identity impersonation with asp.net

my pages have an editing function to modify the records in the table of a db using a user login.  So the field can show who modified the record. The login uses windows authentication. I searched from web and it said add "<identity impersonate="true"/> under<system.web>  in the web,config. It did not change and still shows "NT Authority/System" on the field for any user's logon. if I added 

<identity impersonate="true" username="DOMAIN\UserName" password="Password" /> then it shows  this user no matter who logins. 

Where is the tip to show user associated the login user?

Thanks.