Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 109

volume. For example, the equipotential lines of larger
magnitude often indicate recharge areas. Equipotential
lines of decreasing magnitude indicate the direction of
groundwater flow toward springs, streams, ponds, wells, or
the ocean.

4.7.1 Infiltration

Groundwater is derived from precipitation, either directly or
through infiltration of surface waters derived from precipi-
tation. Therefore, the factors that permit or exclude infiltra-
tion of water through soil to the water table are important.
Infiltration is the process where water enters the soil zone
but does not penetrate to the depth of the water table. In an
ideal soil with no removal of water by plant roots the
distribution of infiltrating water often is delineated by
measurements of moisture content over space and time.
An increase in moisture content from initially dry to increas-
ingly wet soil results in the development of a wetting front:
we can infer from Seneca's comment mentioned at the
beginning of this chapter that he probably was observing
such a wetting front.
The idealized case for a wetting front assumes that the
upper soil layer is permeable enough to allow infiltration.
Over time, surface soils can become impermeable because
of the movement of fine particles into spaces between coarse
particles from surface impaction. Air entrained in the pore
spaces of dry soils also can decrease water infiltration. The
air is displaced after a wetting front compresses it to the
extent that the air pressure increases. This phenomenon can
be seen when houseplants or lawns are watered and bubbles
appear or can be heard as a popping sound at the soil
surface.
Seneca had observed soil conditions after precipitation.
But we also know from direct observation that the upper
layers of loam soils can be dry and have low moisture
content. This results from evaporation, transpiration, or
lack of infiltration. The wetting and drying and re-wetting
properties of a soil represents a characteristic of particular
soils that is related to sediment texture and pore size. This
relation between soil type and wetting and drying typically is
expressed as a hysteresis curve (Fig. 4.14 ). More energy is
required to lose water from a soil than gain water due to the
surface tension of water.
Moisture in the upper layers of soil in the unsaturated
zone that flows under the influence of gravity is the field
capacity, as mentioned previously. Field capacity is analo-
gous to the specific yield of an aquifer. As might be
expected, water that is bioavailable to plants is present
under moisture conditions that are above the wilting point
but below field capacity (Table 4.1 ).

Fig. 4.14 A representative soil-moisture hysteresis curve that shows
the cyclical wetting and drying of a representative soil (Modified from
Hillel 1998).

Table 4.1 The range of plant bioavailable water for different soil
types.

Soil type Field capacity - Wilting point Bioavailable water

(% Dry weight of soil)

¼

Sand 5 2 3

Loam 19 10 9

Clay 36 20 16

Peat 140 75 65

4.7.2 Recharge and Discharge Areas

Precipitation or surface water of higher elevation than the
groundwater will accumulate in the soil as a wetting front,
overcome the resistance of the soil, and, under gravity,
infiltrate through the unsaturated zone, capillary fringe and
zone, to reach the water table. The process of infiltrating
water that becomes groundwater is called recharge. In
recharge areas, a deep well exhibits a lower groundwater
level compared to a shallow well (Fig. 4.15 ). Conversely, in
discharge areas, the deep well will have a higher groundwa-
ter level compared to a shallow well (Fig. 4.15 ).
Recharge can occur to an unconfined aquifer along its
entire surface and to the updip exposure (unconfined por-
tion) of a deeper confined aquifer, called an outcrop area.
Most recharge is comprised of precipitation infiltration in
higher elevation areas with subsequent discharge in lower
elevations. The location of discharge is controlled either by
differences in geologic strata, or presence of surface-water
bodies. When precipitation infiltrates into porous media that
overlies a less permeable layer, and that layer continues
aerially to intersect land surface, then groundwater is
discharged to the surface as a seep or spring. Recharge also
can occur to confined aquifers away from outcrop areas by
leakage from adjacent confining beds. The occurrence of
recharge is common in humid areas of the world, where