PHP/ExpressionEngine set session variable based on querystring

We have an ExpressionEngine 2.1 website (CentOS5/PHP/MySQL) that we need for a session variable to be initialized and set if the URL contains a particular value. Here is an example: [login to view URL] If the URL contains #adwords, we need to have a session variable AdWords set to yes. This was already working on the site at one point but after some recent modifications it is no longer functional. If any additional information is required, please let me know.