Geoscience Reference
In-Depth Information
the CF can be significant at local scales and in the presence of aquifer heteroge-
neities. Moreover, even a relatively minimal contribution to flow in the CF can
translate into significant transport processes within the CF.
An alternative to continuum-scale models is offered by network models. Net-
work-scale models are particularly useful for investigating small-scale (or pore-
scale) flow dynamics in porous media, including processes such as drainage and
imbibition. Ronen et al. (
1997
) used a detailed network model by Blunt and Scher
(
1995
) to capture key features of the CF. The model accounted for capillary-
controlled water displacement processes in the presence of a gravitational field.
Structures generated by the numerical simulations illustrated the irregularity of the
upper ''surface'' of the CF and regions of trapped air within the water phase above
the water table. Moreover, analysis of horizontal cuts through the three-dimen-
sional CF structure demonstrated that ''islands'' of water and trapped air can
coexist in the CF; connections among islands of water permit horizontal flow in the
CF. In fact, the simulations indicated that island coverage of *50 % leads to a
connected phase, in qualitative agreement with field measurements.
Berkowitz et al. (
2004
) suggest that physical heterogeneity in the sediments
within the vicinity of the water table can lead to (1) increased flow and exchange
of chemical constituents between the CF and the underlying saturated zone; (2)
preferential transport of chemicals moving into the CF during infiltration events;
(3) enhanced horizontal chemical flux above the water table, providing an
opportunity for significant horizontal motion of contaminants without the possi-
bility for sampling, e.g., via groundwater piezometers; and (4) increased contact
between gas (both trapped and free flowing) and water phases in the region
bounding the water table. Such phenomena may drive a number of transport and
reaction processes, including (1) nutrient and oxygen availability to microbes in
both the CF and the upper portion of the groundwater zone, (2) geochemical
alteration of the CF and the shallow regions of the groundwater zone, and (3)
delivery of volatiles within the groundwater zone into contact with the air phase in
the vadose zone.
References
Abdul AS, Gillham RW (1984) Laboratory studies of the effects of the capillary fringe on
streamflow generation. Water Resour Res 20:691-698
Berkowitz B, Ewing RP (1998) Percolation theory and network modeling applications in soil
physics. Surv Geophys 19:23-72
Berkowitz B, Silliman SE, Dunn AM (2004) Impact of the capillary fringe on local flow,
chemical migration, and microbiology. Vadose Zone J 3:534-548
Blunt MJ, Scher H (1995) Pore-level modeling of wetting. Phys Rev E 52:6387-6403
Brooks RH, Corey AT (1966) Properties of porous media affecting fluid flow. J Irrig Drain Div
Am Soc Civil Eng 92:61-88
Flury M, Fluhler H, Jury W, Leuenberger J (1994) Susceptibility of soils to preferential flow of
water: a field study. Water Resour Res 30:1945-1954
