Comparative application and optimization of different single-borehole dilution test techniques
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Comparative application and optimization of different single-borehole dilution test techniques Nikolai Fahrmeier 1 & Nadine Goeppert 1 & Nico Goldscheider 1 Received: 5 March 2020 / Accepted: 9 November 2020 # The Author(s) 2020
Abstract Single-borehole dilution tests (SBDTs) are a method for characterizing groundwater monitoring wells and boreholes, and are based on the injection of a tracer into the saturated zone and the observation of concentration over depth and time. SBDTs are applicable in all aquifer types, but especially interesting in heterogeneous karst or fractured aquifers. Uniform injections aim at a homogeneous tracer concentration throughout the entire saturated length and provide information about inflow and outflow horizons. Also, in the absence of vertical flow, horizontal filtration velocities can be calculated. The most common method for uniform injections uses a hosepipe to inject the tracer. This report introduces a simplified method that uses a permeable injection bag (PIB) to achieve a close-to-uniform tracer distribution within the saturated zone. To evaluate the new method and to identify advantages and disadvantages, several tests have been carried out, in the laboratory and in multiple groundwater monitoring wells in the field. Reproducibility of the PIB method was assessed through repeated tests, on the basis of the temporal development of salt amount and calculated apparent filtration velocities. Apparent filtration velocities were calculated using linear regression as well as by inverting the one-dimensional (1D) advection-dispersion equation using CXTFIT. The results show that uniforminjection SBDTs with the PIB method produce valuable and reproducible outcomes and contribute to the understanding of groundwater monitoring wells and the respective aquifer. Also, compared to the hosepipe method, the new injection method requires less equipment and less effort, and is especially useful for deep boreholes. Keywords Groundwater flow . Karst . Borehole techniques . Dilution test . Single-well method
Introduction Investigation of boreholes with single-well methods plays an important role in hydrogeological aquifer characterization, especially in large and deep aquifers. Single-borehole dilution tests (SBDTs) are an easy-to-apply method for characterizing monitoring wells and boreholes and are based on the injection of a tracer into a borehole and the observation of the decreasing tracer concentration over time and depth. As a result, different flow regimes, in- and outflow horizons, and vertical
Published in the special issue “Five decades of advances in karst hydrogeology” * Nikolai Fahrmeier [email protected] 1
Institute of Applied Geosciences, Division of Hydrogeology, Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany
flow in a borehole can be identified (Halevy et al. 1967; Freeze and Cherry 1979). When vertical flow components are negligible or absent, SBDTs can also be used to determinate horizontal filtration velocities and their v
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