Major and trace elements in surface and ground waters in two near-coastal granitoidic settings in eastern Sweden

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Ronnback, Pernilla
Holding Date 07 October 2008

This study focuses on the hydrochemistry of surface water and groundwater at two different areas in eastern Sweden, Forsmark, which has a carbonate-rich till and Oskarshamn with a carbonate-poor till. In these areas the Swedish Nuclear Fuel and Waste Management Company (SKB) is currently conducting extensive scientific investigations and environmental monitoring. The goal with these investigations is to find out which of these two areas is most suitable for the construction of a bedrock repository for spent nuclear fuel. The hydrochemical data collected within these investigations has been used in this study, in order to increase the understanding of how natural waters in near-coastal granitoidic settings are affected by various processes.
The result showed that in streams there were strong temporal (4 years) patterns of major solutes with some inversely correlated with flow (e.g. Na, Mg, K, Cl- and Br-), others showing summer maxima (e.g. Fe and DOC) and still others summer minima (Mn, NH4+, NO3- and SO42-). A lake (Bolundsfjärden) in Forsmark that was studied in more detail had both similar and dissimilar hydrochemical features to that of its inflowing stream (Kungsträsket). Another stream (Bredviken) in Forsmark had a strongly different hydrochemical signature to that of all other surface waters in both areas. This is interpreted as being the result of oxidation of biogenic metal sulphides in fine-grained sediments and associated increased weathering rates of the surrounding carbonate-rich till. Rare earth elements (REEs), which consist of analogues for several actinides (e.g. Am3+ and Cm3+) were strongly enriched in overburden groundwater as compared to surface water and bedrock groundwater, and their fractionation patterns were diverse indicating complex and spatially variable controls. Speciation modelling predicted strong REE association with dissolved organic matter, which to some extent contrasts previous studies emphasizing carbonate complexation. The uranium data for waters in Forsmark and Oskarshamn were compared with that of other surface and ground waters throughout Boreal Europe. Despite strongly elevated U concentrations at certain depths (100–650 m) in the Forsmark bedrock groundwater, these concentrations were not considered anomalous as similar values are not unusual elsewhere in this region.
To conclude, the patterns identified in this study have increased the understanding of the chemical dynamics of natural waters in near-coastal granitoidic settings in the boreal zone. In addition, since either Forsmark or Oskarshamn will be suggested as a site for the construction of a bedrock disposal for spent nuclear waste, the processes and patterns identified in this study provide valuable information also in that sort of context.