I found and provided a link (http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=53381) that will enlighten you on the subject a bit, but I just wanted you to have a few more perspectives to confirm my assertion that there are no backpacking filters on the market as of now that will actually filter out heavy metals (i.e. mercury, arsenic, etc.). I know many filters, if not all, esp. in the case of Katadyn, come with a sticker when purchasing the filter warning against use in areas where there is a likelihood or confirmation of heavy metal presence. I know that carbon-based filters can assist in the reduction of heavy metal presence should it wind up in the water you gather to filter, but no filter can completely eliminate the risk. However, there is hope: so long as you are skimming off the top of a body of water, you should be able to avoid the metal entirely, or at least minimize the risk to the degree that you won't suffer any prolonged or serious consequences. Ultimately, it comes down to finding sources you know are not tainted and/or generally being smart about how you gather your water. Just remember heavy metals tend to sink, so never gather from regions near the very bottom of the water source.
Hope this helps.