Walker* notes in 1888:

The whole of the ground in the marsh [around Caerwent] is rotten, and before the tunnel was commenced there were enormous springs of bright clear water rising up in several places...

At about 2 miles farther north than Caerwent, the hills of Wentwood are met with, with 'Grey Hill' standing in the foreground. The first spurs of the hills fronting the valley are composed of mountain limestone, the higher parts about Shirenewton of the old red sandstone.
On the east side of the Severn, and for some little distance on the western side, the new red sandstone formation is found in nearly horizontal beds. The first disturbance of this takes place behind Portskewett village, where the mountain limestone has been upheaved and the new red formation denuded. A mile further up the same limestone has been upheaved between Caldicot and Caerwent, and from there to the base of the hills the strata have been much broken, and the consequence has been that all the water from the hills, both from the mountain limestone and the old red sandstone, has found subterranean channels through this broken ground, and, before the tunnel was commenced, flowed out in the valley of the Neddern, and formed the great springs which have been before mentioned...

The Neddern, rising as a small brook in the hills above Llanvair Discoed, sometimes lost the whole of its water in the dry season near the foot of the hills, bursting out again near Caerwent, at a point called by the natives ' The Whirly Holes '.

John Nettleship* notes...

Nowadays the stream disappears for the summer, at the mouth of the Cwm and only has running water again from around its junction with the Crick Brook. The stream must have carried more water past Caerwent in the 1930s as older people refer to bathing in it etc. though this must have been thanks to the culverting, whose benefit has not lasted due to the formation of new swallow holes moving upstream in the Cwm.

Apparently the University of Bristol Spelaeological Society have run tracing experiments between the Nedern sink and the Great Spring in the Severn Tunnel, though confirmation of this is waiting (presumably this is in Drew, Newson, and Smith*).