This study was carried out as a Master of Science Thesis work within the Master of Science Programme for Aquatic and Environmental Engineering at Uppsala University, School of Engineering. The study was initiated by the Swedish Environmental Research Institute (IVL) and was performed by Markus Åhman and Stefan Uppenberg during the period September 1997 to January 1998. Lars Zetterberg has supervised the work at IVL, and Ph.D. Lars-Christer Lundin and Ph.D. Allan Rodhe at the Department of Earth Sciences, Uppsala University, have been responsible for the examination.