Advective transport times in the river basin of lake Vombsjön

Sammanfattning: The raw water source lake Vombsjön is situated in the center of southern Scania and provides drinking water by artificial groundwater recharge for approximately 450 000 inhabitants each day. More than 2800 households within the catchment of lake Vombsjön treats their domestic wastewater with a decentralized sewer system. The configuration of these sewer systems varies, but it is common to have some kind of soil infiltration unit as one of the treatment steps from which treated wastewater infiltrates through a soil bed towards the groundwater table. However, depending on the status of the wastewater treatment systems, contamination in form of nutrients, household chemicals, pharmaceuticals and pathogens originating from private households risk spreading with groundwater transport. This master thesis has been carried out in collaboration with the research company Sweden Water Research (owned by Sydvatten AB among others) and aims to calculate the transport times by advective transport from source to closest water body discharging into lake Vombsjön. This represents a conservative approach of contaminant transport in the catchment area since advective transport describes the average linear velocity of groundwater in one dimension without regarding coefficients for retardation. The advective transport times were computed using Geographical Information System (GIS), in this case the software ArcMap was utilized. Transport times were modeled according to two cases depending on prevailing properties of geological deposits within the catchment, a median case indicating probable characteristics of deposits and a worst case referring to more permeable deposits. The catchment area of lake Vombsjön can roughly be divided into two geological areas; the Vomb basin consisting of permeable glaciofluvial sediments and the shale plateau that is dominated by less permeable till and clay till. The fastest transport times for groundwater, modeled in the worst case scenario, varies between half a day up to one week at locations situated in permeable deposits along the water bodies found in the Vomb basin. Substance transport from any point located in the Vomb basin will reach the stream network within six months at most. However on the shale plateau, velocities up to over 100 years were found. In order to elaborate the modeled transport times it is recommended that the geological and hydrogeological properties within the catchment is further investigated.