An experimental investigation of nonaqueous phase liquid dissolution in saturated subsurface systems: Steady state mass transfer rates

Abstract

Results of an experimental investigation into steady state dissolution of nonaqueous phase liquids (NAPLs) entrapped within water saturated porous media are presented. The influence of porous media characteristics, NAPL type, and aqueous phase velocity on NAPL dissolution rates is explored through evaluation of a series of laboratory column studies. For many of the conditions tested, measured organic solute concentrations in the column effluents were below solubility limits, indicating nonequilibrium conditions. Entrapped NAPL distributions are examined and shown to depend upon porous media grading and mean grain size. Experimental results reveal a dependence of dissolution rate on the distribution pattern of entrapped NAPL, as well as upon aqueous phase velocity. A phenomenological model for the mass transfer process is developed which incorporates grain size parameters as surrogate measures of NAPL distribution. An additional set of column experiments, employing another porous medium and NAPL type, confirm the usefulness of the model as a predictor of steady state mass transfer rates in similar systems.

Effect of groundwater flow dimensionality on mass transfer from entrapped nonaqueous phase liquid contaminants

“…The results indicate potential for significant errors using the one-dimensionally based models for NAPL dissolution in field applications. Recently, researchers have avoided the need to quantify entrapped NAPL geometry by using a lumped mass transfer coefficient K, defined as the product of the interfacial area between the NAPL and groundwater phases (ana), and an average mass transfer coefficient for the NAPL-water surface (kla) [Miller et al, 1990;Powers et al, 1992 Powers et al, , 1994. With the derivation of (1) the value of K, incorporating the unspecified specific surface area between phases, is determined from laboratory measurements.…”

1997 Summer Research Program (SRP), Graduate Student Research Program (GSRP), Final Reports, Volume 7A, Armstrong Laboratory

“…The distribution, size, and shape of these blobs is determined by the physical characteristics of the porous medium (e.g. grain size distribution and pore body-to-throat size ratio) and the fluid properties of the DNAPL, such as viscosity and interfacial tension (Powers et al, 1992). These blobs form immobilized residual zones in the subsurface, which slowly dissolve into the groundwater in saturated layers and evaporate into the pore spaces in unsaturated layers (Geller and Hunt, 1993).…”

“…Furthermore, laboratory experiments suggest that this assumption holds only in limited conditions, and that mass transfer is actually ratelimited. Powers et al (1992) hypothesize that the rate limitation may be a result of shrinking blobs, increased groundwater velocity during pumping, decreased saturation of DNAPL, and/or decreased mass fraction of the more soluble components of mixed DNAPLs.…”

“…Although interfacial area is unknown, this lumped coefficient can be determined experimentally. However, these values are often limited to the experimental system under study (Powers et al, 1992).…”

“…However, quantifying interfacial area is a major difficulty in applying these models (e.g., Geller and Hunt, 1993;Powers et al, 1994Powers et al, , 1992Powers et al, , 1991Miller et al, 1990;Hunt et al, 1990). There have been several different approaches to accounting for this parameter.…”