Investigating hydrogeological controls at the GeoLaB site using ParFlow-CLM

The GeoLaB project (https://geolab.helmholtz.de/en/) aims to establish an underground laboratory in the crystalline rocks of the Odenwald, Germany, to conduct controlled experiments on thermal-hydraulic-mechanical-(bio)chemical processes relevant to the development and operation of enhanced geothermal systems. The facility will provide access to representative reservoir rocks of the Upper Rhine Graben, specifically the Tromm granite located along its northeastern margin.

One of the key questions to be addressed during the exploratory phase of GeoLaB concerns the occurrence and flow rates of groundwater in the area. The geological setting is complex and includes plutonic, metamorphic, and sedimentary rocks, all affected by different degrees of fracturing and intersected by regional faults. These units, particularly the hard rocks, are difficult to investigate directly, which results in a general lack of subsurface information in the study area. In addition, the alluvial and colluvial deposits besides the soils developed under grasslands and forests are expected to exert significant control on groundwater flow, together with the weathered zones and associated saprolites. This combination of factors makes the hydrogeological assessment particularly challenging.

To numerically explore the influence of the different hydrogeological units on groundwater dynamics, we employ integrated hydrologic modelling using ParFlow–CLM. The focus is placed on testing alternative assumptions regarding the hydraulic properties and geometries of the shallow deposits and weathered horizons, as well as different hydraulic conductivities for the underlying hard rock units, all of which are expected to exert strong control on groundwater flow. Although groundwater is the central objective, the use of ParFlow–CLM also provides insights into surface water resources and the complete water balance. As a first step, we simulate a suite of steady state models to assess groundwater table depths under different conceptualizations. Subsequently, we run transient simulations for the most plausible scenarios and compare the results with available observations of discharge, groundwater level and soil moisture profiles. Preliminary results highlight the sensitivity of groundwater table depths to the conceptualization of the shallow and weathered units and illustrate the potential of this approach to constrain hydrogeologic conditions in the GeoLaB area.

Overall, this work provides a first hydrogeological assessment for the GeoLaB site and outlines a modelling framework that can be progressively refined to support future explorations and the design of the laboratory.