Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 171

bronze birch borer, which enters the top of trees and then
feeds on the sap contained in the phloem. To guard against
such infestation, the European white birch ( Betula alba or
Betula pendula ) can be used only in areas where
temperatures go down to 20 F. Such problems with birch
borers are less likely with the Betula nigra , which is native to
the United States as it is more resistant to attack from the
borers than the birch imported from Europe.

area where the fuel tanks previously were located. These
trees were not planted as deeply, only about 4 ft (1.2 m). As
of August 2006, this site was being monitored (Van Epps
2006). Unfortunately, little specific hydrologic data is avail-
able regarding the effect of the planted trees on recharge
reduction. However, data do exist regarding the effect of the
trees on groundwater contamination and are described in
Chap. 13.
Another instance of using transpiration to reduce
recharge through a contaminated area was investigated at
an industrial-waste site in South Africa (Duthe et al. 2005).
The site is located inland from the Indian Ocean. Most of the
annual precipitation occurs during the winter months, with
drier conditions prevailing during the summer. Potential
evapotranspiration also is high during the winter and lower
during the summer and greater than precipitation. An area
greater than 30 ha has been contaminated by chlorinated
solvents and total dissolved solids-materials disposed there
between 1958 and 1994. To decrease potential leachate
formation and additional groundwater contamination,
hydrologic control of the source area was proposed through
a variety of methods, such as mechanical pumping of
groundwater to dewater the contaminated area, control of
surface-water runoff through a drainage network, and reduc-
tion in recharge by plant transpiration. Simulations made by
using the finite element numerical model HYDRUS-1D
(Simunek et al. 2005) indicate that the addition of up to
1,600 plants in the contaminated area would result in limit-
ing recharge to less than 0.3% of annual precipitation (Duthe
et al. 2005).
A typical planting layout to achieve similar recharge
reduction at a fuel-contaminated shallow aquifer is depicted
in Fig. 7.5 (Cook et al. 2010). The trees installed were four
types of hybrid poplar and willow. The trees were planted as
cuttings on 5-ft (1.5 m) centers. No artificial irrigation was
used. Tree mortality was high across the site, and the mor-
tality caused primarily by installing cuttings in contaminated
areas without the addition of clean backfill. Subsequent
reinstallation of cuttings in contaminated areas using clean
backfill decreased mortality. About 1-year after planting, the
cuttings had grown greater than 10-ft (Fig. 7.6 ). The study
concluded that large boreholes backfilled with clean soil and
planting prior to the summer increased the success of
installation.

7.2.3 Recharge-Reduction Design

Plants can be installed with the goal of reducing groundwa-
ter recharge, which can be accomplished in two ways. First,
plants can be used to intercept precipitation and remove it by
evaporation before it becomes infiltration. Second, plant
roots can be used to remove soil moisture and infiltrating
water by transpiration before recharge can occur. Moreover,
the root systems of plants selected to decrease recharge do
not necessarily have to reach the water table; the plants used
can be facultative phreatophytes or even drought-tolerant
grasses. These processes behave in a manner similar to
vegetative caps that have been successfully installed at
older landfill sites.
Whereas plants have been added to decrease recharge at a
number of older, closed landfill sites, there are fewer
examples of this practice for contaminated groundwater. A
study where a primary goal was the planting of trees over a
source area to reduce recharge to the water table occurred
near Milwaukee, WI (McLinn et al. 2001). The site was
a former fuel-tank farm adjacent to the Menomonee
River. Because of the geologic history of this area of the
United States, the shallow aquifer at the site is composed of
low-permeability materials, called till, left behind after the
last glacial retreat, to a depth of about 18 ft (5.4 m). Because
of the site activities, the soil and groundwater were
contaminated by petroleum hydrocarbons, including free-
phase product, with some product trapped in the source-
area sediments above the water table. A phytoremediation
system was designed for the site to decrease recharge to the
water table by planting trees to sequester infiltration. In
2000, following extensive site preparation using some of
the approaches described here, 485 hybrid poplar trees
(Imperial Carolina deltoides x nigra ) were planted at the
site. Of the 485 trees planted, 290 were planted in a row
adjacent to a river near the downgradient boundary of the
site. These 290 trees were installed using a hollow-stem
auger to a depth of 9 ft (2.7 m) to have the roots as close
as possible to the water table. In order to ensure that oxygen
in the unsaturated zone was not limiting for root respiration,
an air-injection aeration system was installed during plant-
ing; the air-injection system was discontinued in 2003, how-
ever. Additionally, up to 195 trees were planted in the source

7.2.4 Hydrologic-Barrier Design

At some sites it may be impracticable to control groundwater
hydrology by using plants to reduce recharge because the
source area may be beneath a building or in an area that is
not suitable for planting. In these cases, plants can be used to
control the flow of groundwater that may already be