Groundwater flow paths in fractured crystalline bedrock : Electromagnetic VLF measurements and modelling of a groundwater basin in Svanberga, Sweden

Abstract: Sweden is a country with a relatively high number of private wells, where about 1.2 million inhabitants in permanent housing and an equal amount in summer housing relies on private wells as their drinking water supply. At the same time the market for drinking water treatment products is unregulated regarding quality and requirements are needed. A testing facility with the aim of providing quality certifications as well as sustainability- and efficiency- tests of small scale drinking water treatment techniques is under development and located in a decommissioned groundwater treatment plant in Svanberga, outside Norrtälje, Stockholm County, Sweden. The groundwater treatment plant has three operational bedrock wells connected to it and the groundwater system is rather unknown with only a few previous studies done in the region. Better understanding of the hydrogeological system would aid in further work of evaluating the risks of contaminant spread in the region. Trough fracture outcrop mapping, geophysical Very Low Frequency measurements and groundwater modelling using COMSOL Multiphysics this master thesis have identified several possible flow paths within the bedrock connected to the GWTP. The results indicates there are one or two approximately east-west striking fracture zones that could play a major role in transportation of contaminants related to road traffic and agriculture to the wells, while several north to south striking fracture zones most likely supplies the groundwater basin with water from the thicker soil layers in the northern part of the region. Recommendations of future studies includes conducting a detailed pumping test of the operational wells in Svanberga old GWTP as well as verification of the presumed most important identified fracture zones to the south east and north in the studied area by additional geophysical investigations or  tracer tests.