What was on top of the yellow water barrels that flew up into the air when the McDowell Gilliland wreck ...

May be...

the lids

Garbage can lids.

... happened and it flew up into the air and hit a pit road light fixture and almost hit the bystanders?

lids on the barrel crash cushions full of sand (not water).

They were lids.

there very large lids just like on a plastic garbage can they showed 1 stuck in the splitter of the 77

The barrels lids.

the lids

From what I saw, it was just the lids of the water barrels that went flying up over the inside pit wall. It definitely had the pit light pole shaking like crazy, though.

If you watch the replay, watch the official standing farther away from pit road, right when the accident begins. He hits the ground like he was being shot at... Scary wreck, but it was kinda funny watching him hit the ground like he did...

EDIT: Here is the link. Click on the 1st video on the right hand column:

http://www.nascar.com/video/cup/2008/08/...