Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 223

unsaturated and saturated hydraulic conductivity, or soil
water availability,

CO 2 , which has a stable carbon isotope signature of about

than the demand for ET from VPD 7 per mil (Ziegler 1995). This observed enrichment of the

light isotope 12 C at the expense of the heavier isotope 13 C
was explained to result from isotopic fractionation reactions
that occur during the reduction of the CO 2 , such that the
kinetics of 12 CO 2 reduction were more favorable, or faster,
than the reduction of 13 CO 2 . Further investigation indicated
that the enzyme rubisco, or ribulose bisphosphate carboxyl-
ate/oxidase, discriminates against

(Persson 1995).

9.3 Integrative Monitoring Methods

As stated earlier, a multiple-lines-of-evidence approach is
encouraged regarding the monitoring of a phytoremediation
system for plant-induced changes in site groundwater
hydrology. This approach provides the confidence level nec-
essary to make statements about processes that are occurring
in the subsurface that often are not amenable to direct obser-
vation. This section describes approaches that contain
aspects of both the plant and groundwater components of
phytoremediation.

13 CO 2 (Park and Epstein

1960).
Photosynthesis was discussed in Chap. 3, and the differ-
ence in how plants fix CO 2 gave rise to the classification of
C 3 and C 4 plants (Fig. 9.11 ). This difference between how
plants fix carbon was elucidated after archaeologists found
that radioactive carbon, or 14 C, used to age-date samples of
organic matter such as corn waste had younger radiocarbon
age dates, in years before present (BP), than wood from the
same site (Bender 1971). This was because corn and other
monocots such as grasses turned out to have higher 14 C
contents (and 13 C) than dicotyledon plants. This difference
was explained in terms of the stable isotope value of the
initial sugar produced during gas-phase diffusion of carbon
fixation in different plants. For example, corn and other C 4
grasses fix CO 2 into a 4-carbon product

9.3.1 Geochemistry

The ambient geochemistry of groundwater, in terms of the
concentration of dissolved solutes acquired along groundwater
flowpaths through interactions with the porous media prior to a
contaminant release, can play a role in the determination of the
types of phreatophytes that grow in an area. For example,
cottonwood and willow typically are not found in areas
characterized by high concentrations of dissolved salts. How-
ever, certain hybrid poplar trees, such as the hybrid OP-367,
have been developed that can live in soils where the pore water
is characterized by high salinities. Some native plants that can
tolerate high-salinity groundwater include greasewood,
saltcedar, and bush pickleweed ( Allenrolfea occidentalis ),
although these plants are rarely used in phytoremediation
applications.

that contains

more 14 C and heavier

13 C, measuring about

14 per mil.
This pre-fixation of carbon in C 4 plants is by the enzyme
phosphoenolpyruvate (PEP) carboxylase, which occurs in
the mesophyll cells of the leaves. Final fixation of carbon
occurs by rubisco in the bundle sheaths of the leaf. Con-
versely, those plants that produce a 3-carbon sugar are the C 3
plants, and they produce isotopically lighter carbon, near

27 per mil.
The fixation of carbon is not limited to the C 3 and C 4
pathways. An additional pre-fixation pathway also discussed
in Chap. 3 is the CAM pathway, or Crassulacean acid

9.3.2 Stable Isotopes

Because plants can take up water from multiple sources,
determination of the proportion due to groundwater is essen-
tial at a contaminated site. This evidence is especially con-
vincing if diurnal fluctuations in groundwater levels are
available. One of the more established methods to determine
the source of water is to compare the stable isotopic compo-
sition of the plant water with that of the potential sources of
water. Some examples will be given below.
Plants interact with many elements, but the primary ones
are carbon, oxygen, and hydrogen. All these elements have
stable isotopes. The stable isotopes of carbon were used in
the early 1950s to show that the stable isotope value of plant
carbon was about , per

thousand, or a tenth of a percent), or contained less percent-
age of the heavy isotope 13 C, than the carbon in atmospheric

27 per mil (also shown as

‰

Fig. 9.11 Stable carbon isotope values and relative abundance for C 3
and C 4 plants.