Environmental Engineering Reference
In-Depth Information
5
Plant and Groundwater Interactions
Under Pristine Conditions
An initial understanding of plant and groundwater
interactions did not follow a straightforward path. For exam-
ple, it took many years and developments in forensic chem-
istry to elucidate that the oxygen released by plants during
photosynthesis was derived from water absorbed by roots
rather than from atmospheric CO
2
absorbed by the leaves.
Also, geochemical techniques that involved stable isotopes
revealed that trees that grow on the banks of rivers tap
groundwater rather than the seemingly more available
source provided by surface water. Moreover, the facts that
groundwater is not readily observed and that plants release
invisible water vapor makes it easy to forget that plants
move enormous volumes of water on a daily basis, a process
that is essentially hidden in plain sight.
The consequences of such hidden interactions between
plants and groundwater can be understood, however,
because the mass of water in a particular basin has to
be conserved such that
P
foundation for the application of these interactions at sites
characterized by contaminated groundwater,
and are
discussed in Parts II and III.
5.1
Plants and Groundwater Recharge
Plant interactions with water are an important part of the
hydrologic cycle as described in Chap. 2. Transpiration
and evaporation return to the atmosphere up to 70% of
the average annual precipitation in a particular basin. Of
the 70% of water removed by evapotranspiration, the com-
ponent of this total driven by transpiration alone can range
from 5% to 80% (Larcher 1983; Moreo et al. 2007). Part of
the transpired water can be composed of recent precipitation,
precipitation that infiltrated into the upper soil layers, water
from deeper within the unsaturated zone, or shallow or deep
groundwater. In many areas, recharge is less than 10% of
annual precipitation. From a mass-balance perspective, the
processes of evaporation and transpiration limit the amount
of water available for recharge. Moreover, the allocation of
precipitation to evapotranspiration decreases the amount of
groundwater available for discharge to either natural areas,
such as springs, lakes, or rivers, or even to wells.
The effect of water availability on plant growth is at least
anecdotally recognized by many laypersons through per-
sonal experience. For example, the relative width of tree
rings, revealed after cutting down a tree, indicates the
gross effect of water availability on annual tree growth.
Within each annual growth ring, the springwood is lighter
and thicker because the water-transporting xylem grew rap-
idly in response to the higher availability of water and other
resources. The winterwood is denser and occupies less
space, because water is less available when formed. These
observations have been used recently by biologists to under-
stand past and future climate conditions and effects on plant
and water use.
R
. The foundation of
phytoremediation of contaminated groundwater is part
of this fundamental interaction as revealed in the parameter
of
T
in
ET
.
Some of the best evidence to support the application of
plants to interact with contaminated groundwater is provided
by plant and groundwater interactions that occur under
natural, pristine environments. Some of these examples,
including perhaps the first recorded observation of plant
and groundwater interactions in 1926, were discussed in
Chap. 2. More recently, the study of the interaction of
groundwater, plants, and other ecological systems has
been called by various terms such as ecohydrology,
hydroecology, among others (Bond 2003; Lubczynski
2009; Lowry and Loheide 2010). The focus of this chapter
is to provide additional examples of naturally occurring
interactions between plants and groundwater and how these
interactions can affect recharge, surface-water flow and geo-
chemistry, and groundwater hydrology and water quality.
This
ET
ΒΌ
information on natural
interactions provides
the
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