Mine water is generally very nasty stuff, and discharge from disused mines is a significant pollution threat. Without a permit from the EA this sort of release is almost certainly illegal under the Water Resources Act 1991, regardless of whether it's been left sat there for ages gently leaching into the river anyway (which it usually is). In the comments the diggers remark that 'the water is pH neutral, just a bit of iron', which is surprising as it's supposedly a copper mine. It's very difficult to predict minewater chemistry, so you can only really know if it's going to be harmful by actually testing it properly. All this being said, if it's not a lot of minewater and it's brief and well diluted the impact is probably going to be quite minimal. Still irresponsible and antisocial in my opinion, and "brave" to post about it on social media (not that the EA have the time or funds to take any enforcement action). But I don't want to be too critical, I don't think they mean any harm, and it does sound like an accident after all.