Environmental Engineering Reference
In-Depth Information
The organic matter, such as gasoline or other fuels, exerts a
demand on DO and often leads to anoxic groundwater. This
scenario does not mean that once contaminated, groundwa-
ter is isolated from additional sources of oxygen. As shown
in Fig. 6.1 , groundwater in a shallow gasoline-contaminated
aquifer rendered anoxic by contamination received DO dur-
ing recharge and was observed by using a DO-sensor placed
in a monitoring well (Landmeyer and Bradley 2003). After
recharge, DO concentrations returned to pre-precipitation
conditions because of oxygen demands in the aquifer exerted
by aerobic bacteria or the chemical oxidation of reduced
inorganic species.
The presence of gases in the unsaturated zone, such as
CO 2 , also can be assessed during a site visit. If high
concentrations of fuel are known or expected to be present
in the groundwater, either as residual contamination or as a
separate-phase product, the soil air may contain CO 2 at a
concentration that can inhibit the respiration of plant roots.
One simple method for determining the presence of high
CO 2 concentrations is to dig a hole about half-way to the
water-table depth and slowly lower a flame source, such as
from a butane lighter or torch, into the hole. If the flame is
extinguished, it may indicate the presence of CO 2 . This
should not be done for an extended period, because oxida-
tion of the butane produces CO 2 , and the presence of explo-
sive gases, such as methane or hydrogen, also may be
present in the hole. A safer approach to assess the presence
of multiple gases in the soil zone during a site assessment
would be to use a hand-held gas analyzer that measures the
percentages of either oxidized or reduced gases.
Plant growth is related to the presence of root-associated
fungal and bacterial communities. Hence, some basic
microbiological work as part of site assessment can lead to
a greater understanding of the microbial health of the
contaminated area. Soil samples can be analyzed for various
aerobic and anaerobic bacteria and the presence and distri-
bution of mycorrhizal communities in areas that contain
native vegetation. Alternatively, the establishment of base-
line conditions in areas containing no native plants but
designated for planting can help determine if plant addition
will result in rhizosphere formation. Collection of this
microbiological data also will help to determine if supple-
mentation by root inoculation may be needed during plant
installation, discussed in Chap. 7.
6.4.3 Plant Laboratory Studies
The presence of vegetation at a contaminated site does not
always indicate that plants interact with contaminants or
groundwater in a manner analogous to phytoremediation.
One way of determining if native plants are interacting
with groundwater, or if the plants proposed for use at the
site can interact with the water table, is to conduct laboratory-
scale experiments. Even if the native plants are not
evaluated, site soil and groundwater samples can be used
Fig. 6.1 The delivery of
dissolved oxygen during recharge
events to anoxic groundwater at a
gasoline-contaminated aquifer,
Laurel Bay, South Carolina. The
depletion of oxygen was caused
by microbial oxygen reduction
rather than physical processes, as
the DO-depletion slope is steeper
than that of the slopes showing
changes in water temperature and
specific conductance (Modified
from Landmeyer and Bradley
2003) . One inch is equivalent to
2.54 cm.
Search WWH ::




Custom Search