1. College of Environmental Science, Peking University, Beijing, 100871 China, Yokohama Waterworks Bureau, 522 Kawashima-cho, Hodogaya-ku 240-0045, Japan, Institute for Environmental Sciences, University of Shizuoka, 52-1, Yada, Shizuoka 422-8526, Japan, and Department of Water Supply Engineering, National Institute of Public Health, 4-6-1 Shiroganedai, Minato-ku, Tokyo 108, Japan