What do they mean by an "intentional" act?
What's the current forensic and motivational analysis for this?
Water sensor data should be able to point roughly to the dumping location, and winding back through the keyhole and ANPR data should bring up candidate vehicles proximate to the river-bank.
Mercury salts aren't a particularly "problematic" thing that requires covert dumping, and moving a large volume of it would seem odd. If you had a quantity of it already loaded in trucks then abandoning the trucks in a remote wood or desert would seem more expedient.
It is however an ideal aquatic poison for agricultural warfare.
I remember in the 70s when Israel was hit by mercury poisoning (deliberately injected) of Jaffa oranges - relatively easy to handle, hard to detect without x-raying all produce.
I don't buy the implied (by absence) narrative that this was some kind of "accident" or motivated by only selfish economic forces.