Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 275

12

Chemical and Physical Properties That
Affect the Interaction Between Plants
and Contaminated Groundwater

Hidden from view, 30 rusty underground storage tanks were
slowly releasing the soluble fuel compound methyl tertiary-butyl
ether (MTBE) to the shallow water-table aquifer above the
wellfields of the City of Santa Monica, California. The city's
water utility was unaware of this invisible threat, and continued
to pump fresh, clean groundwater from deep below the increas-
ingly contaminated shallower aquifer. In the fall of 1995, how-
ever, the first molecules of MTBE were sucked up by the city's
pumps and were detected in the drinking water sent to the city's
permanent and tourist customers. In early 1996, the situation
worsened as increasing concentrations of MTBE were being
measured in the water supply. Eventually, the city had no choice
but to shut down its wellfields, and was forced to purchase
surface water from nearby Los Angeles. Water utility managers
do not expect to be able to use the previously reliable, deep
aquifer system again for many years .

subsurface need to be elucidated. Typically, contaminant
fate is described in terms of the degree of partitioning that
can occur between the contaminant and the major phases of
the subsurface, such as soil organic matter, lipids, water, and
air. This interaction is a function of the physical-chemical
properties of the contaminant, such as water solubility, lipid
solubility, vapor pressure, etc. Next, a determination needs
to be made regarding how the properties of the chemical will
interact with the system in question.
For the purposes of the phytoremediation of
contaminated groundwater, the plant is an additional path-
way of contaminant interaction. Plants can be considered
an extension of the other phases that will interact with a
particular contaminant, because plants offer phases that
include solid, organic matter, organic lipid, and water, as
well as gas phases. In addition, the fact that plants interact
with groundwater adds the additional components of life,
from the rhizosphere to the living tissues of the plants
themselves. As such, the total interactions between contam-
inant and subsurface environment have to include both
purely physical as well as biological interactions, and these
can be defined by contaminant partitioning processes.

This short summary of a true story received national media
attention in January 2000 on the TV show “60 min.”
Although this specific incident occurred in California, the
scenario could have occurred at any of the 400,000 leaky
USTs across the United States in which gasoline that
contained MTBE may have been stored. A leaky UST that
contains gasoline enhanced with the fuel oxygenate MTBE
can be accidentally released to the subsurface through cor-
rosion in the joints of underground piping. After escape from
the UST, gasoline as pure free product can migrate through
the pore spaces of the unsaturated zone under the influence
of gravity until it encounters the water-table surface. There,
the lighter specific gravity of the gasoline will cause it to
float on the water table as a separate phase. MTBE, and other
gasoline compounds such as benzene, will then partition
between the gasoline source itself and the air present in the
unsaturated zone above the free product, the water present in
the unsaturated zone as well as the water table, and the
organic matter present in the soil.
To understand the fate of a particular contaminant such as
MTBE, benzene, or chlorinated solvents in the subsurface,
the potential interactions between the contaminant and the
various physical and chemical components present in the

12.1 Contaminant Partitioning in the

Subsurface and Plant Uptake

In general, plants consist of three phases: organic lipids
and solids, water, and air. The unsaturated zone and most
groundwater also can consist of these three phases. The
additional interaction between groundwater, plants, and
xenobiotics can be described in terms of basic processes
such as advection, diffusion, sorption, and transformation
through metabolism that result from interactions between
these phases. These interactions often can be examined