Modellering av grundvattendynamiken och transport av löst organisk kol i Uppsalaåsen

Uppsala esker is the main source of drinking water in Uppsala city and it provides the city with clean water all year around. The demand of drinking water is growing, and the municipality plans on increasing the artificial infiltration in order to meet future demands. During the last years, the concentration of organic carbon in Uppsala’s drinking water has increased which has raised concerns regarding the future drinking water quality. A decrease in the residence time as a result of increased infiltration may partly cause these increasing concentration levels. The aim of this master’s thesis was therefore to recreate the groundwater dynamics in the Uppsala esker with a model and hence, an improved understanding of the transport of organic carbon in order to predict the consequences of an increased artificial infiltration. The thesis also aimed to investigate the potential risk of the concentration of organic carbon in the drinking water to exceed the reference value from the Swedish food agency. The computer code used in this project was MODFLOW together with GMS which together helped creating a simplified, three-dimensional groundwater model of a delimited part of the Uppsala esker that covers Tunåsen infiltration facility down to the well area in Galgbacken. A conceptual model was constructed in GMS for the model domain and was then converted into a numerical MODFLOW steady-state model. The model was then calibrated after both measured groundwater levels with a 40 cm deviation and after the already known residence time for the distance Tunåsen – Galgbacken. Four scenarios with varying infiltration and outtake were then simulated. Each scenario was then simulated with three different incoming concentrations of organic carbon in the infiltration: 7, 15 and 50 mg/L. Results show that the groundwater dynamics can be reconstructed with a simplified model however, it is likely that the simplifications resulted in a less precise model. The transport simulations indicated that the residence time decreases with increased artificial infiltration and outtake. Transport simulations furthermore showed that residence time is the most crucial factor effecting the transport distance of the organic carbon. Lastly, the result indicated that there is a risk that the reference value for organic carbon will be exceeded for incoming concentrations of 15 and 30 mg/L respectively, mainly in scenario C where the maximum infiltration and outtake capacities were simulated. In