Mathern (in Welsh Merthyr Tewdric "the martyrdom-location of Tewdric") is certainly the most obvious location associated with Meurig where the water may have reached at high tide (see Note). It sits next to a stream called the Meurig (now "Mounton brook")*. and has a number of springs, pools, and wells. However, the exact location of the wonder is tricky to discern, if for no other reason that it is actually quite hard to work out which well might be the one under discussion.
On the various maps of the area there are two wells (one north of village, currently dedicated to St.Tewdric [map with well marked] and Ivy House Well). There are also two obvious springs (the current start of the unnamed stream running passed the church, as shown next to St.Tewdric's well on the 1:50000 map (above), and that 500m east of the village), and two pools/ponds (see Note). None match up consistently with the discription from the Historia.
The unfortunate problem is that the most obvious well in the village is that currently dedicated to St.Tewdric*** (see Myth), leaving us to ask whether there was a additional well explicitly dedicated to Meurig. The Ivy House well, if indeed it ever existed, is not marked on modern maps. The prevailing belief seems to place Meurig's well itself as lost somewhere in Pwllmeyric (for example Thomas*), however it is far from obvious that such a thing even existed (See Notes on Pwllmeyric and the Pwll). One could, for example, imagine the writer meaning "the well named by Meurig".
An alternative translation of the second line of the wonder is given by Davies*: "There is there a well near the valley of the pool Maurit (Meurig) and a log in the middle of the well." This works well for the location as it negates having to find a well dedicated to Meurig, and instead allows us to locate the wonder using Pull Mouric (See Note on Pwll). It is the suspicion that this is true which leads one to suggest the area around Pwllmeyric for the wonder, but unfortunately there seems to be little reason for accepting "pool" for putei (see Note on Translation for original - it's also not clear where "Maurit" originates: for known variations see Note on Alternatives). Given this, the Mathern location can only be held somewhat speculatively.
If, rather than the well, we take the spring as the starting point, we don't get much futher. The 1886 map of the area suggests the unnamed stream running next to St. Tewdric's Well and the church used to run overland much further north than it does now, and that the stream is therefore a re-issue rather than a spring. This is confirmed by the current 1:10000 map which notes it as an "issue" rather than a "spring", and shows other stretches of the same stream further north, albeit not following the course it appears to have in 1886. However, it does appear from the 1:10000 data that the stream is joined by a small outlet from another, real, spring in the estate some 70m or so to the north. The second spring near the Mathern, some 500m east of the village, appears on the current and 1886 maps. While this is closer to a pool (see Note on Pwll), there is no well there. Of course, springs can come and go - a groundsman in the grounds of Pwllmeyric House, when questioned about it, indicated that springs came and went on the land on a frequent basis.
The confusion is added to by the fact that Tewdric's church in Mathern is known under his latinized name of Theodorick, and that the 1886 Map of the area shows a church dedicated to St.Teudric some distance to the north, about 200m south of where the Wyelands road goes under the A466.
Given all this, you may feel that you might as well stick a pin in a map. However, given the absence of any information on the existence of a well in the area 500m east of the town, it seems likely that the wonder, if it was here at all, was somewhere in the inlet to the south-west of the church, between the level of the current golf-course and St.Tewdric's Well. Plainly, though, this is somewhat speculative.