Fundamental Evaluation of Tracer Techniques for Characterizatiion of Non-Aqueous Phase Liquids (NAPLs) Retention Volumes in Heterogeneous Aquifers

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0107095IllangasekareDifficulties associated with the detection and quantification of non-aqueous phase liquids (NAPLs) in the subsurface have prompted the development of innovative, less-intrusive site characterization methods that utilize partitioning and interfacial tracers. Although these techniques have been utilized at many field sites in USA and Europe, current application is limited to NAPLs that are at or near residual saturation. It is hypothesized that the combined use of these tracers may also be useful for identifying occurrences that diverge from cases of residual saturation such as isolated pools and macro-entrapment zones that occur in naturally heterogeneous subsurface formations. The goal of this research is to evaluate the behavior of partitioning, interfacial, and conservative tracers in water-saturated subsurface systems where NAPLs are entrapped under conditions of natural heterogeneity. The following research issues will be addressed: The influence of subsurface heterogeneity on the distribution of NAPL will result in tracer behavior that is not necessarily controlled by local equilibrium, an assumption that is frequently used in analyzing tracer data. However, NAPL entrapment under natural heterogeneity may have unique, identifiable tracer signatures, even under non-equilibrium conditions. These unique signatures may be distinguishable for resolving and characterizing complex NAPL occurrences. The up-scaled field behavior of NAPLs differs considerably from what is observed in one-dimensional column experiments, particularly when media heterogeneity is introduced. The ability to resolve the behavior of tracers at larger spatial scales and added dimensionality may lead to refinements in theoretical constructs and improvements in computational methods for characterizing complex NAPL distribution in the field. The approach consists of both experimental studies conducted in columns, cells and intermediate-scale soil tanks and development of methods for tracer data analysis using improved numerical models. This research has both intrinsic scientific merit in terms of the measurement and evaluation of fundamental parameters as well as practical implication for NAPL detection, remediation, and risk assessment.