How to find url parameter unique to program and not lead?

We use the URL parameter to track unique IDs to track where our leads registered from (i.e. if they registered from a blog post or an email blast, or if Partner A drove them to the event vs. Partner B). Using this field is working except that if a lead has registered for more than one program their url parameter is updated to their latest registration.

For example, on 12/2 Joe registered under the url parameter 'newsletter' for a webcast, when we pull the report it shows Joe registered under 'newsletter. Then on 12/5 Joe registereted for an onsite event under the url parameter 'email1'. Now in both member reports for each program Joe is listed as registering under 'email1'.

This is problematic as the url parameter code associated with Joe's registration in the webcast is wrong. It becomes a real issue when we need to share lead information with partners based on programs. 

We've reached out to support and the only solution they've provided is going into each program and then each lead's Activity Log and cross referencing which url parameter they registered for that particular program, which is extremely time consuming. 

Has anyone discovered a best practice for maintaining url parameters to programs vs. leads?

Thanks!
Samm