Lisa Reyenga is a Senior Engineer with GEI Consultants, Inc. She has ten years of experience in the field of characterization and remediation of NAPL contaminated sites. Lisa’s particular specialty is in the quantification of NAPL recoverability under complex hydrogeologic conditions. She earned in BS from Tufts University and MS from the Massachusetts Institute of Technology.
High Resolution Mobile NAPL Interval Identification and Transmissivity Calculations for DNAPL
Dense Non-Aqueous Phase Liquid (DNAPL) distribution can be complex, with multiple mobile NAPL intervals (MNIs) present. These multiple MNIs can result in perched DNAPL with exaggerated thicknesses of DNAPL in wells relative to actual MNI thicknesses in the formation. Published methods for measurement of DNAPL transmissivity yield artificially low values due to falsely large assumed drawdowns. The authors have developed procedures to identify each individual MNI, and to accurately calculate DNAPL transmissivity values for each MNI and for the well in the aggregate. This provides precise determinations of the elevation and thickness of each MNI as well as their associated DNAPL transmissivity values. This data augments the conceptual site model (CSM) to better quantify potential DNAPL migration risk and pathway identification, and design improved remedies. This data can also be used to justify cessation of DNAPL recovery to achieve site closure where no other risk or regulatory driver exists.
Natural Source Zone Depletion Estimation for Confined LNAPL
Natural Source Zone Depletion (NSZD) is an important process in the remediation of LNAPL. NSZD describes the reduction of LNAPL mass by naturally occurring processes that degrade hydrocarbon constituents. Guidance documents from API and ITRC summarize systematic processes to qualitatively assess and quantitatively measure NSZD through various evaluation processes. NSZD rates have been reported 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. Three methodologies (dissolved gases, surface carbon traps, and thermal profiling) were implemented to evaluate the best methodology(ies) to measure NSZD under confined LNAPL conditions.