water source. The remedy was to be completed as quickly and as efficiently as possible as a property transaction.

Approach/Activities:

Difficult drilling conditions within heterogeneous glacial till soils necessitated the use of Sonic™ drilling methods to install monitoring and injection wells to target depths. Remedial design investigation samples were analyzed for microbial populations and genes associated with BTEX degradation by the Quant Array®-Petro Method by Microbial Insights and total oxidant demand (TOD) by the TOD-Sodium Persulfate Method by Redox Tech, Inc. Remedial action alternatives for addressing on-Site soil contamination evaluated demolition of the garage building to potentially facilitate deep excavation at and beneath the groundwater table to remove residual bound source material. Groundwater remediation alternatives included natural remediation/monitored natural attenuation (MNA), in situ treatment (chemical oxidation and/or aerobic biodegradation), and bioremediation via biostimulation/bioaugmentation and recirculation/reinjection. Groundwater concentrations of benzene and MTBE had persisted and were identified in deep wells set above bedrock at the onset of groundwater sampling, confirming that an active remedy with groundwater control would be necessary to complete the objective.

Slug testing and hydraulic conductivity testing showed soil and groundwater hydraulic conditions were altered as part of prior historic fuel tank remedial excavation and backfilling events and provided an engineered subsurface environment that was quite suitable for an in situ bioremediation program to address residual groundwater contaminants. The observed increases in hydraulic gradient or the horizontal velocity of groundwater from the area of the former gasoline UST toward the area of the former fuel oil UST were leveraged to enhance the effectiveness of the remedy. Furthermore, naturally occurring geological conditions encountered further downgradient and cross-gradient along the plume created an intrinsic hydraulic control barrier that contained the treatment process to the affected area and ultimately sped up the contaminant breakdown process. These conditions supported controlled groundwater withdrawal, oxygenation, augmentation with microorganisms and nutrients as required, and then re-injection into targeted areas beneath the Site. Biostimulation/bioaugmentation limited disruption to the natural system at the site, which was ideal for ensuring success of the remedy. The recirculation system deployed was simply constructed to nurture and foster continuous growth of microorganisms that, in this instance, were already identified to be present within the subsurface to break down the contaminants of concern. The entirety of residual trapped and dissolved petroleum-related contaminant mass was effectively degraded, and groundwater was fully restored to the most stringent NJDEP remedial cleanup levels between 2019 and 2022. Post-system shutdown sampling conducted from all wells at the site verified MTBE was not detected and benzene was either not detected or below 1 part per billion (ppb).

Dustin Kapson

Affiliation

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Polluted to Potable: Key ISB Lessons Learned from a Hard Earned (Groundwater) Par