This guy came to MIT to speak for the IG lectures this weekend (wonderful lectures, incidently). He said that the EPA did get wind of his plans and was not happy about it. So he didn't actually drop the sodium directly into the lake, but rather into a pool of lake water on a floating platform. This didn't do much good, since a chunk of sodium was launched upward by the initial explosion and landed in the lake anyway. Bounced four times. It's in one of the videos which, I'm sure, are quite slashdotted by now.