Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 84

3.5.4 Groundwater Movement

Groundwater is in constant motion, although groundwater flow rates are usually much
slower than those of surface streams. Unlike surface streams, groundwater is pulled down-
ward by gravity through an intricate network of passageways between the pore spaces in
the soil and sediment. This area is called the vadose zone (Heath 1983). The vadose zone
(also called the zone or aeration or unsaturated zone) is the subsurface area where water is
present, but air still occupies part of the overall pore space. Figure 3.19 shows the location
of the vadose zone relative to the water table and saturated zone.
A thin layer of water—called hygroscopic water —will always be attracted to the surface
of mineral grains. This attraction is a result of unsatisfied ionic charges on the surface of
the mineral grains combined with the polar nature of water molecules. This is termed the
force of molecular attraction and is shown in Figure 3.20.
The thickness of the vadose zone varies by location. In desert locations, the vadose zone
may be 100 m to more than 305 m (1000 ft) thick; in humid regions, where there is significant
precipitation, the thickness may be just a few meters or even less. There are also seasonal

Precipitation

Soil zone

Recharge to
water table

Water table

Capillary fringe

Saturated zone
below the water table

(Ground water)

FIGURE 3.19
Diagram of the vadose zone. (From United States Geological Survey, Summary of the water cycle, http://ga/
water.usgs.gov/edu/watercyclesummary.html, 2010a.)

Water free to move
through pore space

Sand grain

Water attracted to grains by
force of molecular attraction

Sand grain

FIGURE 3.20
Force of molecular attraction.