Evaluation of Darcy Flow Velocities Obtained from Borehole Dilution Tests

Abstract: The project is part of a larger investigation of the water protection area Donauried-Hürbe. This area is a recharge zone used by one of Germany’s biggest water supply companies, the Zweckverband Landwasserversorgung (LW). The freshwater supply originates from a 200 m thick karst aquifer in the Swabian Alb (Schwäbische Alb) and flows southest towards the Donauried and into a porous aquifer. The long-term research goal of the investigation is to define the flow time and flow path for the water moving from the recharge area to the main discharge area, to update protection zone boundaries and prevent contamination. Groundwater flow patterns are being studied using tracer tests. However, finding suitable borehole for tracer inputs requires knowledge of subsurface flow behavior in these boreholes. This knowledge can be obtained by carrying out single borehole dilution tests(SBDT). The focus of this project was to determine the flow connections between several candidate boreholes and the aquifer, by calculating the Darcy flow velocity (also called filtration velocity) obtained from SBDTs. In some wells, sections with particularly high groundwater flow were detected. Such identified locations would therefore be recommended for tracer tests. Based on results of the electrical conductivity development, total salt quantity development and Darcy flow velocity, four wells were identified as suitable for tracer tests (measurement wells 5303, 5312, 7721 and 7733). Another studied well measurement well 2301) was identified as less suited for tracer tests due to low Darcy flow velocity. Even though Darcy flow velocity is normally used for groundwater flow in non-karstic aquifers, it seems to work relatively good for the karst aquifer wells 7721 and 7733. Three different injection methods (stocking method, hosepipe method and point injection method), and two different measurement methods (Electrical Conductivity Meter and CTD-Diver) for SDBTs were investigated. Compared to the stocking method, the hosepipe method did not deliver any greater improvements for equally distributed salt injections. Due to this and the advantages that the stocking method has in terms of costs, technique handling, and preparation, it is a preferred injection method. The point injection method worked relatively well, based on problems met in the experiments of this thesis, but will work better in the future by developing an improved point injection device. Handling a CTD-Diver compared to an Electrical Conductivity Meter is more practical and time effective. However, the Electrical Conductivity Meter is preferred for SBDTs in boreholes where the groundwater flow characteristics are still relatively unknown. Based on information from point injection tests in a lab hall, density effects during SBDTs could be neglected. In future experiments, SBDTs should be carried out in the field by combining different factors such as seasonal differences, salt quantity, and injection method. This would increase the understanding of how the filtration velocity may vary depending on different factors. From the results, it seems as if a more reliable model of the salt development quantity might be obtained when the measurements of electrical conductivity are made more frequently and during a longer time.