There's both present. At least one big fresh water spring (presumably groundwater in the bedrock strata flowing along one or more faults) and salt-water ingress either down a fault or because the tunnel is so close to the sediment at the bottom of the river bed. Far more water flows through the sandstone into the Mersey tunnel at high tide than at low tide when the estuary water level is so much lower!I did read recently that the corrosion of the catenary in the Severn tunnel was due to salt water.
I heard that BR used to regularly use a helicopter to drop lots of bags of cement into the river to seal off the downward flow into the Severn tunnel, and were quite concerned when the new bridge was built over or close to the line of the tunnel which would prevent that option being used.
I suspect it's not salt or brackish water dripping through the tunnel roof that is the problem but just the constant high humidity.
p.s. Wikipedia https://en.wikipedia.org/wiki/Severn_Tunnel confirms both:
andIn 1885 the bed of a pool, known as the "Salmon Pool", on the English side of the tunnel broke through [8] the tunnel's continuous brickwork lining... The Second Severn Crossing, built in the 1990s, crosses the tunnel on a "ground level bridge" on the English side, near the Salmon Pool. The bridge is supported so that no load is imposed on the tunnel. During that bridge's construction, the concrete cap above the tunnel in the Salmon Pool was renewed"
https://en.wikipedia.org/wiki/Severn_Tunnel#cite_note-eng_time-3Network Rail has observed that the corrosive atmosphere inside the tunnel, produced by moisture and diesel fumes requires replacing the steel rails every six years.[3]
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