TJ,
Why send the source info through the form? Are you passing it your CRM or something?
I typically just utilize Google Analytics to track the source. You can set a custom variable with the lead's name into GA if you want to match up leads.The avantage is that you can also track other lead sources like organic, social, etc. in addition to PPC.
Of course, if you're doing really high volume, this isn't real efficient.
-Rick