1. Nat. Exposure Res. Lab., U.S. Envir. Protection Agency, Mail-Drop 56, Research Triangle Park, NC 27711 (corresponding author).
2. Captain, U.S. Army, U.S. Military Academy, West Point, 580C Benedict Rd., West Point, NY 10996.
3. Assoc. Prof., Dept. of Civ. and Envir. Engrg., Duke Univ., Durham, NC 27708.