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Title: ENTRAPMENT AND DISSOLUTION OF DNAPLS IN PHYSICALLY AND CHEMICALLY HETEROGENEOUS POROUS MEDIA

Author
item Bradford, Scott
item RATHFELDER, KLAUS - U OF MICHIGAN, DCEE
item LANG, JOHN - U OF MICHIGAN, DEPT CEE
item ABRIOLA, LINDA - U OF MICHIGAN, DEPT CEE

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2003
Publication Date: 12/1/2003
Citation: Bradford, S.A., Rathfelder, K.M., Lang, J., Abriola, L.M. 2003. Entrapment and dissolution of DNAPLs in heterogeneous porous media. Journal of Contaminant Hydrology. 67:133-157.

Interpretive Summary: The improper storage and disposal of organic liquids such as oil, gas, and solvents have resulted in the widespread contamination of the environment. As an organic liquid migrates downward through the soil, a residual portion is retained in the soil pores and on solid surfaces. The slow partitioning of this residual organic liquid to flowing water (dissolution) serves as a persistent source of groundwater contamination. Most research to date on organic liquid entrapment and dissolution has been conducted in uniform soil systems. Natural soil systems are much more complex due to variations in soil texture (size) and composition (chemical properties). This manuscript reports on research designed to explore the role of variations in soil texture and composition on organic liquid entrapment and dissolution. Results indicate that such variations lead to regions of high organic liquid retention. Dissolution of organic liquids in these regions occurs more slowly because water tends to flow around these areas.

Technical Abstract: Entrapment and dissolution of tetrachloroethylene (PCE) in physically and chemically heterogeneous porous media was investigated with a two- dimensional multiphase flow and transport simulator in conjunction with measured hydraulic properties, residual saturations, and dissolution parameters. Flow simulations demonstrate that the spatial distribution of PCE is highly dependent on the subsurface wettability characteristics. Simulation results show that a maximum PCE infiltration depth, for a given soil texture, was obtained with low organic-wet mass fractions (0.25) and minimum for strong wettability conditions (water- or organic-wet). This observation was attributed to differences in the entrapment mechanisms of PCE in soil with different wettability characteristics. In heterogeneous systems the PCE distribution was also controlled by the presence of subsurface capillary barriers. Numerical simulations demonstrate that capillary barrier performance can be enhanced or diminished by the presence of subsurface wettability variations. In homogeneous systems the behavior of PCE dissolution was found to be dependent on the soil wettability and the spatial PCE distribution. Shorter dissolution times occurred when PCE was distributed over large regions because of an increased access of PCE to the flowing water. In heterogeneous systems, the dissolution behavior was found to be highly dependent on the spatial PCE distribution. Spatial distributions of soil texture and wettability which produced capillary barriers and higher PCE saturations tended to exhibit longer dissolution times.