Powder poison lifetime?

The strongest weed killing herbicide produced since 1995 is Mono-Bar-Chlorate. It is used for things like sterilizing the ground under concrete slabs, roads, drive ways, etc. It is marketed under several different brand names and comes in several forms, including powder, and has an effective period of 3 yrs. If it was put down 10 yrs. ago, it has been neutralized and broken down by the microbes in the soil by this time.

Very few herbicides are poisonous after ten years. Practically no herbicide or weed killer is poisonous to animals after it has been applied. The weight of the pool and the abvsence of any sunlight has killed any plants or lawn that was underneath but you can rototill the area and replant right now if you want new plants.

If you cannot tell what poison you applied, no one can advise you if it is safe ten years later. However, most herbicides are not "persistent." That means they break down at some point after they do their job.

Glyphosate (Roundup), a popular herbicide, has a half life of 30 days and a re-entry period of 24 hours. It is a non specific, contact herbicide that is applied to the leaves and works its way to the roots.

If you are concerned, remove any soil that you feel is tainted.

Depends what the "poison" was. If you mean Round-up, it lasts several months. Something like "Ground-Clear", more like a year. Heavy salting can render soil infertile for decades.

Most likely, it's fine. Ten years is a long time.