Detecting NAPLs Heterogeneously Distributed in the Subsurface

A particularly difficult task facing engineers and managers concerned with subsurface spills of nonaqueous phase liquids (NAPLs) is determining where the NAPL is and how much is there. Borrowing from past work in petroleum reservoir engineering, partitioning interwell tracer tests (PITT) were developed for characterizing the NAPL source zone and assessing the performance of remediation technologies. PITTs have been used to determine domain-average NAPL saturations as well as the spatial distribution of the NAPL. While these tracer tests work well when the NAPL is distributed uniformly throughout the domain, if NAPL is located nonuniformly, either as millimeter-scale ganglia or pools that are centimeter-scale and larger, the flow paths of the injected tracer solution may bypass NAPL-contaminated zones. In this case, the transfer of tracer mass from the main flow paths to the NAPL may be slow, resulting in extensive tailing of tracer breakthrough curves and underestimation of NAPL mass. In this work we examined the influence of nonuniform NAPL distribution and local-scale mass transfer resistance on the accuracy of measured NAPL saturations using PITTs. Two mathematical models were used along with laboratory column experiments to explore the influence of tracer partition coefficient, tracer detection limit, and injected tracer mass on NAPL measurement when the NAPL was distributed nonuniformly. When dimensionless mass transfer coefficients were small, NAPL measurement errors decreased with decreasing tracer partition coefficient, decreasing tracer detection limit, and increasing injected tracer mass. Extrapolating breakthrough curves exponentially reduced but did not eliminate systematic errors in NAPL measurement. Although transport in a single stream tube was used in the mathematical models and laboratory experiments, the results from this simplified domain were supported by data taken from a three-dimensional computational experiment, where the NAPL resided as large pool. Based on these results, we suggest guidelines for interpreting tracer breakthrough data to ascertain the importance of mass transfer limitations on NAPL measurements.