Cap-and-trade not good for mercury?

So one of my committe members asked me in my defense day. Why so bother with PCB pollution? Those heavy metals stay at the bottom of the lake. How should that be a concern for homeowners? As I noted in some footnotes, these pollutants may cause cancer and other defects. I should have really moved them from footnotes to the text. Today Krugman writes about policy on mercury, one of the PCB substances. He goes further as to assess the recent policy administered by the goverment, the cap-and-trade policy. He points out that C&T is only good for light pollutants. But it would be wrong to use it to control heavy metals. As for mercury case, polluters would rationally buy the permits and... we're waiting for the next Chernobyl.