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Diffusive Loss of Non‐Aqueous Phase Organic Solvents from Idealized Fracture Networks in Geologic Media
Author(s) -
Parker Beth L.,
McWhorter David B.,
Cherry John A.
Publication year - 1997
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.1997.tb00180.x
Subject(s) - porosity , dissolution , fracture (geology) , matrix (chemical analysis) , sedimentary rock , aqueous solution , mass transfer , phase (matter) , solubility , mineralogy , chemistry , geology , materials science , composite material , geotechnical engineering , chromatography , organic chemistry , geochemistry
The time for disappearance of stationary, single‐component immiscible‐phase liquid (NAPL) from planar fractures due to dissolution and subsequent diffusion is directly dependent on the ratio of mass storage capacity of dissolved and sorbed contaminant in the matrix to the initial storage capacity for immiscible‐phase liquid in the fractures. This was determined from an analytical solution for three‐dimensional mass transfer into water‐saturated matrix blocks of clay or sedimentary rock represented as rectangular parallelepipeds. A ratio greater than one indicates the number of times the fracture void volume can be completely replenished with the immiscible phase before disappearance ceases. However, each successive fracture replenishment requires longer time for disappearance due to consumption of part of the matrix storage capacity caused by previous fracture loadings. Ultimately, with continued NAPL replenishment in the fractures, this mass redistribution causes the dissolved concentration in the matrix to equal the aqueous solubility, at which point continued disappearance cannot occur. Mass storage capacity ratios for matrix and fracture conditions typical of clays and sedimentary rocks are greater than one for a variety of the common chlorinated solvents. The initial disappearance times for a tri chloro ethylene (TCE) DNAPL in a fractured clay of 10 fracture porosity and 35 percent matrix porosity with parallel, planar fractures, range from 0.01 to 113 days for fracture spacings of 1 cm and 1 m, respectively. Disappearance times for TCE DNAPL are much larger for a generic sandstone with the same fracture porosity and geometry and 10 percent matrix porosity, and range from 0.44 to 4400 days, due to reduced storage capacity in the matrix. These disappearance times, which are short relative to the ages of subsurface contamination at many industrial sites, likely cause many DNAPL source zones, or parts of source zones, to have little or no actual DNAPL phase. However, the matrix retains the equivalent contaminant mass acting as a long term source zone.