Potential link between mountain subsidence and water discharge from a tunnel in north Iceland

The Vaðlaheiði tunnel is a 7.4 km long tunnel located in north Iceland, linking the Eyjafjörður fjord and the Fnjóskadalur valley. It goes through the Vaðlaheiði mountain at maximum depth of about 500 m. The tunnel was built in order to shorten the main road around Iceland (road 1) by 16 km and avoid a mountain pass which was often blocked by snow during winters. The drilling started in July 2013. On the 16^th of February, after having excavated about 1.9 km, a water vein was encountered and started to leak in the tunnel at a rate of about 350 L/s. Drilling was complete in April 2017 and the tunnel opened for traffic in December 2018. As of January 2021, about 250 L/s of a mix of geothermal and cold water is still going out of the tunnel.

The Sentinel-1 SAR satellites from the Copernicus mission provide acquisitions over Iceland since summer 2015. InSAR time-series analysis were conducted for four tracks covering Vaðlaheiði: two ascending (T118, T147) and two descending (T111, T9). Results show that part of the hill subsided about 10 mm between summer 2015 and summer 2016. It also appears that the same area was subsiding about 5 mm per year between summer 2016 and summer 2020. Older datasets from the Envisat SAR mission covering 2004-2010 were analysed and show no evidence of subsidence in the same location. Therefore, it appears there could potentially be a link between the water going out of the tunnel and the subsidence. Especially since water withdrawal at depth is known to cause surface subsidence, like in the case of agriculture irrigation or geothermal exploitation. Using numerical modelling, we attempt to explain this relation between water withdrawal and subsidence in the case of the Vaðlaheiði tunnel.