Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 135

The effect of plants on groundwater contaminants is covered
in Part III.
In general, phytoremediation to achieve hydrologic con-
trol will be most effective in aquifers characterized by a
depth to water table that is 30 ft or less. Water-table aquifers
tend to be the most susceptible to contamination, and
typically are the least cost-intensive to remediate using
phytoremediation. Phytoremediation has been used for
contaminated confined aquifers, with the remedial goal
focused on the decrease in contaminant concentrations
rather than to affect groundwater hydrology. The use of
phytoremediation for this special case of confined aquifers
is discussed in Chap. 14.

advantage by purchasing additional land downgradient of
groundwater flow and perhaps at a lower cost than the option
of more expensive site remediation. Moreover, a third party
can view the property boundary as a line that, if crossed by
groundwater contamination, can result in decreased property
values or, conversely, as a potential source of revenue from a
successful resource damage suit against the upgradient prop-
erty owner.
In any case, the importance of property boundaries and
groundwater contamination needs to be considered during
any site assessment and characterization of a site for
phytoremediation. At a minimum, the physical boundaries
of a site provide an upper limit on the size of a phytore-
mediation planting. This size will ultimately affect the quan-
tity of groundwater that can be hydrologically controlled by
trees as is described in Chaps. 7 and 8.

6.2.1 Reduction in Groundwater Flow Across
Property Boundaries

6.2.2 Reduction in Groundwater Flow
to Potential Receptors

Although groundwater flow occurs in the subsurface, its flow
may affect management decisions made with respect to the
land surface. Unlike surface water, such as rivers that pro-
vide a natural boundary between, for example, counties and
states, groundwater flow does not recognize these or many
other types of property boundaries. The relation between
groundwater flow and property boundaries becomes rele-
vant, however, when groundwater is contaminated. This
scenario has different implications for different people,
depending on the role that each person plays in groundwater
contamination sites. Much like the fable about the seven
blind men who each tried to determine what kind of animal
an elephant was although only touching a separate part, each
person who is involved as regulator, environmental consul-
tant, responsible party, or a third party often see this relation
between property boundaries and groundwater contamina-
tion from a unique perspective. For example, regulators are
concerned with the spread of dissolved-phase contaminants
in groundwater to off-site areas and may define a property
boundary as a line in the sand that the migration of ground-
water contamination will not be permitted to cross. This
viewpoint is supported with respect to notable cases of
third-party effects, such as occurred in Woburn, MA, where
industrial solvents seeped into groundwater and migrated
off site to reach downgradient municipal drinking-water
wells. This incident was described in the topic Civil Action
(Harr 1995).
On the other hand, an environmental consultant hired by
the responsible party may see a property boundary as a
location that legally permits the maximum extent that per-
missible levels of contamination can exist in groundwater;
such areas between a contaminant source and a property
boundary are called mixing zones, where natural attenuation
processes can be used to assimilate the contaminants. The site
owner sometimes can use the defined property boundary to

Potential receptors of groundwater contamination include
wells, lakes, streams, or springs. These features can be
located on the same property as the contaminant release or
maybe be located off site. A reduction in the flow of ground-
water to these receptors can be a goal of the property owner
who is concerned about future land use, of trustees responsi-
ble for the stewardship of public lands, or of regulators
tasked with the protection of groundwater and surface
water used for water supply and ecosystem resources, or
recreational use.
Groundwater flow was conceptualized in Chap. 4 at three
generalized physical scales—local, intermediate, and
regional. Some knowledge of the scale of groundwater
flow that might be expected at a contaminated site can be
obtained prior to a site visit by determining the location of
the site relative to the aerial or vertical extent of previously
delineated major aquifer systems. This information is avail-
able from most USGS Water Science Centers across the
United States. Because the root systems of the most com-
monly used phytoremediation plants rarely exceed 30 ft
(6-9 m) below land surface, most sites that will benefit
hydrologically from a phytoremediation system are those
sites characterized by contamination in the water table and,
therefore, represent predominately local flow systems. Local
groundwater flow can be connected, however, to deeper,
more intermediate and regional aquifer systems, depending
on the hydrogeological framework of the site.
The presence of a receptor, such as a lake, pond, or
wetland, at or near the contaminated site usually indicates
a location of shallow groundwater discharge. As long as
it can be documented that groundwater flow beneath
a surface-water body does not occur,

then most local