Natural Source Zone Depletion (NSZD) has emerged as an important process in the remediation of LNAPL, and it begins to occur the moment LNAPL is introduced to the environment. NSZD is the term used to describe the reduction of LNAPL mass due to the collective naturally occurring processes of dissolution, volatilization, and biodegradation that result in the loss of hydrocarbon constituents from the subsurface. Guidance documents have been developed by the American Petroleum Institute (API) and the Interstate Technology and Regulatory Council (ITRC) that summarize systematic processes to qualitatively assess and quantitatively measure NSZD through various evaluation processes to measure compositional changes within the LNAPL source and the dissolved- and vapor-phase pathways. NSZD rates have been reported to be in the range of 300 to 7,700 gallons/acre/year. However, these rates are reported from sites with unconfined LNAPL where there are no barriers to vertical transport of gases produced by biodegradation processes. Where LNAPL is present under confined conditions, saturated soils above the source zone may interfere with the vertical transport of gases and require modifications to the standard methods for analyzing NSZD of LNAPL. Three methodologies (dissolved gases, surface carbon traps, and thermal profiling) were implemented to evaluate the best methodology or combination of methodologies to assess and measure NSZD under confined LNAPL conditions.
Primary Author / Conference Presenter:
Fort Collins, Colorado, USA
Lisa A. Reyenga, P.E., GEI Consultants, Inc., Denver, CO
J. Michael Hawthorne, P.G., GEI Consultants, Inc., Denver, CO