Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 159

(2004), the poplar is the internationally accepted model for
molecular studies of tree biology. A Web-based database,
called Populusdb, contains expressed sequence tags (EST)
for 18 tissues that represent various plant organs. Such work
led to the insight that angiosperms and gymnosperms have
a high degree of similarity in genetic information. The
genome of the black cottonwood, Populus trichocarpa ,
was found to contain more than 45,000 genes (Tuskan 2006).
The widespread distribution of poplars provides the best
evidence that water-loving phreatophytes are not simply
limited to groundwater in arid areas with little precipita-
tion (Dickmann and Stuart 1983). For example, Populus
deltoides grows to very large heights east of the Mississippi
River. Also, Populus heterophylla can be found in most low-
lying areas near springs or streams east of the Appalachian
Mountains. Because all of these species tend to grow near
surface water where the depth of the regional water table is
shallow, they are dependent on groundwater for survival.
These plants predominately use groundwater, but can also
use rainwater and, therefore, are an example of facultative
phreatophytes. These plants are facultative in that the depth
for most cottonwood roots is no greater than 35 ft (10 m;
Meinzer 1927; Robinson 1958). The examination of the use
of groundwater by poplar trees began in the mid-1940s, as
we saw in Chap. 1. For example, Gatewood et al. (1950)
grew cottonwood trees ( P. fremontii ) in tanks and deter-
mined that the water use was about 7.6 ft (2.3 m) between
October 1, 1943, and September 22, 1944, when the depth to
water table was constant at 7 ft (2 m).
A possible explanation for why phreatophytes, such as
poplar trees, have come to rely on groundwater and have
established such a large range of growth may be related to
events that occurred some 65 MYa. In general, dinosaurs
were around as recently as the Cretaceous Period, some
90 MYa, as is evident from the fossil record. Such fossils
are abruptly absent, however, in the younger Tertiary
sediments directly overlying the Cretaceous sediments.
This change is accompanied by an abrupt change in plant
fossils as well. Geologists have identified this dramatic
change in fossil assemblages, especially the lack of large
dinosaur fossils in the Tertiary, for many years. It wasn't
until the 1980s that Luis and Walter Alvarez detected high
concentrations of iridium in the sediments at this interface
between the Cretaceous and Tertiary rocks, called the K-T
boundary, at many sites around the world. While iridium is
present in the earth's crust, the stable isotope signature of the
iridium measured at the interface was similar to the stable
isotope signature of iridium present in asteroids. Alvarez
suggested that the lack of dinosaur fossils in the Tertiary
sediments could be explained by a mass extinction that
followed an asteroid impact with the earth. The impact
must have sent ash into the atmosphere which blocked
light, changed the pattern of precipitation, and caused plants

to die and the animals that fed on these plants and the
animals that fed on these animals to die as well. Up to
50-80% of all plant and animal genera perished and is called
the K-T extinction (Plummer and McGeary 1985).
What does this event have to do with the phyto-
remediation of contaminated groundwater? Interestingly,
some plants and animals did make it through this mass
extinction event, and they all share a common trait.
Examples of the life forms that survived the K-T extinction
are present today, essentially in an unchanged form, in
swamps and wetlands. For example, it is possible to trace
the lineage of turtles and alligators back to before the K-T
extinction event. Plants that co-inhabited low-lying areas
near swamps, streams, and lakes, also survived the extinc-
tion. These plants were phreatophytes or had characteristics
of phreatophytes. So, the assumption is that these plants and
animals used low-lying areas as a source of water and pro-
tection during the K-T extinction event, and the ultimate
source of water in low-lying areas is groundwater.
Many studies across a diverse range of natural sciences
indicate that poplar trees use groundwater, which provides a
firm foundation for their application to contaminated
groundwater as is stated throughout this topic. Zhang et al.
(1999) measured sap flow in poplar trees growing in a
riparian setting in England and reported that 15-60% of
the sap flow was composed of groundwater. At the study
site, the aquifer material consisted of sandy loam, and the
water table was about 4 ft (1.25 m) below ground. The trees
investigated were Populus trichocarpa Torr. & A. Gray x P.
tacamahaca L. (Clone TT32), which were about 6 years old
and roughly 18 ft (5.5 m) tall. Sap flow was found to be
directly related to solar radiation and vapor pressure deficit,
with little sap flowing on cloudy and rainy days. Total water
use by the trees approximated ET p , as calculated by using the
Penman equation. Transpiration rates between 13 and
200 mm/day have been reported for poplars (Interstate Tech-
nology Regulatory Council 2009).
With respect to the source of water in the sap in the trees,
Zhang et al. (1999) recorded the drying out of the upper soil
layers to a depth of 39 in. (100 cm). Because the trees
continued to transpire, as indicated by positive sap flow,
the authors suggested that water must be derived from
deeper, more saturated soil layers near the water table. In
fact, water removed directly from the capillary fringe or
water table was estimated to increase from 15% in June to
between 45% and 63% in July and August, respectively.

7.1.2.2 Willows
The genus Salix of the family Salicaceae also has examples
of phreatophytes that can be found in humid climates. These
trees, commonly known as willows, inhabit low-lying areas
near streams where the depth to groundwater is shallowest,