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Preliminary geochemical investigations of the Aisch river in Northern Bavaria show groundwater influences

Simone Hintze1, Anne Batz1, Christian Hanke1, Robert van Geldern1, Johannes A. C. Barth1
1 Lehrstuhl für Angewandte Geologie, Universität Erlangen-Nürnberg

P 6.9 in Grundwasser-, Boden- und Oberflächenwasser-Interaktionen

 

The Aisch is a small, 70 km long, meandering river that flows through Middle and Upper Franconia and discharges into the Regnitz river. Already shortly after its source, the Aisch shows a low average morphological gradient of 1.1 ‰. The catchment area belongs to the “Fränkisches Schichtstufenland” with a geology characterized by “Gipskeuper” in the Southwest and “Sandsteinkeuper” in the North-East. In two sampling campaigns, in February and April 2013, the Aisch and some of its tributaries were sampled and analyzed for major ions (sodium, ammonium, potassium, calcium, magnesium, bicarbonate, fluoride, chloride, nitrite, nitrate, phosphate and sulfate) and for stable isotope compositions of carbon (δ13CDIC, δ13CDOC, δ13CPOC) and water (δ2HH2O, δ18OH2O). The downstream trend of major ions reflects decreasing weathering rates of the geological formations along the Aisch as the river flows into the sandstone terrain. Conductivity also confirms this trend. The dominant ions in the Aisch are sulfate and calcium with average values of 359 mg/L and 188 mg/L. Both originate dominantly from the dissolution of the gypsum-bearing rocks named “Grundgips” within the “Myophorienschichten”. The source of the river is a saturated gypsum solution. The dissolution of the “Grenzdolomit” also influences the calcium concentrations. In contrast, the nutrients mirror the change of the land use and biological activity. The headwater showed higher nitrate concentrations than in the lower course of the river and was most likely dominated by fish farming. The ponds influence also the ammonium concentrations in the river. Water isotopes (δ2HH2O= -60 to -69 ‰; δ18OH2O= -7.4 to -9.7 ‰) show that in winter the groundwater inflow dominates the Aisch, whereas in April snowmelt and more intense precipitation had stronger influences. The water isotopes of a small tributary system, the “Aischgraben”, were more enriched when compared to the main river due to evaporation in a series of upstream ponds. The δ13CDIC values of the entire river system were in the range of   -15.8 to -6.5 ‰. In April the values were more negative, likely due to more active turnover of organic material. The δ13CDOC and δ13CPOM values were mostly influenced by C3 plants and showed a range of -27.4 to -22.0 ‰ for DOC and -30.3 to -21.7 ‰ for POC.

 

Letzte Änderung 24.10.2013