Single-Well “Push−Pull” Partitioning Tracer Test for NAPL Detection in the Subsurface

Abstract

Previous environmental applications of partitioning tracer tests to detect and quantify nonaqueous phase liquid (NAPL) contamination in the subsurface have been limited to well-to-well tests. However, theory and numerical modeling suggests that single-well injection-extraction ("push-pull") partitioning tracer tests can also potentially detect and quantify NAPL contamination. In this type of test, retardation factors for injected partitioning tracers are estimated from the increase in apparent dispersion observed in extraction-phase breakthrough curves in the presence of NAPL. A series of laboratory push-pull tests was conducted in physical aquifer models (PAMs) packed with natural aquifer sediment prepared with and without the presence of trichloroethene (TCE) NAPL. Field tests were conducted in an aquifer contaminated with petroleum hydrocarbon NAPL. Injected test solutions contained a suite of partitioning and conservative (nonpartitioning) alcohol tracers. Laboratory push-pull partitioning tracer tests were able to detect and quantify sorption of partitioning tracers to aquifer sediment (in the absence of NAPL) and to detect NAPL when it was present. NAPL saturations computed from estimated retardation factors bracketed those computed from known volumes of emplaced NAPL in the sediment pack. However, numerical modeling with assumed homogeneous NAPL distribution and linear equilibrium partitioning of tracers between aqueous and NAPL phases was unable to reproduce all features of observed breakthrough curves. Excavation of the sediment pack after all tests indicated that a portion of the emplaced NAPL had sunk to the bottom of the PAM invalidating the modeling assumption of homogeneous NAPL distribution. Moreover, the apparent dispersion in extraction-phase breakthrough curves decreased when the injection-extraction pumping rate was decreased, suggesting that mass transfer limitations existed during laboratory tests. Field push-pull partitioning tracer tests were able to detect NAPL in a portion of the aquifer known to contain NAPL; computed NAPL saturations were comparble to those obtained from sediment coring and the results of a partitioning interwell tracer test conducted in the same location. This study clearly demonstrates that the single-well partitioning tracer test can detect NAPL under both laboratory and field conditions. However, additional research is needed to verify the ability of the test to quantify NAPL saturations.