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