1. Theo Olsthoorn Waternet (Amsterdam Water Supply) / TU-Delft
Brackish groundwater as a new resource for drinking water ? Consequences of density flow. Benefits for the environment ?
Theo Olsthoorn
Waternet (Amsterdam Water Supply) / TU-Delft
home.planet.nl/~hans.farjon/nederland.htm

6. peat

9. Amsterdam

11. Amsterdam -3 -4 -5 -2 -6 -2 -5 -3 -4 -3

12. Schematic cross section

13. 6 m
fresh
fresh
Brackish upflow

15. Water management of the Polder Groot Mijdrecht North Algae blooms,
Salinity fluctuations
Aquatic ecosystem is frustrated in summer
Brackish-brown water with nutrients
Lake ecosystem is also frustrated in summer

18. Ground surface elevations in cm above current surface water level
Ground level 1960
Ground level 2004
Ground surface elevations in cm above current surface water level
Average settling + shrinkage + oxidation = 30 cm in 40 yrs

19. Water management of the Polder Groot Mijdrecht North
Algae, ecosystem frustrated in summer
Brackish-brown water with nutrients
Lake ecosystem frustrated in summer
Settling, shrinkage and oxidation of peat 7 mm/yr
Spontaneous break-through of covering peat layer
Too wet and saline for agriculture over time

20. What about the future of this polder ?

22. Prime Minister

23. cow or jetski ?
no lake
no marsh
but grass !

24. crown prince and water manager of the Netherlands
Investigation Committee Report handover

25. Cross section
Google

26. infiltration
infiltration
Polder with low surface water level

27. Polder with low surface water level
infiltration
infiltration
Qbrackish=3.95m2/d

28. Polder  lake  stagnant salt bulge

29. Polder with low head and extraction of brackish water
infiltration
infiltration

30. fresh
fresh
Brackish
Brackish / saline

32. Jan van Dam
SWIMMER from

34. Conclusions
Sustainability of deep polders with a peat soil and brackish seepage are officially questioned, they may be turned into lakes again.
The flow analysis for like brackish systems can no longer be done without variable density modeling (enquiry committee report).
Brackish water may be a good source for drinking water and its extraction may solve brackish seepage, but it cannot solve subsidence and peat oxidation.
Brackish water extraction increases fresh equilibrium seepage to the polder, which is a density effect.
However, increasing seepage is nowadays politically unacceptable.