Pumping Test in Drillhole OL-KR29 in Summer 2016

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POSIVA OY

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FI-27160 EURAJOKI, F INLAND

Phone (02) 8372 31 (nat. ) , (+358-2-) 8372 31 ( int. )

Fax (02) 8372 3809 (nat. ) , (+358-2-) 8372 3809 ( int. )

May 2018

Working Report 2017-48

El ias Pentti , Perttu Pulkkinen, Kyösti Ripatt i

May 2018

Working Reports contain information on work in progress

or pending completion.

El ias Pentti , Perttu Pulkkinen, Kyösti Ripatt i

Pöyry Finland Oy

Working Report 2017-48

Pumping Test in Drillhole OL-KR29 in Summer 2016

PUMPING TEST IN DRILLHOLE OL-KR29 IN SUMMER 2016

ABSTRACT

Posiva Oy is starting the excavation of the first deposition panel of its repository for spent nuclear fuel in Olkiluoto. To investigate hydraulic connections in the rock volume reserved for the first panel, a pumping test was conducted in drillhole OL-KR29 in 2016. Inflatable packers were installed to isolate the pumping section between 562–570 m of drillhole length, chosen to focus the effect of the pumping to zone HZ039 of the hydrogeological structure model.

Technical problems delayed the start of the test, and the formation of bubbles from dissolved gasses in the pumped groundwater eventually necessitated a change of pumping method from the planned submersible pump to the less effective airlift pumping. This reduced the achievable drawdown in the pumping section from the planned 20 m to below 10 m.

Two long-term pumping periods were conducted, the first one lasting for about 9 days and the second one for 21 days. Their effect was monitored by means of the regular hydraulic head monitoring in the nearby deep drillholes. Drawdown caused by the test was detected in the deepest monitoring sections of OL-KR4 and -KR7. Both affected sections are relatively long and intersect the deeper zone HZ21 as well as the assumed extension of HZ039, so that it remains uncertain, which hydraulic connection explains the observations. In addition to data from deep drillholes, the results of the pressure monitoring of the packed-off drillholes in the ONKALO were examined, without finding any indication of an effect of the pumping test. Keywords: hydrogeology, pumping test, final deposition of spent nuclear fuel

KAIRAREIÄN OL-KR29 PUMPPAUSKOE KESÄLLÄ 2016

TIIVISTELMÄ

Posiva Oy on aloittamassa ensimmäisen loppusijoituspaneelin louhintaa Olkiluotoon rakennettavaa käytetyn ydinpolttoaineen loppusijoituslaitosta varten. Paneelille varatun kalliotilavuuden hydraulisten yhteyksien tutkimiseksi vuonna 2016 tehtiin pumppaus-koe kairareiässä OL-KR29. Pullistettavien tulppien avulla reiästä eristettiin pumppausta varten väli 562–570 m, joka oli valittu siten, että pumppauksen vaikutus kohdistuisi hydrogeologisen rakennemallin mukaiseen vyöhykkeeseen HZ039.

Tekniset ongelmat viivästyttivät kokeen alkua, ja pumpattavaan pohjaveteen liuen-neiden kaasujen kupliminen pakotti lopulta vaihtamaan pumppausmenetelmän suunni-tellusta uppopumpusta tehottomampaan mammutointipumppaukseen. Saavutettavissa oleva alenema pumpattavassa tulppavälissä aleni siksi 20 metristä alle 10 metriin.

Pitkäaikainen pumppaus koostui kahdesta jaksosta, joista ensimmäinen kesti yhdeksän ja toinen 21 päivää. Pumppauksen vaikutusta seurattiin lähistön muissa syvissä kaira-rei’issä tavanomaisen painekorkeusseurannan avulla. Kokeesta johtuvaa alenemaa havaittiin reikien OL-KR4 ja -KR7 syvimmissä tulppaväleissä. Ne ovat molemmat verrattain pitkiä ja leikkaavat syvempää HZ21-vyöhykettä oletetun HZ039:n jatkeen ohella, joten ei ole ilmeistä, mikä yhteys selittää havainnot. Lähimpien syvien kaira-reikien painekorkeusdatan lisäksi ONKALOn tulpattujen reikien paineseurannan tulok-set tarkastettiin löytämättä viitteitä pumppauskokeen vaikutuksesta. Avainsanat: hydrogeologia, pumppauskoe, käytetyn ydinpolttoaineen loppusijoitus

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TABLE OF CONTENTS

ABSTRACT TIIVISTELMÄ

1 INTRODUCTION................................................................................................... 3

1.1 Background ................................................................................................. 3

1.2 Principle of the test ...................................................................................... 5

2 PUMPING TEST ................................................................................................... 7

2.1 Equipment ................................................................................................... 7

2.2 Course of the test ...................................................................................... 10

2.3 Other events with an effect on groundwater pressure ................................ 12

3 RESULTS ........................................................................................................... 13

3.1 Effects in drillhole OL-KR29 ....................................................................... 13

3.2 Other deep surface drillholes ..................................................................... 13

3.2.1 OL-KR1 ......................................................................................... 16

3.2.2 OL-KR4 ......................................................................................... 17

3.2.3 OL-KR7 ......................................................................................... 18

3.2.4 OL-KR39 ....................................................................................... 19

3.2.5 Below hydrogeological system HZ20 ............................................. 20

3.2.6 The HZ20 system .......................................................................... 23

3.2.7 Above HZ20 ................................................................................... 24

3.3 Packed-off drillholes in the ONKALO ......................................................... 26

4 THEORETICAL TREATMENT ............................................................................ 31

5 SUMMARY.......................................................................................................... 33

REFERENCES ........................................................................................................... 35

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1 INTRODUCTION

1.1 Background

After completing the underground rock characterisation facility ONKALO in Olkiluoto, Finland, Posiva Oy is proceeding towards the construction of the actual final disposal facility for spent nuclear fuel. The first disposal panel is planned to be excavated south-west of the ONKALO in a rock volume intersected by drillhole OL-KR29. According to the hydrogeological structure model of Olkiluoto, zone HZ039 is located near the panel (Vaittinen et al. 2017). The zone is modelled to intersect drillhole OL-KR29 at monitoring section L3 (see Figure 1-1), about 525 m below sea level. In order to acquire additional information on possible hydraulic connections within the area of the first disposal panel, along zone HZ039 in particular, a pumping test in OL-KR29 was planned and conducted in 2016.

On the basis of the assumed position of zone HZ039, it can be estimated that its extension would intersect other deep drillholes near OL-KR29 as shown in Figure 1-2. The zone could thus form hydraulic connections to monitoring sections L5 or L6 of OL-KR1, L1 of OL-KR4, L1 of OL-KR7, L6 of OL-KR39, and in ONK-KR13 and -KR14 to the parts closest to the mouths of the drillholes in the ONKALO.

Figure 1-1. The relative locations of drillhole OL-KR29, the first disposal panel (red) and zone HZ039 (green). HZ039 intersects OL-KR29 in monitoring section L3.

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Figure 1-2. The intersections of the estimated extension (orange arrows) of zone HZ039 (green) in drillholes OL-KR1, -KR4, -KR7, and -KR39 (upper pane) and the monitored drillholes in the ONKALO (lower pane).

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1.2 Principle of the test

The idea of the pumping test was first to isolate the depth interval 562–570 m in drillhole OL-KR29 with inflatable packers, and then to pump groundwater from between the packers so that the hydraulic head would be lowered by about 20 m for several weeks. The selected drillhole section lies at the depth range 519–526 m below sea level and is intersected by two water-conductive fractures, modelled to represent zone HZ039 and having a total transmissivity of 6.4×10−6 m2/s. The effect of the pumping mediated by hydraulic connections would be observed by means of the regular monitoring of hydraulic head in other packed-off drillholes, both deep characterisation holes drilled from the ground surface (OL-KR) and the monitored packed-off drillholes in the ONKALO.

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2 PUMPING TEST

2.1 Equipment

Figure 2-1 presents a schematic drawing of the equipment used in the pumping test. The pumping was carried out from a pumping vessel placed in the wider upper part of the drillhole. The pumping vessel was connected to the pumped packer section by two tubes with an inner diameter of 20 mm. The equipment contained three pressure sensors for measuring the water pressures within, below, and above the pumped section of the drillhole.

Figure 2-2 shows the Grundfos MP1 submersible pump that was placed in the pumping vessel. The pump was controlled with an ABB ACS350 frequency converter (see Figure 2-3). Pressures within, below, and above the pumped section were measured with GE Druck PTX 1830 pressure sensors (see Figure 2-4).

The frequency converter and the pressure sensors were connected to a computer that stored the measurement data. The computer was also programmed to adjust the frequency of the pump on the basis of the pressure data from the pumped section so that the water level would remain as stable as possible at a desired level.

However, the arrangement described above could not be made to operate properly because of insulation problems of the control cable and dissolved gasses in the groundwater. In addition, there were problems with the mains voltage at the test site. Because of these problems, the pumping setup was altered by changing the pumping method to airlift pumping.

Figure 2-5 presents the principle of airlift pumping and a picture of the tubing made for the test and installed inside the pumping vessel. Airlift pumping was applied by pumping air into the narrower white plastic tube with a compressor. The pressurized air was mixed with water, and rising upwards through the wider white tube, brought water along with it.

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Figure 2-1. The schematic plan for the pumping equipment for the pumping test in drillhole OL-KR29 (in Finnish).

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Figure 2-2. Grundfos MP1 submersible pump.

Figure 2-3. ABB ACS350 frequency converter for controlling the pump.

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Figure 2-4. Pressure sensor Druck PTX 1830 used for water level measurement.

Figure 2-5. Principle of airlift pump and the tubing used in the pumping.

2.2 Course of the test

Pressure measurement with the equipment was started on May 25, 2016, and the packers were pressurized on May 27. The natural development of pressure in the measuring sections was monitored until May 31.

The first attempt to start the pumping in the drillhole was made on May 31. Switching on the pump immediately caused a ground fault (one or more of the phase conductors in direct electrical contact with the ground potential) in the frequency converter controlling the pump. The probably faulty MP1 pump was replaced by another that was immediately available, but the ground fault did not disappear. However, no fault was found in either of the tested pumps or the pump cable by inspecting them with a multimeter.

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During May 31 – June 6, a number of attempts were made to repair the setup, for example by replacing the pump cable and the frequency converter and by using the pump with an isolation transformer and an aggregate to exclude possible power supply flaws.

Since the power supply at OL-KR29 was known to be weak because of the long distance to the power network, the pumping equipment was also tested at another drillhole by simply connecting the pump and frequency converter in a mains socket on ground surface. Then, the frequency converter indicated a ground fault again.

During June 6–10, a connector in the pump cable was removed to eliminate one possible faulty point in the circuit. The pump was connected by soldering directly in the cable and insulating the connection with two-component epoxy resin. The connection had to be remade once after the first casting failed.

When a Fluke 1587 insulation resistance meter became temporarily available for the project, it was discovered that two submersible pumps and one cable were faulty. During June 13–17, the frequency converter was replaced with another unit that had been in use at another drillhole and was known to be certainly in order. Furthermore, a new cable with protection against interference and an MP1 pump that had earlier been found to be working were taken into use.

The cable connection by casting failed again because of too short application time of the epoxy resin. Air channels were left inside the cast, which eventually caused a short circuit between the wires during operation. After remaking the cast and installing the equipment into the drillhole, the pump was still found to be faulty; all phase conductors were short-circuited. The last reserve pump was also tested and was found to have the same fault: electrical leaks of the phase conductors. On June 16, yet another unused MP1 pump became available and was connected directly to a cable that had been found to be in order. Before installation into the drillhole, the pump was tested on the surface in a water container. In the test, the pump and the cable worked faultlessly.

As the electrical faults in the pumping equipment had finally been found and fixed, it turned out during the subsequent pumping attempts that the submersible pump could not be used continuously for long periods of time because of dissolved gasses in the groundwater. Namely, the structure of the pump was such that gas bubbles that form in the water when its pressure decreases cannot get out of the pump but accumulate inside it. The gas gradually replaces the water between the blades of the pump and finally causes the pump to stop working.

To solve the gas problem, the equipment was first altered by adding two steel strainers (with 100 and 140 µm mesh size) around the water intake of the pump, and by installing a plastic collar around the pump, forcing the water entering the pump to flow downwards, thus preventing the effect of gas bubbles. These actions did not, however, prevent the gas from ending up in the pump, so that it was only able to run continuously for a few minutes at a time. The arrangement was altered further by stopping the pump programmatically roughly once in a minute for a short time. A back-pressure valve was added in the outlet hose on the surface to prevent the draining of the hose during pumping pauses and to quicken the restoration of water level to the desired value. These

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modifications done, two continuous pumping periods of about two days were successfully carried out on June 22–24 and 28–30. Although the modifications made continuous pumping possible, the water level in the pumped section could not be kept stable with the required accuracy.

To summarize the actions during the first three weeks of the test, attempts to fix the electrical failure of the equipment were made by trying different combinations of a total of three MP1 pumps, three cables, and two frequency converters. Three pumps and one cable were found to be faulty, including one new, previously unused pump. Several faults occurring simultaneously and partly contradictory error messages of the frequency converter delayed the solution of the problem. It is also noteworthy that the faults in the pumps and cables were detectable only in wet conditions, either installed in the drillhole or in a test container on the surface. The unavailability of the insulation resistance meter, necessary in detecting small electric leaks, in the beginning of the tests delayed effective fault diagnosis. A careful examination of the cable faults of the MP1 pumps revealed that their structure, particularly the tight fitting of the parts and the stiff insulation material of factory-assembled wires, had caused most of the faults, including the one discovered in the factory-new pump.

Since the dissolved gasses in the groundwater prevented the originally planned pumping with a submersible pump, it was decided that the pumping test would instead be performed by using the method of airlift pumping, which is not affected by gas. However, the drawdown previously aimed at could not be achieved with airlift pumping. Two pumping periods were carried out with the method, during July 6–15 and July 18 – August 8, which ended the pumping test.

2.3 Other events with an effect on groundwater pressure

At the same time with the pumping test in OL-KR29, there occurred some other activities in Olkiluoto in spring and summer 2016 that affected the pressure of groundwater and had to be taken into account when interpreting the hydraulic head data:

• Pumping related to the PFL flow logging in drillhole OL-KR14 on June 14–29 andJuly 13–14, which caused large drawdowns especially in the HZ19 system.

• Groundwater sampling from packed-off drillholes in the ONKALO:o ONK-KR16 packer section L7: June 28–29

o ONK-PH23 section L6 and ONK-PVA11 section L3: From August 9onwards

• Excavation of Vehicle Connections 16 and 17 and Parking Hall 2: all summerFurthermore, the following events before the pumping test were considered in the treatment:

• Pumping related to the PFL flow logging in drillhole OL-KR32: March 4–16

• Removal of packers in OL-KR29: January 28

• PFL pumping in OL-KR29: March 3–31

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3 RESULTS

3.1 Effects in drillhole OL-KR29

Figure 3-1 presents the pressures measured in OL-KR29 during the brief pumping tests in the early stages of the pumping test (before June 22, 2016), converted into hydraulic head. In comparison with the initial level, drawdowns of about 25 m were briefly achieved in the pumped packer section.

Figure 3-2 presents the measured hydraulic heads in OL-KR29 after June 22, in other words, during the longer pumping periods in the later stages of the test. The pumping rates are also plotted. As to the pumping rate of the submersible pump, the graph presents an hourly average of the automatically measured instantaneous value, which varied between zero and peaks of about 25 l/min because of the periodic use of the pump. During the airlift pumping, the rate could only be measured manually with a measuring jug, so that the data is very sparse. The achieved drawdowns in the pumped section were less than 10 m by both pumping methods. Below the packers, head decreased by 1–3 m during each pumping period and ended up at 4 m below the initial value in the end of the test. Above the packers, head decreased by 0.8 m during the test, but that was entirely or almost entirely due to natural drawdown of groundwater table that is typical for summer.

3.2 Other deep surface drillholes

This section discusses the effect of the pumping test in the four deep OL-KR drillholes where a drawdown mediated by zone HZ039 had been assessed possible. The results of the automatic head monitoring are first presented separately for each drillhole as a change with respect to the values on May 27 at 0:00. Then, the analysis continues by grouping the data according to hydrogeological zones to study the development of head below, within, and above the HZ20 system, starting from the beginning of year 2016 to include the effects of the removal of packers and PFL pumping in OL-KR29. The reduction of the effect of earth tides in use in the hydrogeological monitoring programme has been applied, but not other corrections as they sometimes produce artificial corner points to the data that may appear as responses to pumping.

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Figure 3-1. Hydraulic head in drillhole OL-KR29 until June 22, 2016. Data from above the packers in blue, below the packers in green, and between the packers in red.

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Figure 3-2. Hydraulic head in drillhole OL-KR29 and pumping rate after June 22. Data from above the packers in blue, below the packers in green, and between the packers in red.

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3.2.1 OL-KR1

Figure 3-3 presents the change of hydraulic head in drillhole OL-KR1. The pumping periods in OL-KR29 are indicated by black lines, and the pumping related to PFL measurements in OL-KR14 by red lines. In sections L1–L3 and L5 there is small variation of head, partly caused by the earth tide effect and changes in atmospheric pressure and sea level. The remarkable decrease and later increase of head in section L4 continues the fluctuation started in November 2015. The reason of the fluctuation is not yet confirmed, but it may result from the effect of pilot hole ONK-PH28 on zone HZ056. In the uppermost three monitoring sections L6–L8, head decreased by about one metre during the pumping test because of natural drawdown of groundwater table. In addition to that, both pumping events related to PFL measurement in OL-KR14 caused temporary drawdowns in section L8. No effects of the pumping test in OL-KR29 can be observed in any of the monitoring sections.

Figure 3-3. Change of hydraulic head in drillhole OL-KR1 during the pumping test. Pumping periods in OL-KR29 marked in black and pumping related to PFL flow logging in OL-KR14 in red .

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3.2.2 OL-KR4

Figure 3-4 presents the change of hydraulic head in drillhole OL-KR4. The pumping periods in OL-KR29 are indicated by black lines, and the pumping related to PFL measurements in OL-KR14 by red lines. The decrease of head in section L1 from mid-July onwards is probably a result of the pumping test in OL-KR29 and will be discussed further in Section 3.2.5 . In section L2 that intersects zone HZ056, head was below the measurable range throughout the studied period due to large drawdown caused by the ONKALO. In sections L3–L7, the natural drawdown of groundwater table caused a gradual decrease of head during the pumping test. Furthermore, in sections L5–L7, which either intersect HZ19 zones or are hydraulically connected to it, exhibit an effect of the pumping related to PFL measurement in OL-KR14.

Figure 3-4. Change of hydraulic head in drillhole OL-KR4 during the pumping test. Pumping periods in OL-KR29 marked in black and pumping related to PFL flow logging in OL-KR14 in red. Data from monitoring section L2 is missing, because there head has decreased below the measurable range.

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3.2.3 OL-KR7

Figure 3-5 presents the change of hydraulic head in drillhole OL-KR7. The pumping periods in OL-KR29 are indicated by black lines, and the pumping related to PFL measurements in OL-KR14 by red lines. In section L1, a slow decrease of head started in mid-July, similar to what occurred in OL-KR4. In section L2, that intersects zone HZ056, hydraulic head was below the measurable range throughout the studied period due to large drawdown caused by the ONKALO. In sections L3–L8, head decreased gradually due to the seasonal drawdown of water table during the pumping test. In addition to that, in sections L6–L8 that intersect HZ19 zones or are connected to it, the effect of the pumping during the PFL measurement of OL-KR14 is clearly observable.

Figure 3-5. Change of hydraulic head in drillhole OL-KR7 during the pumping test. Pumping periods in OL-KR29 marked in black and pumping related to PFL flow logging in OL-KR14 in red. Data from monitoring section L2 is missing, because there head has decreased below the measurable range.

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3.2.4 OL-KR39

Figure 3-6 presents the change of head in drillhole OL-KR39. The pumping periods in OL-KR29 are indicated by black lines. In the three deepest monitoring sections head is below the measurable range due to drawdown caused by the ONKALO. The pumping test in OL-KR29 did not have any observable effect in any of the presented sections. The general decreasing trend is a result of the seasonal drawdown of water table during summer.

Figure 3-6. Change of hydraulic head in drillhole OL-KR39 during the pumping test. Pumping periods in OL-KR29 marked in black. Data from monitoring sections L1–L3 is missing, because there head has decreased below the measurable range.

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3.2.5 Below hydrogeological system HZ20

In the following sections the treatment of head data from OL-KR29 and nearest other drillholes continues by grouping the monitoring sections on the basis of the intersecting hydrogeological zones.

Figure 3-7 presents the change of hydraulic head in monitoring sections below the HZ20 system, of which OL-KR1 L2, OL-KR4 L1, OL-KR7 L1, and OL-KR29 L2 intersect zone HZ21 of the hydrogeological model, OL-KR1 L3 intersects zone HZ099, and OL-KR1 L4 zone HZ056. The graph also presents the change of head during the pumping test in OL-KR29 below the pumped packer section, in other words, in the part of the drillhole that intersects zone HZ21. Figure 3-8 presents a 3D visualisation of the location of zones HZ039 and HZ21 with respect to the studied drillholes and the ONKALO.

Figure 3-7. Change of hydraulic head in monitoring sections below the HZ20 system and in OL-KR29 below the pumped packer section during 2016. Pumping periods related to PFL measurements in OL-KR29 are marked by a green line at the beginning and a red line at the end, and the four pumping periods in the last half of the pumping test by the yellow bands.

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Figure 3-8. Drillhole OL-KR29, neighbouring drillholes, and the modelled hydrogeological zones HZ21 and HZ039 intersecting the deepest monitoring sections. The ONKALO is shown in the extent after the excavation of the first deposition panel (red part). The packer section installed in OL-KR29 for the pumping test and head monitoring sections of other drillholes are presented in blue. View towards east.

Among the presented monitoring sections, the variation of head is largest in sections L4 and L5 of OL-KR1, but instead of events in OL-KR29, it is probably related to varying inflow into the ONKALO via zone HZ056. In the deepest sections L1 and L2 of OL-KR29, head decreased momentarily by almost 2 m as the packers were depressurized in late January.

In the deepest sections of OL-KR4 and -KR7 intersecting zone HZ21, a drawdown has developed during year 2016 that appears to be temporally connected to the events in OL-KR29. These are the sections where an intersection of the extension of zone HZ039 was assessed possible. First, after the removal of packers from OL-KR29, head in section L1 of OL-KR7 decreased by about 1 m in three months, and in section L1 of OL-KR4 by about 0.3 m, in comparison with section L2 of OL-KR1. During the first airlift pumping of OL-KR29 in early July, heads in L1 sections of OL-KR4 and -KR7 started to decrease again.

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The changes apparently related to the events in OL-KR29 become even more clearly visible in Figure 3-9, which presents the changes of head in L1 sections of OL-KR4 and -KR7 and, for comparison, sections L2 and L3 of OL-KR1, as well as water level and head data from OL-KR29 during the PFL pumping in March and the pumping test in June–July. The head monitoring data from OL-KR1, -KR4, and -KR7 is plotted with corrections of natural fluctuation and relative to the value in the beginning of July. The dashed line presents the variation of head in OL-KR29 during the PFL pumping at the bottom of the drillhole where the intersections of HZ21 and HZ039 have been modelled. The variation is less than the applied 10 m drawdown of water level, because the saltier and, consequently, denser water rising along the drillhole partially cancelled the effect of pumping on pressure. Because of different orders of magnitude of variations, the data from OL-KR29 is plotted with the scale on the right, while the scale on the left is used for head data from other drillholes.

Figure 3-9. Change of the corrected hydraulic head in monitoring sections intersecting zones HZ21 and HZ099 (scale on the left) and head measured in OL-KR29 during the PFL measurement and pumping test (scale on the right).

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On the basis of the graph, it is likely that the PFL pumping in March made head in section L1 of OL-KR4 decrease by about 0.25 m. Head in section L1 of OL-KR7 was still relaxing towards equilibrium after packing-off in November 2015, so that a corresponding effect is not observable there. The drawdown in OL-KR4 L1 was only partially recovered before the airlift pumping in July, which gave rise to an additional drawdown of more than 0.3 m in OL-KR4 L1 and about 0.2 m in OL-KR7 L1. All these drawdowns started to develop only after a couple of days after the pumping in OL-KR29 had started, increased slowly, and recovered even more slowly. This kind of behaviour has been found to be typical for zone HZ21 in earlier investigations. Since the estimated intersections of zone HZ039 both in OL-KR4 and in OL-KR7 are in the same section L1 as HZ21, it is impossible to infer definitely from the result whether the drawdown is caused by HZ21, HZ039, or some other hydraulic connection. It is notable, however, that the effects on head in OL-KR4 section L1 that resulted from the pumping test and the PFL pumping in OL-KR29 in March were roughly equal in magnitude, and, on the other hand, the drawdowns at the depth of HZ21 in OL-KR29 during those events were also comparable. This suggests that the effect might have been mediated by HZ21 in both cases.

3.2.6 The HZ20 system

Figure 3-10 presents the change of head in the monitoring sections with a modelled intersection of HZ20 zones, and in sections L6–L8 of OL-KR39 which are known to be hydraulically connected to the HZ20 system. The four long pumping periods in the second half of the pumping test are shown in the graph by black lines, and the pumpings related to PFL measurements in OL-KR29 and -KR32 by a green vertical line at the beginning and a red one at the end of each pumping period.

In late January, the packers in OL-KR29 were depressurized so that they could be removed later to enable the pumping test. Monitoring of head was continued for about a week after depressurization, and according to the acquired data, head in section L4 increased by almost 4 m, most of the change occurring abruptly at the time of the depressurization. During the days after depressurization, heads in the HZ20 sections of other drillholes also increased by 0.5–1 m. The pumping related to the PFL measurement of OL-KR29 in March caused drawdowns of more than 2 m in the HZ20 sections of OL-KR4 and -KR7. In OL-KR1 and -KR39 its effect was smaller, but there occurred additional drawdown due to the overlapping, shorter pumping period related to the PFL measurement of OL-KR32.

During the pumping test, hydraulic head decreased gradually by almost one metre in all monitoring sections with a connection to the HZ20 system, including the part of OL-KR29 above the pumped packer section that intersects HZ20. At the same time, the reference groundwater level, calculated from water level data from shallow bedrock holes in Olkiluoto to describe the general natural fluctuation of water table at the site, also fell by a comparable amount. Therefore, the observable head variation in the HZ20 system during the pumping test in OL-KR29 was due to the natural drawdown of water table typical for the period between late spring and middle of summer. On the other hand, after the pumping test, two relatively brief pumping periods related to PFL measurements of OL-KR29 caused distinct drawdowns in the HZ20 sections of OL-KR4 and -KR7, because then the pumping affected the entire drillhole.

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Figure 3-10. Change of hydraulic head during 2016 in monitoring sections intersecting the HZ20 system. Some events affecting groundwater pressure and the calculated reference groundwater level are also presented.

3.2.7 Above HZ20

Figure 3-11 presents the change of head in monitoring sections above the HZ20 system. They are all considered as intersecting or being otherwise connected to the HZ19 system. The removal of packers from OL-KR29 decreased the head in the uppermost part of the drillhole by 5–6 m in January as the pressure difference between HZ19 and HZ20 systems became balanced, but in the HZ19 sections of other drillholes, there is no corresponding effect, but head in them increases with the natural groundwater fluctuation. The likely reason for the head decrease at the depth of the HZ19 system in OL-KR29 not having an effect at distance is that the transmissivities of fractures representing HZ19 in OL-KR29 are a least an order of magnitude smaller than in the system on average.

In early spring, the PFL pumpings in OL-KR29 and -KR32 caused drawdowns mostly in drillhole OL-KR1, while the PFL pumping in OL-KR14 during June and July affected OL-KR7 the most but also OL-KR1 and -KR4. In a similar manner as in the HZ20 system discussed above, there occurred a gradual decrease of heads during the pumping test as a result of the natural fluctuation of water table. The brief PFL pumping periods in late summer had no observable effect in the HZ19 sections of other drillholes, where heads were rising at the time because of heavy rains.

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Figure 3-11. Change of hydraulic head during 2016 in monitoring sections above the HZ20 system. Some events affecting groundwater pressure and the calculated reference groundwater level are also presented.

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3.3 Packed-off drillholes in the ONKALO

During the pumping test in OL-KR29, there were nine drillholes in the ONKALO equipped with a multi-packer installation and an automatic groundwater pressure monitoring system (see Figure 1-2). This section discusses data measured during the pumping test in all of them except drillholes OL-PP262 and -PP274. The measured pressures are presented in terms of change of hydraulic head with respect to the value in the beginning of May 27, 2016. This differs from the practice adopted in the pressure monitoring of the packed-off ONKALO drillholes, which is to deal with the measured absolute groundwater pressure instead of hydraulic head, and was chosen in this case to make it easier to compare possible effects of the pumping test with hydraulic head results from OL-KR drillholes on the surface.

Figure 3-12 presents graphs of the change of head in drillholes ONK-KR13 and ONK-KR14. They exhibit no variation that could be related to the pumping in OL-KR29. Nor do the head results from drillholes ONK-KR16 and ONK-PVA11 presented in Figure 3-13 indicate any effect of the pumping test. The pressure decrease that begins in ONK-KR16 in mid-June possibly results from the excavation of Vehicle Connection 16, and the August drawdown that is most clearly visible in packer section L5 is likely to be due to the excavation of Vehicle Connections 16 and 17 only a few dozen metres away from the drillhole. The excavation also appears to have caused the small stepwise decreases and increases of pressure that are the largest in section L1 but also occur in all other sections. This is suggested by the fact that the times of stepwise changes coincide with the blasting in the tunnels. Groundwater sampling from section L7 during June 28–29 also gave rise to a decrease of pressure in all other sections except L1. In ONK-PVA11, the increasing of packer pressure caused the peak in the graphs in mid-June, and groundwater sampling from section L2 the decrease in other section starting on August 9.

Figure 3-14 presents graphs of the change of head in drillholes ONK-PH21 and ONK-PH22. They do not exhibit any indications of an effect of the pumping test in OL-KR29. In ONK-PH21, the changes in pressures from mid-June onwards started when the packer pressure was increased to improve the hydraulic isolation of the packer sections. In ONK-PH22, pressures in sections L4–L6 started to fall on August 9 because of groundwater sampling from section L6 of the nearby drillhole ONK-PH23.

Figure 3-15 presents a graph of the change of head in drillhole ONK-PH23. The increase of packer pressures in mid-June appears as a pressure peak, and groundwater sampling from packer section L6 from August 9 onwards as a decrease of pressure in other sections, L5 in particular. The results do not exhibit any effect of the pumping test in OL-KR29.

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Figure 3-12. Change of hydraulic head in drillholes ONK-KR13 (upper pane) and -KR14 (lower pane) during the pumping test. Pumping periods in OL-KR29 are marked by black lines.

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Figure 3-13. Change of hydraulic head in drillholes ONK-KR16 (upper pane) and -PVA11 (lower pane) during the pumping test. Pumping periods in OL-KR29 are marked by black lines, groundwater sampling from section L7 of ONK-KR16 by the red-and-yellow bar, and groundwater sampling from section L3 of ONK-PVA11 by red lines.

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Figure 3-14. Change of hydraulic head in drillholes ONK-PH21 (upper pane) and -PH22 (lower pane) during the pumping test. Pumping periods in OL-KR29 are marked by black lines, and simultaneous groundwater sampling from sections L6 of ONK-PH23 and L6 of ONK-PVA11 by red lines.

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Figure 3-15. Change of hydraulic head in drillhole ONK-PH23 during the pumping test. Pumping periods in OL-KR29 are marked by black lines, and groundwater sampling from section L6 of ONK-PH23 by red lines.

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4 THEORETICAL TREATMENT

Drawdown of hydraulic head h in a homogenous, infinite, planar aquifer is described by function (Theis 1935)

( ) ,

24,

2

=

trW

TQtrh

ηπ (1)

where r is the distance from the pumped hole, t time, Q pumping rate, T and η the transmissivity and hydraulic diffusivity of the aquifer, respectively, and function W the Theis’ well function

( ) .∫

∞ −

=x

t

tdtexW (2)

Hydraulic diffusivity depends on transmissivity and storage coefficient S by

.

ST

=η (3)

Figure 4-1 presents an application of Theis’ solution (Eq. 1) to interpreting the observed drawdown in L1 monitoring sections of drillholes OL-KR4 and -KR7. The graph shows calculated drawdowns during the final stage of the pumping test in these sections and in the pumped section of OL-KR29. The beginning of the first period of airlift pumping has been defined as the origin of the time axis. The graph also includes two theoretical drawdown curves based on the equations above for two values of distance r: 0.038 m, which is the radius of the pumped drillhole, and 420 m, which is the approximate distance from the pumped packer section to the deepest packer sections of OL-KR4 and -KR7. Both curves have been calculated using the same parameter values Q/T = 8 m and η = 0.05 m2/s, and they fit the experimental data reasonably well.

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Figure 4-1. The measured drawdown in the final stage of the pumping test and approximating theoretical curves.

As pumping rate Q is known to have been about 2 l/min or 3.33×10−5 m3/s, the properties of the assumed aquifer can be solved as follows:

/sm 1017.4m 8

26−×==QT

and

.1033.8 5−×==ηTS

The calculated transmissivity is in fair agreement with transmissivity data from previous measurements. The sum of the transmissivities of the two fractures within the pumped section, determined by PFL measurements, was 6.4×10−6 m2/s in autumn 2004 (Pöllänen et al. 2005) and 3.6×10−6 m2/s in spring 2016 (Pekkanen & Komulainen 2017).

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5 SUMMARY

A pumping test was conducted in drillhole OL-KR29 during summer 2016 by pumping a section between 562–570 m of drillhole length, isolated with packers. The purpose of the test was to investigate hydraulic connections within the rock volume where the first deposition panel is projected to be excavated to the south-west of the present ONKALO. The packer section was chosen to focus the effect of the pumping to zone HZ039 of the hydrogeological structure model (Vaittinen et al. 2017).

Because of electrical faults of the pumping equipment and dissolved gasses in the pumped groundwater, the planned pumping test could not be started before July. The pumping method had to be changed to airlift pumping, which reduced the attainable pumping rate from that of the intended submersible pump. Therefore, drawdown in the pumped packer section fell short of the planned value, being between 5 and 10 metres. Pumping periods of several days during the second half of the test also affected the drillhole section below the packers, giving rise to a drawdown of about 4 metres. Above the packers, hydraulic head in the drillhole followed the natural seasonal drawdown of the water table.

Before the pumping test, it was estimated that effects mediated by zone HZ039 could be possible to observe in four other deep drillholes closest to OL-KR29: OL-KR1, -KR4, -KR7, and -KR39. These drillholes are all packed-off and subject to continuous head monitoring. The only monitoring sections where drawdown caused by the pumping test could be detected were L1 sections of OL-KR4 and OL-KR7. Since these sections are long and both intersect the deeper zone HZ21 in addition to the assumed intersection with HZ039, it remains uncertain which connection caused the observed drawdown. The scale and delay of the drawdown can be roughly explained by the Theis’ solution, assuming a single homogeneous planar structure whose transmissivity is comparable to the fractures observed in the pumped section and associated with zone HZ039 in hydrogeological modelling.

In the monitoring sections closer to the surface, intersecting HZ19 and HZ20 systems, the pumping periods of the entire OL-KR29 during PFL flow logging measurements caused clearly observable responses, and the natural seasonal drawdown of the water table gave rise to a gradual decrease of head. No effects of the pumping test could be detected in these monitoring sections.

The pressure data from all monitored packed-off drillholes in the ONKALO were also inspected, without finding any indications of an effect of the pumping test in OL-KR29.

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35

REFERENCES

Pekkanen, J. & Komulainen, J. 2017. Monitoring Measurements with the Difference Flow Method in Drillholes OL-KR14, -KR29, -KR32 and with the Transverse Flow Method in Drillholes OL-KR14, -KR31, -KR33, -KR35 and -KR36 during the Year 2016. Working Report 2017-xx (in prep.)

Pöllänen, J., Pekkanen, J. & Rouhiainen, P. 2005. Difference Flow and Electric Conductivity Measurements at the Olkiluoto Site in Eurajoki, Boreholes KR29, KR29B, KR30, KR31, KR31B, KR32, KR33 and KR33B. Working Report 2005-47. Eurajoki, Finland: Posiva Oy.

Theis, C.V., 1935. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage. Transactions of the American Geophysical Union, Vol. 16, p. 519–524.

Vaittinen, T., Ahokas, H., Nummela, J., Pentti, E. & Paulamäki, S. 2017. Hydrogeological Structure Model of the Olkiluoto Site – Update in 2015. Posiva Report. Eurajoki, Finland: Posiva Oy (in prep.).

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