Environmental Engineering Reference
In-Depth Information
5.1.2 Reduction in Recharge
trees planted in the 1980s by local government officials
around the city of Khartoum following a drought in the
mid-1970s. The idea was the drought-tolerant, groundwater-
using mesquite would slow the advance of the surroun-
ding desert sands, which were encroaching at a rate near
10 mi/year (16 km/year). Because mesquite seeds are
spread easily by birds and animals, however, the species
invaded areas used for agriculture and has spread about
1,000 acres per year (4.0
Phreatophytic plants under natural settings often provide
little or no economic value. On the other hand, such plants
can be of economic value when used as short-rotation cop-
pice (SRC) for wastewater treatment, pulp production, or
biomass production as an alternative energy supply. The
effect of SRC on the water budget of two crops grown in
different areas with different plants and, therefore, poten-
tially different water-budget implications were examined by
Allen et al. (1999), and their monitoring data are applicable
to the phytoremediation of contaminated groundwater. The
site was a 1.8-ha plantation in England. Six poplar clones
were planted 3 ft (1 m) apart. This planting was useful for
studying not only the effect of plants on water budgets but
the effect of different plant physiologies, because one clone
(Beaupr´,
P. trichocarpa
Torr & A. Gray x
P. deltoides
Bartr.
ex
Marsh) had twice the leaf area and was taller than
the other clone (Dorschkamp,
P. deltoides
x
P nigra
L.). As a
result, the authors were able to show that the Beaupr
´
clone had higher transpiration rates during June 1994—a
mean of 5.0 mm per day compared to 2.4 mm per day for
the other clone. Because of these high transpiration rates,
Allen et al. (1999) concluded that such a rate of removal of
soil water would adversely affect water resources in the
United Kingdom.
In some cases, the plant-facilitated reduction in recharge
was used to solve rather than create water-quantity
problems. Recharge reduction was observed, for example,
following the introduction of mesquite (
Prosopis julifora
)
10
6
m
2
/year). As a result,
recharge has declined and groundwater levels in wells have
decreased from levels prior to those before the spread of the
mesquite.
In some ephemeral river systems typical in arid climates,
recharge areas often are constrained to dry river beds that
contain the most permeable sediments. Phreatophytes
located in these dry river beds, however, often intercept
infrequent
infiltration before it can become recharge
(Fig.
5.2
).
5.2
Plants and Groundwater Discharge
to Surface Water
Trees located along streams use groundwater and not neces-
sarily streamwater to meet evapotranspiration demands
(Dawson and Ehleringer 1991) and at first may seem coun-
terintuitive. After all, the density of trees increases near most
surface-water bodies. This occurs in arid areas along stream
channels and in humid areas in flood plains, swamps, and
oxbow lakes. From an ecological
standpoint,
it
is
Fig. 5.2
Phreatophytes, such as
these saltcedar (
Tamarix spp
.)
growing in a dry river bed in
Tucson, Arizona, intercept
infrequent infiltration before it
can become recharge (Photograph
by author).
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