LSRP Corner: Capping of Volatile Contaminants for the Impact to Groundwater Pathway

Synergy Environmental, Inc.
Dennis Libenson, LSRP

September 18, 2018

The New Jersey Department of Environmental Protection (NJDEP) Soil Remediation Standards addresses the soil to groundwater pathway in unsaturated soils (i.e., the vadose zone) by providing Default Impact to Groundwater Soil Screening Levels, along with several options for establishing site-specific Impact to Groundwater Soil Remediation Standards. The NJDEP also issued guidance regarding compliance and/or remediation options to address the contamination.  One such option is the installation of a low-permeability cap to prevent migration to groundwater.

As recently as 2014, the NJDEP explicitly stated in its Frequently Asked Questions for the Impact to Ground Water Pathway in Soil Remediation Standards that “Capping as a remedial option is only allowed for inorganic and semivolatile contaminants under certain conditions.” However, the NJDEP recently issued guidance entitled “Capping of Volatile Contaminants for the Impact to Ground Water Pathway” which allows capping to address the Impact to Groundwater pathway even when contaminants include volatile organic compounds.

The new guidance provides flow charts for sites where groundwater contamination is already present and for sites where groundwater contamination is not present. Capping is considered as a remedial action; it is assumed in the following discussion that the remedial investigation was completed to allow for proper cap design.

Where groundwater contamination is present, you are now allowed to install the impermeable cap. This is followed by performing eight quarters of groundwater monitoring to establish that natural attenuation is taking place.  In addition, the vapor intrusion pathway must be addressed.  If results are favorable, you should obtain both a Remedial Action Permit for Soil and a Remedial Action Permit for Groundwater and must follow permit requirements.  The cap will need to be maintained until Soil Remediation Standards are met.

Where groundwater contamination is absent, the key factor in most cases will be soil vapor concentrations. Soil vapor screening concentrations are provided in the guidance.  Soil vapor samples will need to be collected in accordance with applicable guidance.  The capping guidance includes scenarios where soil vapor samples are collected in the most contaminated zone and scenarios where soil vapor samples are collected directly above the capillary zone.  The impermeable cap needs to be in place if the soil vapor samples will be collected directly above the capillary zone.  In any event, if results are favorable, you should obtain a Remedial Action Permit for Soil and must follow permit requirements.  The cap will need to be maintained until Soil Remediation Standards are met.

The NJDEP Webpage for Soil Remediation Standards, including guidance for Impact to Groundwater Soil Remediation Standards, is: https://www.nj.gov/dep/srp/guidance/rs/

Feel free to contact Synergy at experts@synergyenvinc.com.

Mr. Libenson is an LSRP in Synergy Environmental’ s Cherry Hill, NJ Office. He can be reached via e-mail at dlibenson@synergyenvinc.com

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