Vulnerability Assessment of Groundwater Aquifers during the Construction of the Citytunnel in Malm

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Vulnerability Assessment of Groundwater Aquifers
during the Construction of the Citytunnel in
Malmö, Sweden 

• Alina Meyn, Kenneth M Persson and Bo Leander
• SWECO, Malmö Lund university, Lund
• Sweden

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The Citytunnel Project

• Citytunneln is a rail project
• Three new train stations
• 17 km new track 6 km two parallel tunnels with
  single tracks below Malmö 4.5 km bored, 1.5 km
  cut and cover construction
• Appr cost 1 B EURO Ready 2011

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Three aquifers
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Vertical distribution of hydrological flow zones
Zone Hydro-logical unit Geological formation Geological subdivisions Thick-ness (m)
A Soil unconfined aquifer Barrier 1 Quaternary soils Fill Peat material Postglacial deposits Upper moraine Glacial sediments Under moraine 2-5 2-3
I Upper limestone aquifer Copenhagen limestone Cop. Limestone 3-8
II Barrier 2 Low permeable layer Top layer of Bryozoan limestone 35
III Lower limestone aquifer Barrier Bryozoane limestone . More compressed Bryozoan limestone. 10 50
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Water balance
mm/year Mm3/year
Precipitation 675 106
Evapotranspiration 545 86
Groundwater recharge 130 20
Pumping for Malmö city -6,6
Groundwater runoff 15
Pumping for Citytunneln 15,48
Re-infiltration 13,24 netto -2,24
Leakage to limestone 20 2,9
Netto 11,1
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Actual pumping at Malmö C and Triangeln
(September 2005 - March 2008)
Days Average m3/hour Accum. m3 Mm3/ year
Triangeln Pump 907 125 2,7 1,09
Reinfiltration. 883 132 2,99 111 1,24
Tapwater 582 28 0,41 0,26
Discharge to harbour 601,0 0,13
Netto 0,29 0,15
MC Pump 777 342 6,4 3,00
Reinfiltration. 777 264 4,95 77 2,32
Netto -1,4 -0,68
Netto -1,2 -0,53
Qpump 2,7 Mm3 for 907 days, or 1,09 Mm3
per year. Allowed Qpump 400m3/hour not
reached. Max 125m3/hour
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Conclusions

• Three main hydrological units unconfined soil
  aquifer, upper limestone aquifer and lower
  limestone aquifer
• Separated by barriers.
• Hydrological properties, location close to the
  Sound and low topographic gradient make the
  aquifers vulnerable to salt water intrusion.

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Conclusions

• During undisturbed (i.e. no contractors present)
  conditions, flushing of all aquifers takes place
• Highest chloride content observed in areas
  with lowest permeability

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Conclusions

• At some points of Malmö Central station area Cl
  gt than in present seawater
• Hydraulic contact with relict saltwater over
  fissures.
• Upconing (pumping for heating and cooling)

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Conclusions

• Citytunnel Project follows strictly the
  Environmental Permit
• Re-infiltration exceed pumping rate at
  Triangeln
• Reached 80 of pumped water at Malmö C.
• No visible changes in chloride content of
  the groundwater at Malmö C area and Triangeln
• Some dilution of Cl due to re-infiltration with
  tapwater at Triangeln
• Increase of chloride content at Kung Oskars
  street.

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Conclusions

• Some flushing / ion exchange at Triangeln
  Station area
• No visible changes in Malmö Central area
• Some saltwater intrusion process in Kung Oskar
  street area.

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Conclusion

• Citytunnel Group managed in general to preserve
  groundwater level
• Flushing/dilution at Triangeln station area.
• Some very high horizontal and vertical
  permeability could still cause salt water
  intrusion, even if hydraulic head were minimal

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Thank you

• Questions, anyone?