Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 319

Fig. 13.8 The preferential loss
of naphthalene from groundwater
over time relative to total PAHs
(as TPAH) present at different
locations at the phytoremediation
site in Tennessee (Modified from
Widdowson et al. 2005a).

observed, even though both plants had similar higher PAH
losses after the 2-year incubation. Nitrogen-cycle bacterial
populations increased by two orders of magnitude in the
planted versus unplanted pots. The number of PAH-
degrading bacteria, or the phenanthrene-degrading bacteria
used as surrogate, increased in the alfalfa-planted pots up to
seven times more than as measured in the controls. Interest-
ingly the number of PAH degraders present in the
contaminated soil grown with the reeds actually decreased.
The fact that PAH decreases were seen over time in these
pots may be because reed can transport oxygen to the root
zone, and this is sufficient alone to cause contaminant
remediation.
Chen et al. (2003) investigated the fate of pyrene in
the rhizosphere of tall fescue ( Festuca arundinacea ) and
switchgrass ( Panicum virgatum ). They added 50 mg/kg of
14 C-pyrene and cold (nonlabeled) pyrene to uncontaminated
soil which contained fescue and switchgrass. The fate of the
14 C-pyrene was traced in the soil, plants, and headspace.
They reported that for tall fescue, 37% and 30% of the
14 C-pyrene was mineralized to 14 CO 2 by the tall fescue and
switchgrass, respectively, relative to 4% mineralization in
the unplanted control. Radioactive 14 C that remained as
plant biomass was 8% and 5% for tall fescue and switch-
grass, respectively. Radioactive 14 C that remained in the soil
unaffected by mineralization was 58% and 55% for tall

fescue and switchgrass, respectively. In part, the high per-
centage of pyrene that remained in the soil can be explained
by the bound residue, probably to soil humic and fulvic
acids, that made it unavailable to plant uptake. The impor-
tance of this work is that pyrene concentrations decreased
more in planted versus unplanted treatments and that greater
than 30% of the loss was to CO 2 .
An interesting outgrowth of this and other similar studies
is the distribution of the pyrene in the bound soil residual
organic matter and the implications for contaminant fate
over time. Plants take up organic matter in the root zone
but also release living and dead organic matter to the soil.
Rhizospheric bacteria associated with this zone can help
degrade these root exudates as well as contaminants. The
contaminants also have absorption sites that can help immo-
bilize the contaminant. This stabilization alone is important,
even if such “aged” contaminant is less bioavailable to plant
uptake or microbial mineralization. Guthrie et al. (1999)
indicated that 13 C-pyrene added to soils remained in the
soil humic fraction without undergoing mineralization.
Schwab and Banks (1994) also reported in a laboratory
study the increased loss of PAHs in contaminated soil rela-
tive to unplanted soils. This loss was attributed to the 7-200
times greater number of microbes in the planted soils rela-
tive to unplanted soils. Most importantly, they added 14 C-
pyrene to soils with and without surrogate plant exudates,