Plant and Groundwater Interactions Under Pristine Conditions - Introduction to Phytoremediation of Contaminated Groundwater - page 126

widely recognized that most of the flow in rivers and streams
during periods of low precipitation consists of ground-
water discharge. In many cases, because the geochemistry
of groundwater often is different than surface water, the
types of plants that can grow in these areas provide a visible
surrogate of the occurrence of groundwater discharge to
rivers.
Rosenberry et al. (2000) were interested in the determi-
nation of groundwater discharge to a lake in Minnesota in
order to estimate the lake's water budget. The researchers
used both the presence and absence of certain plants to
indicate specific locations of groundwater discharge. For
example, the location of groundwater discharge to the lake
was confirmed based on cooler water temperatures than the
prevailing surface water temperature at the same location
and absence of floating leaf and emergent vegetation but the
presence of marsh marigold ( Caltha palustris L.). The yel-
low flowers of the marsh marigold made it a visual indica-
tion of zones of groundwater discharge.
Most groundwater in discharge areas tends to be more
mineralized than dilute infiltration water in recharge areas.
In reporting results of a wetland study in Spain, Bernaldez
and Benayas (1992) indicated that xerophytes were found in
recharge areas in topographic highs. Conversely, in the
lowland discharge areas characterized by groundwater of
higher mineral content, both in salinity and alkalinity, the
plants that predominated tended to be able to adapt to the
higher salinity of the groundwater. Hence, a key control on
the distribution of plant types can be related to the supply of
groundwater and differences in the geochemical composi-
tion of groundwater, depending on its relation to the overall
flow path from recharge to discharge areas.
Benayas et al. (1990) also investigated the interaction
between riparian plant distribution and groundwater geo-
chemistry. The researchers reported that mineralization of
older groundwater along flowpaths in the aquifer system
controlled the distribution of riparian vegetation in the
study basin in central Spain. Groundwater samples collected
farther downgradient from recharge areas were more
mineralized; that is, the samples had higher specific conduc-
tance, pH, sodium and chloride concentrations, and the veg-
etation tended to be composed primarily of halophytic
plants. In areas where the groundwater flow path was shorter
the groundwater was characterized by lower concentrations
of specific conductance, pH, and minerals, and the plants
tended to be glycophytic and adversely affected by salts.
An interesting relation between a plant's need for
dissolved chemicals and its ability to acquire them is
illustrated in the Tree Islands in the Everglades of southern
Florida. These Tree Islands, mentioned previously, form
ridges and the surrounding lower areas result in sloughs.
Tree Islands are geologically relatively recent, circa 5,000
year BP (Gleason and Stone 1994). The woody tree growth

on these tree islands is extensive where land elevations are
high and flooding is not continuous enough to kill roots by
lack of oxygen. But where do these isolated trees get their
necessary minerals? The slash (tree parts that fall to the
ground) and old leaf litter are a potential source but are
depleted of nutrients. Ross et al. (2006) hypothesize that
transpiration decreases the underlying groundwater table,
which focuses the discharge of adjacent surface water that,
although contains naturally low concentrations of phospho-
rus, can have elevated concentrations of phosphorus from
agricultural land-drainage canals—this process provides
phosphorus to the root zone. This may provide a partial
explanation for the observation that Tree Island soils are
higher in phosphorus than the surrounding marsh soils.
Groundwater and surface-water data from a basin in
South Australia also confirm the strong relation between
transpiration and groundwater chemistry (Poulsen et al.
2006). Areas where massive amounts of groundwater dis-
charge leave behind increased soil salinity are referred to
as dryland salinity. In most cases, this increased salinity in
shallow soils occurred following the removal of deep-
rooted, perennial vegetation that was native to the area in
order to plant shallow-rooted, annual, agricultural crops.
Because the deep-rooted, native vegetation kept the
water table low, the salinity was concentrated in the vadose
zone by transpiration and evaporation enrichment was
immobilized at depth (Barrett-Lennard 2002). Following
the planting of agricultural crops, however, the increased
water table from increased recharge and decreased transpi-
ration mobilized the salts into solution: groundwater dis-
charge to surface water increased the salinity of the surface
water as well.

5.5 Summary

It is evident from a wide range of naturally occurring
systems that phreatophytes can affect groundwater and, as
a result, surface-water bodies. Phreatophytes can reduce the
amount of recharge by uptake of either infiltrating water or
groundwater. Conversely, the removal of phreatophytes can
increase the amount of recharge. Phreatophytic use of water
also can cause the water table to rise, such as when the
unsaturated zone is large and depth to water table is great,
or by direct uptake from the capillary fringe. These pro-
cesses can occur in arid areas or in humid areas that are
characterized by riparian plants, which can use surface water
and groundwater to meet evapotranspiration demands. Phre-
atophytic use of groundwater prior to discharge to surface
waters can affect surface-water flows, levels, and chemical
composition. Conversely, some riparian systems use surface
water that has entered the local groundwater-flow system,
and upstream changes in surface-water flows can affect the