The following is oversimplified, but gets the idea across:

Most environmental damage estimates are based on the concept that the toxicity of something is a linear function, that is, if a gram of something is deadly, 1/10th of a gram is exactly 1/10th as deadly. And so on, down to the molecular level. Think homeopathic medicine.

If you take a level of arsenic in the water that has been proven to have killed 1,000 people, it means that if you reduce it by half, this thinking means that you can calculate that 500 people will die from it. And so on.

According to this thinking, there is no "safe" level of anything. Microscopic levels of arsenic do not become harmless, it just reduces the official "fraction" of people who die from it. This is often used for radiation safety as well.

It's worth noting, that in small enough quantities, arsenic is actually a nutrient in animals.