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Abstract "Heft 17"


Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich


Heft 17


Karl-Heinz Feger

Biogeochemische Untersuchungen an Gewässern im Schwarzwald
unter besonderer Berücksichtigung atmogener Stoffeinträge



Freiburg im Breisgau 1986

ISSN 0344-2691


Summary:

According to investigations in Scandinavia and North America many freshwaters became noticeably more acid during the last two or three decades and partly lost their natural fish populations. These waters are located in areas where the parent material is poor in bases and the soils are acid. These changes are mainly ascribed to the increase of atmospheric depositions. With the current forest decline it has been discussed recently, if also surface waters in forested areas of Central Europe are affected by atmospheric depositions, especially by acids and heavy metals. First regional surveys found very low pH values also in some surface waters of German mountain regions with soils derived from parent material poor in bases. Therefore, the question is, if these waters are acidified by anthropogenic depositions or if they are naturally acid. Another question is, if waters acidified by atmospheric depositions can be identified by means of simple models developed in Scandinavia.
The aim of this investigation is to characterize the current chemical balance of four selected lakes in the Black Forest. Which are the current biogeochemical processes determining the physico-chemical conditions of these lakes ? Is there any evidence for direct effects by atmospheric depositions ? Additionally, the chronological reconstruction of the chemical input using the chemical composition of the lake sediments is discussed.
From June 1984 to June 1985, four Black Forest lakes were examined on chemical input-output. Furthermore, the chemical composition of dated sediment cores was studied. The lakes Feldsee, Blinder See near Schonach, Vildsee at the Ruhestein and Herrenwieser See are all located in the higher parts of the Black Forest. The catchments are almost completely forested without habitation.  Inlets,  outlets and precip-itation were analyzed for conductivity, pH value, alkalinity, UV-absorbance (? = 254 n), color (? = 436 ni), DOC, PO4-P, NH4+, Cl-, N03-, SO4-2, Na+, K*, Ca2+, Mg2+, Al, Fe, Mn, Zn, Cu, Cd and Pb. Ion balances were calculated. Furthermore, cluster and factor analyses were carried out. Dating of the sedimentary cores was done using the isotopes 210Pb and 137Cs.
The temporal variations of the concentrations in the surface waters, especially in the inlets of th<= lakes, reflect the biogeochemical processes in the lake catchments. The factor analysis demonstrates strong influences of processes in the soils such as mineralization, podsolization and weathering. The surface water pH values strongly depend on the soil types and the hydrological runoff patterns in the catchments. Under high flow conditions, especially in the podsolic and partly hydromorphic soils of the catchments in the Buntsandstein area of the northern Black Forest (Wildsee and Herrenwieser See), lateral flow through the upper soil horizons causes strong pH depressions and elevated concentrations of the metals Al, Fe, and Mn. High concentrations of potentially toxic metals, mainly aluminum, are always accompanied by high concentrations of complexing organic substances. Hence, a greater part of these elements apparently is organically complexed. But this is decisively important for the ecotoxicological evaluation of the metal concentrations, as only ionic species cause toxic effects. The heavy metal concentrations in the analyzed waters are far below the limits established for drinking water.
Organic anions are only important in the acidity of Blinder See. The low pH values of 4.0-5.0 in the waters of the Bundsandstein area are assumed to be caused by protons exchanged in the strongly acid organic soil horizons. The concentrations of the anions of strong acids, nitrate and sulfate, are determined by the pedological-geological, hydrological, and biological conditions and processes within the catchments. This concerns both the level and the temporal variation. A direct dependence on the atmospheric N- and S-input is not evident. Using the sum (SO42- + NO3-) or other simple chemical models for the identification of freshwaters acidified by anthropogenic acidic depositions seems therefore not advisable. The current chemical composition of the waters in the Feldsee area do not show any changes compared to measurements in the early 1950s, if comparable at all. Changes of the chemical composition of Wildsee and Herrenwieser See, which were documented by lake sediment diatom analysis in other investigations, are mainly the result of the recent forest management history.
The chemical input by precipitation is low compared to other regions in Central Europe and differs only slightly between the four study areas. The temporal variations in the rain water concentrations are very similar, too. For most areas and elements the direct atmospheric input onto the lake surface plays only a minor role compared to the input from the catchments. This input, on the other hand, largely depends on the pedological-geological, hydrological, and biological processes within each catchment. Due to the short water residence times, most substances show only a slight retention in the lakes, except for the heavy metals which are retained to a higher degree. The retention of heavy metals is however slight in comparison to other lakes. This may be caused by the low pH values and the high concentrations of organic-complexing agents.
The dating of the sediment cores with 210Pb and 137Cs yields very low sedimentation rates. The sedimentation rate in Herrenwieser See, for example, is 1.2 mm. a-1 in the 1-2 cm layer. In deeper layers, it is on average only 0.4 mm.a-1 due to compaction. Additionally, the radiometric dating at this lake is supported by a time mark due to forest road construction. The chemical composition of the sediments  is  strongly  related  to  the  mineralogical-petrographical conditions of the lake catchments. Especially the heavy metals and sulphur show distinct enrichments in the most recent sediment layers. Although the "background"-levels differ highly from lake to lake, the increase of the heavy metal contents uniformly starts in the last quarter of the 19th century. The current retention of heavy metals and sulphur in the lakes also agrees with the sediment influx rates calculated from the uppermost dated sediment layers. For the interpretation of depth functions of the analyzed elements in the sedimentary cores besides the deposition history one also must consider postsedimentary mobilization processes which are especially important for iron, manganese and sulphur. Changes in the depth functions of the elements Na, K, Mg, Al, and Fe reflect erosion events but not a mobilization of these elements caused by acidification.



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