If the sand was shipped in a truck with bags of weed killer, bags of bug killer, or some other toxic chemical also delivered to the same place, on top of the bags of sand, or situated near it and sifted down, or blew on the breeze, all the rinsing in the world probably wouldn't get rid of it. And all it takes is a few grains.
It might even be that the truck carried something on the previous trip and left a residue.
It also could be the quarry it came from was down wind from some spraying.
Possibilities are endless.