Environmental Engineering Reference
In-Depth Information
The interaction of plant roots, the rhizosphere flora, and
contaminants can be evaluated using stable isotopes as
tracers. This approach, called stable isotope probing, is
based on following a 13 C-labeled compound from the soil
to microbial DNA. This approach has been used to under-
stand which soil bacteria can degrade xenobiotics such as
PCBs, and has been shown to validate the degradation of
these compounds by plant-produced enzymes that are used
by the plants naturally to degrade plant-derived aromatics.
15.3.4 Tree-Ring Chemistry and Aquifer
Properties
The annual formation of tree rings is a record of the avail-
ability of environmental factors that affect growth, espe-
cially the availability of water. Studies described above
also indicate that annual tree rings record the uptake of
certain elements, such as chloride, originally dissolved in
groundwater. Yanosky and Vroblesky (1995) suggested that
if the chloride concentration of tree rings was measured in a
single tree over time or at the same time but at multiple
locations, this would provide an indirect method to deter-
mine the velocity of groundwater flow. If the hydraulic
gradient is measured and effective porosity can be estimated,
then the hydraulic conductivity, K , can be estimated using
this approach. It may be most useful in those contaminated
aquifers where pumping tests to determine hydraulic con-
ductivity would not be feasible.
Fig. 15.9 Push-pull test results for the loss, presumably by biodegra-
dation, of PAHs over time in planted ( treed ) and unplanted ( untreed )
areas of a phytoremediation site in Tennessee (Modified from
Widdowson et al. 2005a, b).
15.3.5 Lysimeters
Lysimeters are devices that can be used to collect water
samples from the unsaturated zone for geochemical analysis.
They were first used in the early 1960s. They are similar in
design to the tensiometers used to measure soil water ten-
sion. For a lysimeter to collect water through its initially
empty porous cup, however, a vacuum is drawn on the
sample chamber, and the water sample is pumped to the
test vial. The material of the porous cup should be selected
based on the contaminant of interest at a site. Water samples
collected from lysimeters installed near tree roots at
phytoremediation sites provides perhaps the closest sample
of the water quality that those roots are using.
more accurately describe aerobic microbial processes in the
aquifer and may not be related to the effect of plants. This is
because the wells that were used at the site were deep wells,
on account of drought conditions having lowered the water
table below the screened interval of the shallow wells
located closer to the root zone of the planted poplar trees.
15.3.3 Stable and Radioactive Carbon Isotopes
Stable isotopic concepts related to plant sources of water of
different isotopic composition were introduced in Chap. 9.
To summarize, source waters for plants have different isoto-
pic hydrogen and oxygen values to the extent that kinetic
fractionation occurred during evaporation. If a sample of the
xylem water is taken for isotopic composition evaluation,
along with the compositions of the potential water sources,
then the source(s) can be identified, as well as the extent of
mixed sources.
15.3.6 Passive Soil-Gas Methods
The primary goal of phytoremediation of contaminated
groundwater is to decrease the total mass of contaminants
in groundwater or the vadose zone. If groundwater is
contaminated with
volatile
or
semivolatile
organic
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