Evaluating hydrochemical data from shallow groundwater in Laxemar from a microbiological perspective

Oxygen is one of the chemical species that can corrode a copper canister in a KBS-3 repository. It is therefore important to determine whether oxygen dissolved in precipitation or groundwater could reach repository depth by means of groundwater transport. This can be determined by gaining an understanding of the oxygen-consuming microbial processes that take place in shallow groundwater in the area of interest. This report evaluates hydrogeochemical data from shallow groundwater in the Laxemar area from a microbiological perspective. Hydrogeochemical data were gathered from soil pipes at depths from 1.6 to 16.5 m and from percussion-drilled boreholes having mid-point depths of between 28.5 and 131 m. Only a few of the percussion-drilled boreholes had packers installed; the sampled sections were therefore very extended, allowing groundwaters from many different depths to mix. Oxygen and oxygen reduction potential (ORP) were not measured in groundwater from soil pipes or percussion drilled boreholes. The report therefore focuses on parameters that indicated ongoing anaerobic microbial processes, such as nitrite, ferrous iron, dissolved manganese, and sulphide, that were found in many soil pipes. Even though many of the soil pipes were located in similar environments and at relatively similar depths, ranging from 3.5 to 6 m, they displayed individual chemical profiles in terms of chemical species related to microbial activity. The microbial activity could not be linked to the classes of soil pipe, i.e. recharge, discharge, or intermittent. Existing soil pipes and percussion-drilled boreholes could be used for additional sampling to measure microbial parameters. Such sampling would benefit from the careful hypothesis-driven description of the sampling parameters and experience-guided choice of sampling methods.