Influence of heterogeneity and sampling method on aqueous concentrations associated with NAPL dissolution

The purpose of this work is to examine the effects of nonuniform distributions of nonaqueous-phase liquid (NAPL) saturation, porous-media heterogeneity, and sampling method on the magnitude of aqueous concentrations measured under dynamic conditions of flow and transport. Dissolution experiments were conducted in an intermediate-scale flow cell packed with sand in which two zones of residual trichloroethene (TCE) saturation were placed. One was created in the same medium-grained sand as used for the flow cell matrix (zone 2), and the other was created in finer sand (zone 1), Aqueous samples were collected using depth-specific sampling ports, vertically integrated sampling ports, and at the fully screened extraction well. A dual-energy gamma-radiation system was used to measure TCE saturation before and after the experiment. The results indicate that mass removal occurred relatively uniformly across the upgradient edge of zone 2 and continued progressively along the longitudinal axis of the zone throughout the course of flushing. Conversely, mass removal was confined primarily to the perimeter of zone 1. The magnitude of the aqueous-phase TCE concentrations varied as a function of location and sampling method. The concentrations measured at the point-sampling ports downgradient of zone 2 were close to the value of aqueous solubility. Conversely, the concentrations measured at the point-sampling ports downgradient of zone 1 were about one-fourth of solubility. The TCE concentrations measured at the vertically integrated ports and at the extraction well were significantly less than the concentrations measured at the point-sampling ports. Given that substantial TCE saturation remained at the end of the experiment, the less than solubility concentrations observed for zone 1 and for the extraction well appear to reflect in part a steady-state dynamic equilibrium with the physical heterogeneity-induced nonuniform flow field. The less than solubility concentrations, especially for the integrated ports and the extraction well, were also influenced significantly by sampling-associated dilution related to the nonuniform NAPL distribution. These observations are supported by the results of a nonreactive tracer test, by the results of a dye-tracer test, and by a quantitative analysis of flow and tracer transport obtained using a three-dimensional mathematical model.