Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 232

model is a one-dimensional model developed to investigate
the effect of snowmelt and soil freeze-thaw cycles on soil
moisture levels. The model simulates heat, water, and solute
transport in a profile from land surface to depth, and includes
terms for plant and water interactions. The model is based on
the Richards equation to describe water flow in the unsatu-
rated zone. The plant interactions are simulated to specifically
account for the effect on the water budget in the soil profile
due to differences in rooting depth, plant size, and LAI .
Transpiration is simulated as being controlled by the balance
between stomatal conductance and leaf water potentials.
Preston and McBride (2004) used the SHAW model to assess
the impact of planting poplar trees over a decommissioned
landfill in Ontario, Canada. Predictive simulations indicated
that the poplars trees affect the site water budget by taking up
precipitation and decreasing recharge.
Another plant-water-soil profile model is called
UPFLOW (Raes and Deproost 2003; Raes 2004). This
model can be used to estimate the steady-state amount of
water that moves from the water table to the root zone under
a variety of environmental conditions. The model is based on
soil-water retention curves that represent the various soil
types encountered in the unsaturated zone. The resultant
profile is dependent upon the ET demand, the soil water
content, depth to water table, plant root characteristics, soil
properties, and salt content. The model will also calculate
the depth when aeration levels may decrease to the point of
anoxic conditions, which would jeopardize those roots con-
tinually submerged. The model assumes that higher plant-
water uptake rates occur in the shallow soils associated with
increased root density; this may not always be the case for
obligate phreatophytes installed at sites where there may be
lower root density with depth whose uptake will be offset by
higher root hydraulic conductivities.
Hopmans and Van Immerzeel (1988) investigated the
interconnection between groundwater movement

capillary fringe under the influence of ET at a site in The
Netherlands characterized by a shallow water table using the
SWATRE model (Belmans et al. 1981). They used the
model to reproduce field data and concluded that ET was
controlled by the hydraulic conductivity of the soil profile
through the capillary fringe. At locations where the hydrau-
lic conductivity was lower, the ET demand could not be met
by capillary rise.
The Hydrologic Evaluation of Landfill Performance
(HELP, Schroeder et al. 1994) model has long been used to
evaluate conventional and ET covers over decommissioned
landfills. The model simulates the complex relation between
hydrology, soil, plants, ET , and climate using a water-
balance approach. Rather than being based on the Richards
equation, such as SHAW, it is based on a water balance.

9.4 Summary

Water losses by evapotranspiration can approach nearly 70%
of annual precipitation in most areas. Because phreatophytes
effectively couple groundwater to the atmospheric demand
for water, the hydrologic and meteorologic conditions of a
site can affect the ability of plants, such as poplar trees, to
remove soil moisture or groundwater. Conversely, trees also
can affect groundwater levels, flow directions, discharge,
and recharge.
Why is this information important to the
phytoremediation of contaminated groundwater? The
hydrologic changes that indicate plant and groundwater
interactions can be observed and monitored by using both
plant-based and hydrology-based methods. When combined
with geochemical methods, such as the stable isotopes of
water, plant and groundwater interactions at contaminated
sites can be unequivocally demonstrated.

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