If you have a permit to dump wastewater into a river, you are not allowed to dump your wastewater wherever in that river's basin on the assumption that it will eventually flow into the river. You are supposed to use a pipeline for wastewater transfer.
They need to get it to a body of water, not a ditch. Dumping into the drainage ditch or running a pipe in the drainage ditch requires a separate permit.
The water quality is also questionable. Tesla and the drainage company are at odds on the testing method.
At least that’s my understanding of the situation
Edit:
Found the permit in the news article linked from the original article. It seems like the wording comes directly from from the permit (https://ewscripps.brightspotcdn.com/14/fb/2a0cdfa24e8791af37...). That reads to me though that this is then the second case (this specific ditch, which is unnamed) and not “an unnamed ditch, of which this is the wrong one”