DIFFUSIVE LOSS OF NONAQUEOUS PHASE ORGANIC-SOLVENTS FROM IDEALIZED FRACTURE NETWORKS IN GEOLOGIC MEDIA

The time for disappearance of stationary, single-component immiscible- phase liquid (NAPL) from planar fractures due to dissolution and subse quent diffusion is directly dependent on the ratio of mass storage cap acity of dissolved and sorbed contaminant in the matrix to the initial storage capacity for immiscible-phase liquid in the fractures. This w as determined from an analytical solution for three-dimensional mass t ransfer 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 complete ly replenished,vith the immiscible phase before disappearance ceases. However, each successive fracture replenishment requires longer time f or disappearance due to consumption of part of the matrix storage capa city caused by previous fracture loadings. Ultimately,,vith continued NAPL replenishment in the fractures, this mass redistribution causes t he dissolved concentration in the matrix to equal the aqueous solubili ty, at which point continued disappearance cannot occur. Mass storage capacity ratios for matrix and fracture conditions typical of clays an d sedimentary rocks: are greater than one for a variety of the common chlorinated solvents. The initial disappearance times for a trichloroe thylene (TCE) DNAPL in a fractured clay of 10(-4) fracture porosity an d 35 percent matrix porosity with parallel, planar fractures, range fr om 0.01 to 113 days for fracture spacings of 1 cm and 1 m, respectivel y. Disappearance times for TCE DNAPL are much larger for a generic san dstone with the same fracture porosity and geometry and 10 percent mat rix porosity, and range from 0.44 to 4400 days, due to reduced storage capacity in the matrix. These disappearance times, which are short re lative to the ages of subsurface contamination at many industrial site s, 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.