Hydrogeological overview map of Lower Saxony 1 : 500 000 - Extraction conditions in groundwater-bearing rocks
Description
The abstraction conditions (extraction potentials) are estimated to assess the aquifers. This refers to the energy efficiency or the economic efficiency of groundwater abstraction, whereby a sufficient supply of groundwater is assumed.
Unfavourable abstraction conditions are present in low-powered or poorly permeable aquifers, good conditions with correspondingly greater thickness or better permeability. Transmissivity (product of permeability coefficient and thickness) is used as a measure in loose rock. Within the scope of the representation possibilities, only the total transmissivity is taken into account, even if two or more separate aquifers are formed. There are not enough transmissivity determinations from hydraulic investigations for an area-by-area statement, so an estimation is made on the basis of strata inventories of the numerous digestion and well drillings. Standard permeability coefficients (kf values) were used for the main petrographic components, which were determined by evaluating a larger number of pumping tests. For the groundwater deposits in the solid rock, Brunnenergiebigkeiten are used for comparative estimation.
The rocks found in Lower Saxony are divided into four classes that indicate how good the conditions for the abstraction of groundwater are.
Very good extraction conditions correspond to transmissivities of over 100 m2/h. They prevail wherever very powerful, well groundwater-conducting layer packages are available. These conditions are found in Lower Saxony in the areas of Lüneburg and Aurich, where quaternary and tertiary predominantly sandy aquifers form a powerful conductor complex, which is still deepened in the area of quaternary gutter systems and filled with coarse sediment material. In these areas, very large quantities of groundwater can be supported in the long term with relatively little reduction, because the corresponding supply from groundwater recharge is also guaranteed.
Good extraction conditions are present for transmissivities between 20 and 100 m2/h. Such conditions exist throughout the distribution area of the quaternary glacifluviatile loose rocks in northern Germany, including the gravel fillings of the river valleys of the larger rivers. Also in the hard rock in regionally narrow, strongly rugged and karstized zones, quite good extraction conditions can prevail (e.g. Pöhlder Basin).
Aquifers of this category are potentially suitable for the abstraction of larger quantities of groundwater.
Unfavourable extraction conditions are present for transmissivities below 20 m2/h. In loose rock, these conditions are mainly found where small quaternary sediments lie over clayey tertiary layers or in the boundary area to the solid rock. In the solid rock areas, the extraction conditions are often unfavorable. The aquifers of this group are generally not suitable as well sites for larger groundwater abstractions.
Highly varying extraction conditions are present in areas where no uniform characteristics of the extraction conditions can be observed, for example in the area of submerged moraines and partly also in solid rock.
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