Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 110

have shallower, less developed roots. As the trees grow, they
switch from surface water to groundwater as a source to
meet water demands. The use of stable isotopes of water
for monitoring the effect of phytoremediation on groundwater
is discussed in Chap. 9.

4.7.3 Steady-State Flow and Transient Flow

In an aquifer system where the amount of recharge is equal
to the amount of discharge, groundwater levels do not
change over time and groundwater-flow conditions reach
steady state. We saw this in Chap. 2, in Eq. 2.1 where
water inflow ¼ water outflow . In reality, however, steady-state

flow conditions usually are not attained, because evapotrans-
piration demands for water limit infiltration and recharge.

Fig. 4.15 Recharge and discharge areas defined using vertically
discretized or nested well pairs. In the recharge area, the groundwater
level is lower in the deep well relative to a shallow well. In the
discharge area, the groundwater level is higher in the deep well relative
to a shallow well.

4.8 Groundwater and Surface-Water

Interactions

Groundwater not in storage tends to discharge to sinks such
as surface-water bodies. During time of seasonally lower
precipitation or drought, many streams can continue to
flow at full stage for extended periods of time. Such baseflow
is caused by groundwater discharge. This relation between
groundwater and surface water was not widely recognized,
however, until relatively recently (Winter 1999; Conant
2004, references therein) even though early hydrology
textbooks hinted at the interaction (Wisler and Brater
1956). As we saw in Chap. 2, after plant transpiration and
evaporation demands are met, the balance of groundwater
enters surface-water sinks.
Many techniques have been developed to investigate the
interaction between groundwater and surface water. These
range from being as simple as using a shovel or boot heel to
dig a depression, called a piezopit, in the stream bank near
the level of surface water, allowing the piezopit to fill
with water, and then placing small particles on the water
surface in the hole and observing their movement (source:
Dr. Samuel S. Harrison, Professor of Hydrogeology (ret.),
Allegheny College, oral commun. 1988). Alternatively, sim-
ple iron or polyvinyl chloride (PVC) pipes can be pushed
into the streambed sediment at various depths to determine
the magnitude and direction of the vertical head gradient.
More complicated devices called seepage meters also can be
installed to determine the volume of groundwater discharge.
The simplest of these devices, a cut-off 55-gal drum, is
placed in the bed sediments of the surface-water body as
described in Lee (1977); refer to Rosenberry and Menheer
(2006) for a history of this meter. A valve is attached to the
top of the drum lid, and a plastic bag filled with a known
volume of water is attached to the valve. As groundwater

the unsaturated zone is often less thick relative to the thick-
ness of the water-table aquifer. In arid areas where precipi-
tation is less than 10 in./year, the most recent recharge of
extremely deep water tables may have happened more than
100,000 years ago (Alley et al. 2002).
Compared to the range of groundwater-recharge rates that
tend to fluctuate over time due to differences in precipita-
tion, groundwater discharge tends to remain relatively con-
stant. This is why streams and rivers have relatively constant
flow rates between precipitation events or during the early
part of a drought. Also, recharge tends to be aerially
distributed over a much larger area than more localized
discharge areas, whereas discharge areas integrate ground-
water flow lines from across the saturated aquifer thickness.
The rather consistent supply of groundwater discharge is
one reason that phreatophytes are competitive with other
plants, especially in areas that have frequent droughts. For
example, trees that grow along streams have been observed
to obtain greater heights and develop larger trunk diameters
than upland plants of the same species (see Chap. 1). Con-
ventional thought was that these riparian plants used surface
water as their water source. However, it was shown using
variations in the stable isotopes of water that exist between
surface water exposed to evaporation and groundwater, that
the large trees, such as maples ( Acer negundo ), contained
water that had isotopic values similar to those of the ground-
water, but not surface water (Dawson and Ehleringer 1991).
Small trees contained a greater proportion of surface water,
as shown by the isotopes, presumably because small trees