Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 222

drop in groundwater level for plants grown in each type of
aquifer will yield different results for the amount of ground-
water transpired.

downgradient discharge of groundwater after planting is
decreased by an agreed-upon percentage of the average, pre-
planting upgradient flux of groundwater. Some examples of
this method exist at the Carswell Air Force Base site in Texas
(Eberts et al. 1999).

9.2.5 Groundwater Discharge

9.2.6 Soil Moisture

Phreatophytes also can affect the discharge of groundwater
in an aquifer. The change in groundwater discharge at
contaminated sites after planting is a contaminant- indepen-
dent process as it is a function of the local groundwater
hydrologic conditions, as long as the concentration of the
contaminant does not reach toxic levels. The discharge of a
substance can be defined as that quantity flowing across a
given area, perpendicular to flow direction, per unit time. In
aquifers, Darcy's Law, or Q

Moisture present in the unsaturated zone above the water
table provides an alternative source of water to plants, even
occasionally to those plants considered to be obligate
phreatophytes. When soil moisture levels decrease to
tensions that are equal to or greater than the wilting point,
deeper sources of water under less negative tension are
needed to sustain plant survival. As such, the monitoring of
soil moisture around plant roots above the water table can
provide important information for what component of the
total water transpired is derived from soil moisture during
wet periods and for when plants are using groundwater
during drier periods. Soil moisture does not indicate, how-
ever, the potential for water movement or flow direction.
Soil moisture can be determined using a variety of
approaches. Electrical resistivity methods, such as the
Beckman soil moisture meter, can be used. Soil moisture
also can be measured by time-domain reflectometry (TDR;
Campbell Scientific, Logan, Utah). Changes in soil moisture
using TDR were measured at the Air Force Plant 4, Fort
Worth, TX, phytoremediation site and related to input by
precipitation and removal by transpiration (Vose et al.
2000). Soil moisture was highest during the spring when
ET was lowest and immediately after precipitation events.
At a given time, soil moisture was lower within the planta-
tion relative to an open area populated by grasses and no
trees.
Soil moisture also can be measured using a neutron meter
or probes. Nnyamah and Black (1977) used neutron meters
and tensiometers to investigate the depletion of soil moisture
beneath thinned and unthinned forests that consisted of
Douglas fir for a period of 4 weeks during which no precipi-
tation occurred. Changes in soil moisture over the 4-week
period agreed well with total ET measurements made using
the energy-balance approach. During the drier period, water
movement into the root zone increased from about 8-15% of
the total water removed by ET . Conversely, less than 2% of
ET was removed from trunk storage.
The sediment hydraulic conductivity not only affects
infiltration of water downward but also the removal of
water upward by evapotranspiration. Given the same
conditions of sun, weather, and plants, ET will be high in
the areas where the hydraulic conductivity is high and low
where the hydraulic conductivity is low. As such, maximum
rates of transpiration are as equally controlled by the

( A )( i )( K) , can be used to
examine the effect of plants on groundwater discharge.
Prior to the addition of phreatophytes, groundwater entering
the area, Q up , would be equal to groundwater discharge from
the area, Q down , assuming no loss by evaporation or gain by
recharge. This condition may exist after young trees are
planted or during dormancy.
Groundwater discharge that exits the planted area can,
over time, become less than the amount that enters the
planted area, or Q up >

¼

Q down , for at least two reasons: first,
groundwater discharge must be conserved, and second,
groundwater levels will decline. It follows, therefore, that
the term Q in the Darcy flow equation is decreased as the size
of A decreases. Moreover, the rate of groundwater flow will
increase from upgradient areas as the water table drops in the
planted area (Eberts et al. 1999).
As indicated in the Darcy equation, few input parameters
are required and can be easily obtained. For example, wells
can be installed upgradient and downgradient of the planted
area. The cross-sectional area, A , of groundwater flow can be
estimated from well-construction data, such as depth, L , and
width, W , from the scale of the site. These wells could be
used to determine the hydraulic conductivity, K , of the
aquifer, as described in Chap. 6, such as using the Hvorslev
method (Fetter 1988). The hydraulic gradient can be calcu-
lated by dividing the vertical change in groundwater eleva-
tion between the two wells, D h , by the lateral separation

distance, i l .

The groundwater discharge method can provide an objec-
tive criterion to determine if and when hydrologic containment
or control occurs. Ideally, complete hydrologic containment
or control would be implemented when Q down ¼

¼ D h / D

0. This sce-
nario, however, is not realistic, and could only be met if
a trenchwere dug to the bottomof the aquifer and groundwater
discharged by a pump at rates that exceeded the flow into the
trench. At sites where this has occurred it typically has not
been successful (see Widdowson et al. 2005a for site history).
A more realistic objective criterion would be when the