Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 149

hydrogeologic properties of an aquifer. As depicted in Chap. 4,
the groundwater pumped from a well is derived initially
from stored groundwater that previously entered the well.
The time for this stored water to accumulate can be a few
days in low-permeability aquifers to a few minutes for wells
in high-permeability aquifers. Once stored water has been
removed, the groundwater level in the well declines to lower
levels than the groundwater level in the adjacent saturated
sediments of the aquifer outside the well. This scenario
creates a hydrologic gradient from the aquifer to the well,
and groundwater flows into the well.
The water level in the pumped well continues to decline
until the rate of groundwater that enters the well screen from
the aquifer is in equilibrium with the rate at which it is
pumped. The difference between the groundwater level in
the well before pumping and at any time after pumping is
called drawdown. The relation between pumping rate, time,
and groundwater-level drawdown reveals much information
about the hydrogeologic properties of an aquifer. The
groundwater level can be measured in the pumped well,
but usually it is more desirable to measure the groundwa-
ter-level response in a nearby unpumped monitoring well.
As discussed in Chap. 4, in the 1930s the first person to
link the relation among pumping, groundwater-level draw-
down, and aquifer properties for confined aquifers was
C.V. Theis of the USGS (Theis 1935). His equation shows
the relation between pumping, Q , and drawdown, s , over
time, t ,as

to occur into the well screen from upgradient areas and out
of the well to downgradient areas under ambient unpumped
conditions. The rate of groundwater flow can be determined
by using various approaches called flowmeter tests. Older
flowmeter methods used an impeller placed in the well and
the rotations around a fixed shaft were counted over time.
Newer flowmeter methods measure flow by using the rate of
the heat dispersal from a heat source placed into the well.

6.5.10 Ground-Penetrating Radar

As discussed in Chap. 3, the root systems of small or large
trees that can be installed at phytoremediation sites, or of
most plants in general, mostly are hidden from view. Roots
often are seen at erosive features, because the presence of
plant roots limits the further advance of erosion. However,
for most tree roots to be observed, such as when measuring
root penetration to the water table, takes considerable effort.
A common method involves collecting sediment cores in a
grid pattern near trees but away from the tree trunk, or bole,
and observing the presence of roots in the recovered core
material. Trenches can be dug alongside the tree to be
examined, and examples are described in case studies in
Chap. 8. Roots also can be unearthed by using water pressure
and lifting the entire plant from the ground, although this is
rarely done. Roots and other rhizosphere materials also are
viewed by using specially manufactured digital cameras,
called rhizotron cameras, that can be lowered down
boreholes in the root zone. Drawbacks to these methods
include time, expense, and the fact that the root tips and
hairs may be lost.
Geophysical techniques have advanced enough to allow
basic geophysics to be applied at many groundwater con-
tamination sites and, therefore, have some application to site
assessments for phytoremediation. They were developed
primarily for finding oil in subsurface strata in the early
1960s. It came to be recognized that in many instances,
these same technologies have applications for groundwater.
The most commonly used geophysical method that can be
applied to sites with contaminated groundwater where
phytoremediation is being assessed is ground-penetrating
radar.
The application of ground-penetrating radar (GPR) to
identify and map below-ground root structure and distribu-
tion provides a non-invasive technique to study roots. The
use of GPR for this purpose was investigated by Hruska et al.
(1999). The site studied consisted of humus-rich surficial
soils that graded to more loamy soils at depths less than
1 m from land surface where weathered bedrock was
encountered. The forest trees at the site included 50-years-
old oaks ( Quercus petraea (Mattusch.) Liebl.). The GPR
unit was run by two of the trees along straight transects,

T ¼ QW ð u Þ= 4 p s (6.1)

where the term W ( u ) is the well function and represents an
infinite series, and the value of W ( u ) is obtained from a table
of values of u . Theis developed this equation to describe the
flow of groundwater through confined porous media by
adapting existing equations used to explain the flow of
electrons through conductors.
The time required for an equilibrium to be established
between the pumping rate and groundwater level will be
different depending on the sediment composition of the
aquifer being pumped. For example, a coarse-sand aquifer
may reach equilibrium conditions much more rapidly than
an aquifer composed of fine silts and clays, because water
stored in a well completed in fine silts and clays takes longer
to accumulate than it does in a sandy aquifer.

6.5.9.2 Flowmeter Tests
An aquifer test and a slug test are conducted differently, but
they share the common requirement of artificially moving
groundwater through a well after measurement of the static
groundwater level. The static groundwater level, however, is
static only in the sense that the groundwater level is not
moving vertically. The flow of groundwater may continue