Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 317

Fig. 13.6 Concentrations of
benzene ( ● ), toluene ( ○ ),

▾

naphthalene ( ), and dissolved

oxygen (DO) ( ) in wells USGS-

A( top ) and USGS-C ( bottom ),
2001-2007, at the former MGP
site, Charleston, SC. Vertical
dashed lines represent calendar
year divisions.

□

concentrations collected in monitoring wells between 1997
and 2004, the concentrations remained high and stable; the
average maximum was 20,000

interceptor trench at this site assists in decreasing the con-
taminant migration potential of the dissolved phase

g/L. This elevated value is
in part due to the presence of coal-tar DNAPL at depth
below the trees. This caused the researchers to look at the
PAH concentrations at various depths in the shallow aquifer,
rather than total PAH, to determine the effect, if any, of the
trees on PAH groundwater contamination.
Samples were collected between 1998 and 2003. Sample
results indicate that after 2001, the concentrations of total
PAHs near the top of the water table had decreased
(Fig. 13.7 ). Conversely, PAH concentrations at greater
depths remained unchanged. Similar decreases were
observed in the ratio of naphthalene concentration to total
PAH (as TPAH) concentration with depth (Fig. 13.8 ).
Widdowson et al. (2005a) suggest that these concentration
decreases correlate with the interaction of the poplar tree
roots and the water table. Moreover, the installation of an

m contaminants to the creek.

Marr et al. (2006) studied the relation of the
phytoremediation plot at Oneida to the volatilization of
naphthalene as a loss mechanism from the contaminated
aquifer. The flux of naphthalene from the unsaturated zone
to the atmosphere was measured over the year, both when
the trees were actively removing groundwater and the water
table was lowered and when the trees were dormant and the
water table higher. Naphthalene flux was measured using a
flux chamber. The highest flux of naphthalene to the atmo-
sphere near 23 gm 2 /h was measured during the month of

August 2004. The higher flux was attributed to a thicker
unsaturated zone after the water table dropped about 1 m.
The case could have been made stronger for the
phytoremediation-induced increase in contaminant volatili-
zation between the water table drop and plants, however, by

m