Formatted Title

In Search of Correlations between Natural Source Zone Depletion Rates, Site Conditions, and Underlying Processes in NAPL-Impacted Systems

Background/Objectives

Natural source zone depletion (NSZD) has evolved as a viable solution for remediation of sites contaminated with non-aqueous phase liquid (NAPL) and refers to the restoration of NAPL source zones through combination of physical, chemical, and biological processes. Once released near ground surface, NAPL will be distributed in the subsurface as both mobile and residual/immobile fractions where NSZD can occur over time. This will result in progressively reduced contaminant mass, saturation, and mobility via various mechanisms including dissolution, volatilization, biodegradation of both dissolved and volatilized constituents, and direct biodegradation of the NAPL itself. Degradation of organic contaminants can generally proceed under anerobic condition through diverse and complex pathways including fermentation, reductive dechlorination, and redox processes. Methanogenesis is recognized as a predominant process during NSZD of organic contaminants that produces methane and carbon dioxide (CO₂) while methane subsequently oxidizes to CO₂ via an exothermic process in the vadose zone. The rate of NSZD is influenced by many factors, including the availability and type of electron acceptors present in the soils and groundwater to enable microbial enzymatic activity and/or inorganic processes. This can lead to differential degradation processes and rates, as well as variability in contaminant fate, transport, and mass fluxes within the subsurface system. Complex combinations of the interacting attenuating mechanisms during NSZD of NAPL source zones are difficult but important to quantify. The primary objective of this study is to elucidate the relative influence of various parameters and mass transfer processes on NSZD under different field conditions that should be considered for the design and implementation of remediation practices and is vital for mitigating water pollution and crucial in safeguarding ecosystems and public well-being. 

Approach/Activities

NSZD rates at 20 NAPL-contaminated sites located in Canada, Australia, and the United States are compared quantitatively. The NSZD assessments at these sites have been performed using a combination of different approaches including surficial CO₂ efflux via dynamic closed chamber, surface CO₂ traps, subsurface gradient/soil gas monitoring, and biogenic heat method. Various geological, hydrological, hydrogeological, and contaminant compositional aspects of each site including NAPL type and contaminants of concern, contaminant depth and thickness, plume length, groundwater chemistry, flow properties, and field parameters (i.e., pH, temperature, oxidation-reduction potential) are evaluated to identify correlations between NSZD rates and underlying processes/conditions affecting NSZD.

Results/Lessons Learned

Comparison of NSZD rates under various field conditions in terms of geological, hydrological, hydrogeological, and contaminant compositional parameters will provide insight into biogeochemical processes and how these can be enhanced in a complementary fashion to achieve complete remediation and site closure. The study will quantify the interplay between the physical, chemical, and biological attenuation processes for enhanced containment in short term. Furthermore, it will provide greater understanding of how to improve natural attenuation and NSZD processes as a standalone, long-term management strategy and/or practical endpoint where the remaining NAPL predominantly exists in a residual state that is hydraulically immobile and recovery from the subsurface is technically impracticable.