Piezometric Measurements at the Mol-Dessel Site

Transcription

1 EXTERNAL REPORT SCK CEN-ER-57 08/SLa/P-11 Piezometric Measurements at the Mol-Dessel Site Site Characterisation for Disposal of Category A Waste Serge Labat Topical report to ONDRAF/NIRAS prepared in the frame of contract KNT April, 2008 SCK CEN Boeretang 200 BE-2400 Mol Belgium IPA-PAS

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3 EXTERNAL REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE SCK CEN-ER-57 08/SLa/P-11 Piezometric Measurements at the Mol-Dessel Site Site Characterisation for Disposal of Category A Waste Serge Labat Topical report to ONDRAF/NIRAS prepared in the frame of contract KNT April, 2008 Status: Unclassified ISSN SCK CEN Boeretang 200 BE-2400 Mol Belgium

4 SCK CEN Studiecentrum voor Kernenergie Centre d étude de l énergie Nucléaire Boeretang 200 BE-2400 Mol Belgium Phone Fax Contact: Knowledge Centre library@sckcen.be RESTRICTED All property rights and copyright are reserved. Any communication or reproduction of this document, and any communication or use of its content without explicit authorization is prohibited. Any infringement to this rule is illegal and entitles to claim damages from the infringer, without prejudice to any other right in case of granting a patent or registration in the field of intellectual property. SCK CEN, Studiecentrum voor Kernenergie/Centre d'etude de l'energie Nucléaire Stichting van Openbaar Nut Fondation d'utilité Publique - Foundation of Public Utility Registered Office: Avenue Herrmann Debroux 40 BE-1160 BRUSSEL Operational Office: Boeretang 200 BE-2400 MOL

5 5 Table of content Abstract INTRODUCTION EVOLUTION OF THE OBSERVED GROUNDWATER LEVELS POTENTIOMETRIC MAPS REMARKS CONCLUSION RECOMMENDATIONS REFERENCES... 26

6 6 Abstract Context Since May 2000 SCK CEN measures monthly the piezometric levels in the framework of the "Site characterisation at the nuclear site Mol-Dessel for disposal of category A waste". The network currently comprises 48 sites: 43 piezometer sites and at 5 sites (sites 43, 44, 45, 46, and 57) the water level of a watercourse is monitored. Objective The observed groundwater levels can be used for the comparison and verification of water heads calculated with the hydrogeological models and for the analysis of the temporal and spatial evolution of the measured piezometric levels. Therefore, continuity in the measurements is an essential requirement as well as the maintenance of the piezometers. The work is performed as part of the programmes on a)geological disposal of high-level and long-lived waste (category B and C waste), and b)short-lived low and intermediate level waste (category A waste), that is carried out by ONDRAF/NIRAS, the Belgian agency for radioactive waste and fissile materials. Scope This report analyses the evolution of the piezometric levels from May 2000 until December Conclusion The piezometric network that is the subject of this report now covers a timeseries of eight years of monthly head measurements at 48 different sites The data has been extensively used to develop understanding of the hydrogeology of the nuclear site Mol-Dessel. In general, all piezometers show typical seasonal variations. Also, the potentiometric maps for a wet and dry period show a similar pattern with regard to the water flow direction. KEYWORDS piezometer; groundwater level; hydrogeology; Neogene aquifer; category A waste

7 7 1. Introduction Within the framework of the so-called "Preliminary site characterisation at the nuclear site Mol-Dessel for disposal of category A waste" piezometers were installed at 32 sites in 1999 (sites 1 through 32). All these piezometers consist of a PVC casing, a PVC filter, and a PVC "end cap". This "end cap", located at the bottom of the piezometric tube, collects the possible sludge inside the piezometer and prevents that this can obstruct its filter. At the surface all piezometers are protected by a steel casing that is equipped with a detachable cover plate. In 2002 a second drilling campaign was launched. It comprised: the installation of 10 new piezometers (site 42 and sites 48 through 56) an already existing piezometer (site 58) was included in the network, whose technical characteristics are not known (probably an open yet shallow tube without filter) the installation of 3 staff gauges to monitor the level of the Breiloop (site 43), the Hooibeek (site 44), and the Kleine Nete (site 45) the determination of a reference point to monitor: o the level of the Witte Nete in Dessel (site 46) o the level of the Witte Nete in Retie (site 57, the topographical co-ordinates were determined by SCK CEN) The filters of the piezometers that are installed at sites 9, 48, 50, 52, and 56 are located in the Diest aquifer (Miocene), while the filter of the other piezometers is located in the Pliocene aquifer 1. The geographical location of all sites is presented in Figure 1, while Table 1 presents an overview of their technical characteristics. 1 In the Belgatom report "Preliminaire geologische, hydrogeologische en geotechnische verkenning van de nucleaire site te Mol-Dessel-Geel ten behoeve van de definitieve oppervlakteberging van laag radioactief afval" (contract N CCHO , augustus 2000) is mentioned that these filters are located in the Kasterlee aquifer. However, some of them might be situated in the Mol Sands. Therefore, we prefer to use Pliocene aquifer.

8 8 Figure 1: Geographical location of the different sites Table 1: Technical characteristics of the piezometers Site Drilling depth (m) Filter depth (m) Stratigraphy X Y Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Miocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene

9 9 Site Drilling depth (m) Filter depth (m) Stratigraphy X Y Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Pliocene Congovaart (canal) Pliocene Breiloop (river) Hooibeek (river) Kleine Nete (river) Witte Nete (river) Miocene Pliocene Miocene Pliocene Miocene Pliocene Pliocene Pliocene Miocene Witte Nete (river) not known not known Pliocene

10 10 2. Evolution of the observed groundwater levels Since May 2000 SCK CEN measures monthly the groundwater levels at each site. A first evaluation of their evolution with time until December 2001 was reported by Labat (2002). The evolution until December 2007 is revaluated in this report and presented in Figures 2 to 11. For each graph a brief comment about the observed profiles is given. The observed groundwater levels are expressed in metres Second Ground Levelling (S.G.L.) Water level (m S.G.L.) Site 1 Site 2 SCK-CEN/08/SL Date (year) Figure 2: Piezometric evolution at sites 1 and 2 Comment Both sites show seasonal variations (approximately 1 m for site 1 and 0.5 m for site 2). The smoother variations observed at site 2 are probably due to its location in the vicinity of a creek. The unexpectedly low value observed in June 2007 is caused by road works for which pumps were installed to lower the groundwater table to a sufficient depth. 2 Tweede Algemene Waterpassing (m TAW)

11 Site 3 Site 4 Site 5 Water level (m S.G.L.) SCK-CEN/08/SL Date (year) Figure 3: Piezometric evolution at sites 3 through 5 Comment A typical seasonal variation with an amplitude of about 30 centimetres is observed for the three piezometers.

12 Water level (m S.G.L.) Site 6 Site 7 Site 8 Site 9 Site 10 Site 11 SCK-CEN/08/SL Date (year) Figure 4: Piezometric evolution at sites 6 through 11 Comment All levels show the same typical seasonal variations. At site 7 and to a lesser extent at site 9 a more explicit decrease of the water level is observed in the summer. These phenomena are most probably accentuated by temporary pumping in the Pliocene (site 7) and Diest aquifer (site 9) for agricultural reasons in a well nearby these sites. Note also that the observed levels at sites 10 and 11 sometimes show a divergent behaviour (rise/decrease when the other levels decrease/rise). The origin of these phenomena is not known, but at the same time the abnormalities remain limited to a few centimetres. Finally, piezometers 9 (Miocene) and 10 (Pliocene) allow estimating the vertical hydraulic gradient between the Pliocene and Miocene aquifer, which is about 1.0 m (downwards).

13 Water level (m S.G.L.) Site 12 Site 13 Site 14 Site 15 Site 16 Site 17 Site 18 SCK-CEN/08/SL Date (year) Figure 5: Piezometric evolution at sites 12 through 18 Comment Although all sites show the same typical seasonal variations, the amplitude ranges between 0.6 m (site 12) and 1.4 m (site 15). Again, some of the observed levels at sites 14 and 15 sometimes show a divergent behaviour (rise/decrease when the other levels decrease/rise), but they remain limited to a few centimetres.

14 Site 19 Site 20 Site 21 Water level (m S.G.L.) SCK-CEN/08/SL Date (year) Figure 6: Piezometric evolution at sites 19 through 21 Comment The curves show typical seasonal variations yet the amplitude is less pronounced at site 21, because it is located in the vicinity of a pond.

15 Water level (m S.G.L.) Site 22 Site 23 Site 24 Site 25 Site 26 Site SCK-CEN/08/SL09 Comment Date (year) Figure 7: Piezometric evolution at sites 22 through 27 All levels show typical seasonal variations with amplitudes varying between 0.1 m (sites 25 and 26, where the level is (indirectly) influenced by the nearby canal Bocholt- Herentals) and 1.0 m (sites 23 and 27). Also, a significant difference of about 3 m is noticed between the level measured at sites 22 and 24. This decreasing gradient towards the north is most probably caused by a combination of a)the topography of the area characterized by a topographic height at site 24 and b)the influence of the water inflow from the canal Bocholt-Herentals into the Hooibeek, which establishes an artificially high groundwater level near to site 26.

16 Site 28 Site 29 Site 30 Site 31 Site 32 Water level (m S.G.L.) SCK-CEN/08/SL10 Comment Date (year) Figure 8: Piezometric evolution at sites 28 through 32 All levels show typical seasonal variations, although the amplitude is more weakened in the direction of the Hooibeek (site 31 and 32).

17 Water level (m S.G.L.) Site 42 Site 43 Site 44 Site 45 (2nd Y axis) Site Date (year) Figure 9: Water level evolution at sites 42 through 46 SCK-CEN/08/SL Comment Only site 42 is a piezometer, while at the other sites the water level in a watercourse is measured. All levels show typical small seasonal variations, except for site 44 (Hooibeek river). Here, the seasonal variations are not so visible because its level is regulated by: a water inflow from the canal Bocholt-Herentals upstream; an adjustable water trap downstream in order to regulate the water flow towards the provinsial domain Prinsenpark. At site 43 a staff gauge was installed in the Breiloop, but it disappeared in December 2006, and measurements have been discontinued since then. In agreement with ONDRAF/NIRAS, it was decided not to replace it because the available dataset covered a time period of 4.5 year. A similar problem occurred earlier (June 2004) at site 45, where a staff gauge was installed in the Kleine Nete. However, a new reference point was chosen and its Z level was measured by SCK CEN. Therefore, the monthly observations of the water level of the Kleine Nete were resumed in May Also, the level remains nearly constant due to the location of the observation point (in the vicinity of a water trap) and to the width of the river. Therefore, even a small rise/descent can result in major changes in terms of water flow.

18 Water level (m S.G.L.) Site 48 Site 49 Site 50 Site 51 Site 52 Site 53 Site SCK-CEN/08/SL12 Comment Date (year) Figure 10: Piezometric evolution at sites 48 through 54 All sites are very near to each other. Levels show typical seasonal variations, but the observed amplitude of the Pliocene aquifer (sites 49, 51, 53, and 54) is smaller than the one that was observed in the Miocene aquifer (sites 48, 50, and 52). The hydraulic head difference between Plionene and Miocene aquifer is on average about 2 m (downwards).

19 Water level (m S.G.L.) 18.0 Site 55 Site 56 Site 57 Site 58 (2nd Y axis) SCK-CEN/08/SL Date (year) Figure 11: Piezometric evolution at sites 55 through Comment All levels show typical seasonal variations. Note also the upward gradient that is observed at sites 55 (Pliocene aquifer) and 56 (Miocene aquifer) because of their location close to the Witte Nete at Retie.

20 20 3. Potentiometric maps In the framework of the performance assessments performed by SCK CEN, a local groundwater model is developed to calculate the groundwater flows and velocity distributions in the Pliocene and Miocene aquifers. The piezometric measurements are used to validate the model results. The piezometric measurements at a given time, represented in maps with point values and isolines calculated by interpolation between the measured values, provide information on the appearance of the piezometric surface and on the groundwater flow in the considered aquifer. 3.1 Remarks As an example, maps are shown for a wet time period (January 2003) and for a dry time period (August 2003). The isolines have been drawn by using the Kriging interpolation technique and should therefore be treated with caution. The resulting potentiometric maps for the Pliocene aquifer are presented in Figures 12 to 15. To allow the reader to evaluate the reliability of the interpolated isolines Figures 13 and 15 only show the isolines and the measured point values. Potentiometric maps for the visualisation of the global piezometric trends in the Miocene aquifer are not drawn up. Not only is the number of observation wells insufficient (5 in total) for a reliable spatial interpolation, moreover is their geographical distribution unfit for interpolation.

21 Figure 12: Potentiometric map for a wet period (January 2003) (isolines and location of the piezometers) 21

22 Figure 13: Potentiometric map for a wet period (January 2003) (isolines and point values) 22

23 Figure 14: Potentiometric map for a dry period (August 2003) (isolines and location of the piezometers) 23

24 Figure 15: Potentiometric map for a dry period (August 2003) (isolines and point values) 24

25 Conclusion Analysis of 8 year long timeseries of head observations revealed interesting insights into the hydrogeological system of the area around the Mol-Dessel nuclear site; for example, existence of downward gradients across the KDTL in the central part of the domain and upward gradients in the vicinity of the Kleine Nete river. Potentiometric surfaces typical of summer and winter conditions were derived using all piezometers. When both summer and winter potentiometric maps are compared, nearly the same pattern is observed for the isopiestic lines (i.e. lines of equal hydraulic head). Water flow directions are mainly oriented from southeast to northwest, yet an "island" with high water level can be observed at the east side of the measured area.these higher water levels are probably influenced by water inflow from the canal Bocholt-Herentals. This is more obvious when, looking at the wet period, one compares these high levels with the high levels located in the east and south of the canal. Both piezometric heights also coincide with topographic heights, the most southern piezometric height however disappears in the summer, while the levels near the canal remain high. During the wet period (see Figures 12 and 13) an additional feature appears at the centre right side with locally lower water levels. This is probably due to a limiting effect of the topography on the water levels. 3.3 Recommendations Given the importance of this information for the performance assessments of the planned low-level radioactive waste disposal facility at Dessel, it is strongly recommended to continue the monitoring of the groundwater levels at the Mol-Dessel site on a monthly schedule. Special attention should thereby be given to the observed high levels in the east, the drainage effect of the rivers, the vertical gradients between the Pliocene and Miocene aquifers, and the anomalies caused by the canal. After all, the gathered data is used for the calibration and validation of groundwater models that are used in PA calculations.

26 26 4. References Bontemps T., Demoulin H., Schittekat J., Van Echelpoel E., 2002, Aanvullende preliminaire geologische, hydrogeologische en geotechnische verkenning van de nucleaire site te Mol-Dessel-Geel ten behoeve van de definitieve oppervlakteberging van laag radioactief afval. Belgatom, Brussels Labat S., 2002, Piezometric Measurements at the Mol-Dessel Site. SCK CEN, report R Schittekat J., Van Echelpoel E., Bontemps T., 2000, Preliminaire geologische, hydrogeologische en geotechnische verkenning van de nucleaire site te Mol-Dessel-Geel ten behoeve van de definitieve oppervlakteberging van laag radioactief afval. Belgatom, Brussels Berckmans A., Syntheserapport van een bibliografische studie en twee geologische terreinverkenningen die de impactevaluaties ondersteunen van een oppervlakteberging van categorie A-afval in Mol-Dessel. ONDRAF/NIRAS, report NIROND N

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