Refute B please
As per the argument the pollution is kept under control by limiting the discharge to such an amount that it is diluted to an harmless extent. The question does not ask for weakening or strengthening. If the chemical is not diluted then it is not serving the purpose. This has to be the assumption to connect the conclusion and premise.
A is out of scope though it would sound logical for a question "which one of the following if true would weaken authors argument"
B tells that whatever the authorities expect is indeed happening.
The best way to find out an assumption is to negate it.
Lets negate B.
We get -
The flow of water is not enough to "disperse" the chemicals.
But the important point to note is that the argument is based on "dilution" not "dispersion". A chemical can be diluted in stagnant water. Can't it ?
So, even if you negate Option B the argument still stands.
Now, lets negate A.
We get -
The chemicals interact with each other and create other harmful chemicals.
Even if there is flow of water, or even if the chemicals are diluted, if the chemicals can potentially interact to form harmful compounds then the argument falls apart. Doesn't it ?