Page 46 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 29

Discussion:
1. The mass fraction of each COC can also be directly determined
from the ratio of the COC concentration and the TPH concentra-
tion. Using benzene as an example, mass fraction of benzene =
(20 mg/kg)/(1,000 mg/kg) = 0.020 = 2.0%.
2. The mass fractions of B/T/E/X are all the same (2.0%) because
they have the same concentration (20 mg/kg). On the other hand,
their mole fractions are different because of the differences in
their molecular weights.

2.2.5 Height of Capillary Fringe
The capillary fringe (or capillary zone) is a zone immediately above the water
table of unconfined aquifers. It extends upward from the top of the water
table due to the capillary rise of water. The capillary fringe often creates com-
plications in site-remediation projects. In general, the size of the plume in
the aquifer would be much larger than that in the vadose zone, because of
the spread of the dissolved plume in the aquifer. If the water table fluctu-
ates, the capillary fringe will move upward or downward with the water
table. Consequently, the capillary fringe above the dissolved groundwater
plume can become impacted. In addition, if free-floating product exists, the
fluctuation of the water table will cause the free product to move vertically
and laterally. The site remediation for this scenario will be more complicated
and difficult. In addition, most of the commonly used technologies cannot
effectively remediate the impacted capillary zones.
The height of capillary fringe at a site strongly depends on its subsurface
geology. For pure water at 20°C in a clean glass tube, the height of capillary
rise can be approximated by the following equation:

0.153
c h = (2.11)
r

where h is the height of capillary rise in cm, and r is the radius of the capil-
c
lary tube in cm. This formula can be used to estimate the height of the capil-
lary fringe. As shown in Equation (2.11), the thickness of the capillary fringe
will vary inversely with the pore size of the formation. Table 2.3 summarizes
the information from two references with regard to capillary fringe. As the
grain size becomes smaller, the pore radius often gets smaller, and the capil-
lary rise increases. The thickness of the capillary fringe of a clayey aquifer
can be as large as 10 ft.