Geoscience Reference
In-Depth Information
In contrast, flow in the groundwater zone generally is characterized by pores that
essentially are saturated with water. The water within the pores typically is at
pressures greater than atmospheric. Flow no longer is predominantly vertical but
reacts to local or regional recharge and discharge fluxes, including natural fluxes
such as recharge from precipitation or discharge to surface water.
The CF constitutes a relatively narrow zone that is the critical connection
between the vadose zone and aquifers. Sediments immediately below the water
table, for example, are characterized by pore-water pressures greater than atmo-
spheric pressure and nearly full saturation by the water phase in most portions of
the medium. Also present are local zones of entrapped air in the form of bubbles or
small inclusions of sediments at low moisture content. The region immediately
above the water table contains water that essentially saturates the pores, although
the pore-water pressures are less than atmospheric. The CF, combined with the
region immediately below the water table, may affect, far more significantly than
usually is assumed, the natural geochemical and microbial conditions present in
the region of transition from the vadose zone to the saturated groundwater zone.
The literature demonstrates the potential impact of flow within the CF on
hydraulics in the subsurface. Most studies of the CF have focused on responses to
pumping from groundwater wells, hydrologic response of streams, behavior of
seepage faces, and exfiltration to excavations (e.g., Abdul and Gillham
1984
;
Gillham
1984
; Mixon
1984
; Nwankwor et al.
1992
; Zhang et al.
1999
). The CF
also has been shown to affect groundwater response to water table dynamics,
particularly in response to infiltration events (e.g., Rosenberry and Winter
1997
;
Nielsen and Perrochet
2000
). However, to date, little information is available on
the impact, at the local scale, of the CF on exchange of water across the water table
or the influence of heterogeneity of grain size on behavior at the microscale within
the CF.
The dynamics of water flow therefore are a combination of those governing
flow in the partially saturated zone (essentially vertical, downward flow) and flow
in saturated zones (aquifers), which can be fully three-dimensional. In general, the
modeling approaches mentioned in
Sect. 9.1
are applicable here—continuum
models and pore-scale network models—although detailed quantification of flow
and transport in the CF has received only limited attention.
Silliman et al. (
2002
) use a fully saturated-unsaturated model of flow within the
combined vadose zone, CF, and saturated groundwater zone to reproduce patterns
of tracer transport in a laboratory flow cell. Extrapolation to the field scale indi-
cates that CF flow, while of local importance, contributes insignificantly to
regional volumetric flow. This was demonstrated quantitatively by Silliman et al.
(
2002
), who simulated horizontal flux through a coupled saturated and partially
saturated medium, for a series of (homogeneous) sediment types ranging from
sand to clay. The total horizontal flux above the water table was estimated and
compared to the flux observed below the water table for aquifer thicknesses of
1-100 m. The calculated percentages of total horizontal flux in the CF, relative to
the region below the water, were as high as *6 % for a 1-m-thick loam aquifer
and as low as *0.04 % for a 100-m-thick sand aquifer. However, lateral flow in
