Hydrochemistry and isotope geochemistry of the upper Danube River |
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| Authors: | Frank Pawellek Frank FrauensteinJán Veizer |
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| Institution: | 1 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, 44801 Bochum, Germany
2 Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada |
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| Abstract: | The upper Danube River and 19 of its major tributaries were monitored for 1046 km downriver and seasonally for Ca, Mg, Sr, Na, K, HCO3, CO2, Cl, PO4, SiO2, NO3, NO2, NH4, SO4, δ13CDIC, δ18OH2O, δDH2O, δ34SSO4, δ18OSO4, and 87Sr/86Sr. Hydrological considerations and δ18O/δD data show that the water balance in the river, particularly after its confluence with the Inn, is controlled by the southern tributaries draining the Mesozoic carbonate complexes of the Alps. As a result, chemical balance at Bratislava is mostly a conservative product of its tributaries. The concentrations of Ca, Mg, and Sr in the Danube are 6 to 9 times higher than in “pristine” rivers. Although all these cations are derived from dissolution of Mesozoic carbonates, the ultimate cause is likely the enhanced generation of soil CO2 due to agricultural and forestry practices. Dissolution of Triassic sulfates is an additional factor for Ca enrichment in the headwater section of the river. Dissolved sulfate, with a comparable enrichment factor to that of alkaline earths, appears to be derived mostly from atmospheric deposition, a proposition based on consideration of its sulfur and oxygen isotopic compositions. Na, K, and Cl are enriched by a factor of 2.5 to 4 times, mostly as a result of industrial and municipal pollution sources.In contrast to the above components, which behave mostly conservatively during the downriver flow of the Danube, biogenic elements such as nutrients and Si are influenced by in-river processes. The photosynthesis/respiration balance that impacts the carbon cycle and the oxygen balance has been discussed elsewhere (Pawellek and Veizer, 1994). NO3, NO2, and NH4 are enriched by a factor of 10 to 16 times from point and diffuse sources along the watercourses, generating at times downflow nitrification plumes from NH4 to NO3. PO4 varies seasonally, chiefly as a result of biologic demand during the warm periods. For SiO2, the biological uptake (mostly for secretion of diatom frustules), combined with the deficiency of this compound that results from the predominantly carbonate lithology of the catchment, results in concentrations that are below those of pristine rivers. Overall, the present-day “salted” characteristics of the river are chiefly a consequence of the long habitation history of the upper Danube watershed. |
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