Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 310

source area. Some of these processes are the same that cause
rock to become weathered into soils. In fact, in some cases
the byproducts of contaminant weathering can become part
of the soil organic horizon through the humification pro-
cesses (Gregory et al. 2005), where they are irreversibly
bound, adsorbed or absorbed, to sediment and soils.
In a related process, the presence of plants also affects the
composition of soil horizons. This is caused by the release of
plant root material either through exudation, root sloughing,
or root turnover, which increase the sorptive capacity of the
O-horizon over time. This may seem as counterproductive at
phytoremediation sites, where trees are planted to remove
contamination but over time will act to reduce the efficiency
of uptake because contaminant uptake is decreased as
contaminant bioavailability decreases and soil sorption
increases. This decrease in bioavailability, however, actually
helps to accomplish the goal of site remediation, even
though the contaminants are not necessarily taken up inside
the plant.

Some of the common aromatic groundwater contaminants
and their physical and chemical properties related to plant
uptake are described in Table 13.1 .

Table 13.1 Physical and chemical properties of common aromatic
groundwater contaminants with importance to plant bioavailability
and phytoremediation.
Contaminant Water
solubility
(mg/L)

Henry's constant (Pa m 3 /
mol)/dimensionless (H/RT)

Log
K ow

Log
K oc

Benzene 1,780 2.13 1.5 557/0.22

Toluene 520 2.69 1.75 673/0.24

Ethylbenzene 152 3.15 2.94 854/0.35

m -Xylene 160 3.18 2.20 700/0.31

13.2.1 Plant Interaction and Uptake Pathways

Laboratory studies have indicated that both herbaceous and
woody plants can take up a variety of dissolved-phase petro-
leum hydrocarbons. Burken and Schnoor (1997) reported
that the herbicide atrazine was taken up and subsequently
metabolized by poplar trees ( Populus deltoides ). They
extended that work to investigate the uptake, translocation,
and volatilization of BTEX by poplar cuttings grown in
hydroponic solutions. These compounds share the character-
istic of many USEPA priority pollutants, that is, of an affin-
ity for the dissolved phase, even though they have high vapor
pressures. The log K ow for these compounds is, in general,
between 2 and 3.5 (Table 13.1 ).
Jordahl et al. (1997) investigated the influence of poplar
tree roots on the fate of BTEX compounds as a function of
increased microbial activity. As was discussed previously,
root zones are sites of increased microbial numbers. For
remedial purposes, however, the root zone needs to have
bacteria that contain enzyme systems that will degrade the
contaminant of interest. Jordahl et al. (1997) characterized
the microbial populations in the rhizosphere of a mature
hybrid poplar tree ( Populus deltoides x nigra DN-34 “Impe-
rial” Carolina) using the most probable number (MPN) tech-
nique on three soil samples taken in the root zone of poplar
trees and compared to the same for an adjacent corn field
with no tree roots. As would be expected from the “rhizo-
sphere effect,” there were more microbes associated with the
roots of poplar trees than with the controls.
What is more important in terms of contaminant remedi-
ation, however, is not just the numbers of bacteria but the
ability of these microbes to express genes to produce
the enzymes needed to degrade contaminants such as ben-
zene. More work in this area needs to be done to firmly
establish a positive relation between plants, root microbial
communities, and BTEX. It will then become possible to

13.2 Plant Interactions with Aromatic

Hydrocarbons: BTEX

All manmade petroleum hydrocarbons used for a wide range
of purposes are derived from fossil fuels such as crude oil or
coal deposits. These source materials are themselves com-
posed of the remains of mostly plants and animals alive up to
450 MYa, when global temperatures were uniformly warmer
and the continents more coalesced than today; after death,
they were buried by fluvial sediments that removed them
from oxygen and, therefore, slowed their decay back to CO 2
and water. These buried plant remains, after exposure to
geologic time and pressure, have produced the resources of
crude oil and coal that can be pumped or mined. These raw
products have to be modified, and crude oil is refined or
“cracked” to produce a wide range of products based on their
boiling point as the oil is heated. It is ironic, or perhaps
symmetrical, that phytoremediation can be used to clean
up groundwater contaminated by products refined from
the remains of ancient plants, many of which were
phreatophytes themselves.
A common product of the refinery process is aromatic
hydrocarbons composed of a ring of C

H bonds, with three
double bonds. Benzene is the most carcinogenic of its
homologues toluene, ethylbenzene, and the xylenes, but all
can cause cancers such as leukemia. Collectively these
compounds are called BTEX, short for benzene, toluene,
ethylbenzene, and xylenes. Because these compounds are
used in gasoline and it is widely distributed, it is a common
groundwater contaminant and, therefore, its presence drives
the need for remediation at many sites.