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Workreport 2014-44

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Name:

Results of Monitoring at Olkiluoto in 2013 Hydrogeochemistry

Writer:

Lamminmäki, T., Pitkänen, P., Penttinen, T., Loimula, K., Partamies, S., Ahokas, T.

Language:

English

Page count:

304

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 2013.

Shallow groundwater monitoring continued 2013 (41 monitoring and 13 infiltration area samples). The pH values varied from acidic (5.2) to slightly alkaline (7.9). A slightly upward trend in TDS has been still observed near Olkiluoto Natura area (and the old forests preservation area) in OL-PVP4A and OL-PP2. 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 16 different drillholes (26 samples) and from ONKALO (44 samples) during 2013. The results 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 observed in several sampling points during 2013. The high sulphide level in OL-KR13_T360 decreased from initial samplings (2001; 12 mg/L) to near detection limit during years but slightly increased to value of 0.57 mg/L again in 2013. Another sampling section of the same drillhole, OL-KR13_T405, was sampled for the first time in 2013 and an elevated sulphide concentration (12 mg/L) was measured. These two sections are in hydraulic connection with each other. The multipacker system was installed in late 2007 which has prevented artificial mixing along the drillhole OL-KR13. OL-KR46_570 was also sampled for the first time in 2013. An exceptionally high sulphide concentration (31 mg/L) was measured. The section 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 making the conditions favourable for sulphate reduction (sulphide production). Another section of the same drillhole OL-KR46_493 (also contaminated during open drillhole phase) had one of the highest sulphate concentrations (736 mg/L) measured from groundwaters in Olkiluoto. The sulphide content in OL-KR23_T425 (HZ20A) has been increasing since 2010 when it was 0.25 mg/L. In 2013 sampling the sulphide content was 2.5 mg/L. The salinity changes in this section have been minor since the monitoring started. However, small input of SO4 has been observable since 2010. Another important observation at Olkiluoto has been groundwater dilution, especially via hydraulic zone HZ20(A+B). The 2013 results from OL-KR22_T420 (HZ20B) showed that the strong dilution observed earlier was still continuing. However, the section OL-KR20_T110 (HZ20A), father from ONKALO, had been slightly concentrating since the previous sampling in 2008.

The results of the monitoring programme in ONKALO during 2013 have mainly behaved predictably and remained unchanged or in a state of slow change in terms of samples collected from ONKALO. The increasing sulphate levels in ONK-KR1, ONK-PVA3 and ONK-PVA8 stabilised, whereas the sulphate in ONK-KR3 continued to increase (back to the original level). Sulphate in ONK-PVA5 continued to increase via the subvertical fracture zone OL-BFZ100. Elevated sulphide concentrations (few mg/L) have been measured in ONK-PVA8 during the whole monitoring period. The increased sulphate concentrations (above the monitoring action limit) were still observed in ONK-PVA9 and ONK-RV4385 at the repository level. Both of these sampling points are connected to the vertical water conductive fracture zone OL-BFZ045.

The monitoring results of dissolved gases during 2013 (altogether 32 samples), from samples collected with the PAVE technique, corresponded well with the earlier data collected at Olkiluoto. The samples collected with the SWA technique generally gave lower results and the usability of the SWA technique should be paid special attention to in future monitoring samplings.

Gefinex 400S (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 were due to the changes of the groundwater salinity. According to the modelling, systematic increasing resistivity values can be detected in the area to the SE of ONKALO indicating a dilution of groundwater. In addition to this, systematic decreasing resistivity values can be detected mainly to the NW of ONKALO indicating possible rising groundwater salinity along the hydraulically conductive zones. However, upconing of saline groundwater has not been observable in groundwater samplings. The results of SAMPO electromagnetic soundings also showed that the changes in resistivity have been clearly weaker after 2010 and even normalised at some stations.

Keywords:

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

File(s):

Results of Monitoring at Olkiluoto in 2013 Hydrogeochemistry (pdf) (30.8 MB)


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