Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 209

9

Monitoring Plant and Groundwater Interactions

In general, the combined processes of evaporation and tran-
spiration can remove about 70% of annual precipitation from
a basin or, on a smaller scale, a phytoremediation site.
But how much of this water is removed by transpiration?
How much of this transpired water is derived from ground-
water? Fortunately, geochemical methods can be used to
elucidate the various sources of water, including groundwa-
ter, that comprise sap flow. These geochemical methods
can be used in combination with the water-budget methods
discussed previously to decipher plant and groundwater
interactions at contaminated sites.

potentials of the capillary fringe, soil, root zone, plant, and
leaves, can provide an indication of the potential for water to
be transpired under given conditions.
The water potential of each component of the soil-plant-
water-air continuum can be measured, although such
water potentials are not commonly measured at most
phytoremediation sites. Soil-water potentials can be readily
assessed in the field, however, using instruments such as
tensiometers or psychrometers, as described in Chap. 3 and
briefly summarized here. Tensiometers can be placed
directly in the field to measure the water potential of the
water in the soil near plant roots. A tensiometer is essentially
a porous ceramic cup attached to a tube filled with water and
then sealed. If the soil in which the tensiometer is placed is
drier than the water-filled ceramic cup, water will exit the
cup. Because no air enters the water-filled tube to replace the
water that exited from the cup, a negative pressure develops
in the tube. This pressure change can be measured with a
pressure gauge installed in the air space created in the
tensiometer.
With the psychrometer, a piece of plant material of
unknown water potential is placed in a sealed chamber that
also contains a droplet of a solution of known water poten-
tial. If the plant material has a lower water potential than the
droplet of reference solution, and hence a lower vapor pres-
sure by way of a higher solute concentration, preferential
evaporation from the droplet cools the surface of the water;
this temperature difference is measured using a thermocou-
ple. Conversely, if the droplet of reference solution has a
lower water potential than the perhaps less concentrated
sample of plant material that contains water, the sample's
evaporation will warm the reference droplet. Hence, if a
particular solution's water potential is known and it results
in no net movement of water to cool or warm the droplet,
then the sample of plant material containing water must have
the same water potential as the reference sample. Because a
change in temperature can also cause a change in water
potential, where a change in 0.01 C

9.1 Plant Physiologic Monitoring Methods

Various methods based on the fundamentals of plant physi-
ology presented in Chap. 3 can be used to monitor the
interaction between plants and groundwater to meet the
three hydrologic goals presented in Chap. 6. The common
denominator emphasized in most of these methods is the
water status of plants. The measurement of plant-water sta-
tus, or water potential, is only part of the story, however.
Plant-water status does not indicate whether groundwater is
the only source of the water being assessed in various plant
tissues, and this limitation is addressed later in this chapter.

9.1.1 Water Potential

A property that can be used to assess plant-water status with
respect to groundwater interaction for hydrologic control is
water potential, introduced in Chap. 3. The concept of water
potential provides an advantage over soil-moisture content,
because the amount of water in sediment relative to saturated
conditions does not indicate whether (1) the water is bio-
available to plants, or (2) the direction that water will flow.
Because water will flow from less negative water potentials
to more negative water potentials, the measurement of water

¼ 0.1 MPa, the cham-

ber must be kept at constant temperature. As such,