Historical Foundation of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 13

habitats; the study of these interactions is referred to as
hydroecology.
It may seem ironic that initial observation of the interac-
tion between plants and groundwater occurred in the desert
regions of North America. Deserts, however, cover roughly
one-sixth of the land area of the United States. Deserts are
found in greatest abundance west of the Mississippi River
between 15 and 30 latitude. This location is typical for
most deserts around the globe where the weather is
dominated by dry, falling air under high pressure. In con-
trast, humid areas are characterized by moist, rising air under
low pressure.
The plant species that characterize deserts have developed
at least two different strategies for survival in regard to water.
Those that use water derived from relatively infrequent pre-
cipitation are the xerophytes introduced earlier. These types
of plants usually become dormant between precipitation, and
they have developed physical mechanisms to retain the water
they take up, such as highly modified leaves that became
spines, photosynthetic stems, and a unique photosynthetic
process that is discussed in Chap. 3. Therefore, xerophytes
adapt to limited water availability in deserts, or other areas
with limited precipitation or to high infiltration rates through
porous soils, by a strategy of water conservation. The second
type of strategy is discussed in the next section.

1.2.2 The U.S. Geological Survey, O.E. Meinzer,
and Phreatophytes

In the 1910s and 1920s, a hydrogeologist by the name of
Oscar EdwardMeinzer with the USGS traveled from the very
dry lands of southern California to the valleys in adjacent
Nevada. He observed that the apparent location of the water
table in reference to land surface had a direct effect on plant
occurrence and distribution. He summarized his ideas and
observations in USGS Water-Supply Paper 577 (Meinzer
1927; the cover of which is shown modified in Fig. 1.5 );
some of this information was presented earlier (Meinzer
1926). From today's perspective, Water-Supply Paper 577
is regarded as more insightful than even the late author would
have realized because his observations of plant and ground-
water interaction help establish some of the first principles of
the phytoremediation of contaminated groundwater.
O.E. Meinzer noted that certain plant species appeared to
be associated with more consistent sources of water deep
below ground rather than the infrequent, light precipitation
that characterized the area. He apparently had been thinking
about this for some time, as he had coined the term
phreatophytes to categorize such plants in an earlier publica-
tion (Meinzer 1923). Meinzer's definition of a phreatophyte
was a “plant that habitually obtains its water supply from the
zone of saturation, either directly or through the capillary

Fig. 1.5 In the 1920s, Oscar Edward Meinzer of the U.S. Geological
Survey related the presence of certain plants to the depth of groundwa-
ter in desert areas of the southwestern United States and summarized in
his classic 1927 publication, Plants as Indicators of Groundwater, U.S.
Geological Survey Water-Supply Paper 577.

fringe” (Meinzer 1923). He was careful in stating that this
new term was not designed to create a new category separate
from other classifications of plants based on water source, but
to overlap the existing terms already used by plant
physiologists. In contrast to the previously mentioned strat-
egy of water conservation, these plants that have deeper root
systems that tap groundwater employed a strategy of drought
avoidance. Today, the term phreatophyte is used routinely by
both hydrogeologists and plant physiologists to refer to all
plants that tap deep, perennial sources of groundwater.
Phreatophytes have root systems that tap groundwater
from the capillary fringe or deeper as an ecological advan-
tage in arid areas or in humid areas where precipitation is not
constant from year to year or due to geological constraints
such as the presence of lower permeability sediments near