Ed,
If it is in fact PE or, polyethylene which, I'm assuming it is, then yes it's weld-able. The question is, can you get at the injured area(s)? If not, there are a number of newer, epoxy type glues out there that are pretty darn strong at binding to many surfaces. If you absolutely cannot get it welded, then this might be the way to go. You'd still need to get a dremmel in there to do some gouging where the cracks/breaks are so that the epoxy can get a good bite on things.
These darn manufacturers don't make it easy to do things like this, even though there's a fair possibility that work on a water tank may be needed in the future.
Scott