Bedrock structure and ground water migration in the eastern Bavarian crystalline basement

Matthias Zeitlhöfler1, Timo Spörlein2, Bernhard Wagner2
1 IFB Eigenschenk GmbH
2 Bayerisches Landesamt für Umwelt

O 13.9 in Czech-German exchange on environmental-, soil- and hydro(geo)logical issues

30.05.2014, 14:40-15:00, H18, NW II

The Bavarian Environment Agency conducted a statewide survey co-funded by the EU and the Bavarian Ministry of the Environment to compile an inventory of geologic and hydrogeologic maps for the entire eastern Bavarian crystalline basement. One purpose of this project was the deduction of a plausible hydrogeologic conceptual model for ground water flow in this area commonly known for its sparse groundwater resources.

In those per se impermeable crystalline rocks ground water migration predominantly occurs along joints, faults and other discontinuities. A lineament analysis based on a digital elevation model was carried out to identify the underlying structual geologic pattern, which was ground truthed with fault lines depicted on geological maps as well as copious amounts of fracture measurements in the field.

Quantitative data on ground water flow (hydraulic conductivities and specific yields) was gathered from over 280 pumping tests in bedrock wells and piezometers as well as injection tests in boreholes.

The calculated hydraulic conductivities were then attributed to major hydrogeological units in the study area and visualized in histograms and box-whisker-plots. Additionally, the data was depth-correlated up to 260 m below the topographic surface.

Ground water migration and structural geology were correlated by a spatial analysis of the distance of the pumping test sites to mapped topographic lineaments.

As a result, a relationship could be identified between the hydraulic conductivities (kf-values) - and accordingly specific yields - and the distance to the closest lineament to the effect that wells associated with low to very low conductivities (< 10-5 m/s) are generally independent of their distance to the nearest lineament. In contrast, higher kf-values (up to 10-4 m/s) were almost exclusively located < 500 m from the mapped lineaments.

With regard to depth zonation three distinct zones could be discerned: a shallow level where tertiary and quaternary weathering processes took effect on the bedrock, an intermediate level characterized by a somewhat higher degree of fracturing due to unroofing effects and a lower level where discontinuities predominantly form in response to tectonic stress fields. In the latter, ground water migration is progressively confined to fewer but spatially more extensive fractures with increasing depth below the surface. Thereby, the overall bedrock conductivity becomes highly heterogeneous in that the unfractured bedrock is largely impermeable while the few conductive pathways accommodate the bulk of the available ground water.

Thus, chances of obtaining higher yields from bedrock wells are significantly increased if well sites are located in the vicinity of lineaments, or - ideally - intersections thereof. Although this approach cannot substitute classical hydrogeological reconnaissance it provides an additional source of information to be utilized for locating well sites in the crystalline basement of eastern Bavaria.

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Letzte Änderung 07.11.2013