Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 147

the ground. No cuttings are generated. The borehole created
can be used for placement of a monitoring well, some which
can be pre-assembled for rapid installation. Alternatively,
groundwater samples can be taken during direct-pushing
through a slotted or screened metal pipe attached to the
end of the first drill rod. In this manner, groundwater can
be collected rapidly and analyzed for various contaminants
on site by using appropriate field-laboratory equipment. A
variety of probes can be added to the end of the drill rod such
as electrical resistivity probes and sensors that can detect
VOCs. Moreover, aquifer properties such as hydraulic con-
ductivity can be assessed using direct-push rods and tradi-
tional slug testing. This technology has made site assessment
and characterization almost real-time activities.

contamination at one depth in the aquifer can spread to
shallower or deeper parts of the aquifer, which previously
were uncontaminated. For this reason, most state or federal
regulations require the part of the well above the water table
to be sealed with an impermeable material, such as grout or
cement, that impedes such short-circuit flow, and with
appropriate filter pack sediments near the well screen. A
poured-concrete base at land surface also helps prohibit the
entry of surficial contaminants to depth.
Accurate measurement of the depth to and fluctuation of
the water table across the total suspected contaminated area
of a site is warranted, because a uniform depth to the water
table may not indicate uniform contaminant flow beneath the
site. For example, recharge through uncontaminated parts of
the site will deflect horizontal groundwater-flow paths, and
any dissolved contamination in these flow paths will be
pushed deeper below the water-table surface. A consequence
for a phytoremediation project planted in such an area where
the contaminated groundwater-flow paths are deflected
would be that roots would interact with a water table that
is not, however, contaminated.
The depth to the water table can be determined rapidly in
monitoring wells by measuring the depth using either a steel
tape or electronic water-level meter. In areas where few or
no monitoring wells exist, such as at uninvestigated sites, the
hand-auger method can be used to determine an approximate
depth to groundwater, where practicable. Where the water
table is shallow, a steel pipe or steel pipe with slotted screen
called a drive point, can be driven into the ground and used
as a temporary well to measure the groundwater level.
Finally, a series of monitoring wells installed with screens
located at increasing depths below land surface, or nested
wells, should be added at a phytoremediation site in order to
document the presence of vertical gradients or the induce-
ment of vertical gradients by tree uptake of water. This
approach is discussed in Chap. 9.
The depth to groundwater ultimately is the major factor
that controls the success of the installation of a phytore-
mediation system to hydrologically contain and (or) control
contaminated groundwater. In most cases, the shallower the
depths to the water table the better for phytoremediation
establishment and hydrologic interaction. However, there
are exceptions to this rule. For example, because the water
table is a planar surface that moves up and down in response
to the balance between input by precipitation and removal by
ET , its depth below land surface is not constant. In fact, the
depth to the water table can vary considerably over a year,
especially in areas containing natural vegetation, or at sites
near tidally influenced surface-water bodies. An implication
of these groundwater fluctuations for phytoremediation
systems is that trees initially established with a root mass
above the water table can die if inundated by a rising water
table for long periods, which varies from species to species.

6.5.7 Monitoring-Well Installation and Depth
to Groundwater

Surface-water features, such as streams, ponds, or lakes, can
provide an indication of the general depth to groundwater at
most sites. This is because most surface-water systems
receive discharge from groundwater, especially between
precipitation events, as discussed in Chap. 4. For the
purposes of site assessment and characterization activities,
however, a more controlled method of documenting the
depth to water table and its fluctuation is required. This can
be accomplished by installing monitoring wells.
The construction of the ideal monitoring well to deter-
mine the groundwater level in porous media would be simi-
lar to the manometer Darcy used in his laboratory-column
experiments (Darcy 1856). That is, a hollow tube open only
at the end so that the water level inside the tube above the
open end represents the pressure head at that open point. The
pressure head elevation added to the elevation of the open
end above a common datum would represent the total pres-
sure head (see Fig. 4.6). In practice, however, monitoring
wells tend to have an open, or more likely a screened interval
that can be 5-10 ft (1.5-3 m) in length. The larger open or
screened interval is used by convention in unconsolidated
sediments because it integrates groundwater across a larger
part of the aquifer, permits groundwater samples to be col-
lected even as the groundwater level changes, and permits
easier removal of groundwater samples.
Because the installation of a monitoring well requires a
borehole to be created, the presence of the borehole and,
subsequently screened interval, results in an artificial condi-
tion that permits vertical groundwater flow to occur within
the saturated zone that previously did not occur. If not
properly sealed, surficial contaminants can enter the aquifer
by downward leakage. In fact, a poorly installed monitoring
well can short circuit normal infiltration. This vertical trans-
fer of contaminants also can occur below ground, where