Monitoring for Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 360

fact, chlorophyll a sensors that optically measure fluores-
cence are used in many surface-water monitoring programs.

ranged from simple observation of leaf surfaces in response
to difference conditions of VPD to photographic imaging
systems (Weyers and Meidner 1990). Other methods
included the addition of various fluids to a leaf surface and
measuring the time of uptake, presuming that the uptake
occurred through open stomata. Today, the accepted stan-
dard to measure stomatal conductance is to use a gas-flow
porometer.

15.1.7 Compound-Specific Isotope Analysis

Stable isotopes were used to determine that water was the
source of the oxygen released by plants during photosynthe-
sis, as described in Chap. 3. The carbon isotopic value of
various carbon compounds derived from plants also varies,
and can be traced back to the formation of the compound, or
parent compound, by the fixation of carbon during photo-
synthesis. Most plants use the C 3 or C 4 fixation pathways.
Additional carbon isotope changes occur during the refin-
ery process that converts crude oil to usable products. Most
fuel compounds will have carbon isotope values near that of
the plants, near 27 per mil, the fuel is derived from. BTEX
compounds that biodegrade under oxic conditions produce
CO 2 that is not fractionated from that of the compound being
degraded and, therefore, also will be near

15.1.9 Leaf Area Index

The total leaf area exposed to the air has a significant effect
on the volume of water lost by transpiration, and hence,
potential for the uptake of groundwater. The leaf area
index, LAI , is a measure of this potential and is defined as
the ratio of leaf surface area of a plant or grove normalized to
ground surface area covered by the canopy (see Chap. 9).
LAI can vary from a high of 10 down to 0, and is dimension-
less. The magnitude of LAI varies with such factors as the
size and spacing of trees; LAI also tends to increase as trees
age. For example, the LAI for young trees is near 0 and can
approach 10 in dense stands of mature trees. Measurements
of LAI over time may be useful in quantifying the increases
in ET during the development of a poplar grove at
contaminated sites or quantifying the effect of contaminant
concentrations on tree health (Fig. 15.8 ).

27 per mil.
Under anoxic conditions, however, the lighter isotope, 12 C,
reacts faster than 13 C, leaving the undegraded parent com-
pound enriched with the heavier isotope. The application of
carbon isotopic differences, in both stable as well as natu-
rally radioactive and emplaced isotopes, for example, was
used to determine the extent of local recharge of younger
groundwater more recently in contact with the soil zone into
older groundwater in a coastal area that has undergone much
development and groundwater pumpage (Landmeyer and
Stone 1995). In a contaminated aquifer, stable isotopes of
carbon also were used to trace the pathway of biodegrada-
tion of contaminant compounds, such as BTEX, under oxic
and anoxic conditions (Landmeyer et al. 1996a).
A concern with the above stable isotope approach at
groundwater contamination sites that are candidates for
phytoremediation is that CO 2 also can be produced by the
degradation of non-contaminant compounds, which dilutes
the stable-isotopic signature of the C in the CO 2 analyzed.
Also, the approach is based on changes in the isotopic signal of
the products of a reaction, rather than the reactants. Looking at
changes in the reactants can be accomplished, however, by GC
separation, followed by isotope ratio mass spectrometry
(IRMS), and is called compound-specific isotope analysis
(CSIA). This approach has been applied to groundwater con-
tamination sites where fuels or solvents have been released
(U.S. Environmental Protection Agency 2008).

Fig. 15.8 The increase in leaf area index for multiple stands of trees
over time at a phytoremediation planting.

15.2 Hydrogeologic Methods

Processes that occur in the subsurface often can be under-
stood only after sufficient amounts of data are collected. The
conventional approach to sampling groundwater is to use
monitoring wells that are essentially the same design as
those wells intended to remove groundwater for municipal
or industrial use. Such wells consist of slotted screen placed

15.1.8 Stomatal Conductance

In the past, plant physiologists have relied on a number of
approaches to quantify stomatal size and how it relates to
plant-gas exchange (see Chap. 9). These approaches have