Conceptual Frameworks for Phytoremediation to Achieve Hydrologic Goals - Introduction to Phytoremediation of Contaminated Groundwater - page 189

8

Conceptual Frameworks for Phytoremediation
to Achieve Hydrologic Goals

water budget in at least two ways. First, the plants will
decrease the amount of recharge by precipitation. Second,
the plants will increase the amount of one of the outputs,
such as discharge by transpiration from the subsurface. Both
processes lead to a change in the amount of water stored or
released from groundwater.
When a site is to be evaluated for phytoremediation to
control or contain groundwater, various approaches can be
used to estimate the influence of plants on the subsurface
water resource. It is best to start with simple site-assessment
approaches and proceed to more complicated approaches if
necessary. Use of multiple approaches will decrease the
level of uncertainty inherent to each approach. The follow-
ing approaches that can be used for site assessment include
the evaporation rate of water from free, exposed surfaces;
the removal of water from plants caused by meteorological
parameters; changes in groundwater levels caused by tran-
spiration; and tank studies, as described below.

A logical application of the naturally occurring plant and
groundwater interactions elucidated in Part I is at sites
characterized by contaminated groundwater. The data col-
lected during the site-assessment and characterization
approaches described in Chap. 6 can be used also to assess
whether a plant-based system can achieve the three major
hydrologic goals at a given site. Because this assessment is
being made in the context of state and Federal requirements
to protect and restore groundwater resources, the use of
objective approaches to evaluate the benefits of plant and
groundwater interactions is warranted. Objective approaches,
also called frameworks, that can be used are offered in
this chapter.
The primary approach, or framework, to assess plant and
groundwater interactions presented in this chapter is based
on water budgets. At its most basic level, a water-budget
framework states that if groundwater flow is fast relative to
the removal of groundwater by transpiration, then hydro-
logic control will not be achieved. Conversely, if groundwa-
ter flow is slow relative to the removal of groundwater by
transpiration, then some degree of hydrologic control may
be achieved. In both cases, however, complete hydrologic
control by plants may not be reasonable. Because a water-
budget approach may not be useful at all sites characterized
by contaminated groundwater, alternative frameworks also
are discussed.

8.1.1 Free-Surface Water Evaporation

As described previously, the solar energy input to an area
changes liquid water to vapor. This energy amount is fixed
for a particular geographic area, although it varies within an
area with the seasons. Between 540 and 590 cal are required
to evaporate 1 g of water at 15 C. Because less than
400 cal/cm 2 are available on a clear sunny day, the maxi-
mum amount of water that can be evaporated from the
surface of water is equivalent to about 6 mm/day (Kozlowski
and Pallardy 1997). The point here is that the transpiration
rate cannot exceed this evaporation rate, unless drier air is
continuously advected over a planted area.
The outflow of water at a site by evaporation is, therefore,
an excellent indicator of the maximum potential for water
to be evaporated through transpiration and, therefore, at a
phytoremediation site. In general, the free-surface water
evaporation rate or index will be about 70% of the annual

8.1 Initial Approaches to Assessment

For a given basin, the amount of water that enters through
all compartments will equal the amount that exits through
all compartments, assuming steady-state, or long-term, aver-
age conditions. This equality of water was discussed in
Chap. 2 as part of the hydrologic cycle. At a site char-
acterized by contaminated groundwater, the installation of
plants that interact with groundwater will affect the site's