Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 307

13

Plant Control on the Fate of Common
Groundwater Contaminants

When they came to Marah, they could not drink the water
because it was bitter

fate of a wide variety contaminants in groundwater from
point sources. Such contaminants investigated include
chlorinated solvents, petroleum hydrocarbons, tritium, and
heavy metals.
The interaction between plants, groundwater, and
contaminants from either type of source depends in most
part on the relation between the abiological, physical pro-
cesses that affect contaminant concentrations, and the rate of
water flow through plants. For example, the uptake of water
by root hairs is controlled by the physical properties of
water, such as surface tension and capillarity. Uptake of
dissolved solutes, such as contaminants, by root hairs also
is controlled by the physical properties of the contaminant,
such as water solubility and log K ow .
Plants can control the movement of contaminant solutes
through their structures, however, by affecting the rate of
evaporation at the leaf surface and by the initial movement
of water into root hairs in the subsurface. The only part of the
interaction between plants and contaminants that is truly
plant based, in the sense that it is not entirely related to
physical or chemical phenomenon, is the cellular detoxifica-
tion of xenobiotics discussed in Chap. 12. Even the root
zone microbial degradation processes are predominantly
ex-situ processes, more akin to bioremediation than
phytoremediation.
A potential framework to address the interaction between
plants and contaminated groundwater can be based on
whether or not a particular contaminant fits into one of the
following biogeochemical pathways:

...
and the Lord showed him (Moses) a tree,
and he threw it into the water,
And the water became sweet.

Exodus 15: 23-25 (RSV)

In general, groundwater can become contaminated by two
different processes; contaminant release from nonpoint and
point sources. Nonpoint-source groundwater contamination
reflects the widespread release of contaminants from sources
that are dispersed throughout an area or cannot be attributed
to an identifiable location. Such nonpoint sources include
runoff or atmospheric deposition. Between 1985 and 2001,
the USGS National Water-Quality Assessment (NAWQA)
Program analyzed about 3,500 samples of groundwater col-
lected throughout the United States. As part of that study, it
was determined that almost 20% of the ambient groundwater
samples contained 0.2

m

g/L or greater of one or more of
55 volatile organic compounds (VOCs) analyzed (Zogorski
et al. 2006). The most frequently detected VOCs were
trihalomethanes (THMs), such as chloroform, and the
chlorinated solvents PCE and TCE. In some specific areas of
the United States, VOCs such as the fuel oxygenate MTBE,
the fumigant and gasoline additive ethylene dibromide (EDB),
and the soil fumigant dibromochloropropane (DBCP) also
were detected. Encouragingly, however, 90% of the ground-
water samples analyzed had VOCs concentrations less than
1 m g/L. Although some regulated compounds haveMCLs near

1 g/L, such as EDB, and vinyl chloride (VC), the low con-

centration detected for most compounds indicates either the
lack of a constant contaminant source or the cleansing effects
of natural attenuation processes.
In contrast to nonpoint source releases to groundwater,
point-source releases have an identifiable, and often
regulated, release location. Unregulated point sources
occur at spills or accidental releases. For these sources,
contaminants in groundwater often are found at percent-
level concentrations. In the United States, the USGS Toxic
Substances Hydrology Program specifically investigates the

m

• Flowing through one of the Phase I to III detoxification
processes
• Flowing through the plant by transpiration and evapora-
tion, and
• A physical partition into plant tissue

If a contaminant does not enter into at least one of the
above pathways, then that particular contaminant may not be
amenable to phytoremediation. A framework to determine