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Workreport 2015-44

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Name:

Results of Monitoring at Olkiluoto in 2014 Hydrogeochemistry

Writer:

Lamminmäki, T., Pitkänen, P., Penttinen, T., Komulainen, J., Loimula, K., Wendling, L., Partamies, S., Ahokas, T.

Language:

English

Page count:

326

Summary:

The construction work of underground research facility ONKALO started in the June 2004. Possible changes caused by the construction of the disposal facility in the chemical environment in shallow and deep groundwaters are monitored on a regular basis. This report presents the hydrogeochemical monitoring measurements and observations made in 2014.

Shallow groundwater monitoring continued 2014. The pH values varied from acidic (5.9) to slightly alkaline (7.9). High sulphate concentrations were measured in few shallow groundwater sampling points, noteworthy high in OL-PP56 (390−1720 mg/L). The changes in the chemistry of OL-PP56 were related to the construction of the parking area on the ground surface. Most probably the crushed rock from ONKALO used in the construction work contains sulphide minerals which may have oxidized and produced sulphate. Previously observed a slightly upward trend in TDS evened out in OL-PVP4A and OL-PP2 near Olkiluoto Natura area (and the old forests preservation area) during 2014. On the basis of the isotope results δ18O and δ2H, the waters of OL-PVP12, OL-PVP30 and OL-PP3 are affected by the Korvensuo reservoir water.

Deep groundwaters were monitored with samplings from 17 different drillholes and from ONKALO-tunnel during 2014. The results of deep drillholes from ground surface showed some indications of changes in groundwater compositions, which in many cases are caused by high hydraulic gradient due to ONKALO. Elevated sulphide concentrations were again observed. High sulphide concentrations of 30 mg/L and 16 mg/L were measured from OL-KR46_570 and OL-KR13_T405, respectively. The section OL-KR46_570 intersects HZ056 and hydraulic connection to ONKALO has caused drawdown of SO4 rich groundwater along drillhole and mixing with the saline type groundwater at this depth. This has caused favourable conditions for sulphate reduction (sulphide production). Dilution of hydraulic zone of HZ19 was not anymore explicitly observed. The slight dilution in salinity and decrease in sulphate concentration continued in OL-KR25_T91 (HZ19) and the stable isotopic composition trending towards heavier values indicated an influence of increasing amounts of Korvensuo water in the sampling section. An influence of Korvensuo water was also seen in isotopic composition of OL-KR35_57 and OL-KR35_77. The dilution of OL-KR37_T165 (HZ19) had stopped and the chemical composition returned towards the initial composition. In recent years, one important observation has been a strong dilution of hydraulic zone HZ20. In 2014, it was observed in the HZ20 intersections OL-KR22_T419, OL-KR23_T425, OL-KR25_T357, OL-KR25_T337 and OL-KR28_T386. Due to dilution and mixing, sulphide production has launched and elevated sulphide results were measured in OL-KR22_T419 (2.8 mg/L) and OL-KR23_T425 (1.4 mg/L). However, strong dilution of OL-KR25_T337 with increasing DIC and tritium concentrations indicating near surface groundwater addition, might be result of technical failure in packer system (a leakage from one of three packer sections above). The deep groundwater sample OL-KR1_977 (-919 m) had the highest TDS (127 g/L) measured from Olkiluoto so far. It had also exceptionally high sulphate, which may have a mineralogical origin (few baryte observations in fracture fillings).

The results of the monitoring programme in ONKALO during 2014 have mainly behaved predictably and remained unchanged or in a state of slow change in terms of samples collected from ONKALO. ONK-PVA9 started to concentrate during 2014 after the drilling of ONK-KR16 nearby. Both of these sampling points are connected to the vertical water conductive fracture zone OL-BFZ045. However, the drilling induced salinity increase in ONK-PVA9 levelled out by the end of 2014. The most notable increases in sulphate concentrations were observed in ONK-KR1, ONK-PVA3 and ONK-PVA7. High sulphate concentrations in most upper part of ONKALO (ONK-KR1 and ONK-PVA3) were possibly derived similarly from construction works on ground surface as in shallow groundwater data and use of crushed rock from ONKALO, when sulfide minerals are oxidized. The highest sulphide concentration in ONKALO was measured from ONK-PVA8 (2.5 mg/L). This groundwater station has had an elevated sulphide content during the whole monitoring period. Elevated sulphide contents were also measured in ONK-K16_14 (1.6 mg/L) and ONK-PVA5 (0.7 mg/L).

Altogether 35 gas samples were collected from deep drillholes and ONKALO during the year 2014. The Posiva Flog Log Groundwater Sampler (PFL-GWS) method was introduced to the gas sampling programme. One drillhole section (OL-KR47_711) was sampled with the PFL-method along with the previously used PAVE and SWA sampling methods. The samples collected using the SWA method typically yielded lower dissolved gas contents compared to the two other methods. The sample collected with the PFL-method (without pumping) had the highest concentration of H2 indicating that hydrogen is partially lost during pumping and not fully observed with PAVE and SWA techniques. The PFL-method needs more testing and data to evaluate its representativity. Helium contents were depleted in the 2014 ONKALO samples when compared to the baseline data. This may be due to diffusion of lighter gases from groundwater to tunnel in short sampling distances from the tunnel wall, where hydraulic pressure has been substantially decreased.  

The results of the Sampo electromagnetic soundings have given information of the resistivity changes in the bedrock. In spite of the noise caused by the ONKALO construction work and new power lines and cables in the survey area the detected changes in the resistivity values at some monitoring stations are probably due to the changes of groundwater salinity. The stations for which the interpretations indicate increasing resistivities occur mostly to the east of the ONKALO excavation site, whereas the stations mostly with decreasing resistivities occur to the west of the ONKALO excavation site. The new results were consistent with the results in 2013.

Keywords:

Hydrogeochemistry, ONKALO, monitoring, groundwater, gas sampling, analysis results, Gefinex S400.

File(s):

Results of Monitoring at Olkiluoto in 2014 Hydrogeochemistry (pdf) (26.8 MB)


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