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additional water to discharge to the river and increased the
volume of water, and sediment load, that spread out over the
flood plain of the area and, ultimately, into the port.
5.2.3 Changes in Surface-Water Chemistry
The documented interaction between phreatophytes and
surface-water flows also may affect the geochemistry of
surface water. This would especially be the case if the geo-
chemistry of the groundwater is different than surface water.
Perhaps one of the first studies that observed the linkage
between the uptake of water by phreatophytes and decreased
surface-water flow and changes in surface-water chemistry
was conducted in the Moshiri basin on the island of
Hokkaido, Japan. The basin is characterized by oaks with a
bamboo understory. During 1989, streamflow measurements
were made at the basin outlet, and discharge decreased
during the warm, dry summer months as a result of
decreased precipitation and increased evapotranspiration
(Fig.
5.3a
). During July, the period of lowest streamflow,
warmest weather, and least precipitation, the authors noted a
relation between the fluctuation in streamflow and water
chemistry on a daily, or diurnal, basis (Fig.
5.3b
). Each
day, streamflow was highest in the morning and decreased
in the evening, the difference a result of removal of ground-
water by riparian phreatophytes rather than by evaporation
of surface water. This is because the dew point was higher
than the stream temperature. Specific conductance measured
in the streamflow also followed a daily pattern, but
concentrations were low in the morning and higher in the
evening. Groundwater in the area had lower total ions as
total dissolved solids than the stream water, so a decrease in
groundwater discharge would lead to increased influence on
water-chemistry by the surface water.
Clear cutting both riparian and upland forest trees can
increase the rate of nutrient runoff, which can be measured
by increased nutrient concentrations in stream water. For
example, when nutrients, such as nitrate, no longer are
taken up by plants, they become available for transport by
runoff or groundwater. The net loss of nutrients from a clear-
cut area can exceed by a factor of 8 the loss of nutrients from
forests that are not clear cut. This is especially evident as
increased nitrogen loading to streams near clear-cut areas
(Bormann and Likens 1967).
A diurnal variation in trace-metal chemistry in streams
was reported by Nimick et al. (2003). Rather than exhibiting
a constant level of trace-metal concentrations over time, the
streams exhibited a changing cycle of concentrations. The
lowest concentrations of trace metals, such as manganese,
cadmium, and zinc, occurred near the end of each day.
Conversely, the highest concentrations occurred in the morn-
ing. Potential reasons for these fluctuations include sorption,
Fig. 5.3
The influence of plant uptake of groundwater on (
a
) surface-
water flow and (
) geochemistry for a site in Japan. One millimeter is
equivalent to 0.039 in., and one centimeter is equivalent to 0.39 in.
b
diurnal uptake by riparian plants, and decreased input of
groundwater that contains these trace elements during the
day (i.e., a reduction in streamflow; Nimick et al. 2003).
Speiran (2010) reported that groundwater uptake by
riparian phreatophytes effected nitrate concentrations in
groundwater at two sites in Virginia. Nitrate concentrations
in groundwater decreased from 10 to 2 mg/L after flow
through a riparian zone. The riparian forest also focused
local groundwater discharge to wetland systems that led to
significant mass loss of nitrate through denitrification. More-
over, the study reported diurnal changes in groundwater
levels near 0.25 m.
The beneficial effect of riparian plants on surface-water
chemistry has been employed to decrease the discharge
of groundwater contaminants such as nitrate to surface-
water bodies (Tabacchi et al. 2000). In fact, many local
municipalities enforce riparian buffers defined as a fixed
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