Analys av stabila isotoper med beräkning av blandningsförhållande i ett grundvattenmagasin i Älvkarleby-Skutskär

Sammanfattning: Sweden has in recent years suffered record-low groundwater levels with imposed water restrictions. One of the municipalities who have had problems with their drinking-water supply is Gävle. They have hired Midvatten AB to review a possible location that for groundwater extraction in Älvkarleby-Skutskär. The Uppsala Esker, which contains a lot of groundwater, passes through the area. Parallel to the esker the river Dalälven flow. The quality of surface water in general in poorer than groundwater, therefore it is of interest to investigate the hydraulic contact between the groundwater and the surface water. In this study, the stable isotopes [sup]16[/sup]O, [sup]18[/sup]O, [sup]1[/sup]H and [sup]2[/sup]H have been reviewed. These isotopes follow the water unaffected, therefore they can be used as a tool to investigate the origins of water. The method of this study has been to review literature, process data and apply calculations. By applying a mixing-model on isotope ratios and the parameters chloride, conductivity and magnesium , a mixing-ratio could be calculated which quantified how many % of the groundwater that has its origin from surface-water. Results from Dalälven show statistically significant higher δ[sup]18[/sup]O and δ[sup]2[/sup]H than the groundwater, which is due to that Dalälven has more exchange with the atmosphere, and therefore is enriched in the heavier isotopes. This confirms the possibility to use stable isotopes to distinguish surface-water from groundwater. The groundwater in Br1301 was calculated to have over 50 % of its origin from Dalälven. It is not likely that surface water infiltrated to other areas due to long distance, higher groundwater-levels and presence of silt. Results from Br1301 and Dalälven show variations that were interpreted as seasonal. From these variations, a travel-time for the surface water to infiltrate the basin was estimated to 34-130 days. The mixing ratio calculated with the results of magnesium and conductivity showed a surface-water component in the groundwater. The mixing ratio calculated with the results of chloride showed negative surface-infiltration.