Environmental Engineering Reference
In-Depth Information
Table 7.4
Relation between plant type and depth to the water table
(Modified from Meinzer 1927).
Plant Depth to water table, in feet
Rushes and sedges Water at surface or water table within a few feet.
Giant reed grass
Water at surface or within 1-8 ft.
Giant wild rye
Water near surface to 12 ft or more in arid
areas, less relation in humid areas.
Salt grass
Near surface to 12 ft.
Arrow weed
Water at surface, or from 10 to 25 ft.
Willow
From surface to 12 or more ft.
Palm tree
Water within a few ft.
Greasewood
Water table from 3 to 40 ft.
Mesquite
From 10 to 50 ft.
however, the percentage of total plant transpiration from
groundwater decreases (Ayers et al. 1999). In fact, this
relation was stated by Grismer and Gates (1988) as
Q
¼
G
=
ET
¼
a
bD
(7.2)
where
Q
is described as the ratio of the amount of water
supplied by groundwater,
G
, to total evapotranspiration,
ET
;
a
and
b
are constants that relate to the hydraulic properties of
the soil and range from 0.7 to 0.36 and 0.20 to 0.17, respec-
tively; and
D
is depth.
The depth to water table can vary many feet even in
roughly the same region, so it is not surprising that the
lengths of roots of certain plants that use groundwater also
vary in relation to the depth to water table. For example,
desert saltgrass (
Distichlis stricta
) can be found where the
water table is no more than 12 ft (3.6 m) from land surface,
whereas alfalfa (
Medicago sativa
), another member of the
grass family, can have roots penetrate the subsurface to
depths greater than 60 ft (18 m; Robinson 1958). It must
be remembered that even deep-rooted plants, such as alfalfa,
cottonwood, or willow, also can grow in areas where the
water table is much shallower. However, in areas where only
stands of alfalfa predominate at the expense of other plants
that use groundwater, it is most probable that the explanation
is the greater depth to groundwater.
In the early 1920s, O.E. Meinzer of the USGS made an
intensive survey of the groundwater supplies of Sulphur
Spring Valley, AZ, the Tularosa Basin, NM, and Big
Smoky Valley, NV. As part of his investigations, he noted
the relation between the depth to water table and the type of
plant that predominated, as was introduced in Chap. 1.
Although the reader is referred to Meinzer (1927) to see
his entire list, he made some general relations as noted in
Table
7.4
that have some applicability to phytoremediation
of groundwater.
Fig. 7.9
A generalized comparison of the average maximum observed
rooting depths, in feet, of phreatophytes commonly installed for
phytoremediation, such as
Populus
,
Salix
, and
Betula
. These depths
can be exceeded by native plants of
Quercus
and
Pinus
at some sites,
but the total transpiration rate and, therefore, groundwater use is much
lower than the shallow-rooted plants, which makes them less effective
for use in phytoremediation of contaminated groundwater. One foot is
equivalent to 0.304 m.
As stated previously, most sites where phytoremediation
can be applied successfully typically have water tables
between 5 and 15 ft (1.5-4.5 m) below land surface
(Fig.
7.9
). Water becomes more bioavailable closer to the
water table, as water potential is the least negative.
That the depth to groundwater affects phreatophytes
and the water budget with respect to groundwater can be
observed in areas that are characterized by riparian
ecosystems. In Chap. 5, native and invasive woody vegeta-
tion in the riparian zones of rivers in the American South-
west were shown to use groundwater. Many of the naturally
flowing streams have been dammed or impounded in the last
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