asp.net form authentication impersonate with SQL Server

Dear Guys,
I've a problem with asp.net forms authentication (with sqlserver membership provider) that the impersonation is not well for our AD Users, which is required for auditing database transactions
 
when running the web applications sql server system_user becomes NT AUTHORITY\SYSTEM; by the way the connection string is like:
Data Source=10.50.1.21;Initial Catalog=tmpIIMS;Integrated Security=True;Persist Security Info=False;Trusted_Connection=Yes;
whic is authenticated by Integrated security with AD at IIS.
 
so could any one help me to retreive at audit with a desired account for each AD Users
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