The River's Toxicity Levels

Overview
On Beta's River in the Sky (Beginner Stage 46), the STOP II Team claims that the river is toxic. However, since the article for this stage was written, one of Jiquor's friends had mentioned how the river actually might be toxic since it doesn't look to be filled with trash, nor appears to be murky. So, the question is: How is this river toxic, and what is it filled with?

How did this river become toxic?
There are these strange green pipes that are supposedly pumping something into the river, but we can't see what that substance is.

What is the significance of this disaster?
Engineers pump pollutants and toxic waste into the river from factories(?) Only to flow and drain in an unknown location.

So what's in this river anyway?
If we can't see the pollutants nor the toxic waste, it has to be something invisible to the naked eye. Gross.