Evaluating the potential of artificial infiltration to increase groundwater storage using MIKE SHE : A case study in northern Storsudret, Gotland

Sammanfattning: The island of Gotland, Sweden, is struggling with supplying the population with freshwater during summers. At the same time as the demand increases during these months, the warm weather and geological conditions prevent sufficient volumes of groundwater to recharge. The Swedish Environmental Research Institute is conducting a research project together with Region Gotland, aiming to solve the freshwater supply problem in Storsudret, located on southern Gotland, by investigating the suitability of different water management techniques. In this thesis, the impacts of artificial infiltration on the groundwater level have been investigated in northern Storsudret, where a sandy deposit has been identified to possibly be suitable for increased groundwater storage. The modelling software MIKE SHE was used to create a groundwater model to assess the suitability of the area. Further, an assessment of using MIKE SHE for this purpose was done. Electrical resistivity measurements and Ground Penetrating Radar were used in field, to identify the geological conditions and conceptualize the domain. The model was based on the field data, as well as data from e.g. SMHI, Lantmäteriet and SGU. Based on the model simulations, locations were evaluated to find suitable areas for artificial infiltration during spring, and for groundwater extraction during summer. The results indicate that the sand deposit present in the domain is thin, leading to high evapotranspiration during spring and summer. Artificial infiltration elevates the groundwater level in a limited proximity to the infiltration point. However, the difference in groundwater level decreased during July and the conditions were the same as without artificial infiltration in August. Maximal water extraction during June, July, and August, did not seem to impact surrounding groundwater levels significantly. This suggests that the combination of low hydraulic conductivity and inadequate thickness of the sand limits the possibilities to use the deposit as an aquifer. MIKE SHE was assessed to perform well, when comparing the water balance with historic data. It was evaluated to be suitable for projects of this character, but limited research and documentation on artificial infiltration in MIKE SHE was found. However, to validate the groundwater flow simulation, sufficient groundwater observation data is required.