Water Balance and Efficiency of Landfill Covers Composed of Layers of Dredged Material : Results from Long-term Measurements in Northern Germany(Symposium 3.5.2 Risk Assessment and Risk Based Remediation,International Symposium: Soil Degradation Control, Remediation, and Reclamation, Tokyo Metropolitan University Symposium Series No.2, 2010):results from long-term measurements in Northern Germany