Doesn't the stuff flow away, down the river? Test further downstream right?
Maybe the perpetrator stopped dumping it when it became a news story? Like, they started dumping it recently and when they realized they were the cause to a catastrophe they stopped to try to avoid blame.
> At the time, the Lower Silesia Water Authority based in the nearby Polish city of Wroclaw detected a toxic substance in two locations on the Oder that is likely the solvent mesitylene, which is known to have a toxic effect on fish. However, subsequent tests have shown no trace of the substance.