Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 143

saturated) drain by gravity, air enters the soil to replace that
volume lost. These changes occur when the water table
fluctuates, which directly affects groundwater use by plants.
As we saw with the description of Darcy's Law, ground-
water is affected by gravity. Water movement in the unsatu-
rated zone is called gravity drainage. If the hydraulic
conductivity of the soil is higher than the rate of water
entry into roots, the water will drain past the root zone before
plant uptake. After gravity drainage occurs, the remaining
water adheres by tension to the surfaces of soil particles.
This water, called field capacity, has a tension of about

as plant-available water. Typically, plant-available water is
half of the specific retention. The range of plant-available
water is between 1.5 MPa

[megapascal]). For conversion, 1 kg of mass on a unit sur-
face area is equivalent to 9.8 Pa, where 100,000 Pa is
equivalent to the pressure of the atmosphere, 100 Pa is
equivalent to 1 mb (millibar), and 1 atm is equivalent to
1,013 mb.
The relation between water tension and plant-water avail-
ability can be envisioned by a simple analogy using a
sponge. The water absorbed by a sponge after immersion
represents water saturation. The amount of water that drains
by gravity after immersion represents field capacity; when
the sponge is squeezed, the water removed is the holding
capacity, and the water that remains in the sponge represents
water held by tension. Plant-available water is represented in
this analogy, therefore, by the water available that is half of
the water-holding capacity.
Fluctuations in water availability to plant roots produce
different effects on plant growth and overall metabolism and
health. A decrease in water availability, from drought
conditions or insufficient irrigation, can result in stomatal
closure and a reduction in CO 2 uptake. This decreases pro-
duction of carbohydrate and, if continued for a long period,
leads eventually to plant death by starvation. Too much
water, on the other hand, especially if it is stagnant, can
lead to the expulsion of air from the root zone and decreased
root growth from lack of oxygen necessary to support root
respiration. If the inundation lasts too long, the roots will die
from a lack of oxygen. From this discussion, we can see that
the water held by tension in the capillary fringe provides a
major advantage as a source of water to plant roots, espe-
cially in terms of plants used at phytoremediation sites,
because air is available in the pore spaces to support root
respiration, the water will not drain by gravity, and the plants
can overcome water tensions.
The thickness of the capillary fringe changes in space in
different soil types and over time because of variations in
infiltration amounts and rates. Sands have an average plant-
available water thickness of 0.75 in./ft (1.9 cm/0.3 m) of soil
column. This means that for a vertical soil column of 1 ft
(0.3 m), an average of 0.75 in. (1.9 cm), or about 6% of the
water in the sand pores, is available for plant use. Con-
versely, clays have an average of 2 in./ft (5 cm/0.3 m) of
soil column, or about 16% of the total available. This is why
plants grown in sand either need continuous water from
frequent precipitation or a very deep root system that is
exposed to soil moisture throughout the unsaturated zone
down to the capillary fringe and water table; more of these
factors are discussed in Chap. 7.
The widely used concept of the wilting point just
discussed is not valid, however, for the roots of
phreatophytes that are in contact with surface water or

0.33 and 15 bar ( 0.033 and

0.33 atm). Although not affected by
gravity, water held by tension is available for entry into plant
roots, called plant available water, until tensions exceed

0.33 bar (roughly

15 bar. As the soil dries out and tensions increase, plant
wilting occurs, as plants cannot take up water even though
water is present. At the wilting point, surface tension
exceeds osmosis, the root water potential, c r , is equal to

the soil water potential, c s , and no water flow occurs. The

wilting point typically occurs when the volumetric water
content in loams equals 30% (Fig. 6.6 ). The wilting point
in clays is reached when the soil moisture decreases below
about 15%, and in sandy sediments is nearer 5%.
The amount of water in soil or sediment between the
wilting point and field capacity is referred to as specific
retention, also called gravitational water or water-holding
content by plant physiologists. The amount of water under
specific retention that can enter plant root hairs is referred to

Fig. 6.6 Generalized diagram of the zone of plant-available water as a
function of soil moisture, which is represented as the volumetric water
content and depth below land surface. This relation explains why many
plants, including some phreatophytes, have extensive shallow roots.
One centimeter is equivalent to 0.39 in.