When we had a polution insident where I worked, they always said the answer to polution was dilution.
There's no single answer. It depends on what type of pollution. There are some types that cannot be tolerated even in small amounts. Others can be tolerated in small amounts, and dilution may be fine, but often these are the types of pollution where there are many sources. For instance, a stream may be able to safely dilute runoff from a single farm, maybe even a dozen farms, but when it's recieving runoff from thousands you got a problem, there's just not enough water to dilute it. So which one do you go after, as each is the proverbial straw that broke the camel's back?
Today, most of the "death nail" types of pollution have been controlled. We still get accidental spills of some pretty nasty stuff, and we still have problems left over from days of old, such as AMD and seepage of various substances from soil below industrial plants. But we've largely done away with the institutionalized new sources of complete ruinization for waterways. However, we still struggle with the gradual problems. Sewage, treated frackwater, agricultural runoff, acid rain, storm runoff from developed areas, taking away groundwater recharge sites, etc. A little of any of them aren't going to render a stream dead, and unfortunately, to live our way of life we have to put up with some of it. But we have to understand that every little bit degrades water quality a little bit. We have to monitor levels of all of them, and take all reasonable measures to limit the damage as much as we can.
We don't always do a good job of that. It's too easy for someone to say, "this little bit ain't gonna do much in all that water, it'll be diluted to the point it doesn't matter." And they're right, but their little bit is beind added to everyone else's little bit, and collectively, its a problem.