2016 NGWA Groundwater Summit

A Simple Spreadsheet Model to Simulate the Natural Attenuation of Residual Hydrocarbon NAPL

Monday, April 25, 2016: 2:20 p.m.
Confluence Ballroom A (The Westin Denver Downtown)
John Wilson, Ph.D , Scissortail Environmental Solutions, Ada, OK
Ian Hers, PH.D. , Golder Associates Ltd., Burnaby, BC, Canada
Parisa Jourabchi, PH.D. , Golder Associates Ltd., Vancouver, BC, Canada

At most sites, the rate of depletion of residual NAPL hydrocarbon is primarily controlled by the rate of aerobic biodegradation.  The actual rate of aerobic degradation is directly related to the diffusion gradient of oxygen in the unsaturated zone.  The lifecycle of the source zone will be controlled by the concentration of TPH, by the vertical distribution of the TPH, by the air-filled porosity that can supply oxygen by diffusion to the TPH, and by the fraction of the time that the TPH is above the water table and in contact with the soil gas.  The Vadose Zone Biodegradation Loss Model (VZBL) is a screening model that incorporates these site specific parameters and forecasts the lifecycle of TPH in the source zone.  The model forecasts the vertical distribution of TPH, the highest concentration of TPH at any depth interval in the vertical profile and the total amount of TPH remaining in the profile.  The VZBL model was applied to the former Hal’s Chevron Site in Green River, Utah.  A sensitivity analysis showed that the persistence of TPH was most sensitive to the air-filled porosity in the unsaturated zone, and to the distribution of the TPH compared to the distribution over time of the depth to the water table.  The presentation will offer practical techniques that can be used to estimate the air-filled porosity of the soil profile and attain a robust calibration of the VZBL model to a particular site. 

John Wilson, Ph.D, Scissortail Environmental Solutions, Ada, OK
John Wilson recently retired from the position Research Microbiologist for EPA’s Ground Water and Ecosystems Restoration Research (GWERD) Subsurface Remediation Branch. He holds a B.S. in Biology from Baylor University, an M.A. in Microbiology from the University of California at Berkeley, and a Ph.D. in Microbiology from Cornell University. Wilson led research dealing with biological and non-biological processes that destroy contaminants in groundwater and provided training and technical assistance to EPA’s regions and to state agencies on natural attenuation of hydrocarbon components in groundwater.

Ian Hers, PH.D., Golder Associates Ltd., Burnaby, BC, Canada
Dr. Ian Hers is the world-wide vapor intrusion practice leader for Golder Associates. He specializes in the assessment and modeling of vapor intrusion with particular emphasis on vadose zone biodegradation processes. He is working on or has completed applied research and guidance projects for API, Shell, ExxonMobil, USEPA, EPRI, Science Advisory Board for Contaminated Sites in BC, Contaminated Sites Approved Professional Society, NJDEP, Health Canada, UK Environmental Agency and several states and provinces in US and Canada. Ian’s involvement on projects spans the range of issues required on this project: he was the principal researcher for development of the Petroleum Vapor Intrusion database for USEPA and previously contributed to the USEPA “chlorinated solvent” database (and through this work is aware of issues for data analyses); he is highly experienced in data analysis and modeling that may be needed to support this project including evaluation of natural source zone depletion and dissolved plume and vadose zone attenuation processes; and through technical advisory work for Imperial Oil Ltd. and vapor practice leader he is familiar with typical contamination encountered at UST sites and soil vapor data quality issues. He is expert reviewer for the SERDP/DOD VI research program. Overall, he has the breadth of experience and knowledge to lead this project, but also the understanding of the effort and data compilation/analysis needed for this project to be a success.

Parisa Jourabchi, PH.D., Golder Associates Ltd., Vancouver, BC, Canada