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Workreport 2016-44

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Name:

Results of Monitoring at Olkiluoto in 2015 Hydrogeochemistry

Writer:

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

Language:

English

Page count:

332

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 2015.

Shallow groundwater monitoring continued 2015. The pH values varied from acidic (6.1) to slightly alkaline (8.4). The pH values over 8 were measured in OL-PP56. High sulphate concentrations were measured in few shallow groundwater sampling points, noteworthy high in OL-PP56 (700−860 mg/L). However, concentration was clearly lower than in 2014. 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. A slightly upward trend in TDS, observed near Olkiluoto Natura area (and the old forests preservation area), has evened out in OL-PVP4A, but continued to increase in 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 and from ONKALO-tunnel during 2015. 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 concentration of 49 mg/L as well as acetate concentration (81 mg/L) was measured from OL-KR46_570. 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. This has caused favourable conditions for sulphate reduction (sulphide production). The high sulphide level in OL-KR13_T360 (HZ001) decreased notably from initial samplings (12 mg/L in 2001) until 2012, but elevated again in 2015 (2.4 mg/L). In recent years, one important observation has been a strong dilution of hydraulic zone HZ20. Dilution was still observed during 2015. However, the most significant changes were stabilising and dilution was not anymore as strong or salinity started to slightly increase in OL-KR22_T419, OL-KR25_T337 and OL-KR25_T357. In these drillholes water type has changed into fresh/brackish HCO3 -water in abnormally deep depths during previous years. The dilution effect of HZ20 was also seen in brackish Cl-type water samples, although the changes were not as considerable as in HCO3-type waters. The deep groundwater sample OL-KR1_980 (-902 m) had the highest TDS (ca. 130 g/L) measured from Olkiluoto so far, with exceptionally high potassium (94 mg/L) and sulphate (275 mg/L) concentrations. Calculated mineral saturation indices suggested that gypsum and/or barite could be possible sources for sulphate in OL-KR1_980. Barite has been observed occasionally in fractures of the OL-KR1drillcore.

The results of the monitoring programme in ONKALO during 2015 have mainly behaved predictably and remained unchanged or in a state of slow change in terms of samples collected from ONKALO. In general, the slightly increasing DIC concentrations in the upper part of ONKALO, decreasing pH and relatively stable calcium concentration during recent years indicate the buffering role of calcite to prevail calcite equilibrium in shallow hydrogeochemical system. The results for ONK-PVA9 at repository level (-426 m) varied a lot during 2015, but the trend was towards lower salinity and higher sulphate concentration. Also sulphide was observed first time in ONK-PVA9. The sampling location is connected to the vertical water conductive zone OL-BFZ045 in which the groundwater flow and chemistry seem to be strongly responsive to new drillings (ONK-KR16 in 2013 and ONK-PH28 in 2015) intersecting the fracture. Besides ONK-PVA9, sampling points ONK-RV4385 and ONK-KR16_14 connected also to OL-BFZ045 were showing dilution in salinity and an increase in sulphate concentration. In ONK-PH28, also acetate was observed up to concentration of 36 mg/L. Sulphate concentration in ONK-KR1 continued the increasing while the SO4 increase stalled in ONK-PVA3. The increase of sulphate concentration in ONK-PVA7 continued, via the hydraulic connection to OL-HZ056. The highest sulphide concentration in ONKALO was measured from ONK-PVA8 (3.3 mg/L). This groundwater station has had an elevated sulphide content during the whole monitoring period, but the 2015 measured sulphide concentration was the highest. The sampling point ONK-PP321 (REPRO-program) had an abnormally heavy isotopic composition for a groundwater of such depth (-403 m). It represents a low transmissivity fracture and heavy isotopic signature clearly above the GMWL refers to a longer residence time and significant water-rock interactions.

Altogether 21 gas samples were collected from deep drillholes and ONKALO during the year 2015. The samples collected using the SWA method typically yielded lower dissolved gas contents compared to PAVE samples. Helium contents were depleted in the 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. According to the interpretations systematic increasing resistivity values can be detected in the area to the SE of ONKALO indicating groundwater dilution. In addition to this, systematic decreasing resistivity values can be detected mainly to the NW of ONKALO indicating possible increase of groundwater salinity along the hydraulically conductive zones. The results showed also that the changes have been clearly weaker after 2010 and even stabilised at some stations.

Keywords:

Hydrogeochemistry, ONKALO, monitoring, groundwater, gas sampling, analysis results, Sampo electromagnetic soundings

File(s):

Results of Monitoring at Olkiluoto in 2015 Hydrogeochemistry (pdf) (28.3 MB)


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