Investigation of sulphide in core drilled boreholes KLX06, KAS03 and KAS09 at Laxemar and Aespoe: Chemical-, microbiological- and dissolved gas data from groundwater in four borehole sections

This report describes a study performed during 2009 which focused on the production of sulphide (microbial sulphate reduction) in deep groundwater that was implemented in the core drilled boreholes KLX06, -475 to 482 meter above sea level, m a s l, KAS03,-97 to 241 and -613 to 984 m a s l, and KAS09, -96 to -125 m a s l, at Laxemar and Aspo. The study aimed to increase knowledge of background groundwater levels of sulphide and its variations in time and space through the analysis of sulphide and parameters related to sulphide production. Sampling of groundwater was conducted in three core drilled boreholes of varying age as time series with continuous pumping and as single samples. The analysis program covered chemical parameters (pH, chloride, sulphate, iron, and organic carbon), dissolved gas composition, stable isotopes in groundwater (δ2H, δ18O, δ34S, δ13C), stable isotopes of gaseous compounds (δ2H, δ13C, δ18O), microbiological parameters (sulphate- and iron reducing bacteria, SRB and IRB), phthalates and low molecular mass organic acids (LMMOA). The sampling in KLX06 was carried out as time series with a 9 week pause in pumping. When the water volume discharged was about 150 times that of the packer-isolated borehole section, sulphides decreased from 7 mg L-1 to 0.05 mg L-1 and the salinity increased from 740 to 1,480 mg L-1. After a 9 weeks pause in pumping, the sulphide concentration and salinity again approached the original values, i.e. 7 mg L-1 of sulphide and 450 mg L-1 of chloride. The SRB and IRB showed high concentrations that were reduced during pumping in the borehole. The water in the standpipe which has a different water composition than the groundwater, also showed similar high concentrations of sulphide and SRB. The standpipe is a plastic pipe in the wider upper part of the borehole; connected with the tube from the packer of the borehole section and used to accommodate a filter and a groundwater pump when collecting samples. Analyses of δ34S in dissolved sulphide and sulphate showed a fractionation corresponding to about +20 per mille, which is expected for open systems where microbial sulphate reduction occurs. Analyses of dissolved gases showed that those gases that are biochemically active (carbon dioxide, hydrogen and methane) decreased in concentration during pumping, while the concentrations of gaseous compounds such as nitrogen and argon were unchanged. In KAS03 and KAS09, drilled and equipped at the end of the 1980s, the installed equipment was lifted up and inspected visually following completion of sampling. Water standpipes were partially filled with black sludge; connection pipes and tubing were covered with deposits that seemed to be of salt and rust. Analyses of the water in the standpipes reflected the conditions in which water was not exposed to the impact of pumping for a couple of years back. The concentrations of SRB were high (> 104 cells mL-1), especially in KAS09, where sulphides were sometimes also very high . between 92 and 102 mg L-1. A comparison of the ionic product of iron (II) and sulphide with the saturation indices of amorphous and crystalline (mackinawite) mono sulphides shows that the measured sulphide in KAS09 results in significant supersaturation. Given the fast kinetic processes for precipitation of mono sulphides, supersaturation is unlikely. A possible reason for the supersaturation, which was mainly observed in samples with high content of organic matter and particulates, could be that the analysis of sulphide most likely included both dissolved and particulate sulphide. Overall, this study has shown that elevated sulphide concentrations in core drilled boreholes may occur in the periods between pumping. Different chemical and physical conditions prevail in the isolated borehole section, in the tubes and in the standpipe as compared with the surrounding rock fractures. For example, new surfaces and materials are added (drilled borehole walls and installed equipment), chemical gradients may be generated during pumping and by differences in composition between standpipe and section water. Further studies are required in order to understand what parameters are important for the growth of sulphate reducing bacteria in the borehole sections, tubes and standpipe, and to demonstrate the relative importance of the possible reductants, hydrogen, methane, acetate, biogenic carbon, and organic carbon in equipment material, which are involved in the reduction of sulphate.