Environmental Engineering Reference
In-Depth Information
100 years. This change in hydrology has affected riparian
phreatophytes by changing the depth to groundwater. Horton
et al. (2001) investigated the effect of various depths to
groundwater on the physiology of riparian trees, such as
poplars, willows, and tamarisk. One of their observations
was an apparent linear relation between the decrease of
groundwater use by the trees as the water table declines
until it reaches a critical depth after which groundwater use
per unit increase of water-table depth declines exponentially.
A water-based optimal root depth was determined for
plants based on the carbon costs of deep roots relative to
their use for water removal (Guswa 2010). In this investiga-
tion, deeper roots were related to areas where the precipita-
tion amounts were roughly equal to
ET
p
. In areas where
precipitation exceeds
ET
p
there is no benefit to a plant to
support deeper roots and, conversely, in areas where precip-
itation is less than
ET
p
deep roots often are not found in all
instances if groundwater is not available.
An interesting relation was observed between the depth of
root penetration to the water table and how slowly the water
table declined. Fenner et al. (1985) investigated the presence
of root elongation in cottonwood trees in relation to ground-
water levels that were regulated to some extent by regulated
surface-water flows. They observed significant root elonga-
tion in the riparian cottonwoods if the water-table decline
was on the order to 0.83 to 1.1 in./day (2 to 3 cm/day).
Hence, fluctuations in the water table tended to lead to a
more extensive penetration of deeper soil.
If the water table fluctuates, the rate of fluctuation may
have a large effect on the survival of phreatophytes. If the
rate of water-table decline is greater than the rate of terminal
root growth and root-hair development, the plant may
undergo water stress. This is particularly true in coarse-
grained sediments that have a thinner capillary fringe rela-
tive to in fine-grained sediments. The highest density of root
hairs of phreatophytes is concentrated in the capillary fringe.
If the water table declines slowly, root-hair growth can occur
at a similar rate, and the plant will not be water stressed.
Conversely, if the rate of water-table decline is faster than
the rate of root-hair growth, such as rapid drops that
approach 3 ft (0.9 m), the tree will be water stressed and
may die (Scott et al. 1999; Shafroth et al. 2000).
Conversely, a high water table can be detrimental or
beneficial to a phytoremediation planting. It can be detri-
mental if the water has such a low flow rate or is in sediments
that contain either organic or inorganic compounds that
interact with and deplete oxygen to the point that root sur-
vival is diminished for most plants that are not adapted to
such saturated conditions. If the water is oxygenated, how-
ever, the needs of water for transpiration, photosynthesis,
and oxygen for root respiration are satisfied. If the water
table is so high that anoxic conditions occur for long
periods and the water becomes stagnant, the roots cannot
respire and will die from a lack of energy. Plants, such as the
baldcypress and tupelo present in swamps of the Southeast-
ern United States, survive, in part, because the surface water
in which they stand is not stagnant but is continually being
exchanged by evaporation, transpiration, and groundwater
discharge.
In studying the relation between transpiration by native
cottonwood (
Populus deltoides
) and invasive saltcedar
(
Tamarix chinensis
) along the Middle Rio Grande River,
NM, Cleverly et al. (2006) related the depth to groundwater
to transpiration,
LAI
, and groundwater levels during drought
conditions. For the poplars, drought conditions did not affect
evapotranspiration rates and the depth to water table
remained fairly static at 9 ft (2.7 m) below land. For the
saltcedar trees, however, evapotranspiration increased and
the depth to water table decreased during the same period.
The evapotranspiration increased from 0.23 to 0.35 in./day
(6 to 9 mm/day) as the water table decreased about 0.2 in./
day (7 mm/day), which indicates the decrease was caused by
the increase in evapotranspiration. It is possible that the
higher evapotranspiration rate measured for the saltcedar is
driven to a greater extent by the atmospheric conditions than
depth to water table.
Water tables that are within a few feet of land surface
are more apt to receive recharge during precipitation
events than deeper water tables that undergo the same
amount of precipitation. This creates a unique scenario
for a phytoremediation application. Even if the site is
characterized by a relatively high
ET
p
that is near precipita-
tion amounts, the water table will not trend toward ever
lower depths, especially if precipitation and recharge are
frequent. At sites with shallow water tables, a higher per-
centage of the precipitation becomes groundwater than at
sites with deeper water tables, where less of precipitation
reaches the water table. In the first case the water table
actually may rise, as it is being replenished by recharge at
a rate faster than it is taken up by trees. In the other case, the
trees continually take up the groundwater and intercept
infiltration in the unsaturated zone, and the groundwater
level steadily declines.
The relation between the depth to water table and transpi-
ration by phreatophytes was investigated by Gazal et al.
(2006). In general, they found that as the depth to water
table increased from 3.2 to 13 ft (1 to 4 m), the measured
transpiration rate decreased from about 0.2 to 0.07 in./day
(0.5 to 2 mm/day).
7.3.7 Semiconfined to Confined Groundwater
Conditions
At some sites the groundwater to be assessed and
characterized is under semiconfined to confined conditions.
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