Sk. O'Hara et al., Trichloroethene DNAPL flow and mass distribution in naturally fractured clay: Evidence of aperture variability, WATER RES R, 36(1), 2000, pp. 135-147
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.