Qualitative status assessment of the deep geothermal aquifer in the Lower Bavarian-Upper Austrian Molasse Basin

The European Water Framework Directive (WFD) commits EU member states to achieve a good qualitative and quantitative status of all their water bodies. For this purpose, it provides a list of actions to be taken to achieve this goal of “good status”. The directive, however, neglects the specific conditions under which deep ground water aquifers exist, compared to shallow ground water or surface water bodies. For example, deep ground water is influenced by interaction with the rock matrix and other geofluids, and may assume a “bad” status without anthropogenic influences. Thus, the steps offered by the WFD are inadequate in the case of deep ground water, and a new concept for this specific kind of aquifers is needed. Their status evaluation must be based on the effects induced by their exploitation. Here we analyze long-term data series to detect changes in the hydrochemical deep ground water composition which might be triggered by balneological and geothermal exploitation.

We are aiming to use these insights to design a set of criteria with which the condition of deep ground water aquifers can be quantitatively and qualitatively determined. Our analysis is based on a unique long-term hydrochemical data set taken from 8 balneological and geothermal wells in molasse basin of Lower Bavaria, Germany, and Upper Austria. It focused on bicarbonate, sodium, fluoride and chloride values. The data range back to 1937. Our methods are include cluster analyses, correlation and piper diagram analyses, as well as developing threshold corridors within which a good status can be assumed.

The results underline the importance of using long term time series analysis to better inform quality and quantity assessments for deep ground water bodies: most fluctuations would stay undetected within a < 5 year time series window, but become a distinct irregularity when viewed in the context of multiple decades. This will ultimately aide in assessing the sustainability of deep ground water exploitation for balneological and geothermal uses.

Project duration: completed 2021