Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 145

need to be investigated. Phytoremediation could still be used
but would require more costly deep-planting techniques,
such as trees installed in vertical holes using Tree-wells ® ,
or the installation of an irrigation system.
Some of the methods that can be used to assess the
hydrogeologic properties of a site for possible phytore-
mediation are similar to conventional groundwater remedial
strategies. One such approach applied to achieve hydrologic
containment and (or) control has been and continues to be
the mechanical pump-and-treat approach. In brief, the
pump-and-treat approach is based on the installation of
relatively large-diameter wells that are screened or open to
the part of the aquifer that is contaminated, and the ground-
water is pumped and treated to acceptable levels prior to
discharge, usually into the sanitary sewer system. At best,
experience from many sites undergoing pump-and-treat
remediation indicates that a time period on the order of
decades is necessary to remove enough pore volumes of
groundwater to decrease contaminant levels to acceptable
levels (National Research Council 1994), and that the oper-
ation and maintenance costs associated with such long-term
remediation are prohibitive. At worst, the wrong placement
of the wells used for pumping, such as near property
boundaries downgradient of the source area, can accelerate
the spread of contaminants to previously uncontaminated
areas. Finally, this approach of using groundwater as a
vector to remove contaminants would have obvious
limitations at sites characterized by aquifers of low transmis-
sivity and low specific yield.

hydrogeology. Although locations of such groundwater dis-
charge areas generally are depicted on maps, their effect on
local groundwater-flow paths, flow directions, and site-
specific water budgets usually are not clear at this level of
investigation. For example, in many cases the elevation of
these surface-water features may not have been determined.
This lack of information makes an a priori determination
difficult as to whether a surface-water feature is, indeed, a
site of localized groundwater discharge or recharge. In other
cases, the water level of the surface-water body may be
known and shown on maps in site reports, but how contours
of equal groundwater levels interact with the surface-water
body may not be depicted or known.

6.5.2 Depth to Groundwater, the Unsaturated
Zone, and Infiltration

In general, shallow depth to the water table provides many
advantages for the hydrologic control of groundwater by
phytoremediation. When the water table is shallow, plant
installation is relatively easy, and growth is more rapid. This
is because groundwater quickly becomes the primary water
source of the plants. However, a shallow water table also has
disadvantages for phytoremediation. One disadvantage is
that many phreatophytes are facultative and can use sources
of water other than the groundwater that needs to be
controlled.
This does not mean that sites where groundwater is at a
considerable depth cannot be controlled successfully by
using phytoremediation. At such sites, typically in arid
areas or in humid areas characterized by porous sediment,
the effects of plants and plant selection on the site hydrology
may be more important in terms of water removal from the
capillary fringe, a reduction in leachate formation by inter-
ception of infrequent precipitation, or induction of the
upward movement of groundwater toward the land surface.
Between these two water-table extremes are the cases of
deeper semiconfined or confined aquifers.
The characteristics of the components of the unsaturated
zone at a site can be evaluated without collecting sediment
samples, but by observation of what happens to rainwater
during or after precipitation. Areas of standing water after
precipitation often indicate a low infiltration rate. Such
'recharge refusal' typically occurs in areas subject to com-
paction by foot or other heavier traffic. Too much compac-
tion leads to runoff or ponded water. In the latter case, the
water eventually infiltrates to the unsaturated zone and water
table relative to the properties of the soils, such as porosity
and permeability or hydraulic conductivity. In general, slow
rates of infiltration range from 0.2 in./h to less than 0.05 in./h
(0.5-0.12 cm/h), whereas rates above 5 in./h (12.7 cm/h) are

6.5.1 Groundwater System Concept

A common mistake made during site-assessment activities is
to focus on the hydrologic conditions within the site-specific
boundaries to the exclusion of that of the surrounding areas.
Many sites assessed are only a few acres or less. Before
site-specific characteristics are evaluated for the potential
application of phytoremediation for hydrologic control, the
site-specific conditions should be evaluated in context with
the overall regional hydrogeologic framework, of which the
site conditions are but a small part. For example, the
locations of regional groundwater recharge and discharge
areas must be evaluated in relation to the site location. It is
possible that the site may be located in the recharge area of
an aquifer used as a sole source of drinking water farther
downgradient where the aquifer has become confined.
Information on regional groundwater systems can be
found in either previous site-investigation reports, or in
more generalized publications about the study area. Potential
groundwater discharge areas that can receive contaminated
groundwater, such as lakes, ponds, streams, and wetlands,
must be evaluated in terms of their effects on local site