|Paper title||Phasing out long term industrial groundwater pumping in Delft, Netherlands|
|Form of presentation||Poster|
Groundwater has been extracted in the municipality of Delft since 1916. The extraction used to be owned by a privately owned yeast factory, but when the company recently seized production, the extraction was transferred to the municipality of Delft. Though the extraction has no functional use at this time, the city of Delft is worried that due to the size of the extraction, which is currently 1200 m3 (about an olympic swimming pool) per hour, stopping the extraction will have a big effect on the buildings in the historic 16th century city centre. The current annual costs of water disposal for the municipality are €2.5 million.
Since 2017 the municipality is slowly phasing out the extraction. The reduction in groundwater extraction must be carefully controlled to avoid an abrupt rise of the surface level due to swelling of the ground and consequent damage to infrastructure. To monitor the effects of the reduction, extensive measurements like groundwater levels have been collected since 2010.
It is estimated that the shutdown of these wells over a 30 year period could lead to ground swelling of more than 10 cm near the extraction wells. Abrupt and uneven swelling can cause damage to buildings, and infrastructure like tunnels, parking garages etc. There are 70,000 buildings in the 5 km surrounding the extraction site, including a range of irreplaceable historic buildings.
To supplement the groundwater level measurements, InSAR is used to monitor the changes in the uplift of the soil (since 2014) and guide the speed with which the groundwater extraction is reduced. Due to the wide extent of the impacted area, local levelling campaigns did not cover the full extent of the impacted area. Prior to the reduction in groundwater abstraction, the area was subsiding -1 to -2 mm/yr. Between 2016 to 2019 displacement rates remained fairly stable, with a gradual reduction in subsidence rates from -1 to -2 mm/yr to to 0.0 mm/yr.
However, from June 2019, the ground started to swell locally at +1 mm/yr. Using these swelling observations with InSAR, it was decided to pause the phase out during 2021, prolonging the significant costs of extracting water by another year. At the moment the area is being continuously monitored with InSAR measurements every 11 days, and early 2022 it will be assessed whether the phase out can be continued in 2022.