Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 148

Alternatively, trees in humid areas may rely on precipitation
as much as on groundwater to meet ET demands, so shallow
depths to groundwater may not have a positive effect on
phytoremediation of contaminated groundwater. Even
though tree growth at a site may be successful and the trees
documented to transpire water, very little of the transpired
water will consist of groundwater. On the other hand, the
water table may be too deep, as in arid areas or in higher,
well-drained elevations in humid areas, to support the rapid
establishment of trees at the surface installed by using con-
ventional methods, as is discussed in Chap. 7.
A metric that can be used in evaluating phytoremediation
projects is the mean depth to high water table calculated
from seasonal groundwater-level fluctuations. This mean
depth is affected by additional factors, such as tides or
changes in barometric pressure that can be accounted for
by using equations discussed in Chap. 9.
The above problems notwithstanding, phytoremediation
for hydrologic containment or control of contaminated
groundwater generally tends to be applicable and cost effec-
tive in areas where the depth to water table below land surface
is 30 ft (9 m) or less. This would indicate that phytore-
mediation tends to be restricted to shallow aquifer systems. It
is exactly these shallow aquifer systems, however, that are
most vulnerable to contamination because of the proximity to
land surface. Deeper groundwater can be candidates for
phytoremediation for hydrologic control, however, by using
a variety of approaches that are beyond the scope of this topic.
In brief, these approaches include installation of a phytore-
mediation planting in recharge or discharge areas of deeper,
more regional aquifers where groundwater flow is at or near
the surface of the ground. Moreover, even shallow and deeply
confined contaminated aquifers can be accessed by plants,
either under natural or engineered conditions; some examples
are provided in Chap. 8.

dissolved-phase plume in groundwater may reveal a poten-
tial source area using the direction of groundwater flow.

6.5.9 Hydraulic Conductivity and Aquifer Tests

Many methods can be used to measure the hydraulic con-
ductivity, K , of aquifer sediments. The magnitude of hydrau-
lic conductivity is the rate-limiting step that controls the
occurrence of groundwater flow and the potential uptake
rate of groundwater by plants from the unsaturated or
saturated zones. The easiest but least accurate method to
estimate hydraulic conductivity is by using grain-size analy-
sis. Problems of accuracy of grain-size analysis are related to
the assessment being performed on a sample that is not in its
original setting and was disturbed during collection. Another
method that also is used on disturbed sediments and water
are the tests done in the laboratory where the hydraulic
conductivity is determined from the rate that water moves
through a vertical column of saturated sediment.
Tests for hydraulic conductivity done in the field provide
more accurate results, although they require more time to
perform properly. A common method used is the single-well
slug test, in which a volume of water is added to a monitor-
ing well and the rate that the affected water level returns to
static conditions provides an estimate of K . However, a slug
test can sometimes provide a K value of the sand filter
material used to pack the well screen, rather than the K of
the aquifer sediments. These differences in K generally show
up in the groundwater-level-change data plotted over the
time of the test and result in two distinct slopes of change
with respect to time. For example, the slope of the initial
water-level change typically is the K of the well-screen filter
pack material, and the latter slope of water-level change
represents the aquifer hydraulic conductivity.
If water cannot be added to a well to conduct a slug test,
as often is the case at a contaminated site, an artificial slug of
equal volume made from a pipe or other material can be used
to displace the water in the well and then rapidly removed.
The limitation with both slug-test methods is that K is
determined for the immediate area around the well only;
slug tests in many wells need to be conducted to assess the
distribution of K at depth across the site.

6.5.8 Groundwater-Flow Direction

Groundwater flow that occurs at a contaminated site is a
vector of potential dissolved-phase contaminant transport.
This is because many contaminants released to groundwater
are soluble in water, and groundwater in porous media
moves under the influence of the hydraulic head gradient,
as demonstrated by Darcy's Law in Chap. 4. Because
groundwater flow is relatively slow compared to the flow
of surface water, individual particles of water tend to move
by laminar rather than turbulent flow. If the direction of
groundwater flow can be determined, such as by taking
water-level measurements in monitoring wells, the compass
direction of contaminant transport can be assessed with
a high degree of certainty. Conversely,

6.5.9.1 Aquifer Tests
In order to determine a hydraulic conductivity value for an
entire site more efficiently than using slug tests on individual
wells, an aquifer test can be done. Because a well creates an
artificial zone of 100% porosity in an aquifer and is open to
the atmosphere, a nonpumped well can induce groundwater
flow toward it. Under pumped conditions, the removal of
groundwater from a well is controlled by many factors that
can be used in a diagnostic manner to understand the

location of a