Rule of 65
The dilution in the river renders the spill harmless that far down stream
Dilution with respect to water as in this case is C1xV1 = C2xV2. Anything different is just some rule of thumb shorthand when the data is limited or guesstimated.
Of course, what do I know. I just had 40yrs experience in industrial water and wastewater treatment project management, design, testing, pilot plant, field & plant operations and R&D among other things.
Cincinnati and any other municipal water systems downstream of the spill should be following the same prudent course of action until sampling and river modeling gets an all clear from the relevant local and state authorities and the EPA. All WILL be involved.
In addition, there should be a ban on fishing until proven safe. Bioaccumulation in sediments, fish, clams and such is a long term concern until proven otherwise by site assessments. This will prove to be a long term issue to fully scope out.
My opinions….