The usage of Grimer to poison water supplies is an old standby of biological warfare. As a highly toxic pokemon whose liquid body can easily mix with water, a small group swimming around a reservoir or river can force a choice between death from poison or thirst. Because of this danger, sources of water have been well-protected even since antiquity, and deaths by Grimer poison in the old days did not occur in proportion to the fear which these pokemon caused.
And then came the industrial revolution. With the growth of cities and factories came massive pollution, and the city of Celadon soon discovered Grimer in their ponds and rivers in peacetime. At first, suspicion fell upon the reservoir guards and they were executed, for the people of the city were dying in droves. The government went into deep debt to import clean water from afar, for the city's vast population made it difficult to meet its drinking needs. The new guards could do nothing to stem the flow of Grimer, and it soon became clear that the executed guards had been innocent.
Inspections revealed no pathways between the water supply and the sewage system which normally held these poison pokemon. Weeks later it was found that industrial waste from the factories had been leaking into the reservoir, and that this waste was combining with air pollution to form live Grimer right there in the water. A massive cleanup effort was begun, but soon abandoned, for the people of Celadon were unwilling to make the sacrifices to avoid pollution, finding it cheaper to simply import water from abroad and raise taxes accordingly. Today, Celadon is still one of very few places in Kanto (and the only city) where wild Grimer can be found on the surface.