1. Research Laboratory of Water Quality, Ministry of Health, P.O.Box 8255, Tel-Aviv 61080, Israel, Institute of Soil, Water and Environmental Sciences, The Volcani Center, P.O.Box 6, Bet Dagan, 50250, Israel, Private consultant, P.O. Box 9050, Jerusalem, Israel, and Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Israel