Trichloroethene DNAPL flow and mass distribution in naturally fractured clay: Evidence of aperture variability

A cylindrical sample, 0.5 m in diameter and length, was obtained by excavat ion from 3.7-4.2 m depth below ground surface in a surficial deposit of fra ctured glaciolacustrine clay. The sample was enclosed in a triaxial cell in a cold room to impose conditions close to field temperature and stress. Hy draulic testing using a hydraulic gradient of 2 provided a saturated hydrau lic conductivity of 7 x 10(-10) mis, which is only slightly larger than the matrix hydraulic conductivity. The cubic law provided a mean hydraulic ape rture of 5-6 mu m for the four continuous vertical fractures in the sample. A column of immiscible-phase trichloroethene (TCE) imposed incrementally a t the top of the sample provided an entry-pressure-derived aperture equal t o 17 mu m for a parallel-plate fracture. TCE dense nonaqueous phase liquid (DNAPL) that flowed through the sample produced dissolved-phase diffusion h aloes in the matrix that indicated the preferential fracture pathways. Thes e haloes indicated that DNAPL flowed through only 5-15% of the visible, oxi dation-stained fractures; hence natural fractures had variable apertures al ong their length, and the larger aperture regions provided channels for flu id flow. Using the hydraulic test results and applying the cubic law only t o the aperture segments of confirmed DNAPL flow, an equivalent hydraulic ap erture of 8-11 mu m was obtained, which is greater than the conventional me an hydraulic aperture and smaller than the local aperture determined from t he DNAPL entry pressure. These differences are large in the context of flui d flux, which is proportional to the aperture cubed.