Hydrochemical Zoning and Chemical Evolution of the Deep Upper Jurassic Thermal Groundwater Reservoir Using Water Chemical and Environmental Isotope Data

Florian Heine,Florian Einsiedl,Kai Zosseder

doi:10.3390/w13091162

Florian Heine, Florian Einsiedl + Show 1 more

Open Access

https://doi.org/10.3390/w13091162

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Journal: Water	Publication Date: Apr 22, 2021
Citations: 11	License type: CC BY 4.0

Affiliation: Technical University of Munich, Norsk Hydro (Germany)

Abstract

A comprehensive hydrogeological understanding of the deep Upper Jurassic carbonate aquifer, which represents an important geothermal reservoir in the South German Molasse Basin (SGMB), is crucial for improved and sustainable groundwater resource management. Water chemical data and environmental isotope analyses of δD, δ18O and 87Sr/86Sr were obtained from groundwater of 24 deep Upper Jurassic geothermal wells and coupled with a few analyses of noble gases (3He/4He, 40Ar/36Ar) and noble gas infiltration temperatures. Hierarchical cluster analysis revealed three major water types and allowed a hydrochemical zoning of the SGMB, while exploratory factor analyses identified the hydrogeological processes affecting the water chemical composition of the thermal water. Water types 1 and 2 are of Na-[Ca]-HCO3-Cl type, lowly mineralised and have been recharged under meteoric cold climate conditions. Both water types show 87Sr/86Sr signatures, stable water isotopes values and calculated apparent mean residence times, which suggest minor water-rock interaction within a hydraulically active flow system of the Northeastern and Southeastern Central Molasse Basin. This thermal groundwater have been most likely subglacially recharged in the south of the SGMB in close proximity to the Bavarian Alps with a delineated northwards flow direction. Highly mineralised groundwater of water type 3 (Na-Cl-HCO3 and Na-Cl) occurs in the Eastern Central Molasse Basin. In contrast to water types 1 and 2, this water type shows substantial water-rock interaction with terrestrial sediments and increasing 40Ar/36Ar ratios, which may also imply a hydraulic exchange with fossil formation waters of overlying Tertiary sediments.

Highlights

The deep Upper Jurassic carbonate aquifer in the South German Molasse Basin (SGMB)is one of the most important mid-enthalpy geothermal water resources in Europe [1,2].More than 20 hydrothermal doublets and triplets are installed in the Upper Jurassic thermal reservoir and several new power plants are planned in the framework of the heat energy transition to a climate-friendly supply (Figure 1)
An unbiased analysis and interpretation of the groundwater composition can be supported by using multivariate statistical techniques of the hydrochemical data, such as exploratory factor (EFA) and hierarchical cluster analysis (HCA), which have been widely used to enhance the understanding of complex hydrogeological systems [20,21,22,23]
The aim of this study is to provide a comprehensive hydrochemical zoning of the deep Upper Jurassic reservoir in the SGMB, which can be further used to complement with hydraulic and petrophysical data to provide a consistent hydrogeological picture of the exploited geothermal reservoir

Summary

Introduction

The deep Upper Jurassic carbonate aquifer in the South German Molasse Basin (SGMB)is one of the most important mid-enthalpy geothermal water resources in Europe [1,2].More than 20 hydrothermal doublets and triplets are installed in the Upper Jurassic thermal reservoir and several new power plants are planned in the framework of the heat energy transition to a climate-friendly supply (Figure 1). The water chemical composition and especially environmental isotope signatures of δD, δ18 O, 87 Sr/86 Sr, 3 He/4 He, and 40 Ar/36 Ar in the thermal groundwater convey valuable information about the hydrochemical evolution of the groundwater system, water-rock interaction and groundwater recharge conditions, which can be used in the context of 4.0/). The study and combination of these isotopic chemical parameters on groundwater in other important aquifer systems within deep sedimentary basins, such as the Parisian Basin, e.g., [15,16,17], Pannonian Basin, e.g., [8,18], or Baltic Artesian Basin, e.g., [19], led to the successful distinction between younger meteorically recharged and very old, partly highly saline fossil formation waters. An unbiased analysis and interpretation of the groundwater composition can be supported by using multivariate statistical techniques of the hydrochemical data, such as exploratory factor (EFA) and hierarchical cluster analysis (HCA), which have been widely used to enhance the understanding of complex hydrogeological systems [20,21,22,23]

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