Vacuuming Sand with a Shop Vac results in a Van de Graaff generator — shocking

I pulled out my shop vac to clean up some sand left behind from the mastic around my pool. The more I vacuumed, the faster the static charge would build up. At first I didn’t realize what was happening. I kept feeling small zaps on my feet. I was wearing sandals and the shock was arcing from my foot to the ground a half inch away. The more I vacuumed, the stronger and the more frequent the shocks became. At one point I bumped into a low hanging leaf on a tree branch and got a jolt to the top of my head. Ouch. Fun with Science!