Monitoring for Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 353

water uptake (Gough et al. 1979), the concentration of
sodium and chloride in various plant tissues has been of
interest since the mid 1900s. For example, the USGS geo-
chemist Hem (1967) investigated the occurrence of Na and
Cl in the leaves and stems of the riparian phreatophyte
saltcedar ( Tamarix spp. ) growing above salinized ground-
water along the banks of the Gila River, Arizona, and Rio
Grande, New Mexico. The total content of chloride, as well
as calcium, magnesium, and sulfate, in the leaves of
saltcedar ranged from 5% to 15% of the leaf dry weight.
Moreover, the leaves that contained the highest concentra-
tion of sodium and chloride grew above groundwater that
also was characterized by high salinity, even though the
saltcedar is not a halophyte. This study by Hem (1967) is
perhaps the first published study that showed a link between
groundwater quality and the uptake of a contaminant by a
phreatophyte.
Hem (1967) also related the changes in mineral concen-
tration in the leaves over time to the chemical composition
of the groundwater, the time of year, and transpiration rate at
the time of sample collection. In the field, about 25 g of plant
tissue was collected and placed in a bag and then taken to the
laboratory where the tissue was air dried; a smaller subsam-
ple was oven dried. These samples were placed in a beaker to
which was added distilled water, and this extract was
analyzed for the presence of ions. More than 30 trees were
sampled and analyzed in this manner.
In general, the depth to groundwater had an effect on the
mineral composition of the plant-leaf extract (Hem 1967).
The lowest concentrations of calcium, magnesium, sulfate,
and chloride were observed in samples from saltcedar trees
that grew where the depth to water table was the deepest, and
the converse also was true. Interestingly, the composition of
the residue that remained on the leaves of saltcedar also was
analyzed. Saltcedars can survive in salinized groundwater,
even though they are not halophytes, by removing the excess
salt to the outer part of the plant using special salt glands.
These glands are locations of high salt concentrations. This
and the guttation of salinized water may help these plants
tolerate changes in the mineral content of groundwater.
Other plants, such as succulents, can deal with high salinity
by maintaining high internal water concentrations, and do
not excrete salts.
Hem (1967) analyzed the mineral content of water shaken
on leaves and related it to the chemical composition of
groundwater pumped from a nearby well. Even though the
author cautioned that little can be stated about the processes
that led to the observation between solute leaf concentration
and source water, the gross groundwater geochemistry
appeared to be related to the chemistry of the leaf extracts
(Hem 1967). For example, two saltcedar trees that had the
highest chloride concentration grew above groundwater that
was characterized by 3,000-4,000 mg/L chloride. At another

location, shallow saline groundwater was related to the pres-
ence of high salinity in the leaves of saltcedar. Moreover, the
sulfate/chloride ratio in leaves growing above saline ground-
water was nearly the same as the groundwater sulfate/chlo-
ride ratio. Hem (1967) went on to state that lower
concentrations of minerals were detected in young leaves
and higher concentrations were found in older leaves.
The collection of cores of tree material to understand the
presence or absence of the uptake of xenobiotics other than
salt as shown by Hem (1967) from groundwater can be
traced back to initial investigations on metals (Vroblesky
and Yanosky 1990; Vroblesky et al. 1992). These reports
indicated the tree cores contained not only native metals,
such as those essential and trace elements necessary for tree
metabolism, but also excess concentrations of metals related
to the presence of higher than background concentrations of
these contaminants in groundwater. The depth to the water
table where the tree cores were collected ranged from 8.2 to
0.9 ft (2.5-0.3 m) adjacent to surface water. The groundwa-
ter may have been contaminated for some time, at least since
the 1930s, and groundwater samples collected in 1987
contained between 19 and 88 mg/L iron and 52-2,150 mg/
L chloride (Vroblesky and Yanosky 1990), relative to much
lower concentrations of these elements in groundwater from
uncontaminated areas, where these values were 0.1-4 mg/L,
respectively. The concentrations of iron (Fig. 15.2 ) and
chloride in tree rings were reported in rings formed since
the 1930s, and this time period encompasses the time prior to
and during the disposal activities that occurred upgradient of
where the cores were collected.
Vroblesky and Yanosky (1990) did not use tree-ring
chemical data for rings that were formed after 1980. This
was because all trees, including those growing above
contaminated and uncontaminated groundwater, showed
elevated concentrations of iron in rings formed between
1980 and 1987 (Fig. 15.2 ), when the samples were collected.
These elevated iron concentrations may reflect the flow of

Fig. 15.2 The concentration of iron in individual tree rings between
1930 and 1990 (Modified from Vroblesky and Yanosky 1990).