Development of hydro geophysical techniques to characterize water fluxes in the vadose zone of karstified and porous heterogeneous aquifers

Abstract

This research aims to develop in-situ and non-invasive hydrogeophysical techniques to monitor water fluxes within the unsaturated zone. While porous media are more continuous in their flow pattern, no suitable technique so far exists at the field scale for karstifed vadose zones. Motivated by the existing uncertainty in understanding vadose zone water flux patterns at a larger scale our study aims to develop a new technique to invert tracer signals, such as heat, into a 3D flow field with the vadose zone. Up to now, there are only very few studies published that use 3D mechanistic flow models to analyse 3D high- resolution heat as a tracer for water fluxes in the unsaturated zone. Natural daily temperature fluctuations influence soil water temperature down to 1 meter below the surface, while natural seasonal variations extend to depths of 15 to 20 meters. Beyond this depth, in the 'neutral zone,' temperature responses are influenced by rock thermal conductivity and regional heat flux density. We investigate two test sites: one in a porous medium and the other in a karstified aquifer. Our investigations allow us to compare under similar climate water fluxes of a porous with a karstified medium and to assess their higher vulnerability to drought periods. Utilizing Distributed Temperature Sensing (DTS) for continuous monitoring of soil water temperature profiles, we installed the DTS equipment in several boreholes and analyse temperature fluctuations over time using HYDRUS-3D. Distributed Temperature Sensing (DTS) technology will be installed within boreholes positioned in the vadose zone, with each borehole interconnected by a fiber optic cable. The DTS system enables continuous and precise monitoring of groundwater temperature profiles. Over an extensive period of six months, the study will collect comprehensive datasets capturing natural temperature signals within the vadose zone. The successful installation of the fiber optic cable within the boreholes poses another significant challenge. The cable must be carefully deployed to minimize the risk of breakage or damage during installation. Specialized techniques and equipment may be required to handle the cable delicately, ensuring its integrity throughout the entire length. In summary, addressing challenges related to optical fiber reliability, cable installation integrity, and precise warm water injection is critical for the success of the experiment. Implementing proactive monitoring, rigorous installation practices, and contingency plans will contribute to overcoming these challenges and obtaining robust and reliable results from the research study.