5th Swiss Geoscience Meeting, Geneva 2007
Carbon isotope composition of dissolved inorganic
carbon and cation content in the upper reaches of the
Rhone river, Switzerland, as tracers of geo-, bio- and
anthropogenic sources of dissolved ions
Daniela Fontana* and Torsten Vennemann*
* Institute of Mineralogy and Geochemistry, University of Lausanne, L'Anthropole, CH1015 Lausanne, Switzerland
(Daniela.Fontana@unil.ch)
The carbon isotope composition of dissolved inorganic carbon (DIC), as well as
the hydrogen and oxygen isotope composition of water, the pH, temperature,
alkalinity, conductivity, and the major ions in water have been analyzed
seasonally for the Rhone, several of its major tributaries, as well as some
reservoirs used as sources for hydroelectric energy, and waters exiting from two
different waste water treatment plants. The major aim was to evaluate the use
of DIC and its carbon isotopic composition as a tracer for the different geologic,
biologic and possible anthropogenic contributions of carbon to the Rhone in
Switzerland.
The upper reaches of the Rhone are characterized by an Alpine catchment with
sparse vegetation and a thin soil, covering exclusively silicate rocks. Glacial
melt water as well as surface runoff make up the bulk of the water source in this
part. Further downstream other tributaries, themselves being largely melt water
fed but also being exploited for hydroelectric power generation and thus with a
number of dams along their course, join the Rhone. The lithologies become
mixtures of silicates and carbonates and the vegetation and soil cover increases
in parallel with the agriculture exploitation. Further downstream, past the city of
Sion, the lithologies in the catchment are dominated by carbonates and, with
the valley widening, agricultural production increases once more, hence the soil
cover becomes more important.
The difference of the geology in the catchment as well as the soil cover
thickness and hence biologic activity in the soil are generally reflected by the C
isotope composition of the DIC. δ13C values of DIC are as high as –2.5‰ only
in the upper source reaches of the Rhone, reflecting predominant uptake of CO 2
from the atmosphere. Directly downstream from the source region, δ 13C values
of DIC decrease towards –5 to –8.5‰, with generally lower values in autumn
compared to winter, suggesting a higher input of soil-derived CO2 when the soil
is not frozen. Relatively high δ13C values of about –3.5‰ during winter, and
lower values during autumn and spring (about –4.5‰) have also been
measured for the glacial melt water-fed tributaries to the Rhone in the upper
reaches of its catchment, again reflecting a large input of atmospheric CO 2 to
these rivers with only a small contribution of soil-derived carbon dioxide. In the
downstream section of the Rhone, δ13C values of DIC decrease to values of
around –6‰, despite the fact that carbonate rocks become more prominent in
the lower catchment. Furthermore, the δ13C values are now lower in winter
(about –6.8‰) compared to autumn or spring (–5 to –6‰), indicating a larger
5th Swiss Geoscience Meeting, Geneva 2007
influence of carbon derived from organic matter and a higher biological activity
within the water column during spring and autumn compared to winter. In
summer the δ13C values are generally even higher compared to spring and
autumn (0.3 to 6.4‰). There are, however, a few exceptions given by a
number of rivers that enter the Rhone, which may have slightly lower values in
summer compared to spring. The reason for this is not clear but may be related
to variable contributions of water derived from dams used for hydroelectricity.
Contributions of organic carbon from municipal and private wastewater
treatment plants are also possible sources of DIC. In fact, directly downstream
of such plants, the contributions of DIC from the plants (with values as low as –
27‰) to the Rhone water are noticed by 0.5 to 2.6‰ lower δ13C values for DIC.
This additional "anthropogenic" water source is also clearly expressed by
differences in the H- and O-isotope compositions of the waste waters relative to
the river water, suggesting that substantial volumes of the Rhone water are
treated waters from the communities along the Rhone.
The major cation concentrations are typical for rock-dominated fluvial systems
with total cation contents in natural waters of between 25 to about 130 mg/l and
Na/(Na+Ca) ratios of between 0.05 to 0.2. This ratio is marginally higher but
with a low total ion content in the direct source area to the Rhone (siliceous
source rocks only) as well as in summer compared to winter. Ca content varies
between about 0.4 (source waters) to about 120 mg/l for natural waters and it is
generally higher in tributaries draining predominantly carbonate lithologies and
further downstream along the Rhone. As would be expected, the Si content
(between 0.1 and 3.2 mg/l) is higher in drainage basins dominated by siliceous
rocks compared to carbonate rocks. Very high contents of Na and K are noted
for water from the wastewater treatment plants (up to 830 mg/l of Na). These
elevated Na and K concentrations can also be traced downstream of the
entrance of these waters into the Rhone.