Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 340

sites since 1999. It could indicate that responsible parties did
not invest after government initiation of the project, or other
remedial activities were selected or mandated by the regu-
latory agency involved in the project. A low interest in private
investment into phytoremediation at some of these sites also
could have occurred because of a real or perceived lack of
return on capital. Also, at some sites there may be too much
risk involved with environmental variables, such as droughts.
The ever-changing political and environmental arena, how-
ever, may provide additional reasons to increase the involve-
ment of research into new phytoremediation projects. These
incentives include the use of vegetation to achieve carbon
sequestration goals as well as remedial goals; the potential
for cash flows from biomass generation; or as sources of
carbon credit income. The limitation here is that in most
cases phytoremediation sites will be less than 5 acres in size.

Fig. 13.23 The distribution of the types of contaminants being treated
by phytoremediation (Number of sites, with percentage of total sites).

briefly depicted in Chap. 11. After 1995, however, the
majority of sites where phytoremediation was attempted
contained chlorinated solvents and petroleum hydrocarbons
in groundwater and soils (U.S. Environmental Protection
Agency 2005b). This reflects the priority pollutant nature
of these compounds, their toxic effects at low concentra-
tions, and the scientific communities' intensive research into
these compounds with respect to natural attenuation pro-
cesses. An additional class of compounds that received
much interest was the petroleum hydrocarbons released
at AST and UST sites (U.S. Environmental Protection
Agency 2005b)—no data were collected, however, on
these contaminants. Moreover, at some of the sites under
Superfund authority, phytoremediation as a remedial strat-
egy was written in the Record of Decision (ROD) for
the site.
Since the late 1990s, the number of reported new sites has
declined each year according to the USEPA survey
(U.S. Environmental Protection Agency 2005b) (Fig. 13.20 ).
There are many possible explanations of this trend. First, the
lull could be the discontinuation of government-sponsored

13.10 Summary

The plant-uptake of commonly detected groundwater
contaminants is a reproducible fact based on a wide range
of laboratory- and field-based studies. Easy-to-use concep-
tual models, such as TSCF and RCF , have been developed to
guide our understanding of these interactions. At its most
fundamental level, the degree of interaction between plants
and groundwater contaminants is a function of the prevailing
chemical and physical properties of the contaminants and the
subsurface.
Why is this information important to the phytoremediation
of contaminated groundwater? Plants can control the move-
ment of contaminant solutes through their structures by affect-
ing the rate of transpiration at the leaf surface and by the initial
movement of water into root hairs in the subsurface. If a
contaminant does not enter into at least one of the pathways
discussed in this chapter, then that particular contaminant may
not be amenable to phytoremediation.