Environmental Engineering Reference
In-Depth Information
and for the most soluble PAH compound naphthalene, up to
1,000 mg/L (Landmeyer et al. 1998a) are associated with
groundwater in contact with the free-phase form of these
wastes, known as DNAPL, or dense, non-aqueous phase
liquid.
Due to the low groundwater-flow rate and occurrence of
significant microbial biodegradation of these soluble
compounds at the site (Landmeyer et al. 1998a), dissolved-
phase plumes are limited in size to halos around discrete
sources of DNAPL. Many of these discrete DNAPL sources
have been identified and either have been removed by exca-
vation or are being pumped near gravel-lined French drains
installed at the site.
The contaminated groundwater is characterized by low,
and metals (Forget and Zayed 1995). At the study site, cores
were collected at a height of 1 ft (0.3 m) above ground on the
southern side of each tree. Replicate cores taken about 2 in.
(5 cm) apart were collected at each tree.
Results indicate the presence of benzene, toluene, and
naphthalene (as well as other coal-tar related compounds,
such as styrene) in various tree tissues sampled throughout
the planted area in March 2002 (Table
13.2
) that are also
present in the underlying groundwater. These data suggests
that the poplar trees are taking up contaminated groundwater
as part of the transpiration stream during the time of
sampling.
To relate changes in groundwater-level fluctuations with
tree uptake of contaminated groundwater, the flow of water
through representative hybrid poplar trees was estimated
with a Dynagage sap-flow meter (Fig.
13.4
; Flow32,
Dynamax, Houston, TX). The Dynagage is used to heat the
water in a tree using an imprinted circuit to which a constant
low voltage (less than 5 mV) is applied; radial heat losses to
the atmosphere and vertical heat losses are reduced with a
Styrofoam wrap and heat from incident radiation is reduced
by covering the gage with reflective tin foil. The sap-flow
system was run for 1-week intervals during the summer
months, powered by a marine battery. Data on the flow rate
of water in the tree are reported in grams per hour (g/h), as
induced by a change in temperature of the water in the tree,
D
<
1 mg/L, levels of dissolved oxygen, and the redox compo-
sition is dominated by sulfate reduction, with low sulfate and
high sulfide concentrations, and methanogenic conditions
(Landmeyer et al. 1998a). Although groundwater on the
western, inland side of the site is relatively fresh, groundwa-
ter is more saline, being
>
1,000 microsiemens per centime-
ter (
S/cm), as it interacts with the more saline water from
the Cooper River.
As stated in Chap. 8, hybrid poplar trees were planted at
the site in two phases; phase one occurred in November 1998
and included the central to western part of the area along the
southern boundary of the site, and phase two occurred in
May 2000, in the remaining eastern part of the site. Poplar
tree-tissue samples were obtained using an increment borer
(Sunnuto Corporation) between 1999 and 2005 from trees
growing in the area delineated by dissolved-phase ground-
water contamination. Such tree coring methods have been
used previously at other sites to determine the presence of a
variety of chemicals in tree rings, such as chlorinated
solvents (Vroblesky et al. 1999a), petroleum hydrocarbons
related to fuel-oxygenated gasoline (Landmeyer et al. 2000),
m
T
, between a heated and unheated reference area on the
Dynagage.
Sap flow measurements were made on trees growing next
to a series of monitoring wells. For example, a monitoring
well (A3-T1) is located upgradient of the planted area on the
eastern side of the phytoremediation plot. In this well,
concentrations of benzene and naphthalene were greater
than 6 mg/L, and less than 2 mg/L for toluene prior to
2001 (Fig.
13.5
), suggesting a source of DNAPL some
Table 13.2
Detection and relative concentrations, in parts per billion by volume, of benzene, toluene, naphthalene, and other organic compounds
in the headspace of vials containing tree materials from the phytoremediation site at a former MGP near Charleston, SC, March 2002.
Tree cored
Benzene
Toluene
Ethylbenzene
Xylenes
Naphthalene
Styrene
1
19.2
6.4
<
20.0
21.1
<
20.0
12.4
2
20.2
6.2
<
20.0
23.2
<
20.0
108
3
23.5
6.8
<
20.0
29.9
<
20.0
-
4
20.1
5.8
<
20.0
23.7
<
20.0
-
5
21.7
6
<
20.0
24.3
<
20.0
-
6
22.5
5.9
<
20.0
14.3
<
20.0
1,600
7
16.2
5.1
<
20.0
20.3
<
20.0
47.6
8
17.6
5.7
<
20.0
22.7
<
20.0
12.5
10
17.8
6.6
<
20.0
22.3
<
20.0
-
11
20.4
6.2
<
20.0
26.6
<
20.0
-
12
20.6
5.9
<
20.0
22.9
<
20.0
-
13
20.0 -
All results are in parts per billion by volume (ppbv), - not detected,
<
less than, Method detection levels were 10 ppbv (benzene); 5 ppbv (toluene);
20 ppbv (ethylbenzene); 10 ppbv (xylenes); 20 ppbv (naphthalene); 10 ppbv (styrene)
14.6
5.1
<
20.0
17.5
<
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