This is a really solid adventure. I think you do a great job of very concisely setting the premise up and putting obstacles in the players' way in the form of encounters and potential enemies.
I think if I were to run this as a GM, I might try to build in some hints about other ways to tackle the problem. If I were a player, I would definitely want to find a way to purify the elemental, restoring balance and all that good stuff. To that end, as a GM, I might also have notes spelling out:
- Ways that the players may get clues that the blockage is a corrupted spirit before they're directly confronted by the spirit itself
- I might have notes about how to purify the river spirit. What immediately comes to mind is the spellbook you've already put in there (maybe for that exact purpose) and obviously eliminating the source of the corrupting sludge
- Ways to appease a purified, but probably still angry, spirit that will want to ensure this place never becomes corrupted again. Such as: getting rid of all of the miners, closing off the mind entrances for good, etc.
I'd also probably prepare for a less virtuous playthrough by thinking about what would happen if they just decide to kill the spirit and leave it at that. Maybe their village lies downstream and they'll have to live with the consequences of the pollution.
I say all that because I think that even though space is limited, a brief nod to it on your cover page may go a long way in helping the GM to plot out what that might look like.
Great job!