how to access query string in wordpress?

The answer to my general question has a very specific answer, and I found the answer by running across this answer to more or less the same problem. Basically GoDaddy hosting does something special to utm_-type query strings. Tech support could provide no detail.

In case anyone else runs across this, it looks like something mysterious happens to the standard Google Analytics query strings such as utm_source. Example: utm_source will not show up in $_GET (much less using get_query_var), but utm__source (2 underscores) is just fine.