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and thorough field investigation, calling for improved methods in this area. Accurate monitoring of the groundwater flow and quality, including estimate of techniques discharges and storage is also important. Restoring the quality of polluted groundwater entails not only the elimination of pollution sources but also the remediation of contaminated groundwater in both the saturated and unsaturated zones.

The mechanisms of multiphase flow, mixing and mass transfer in porous media are not sufficiently understood and the efficiency of different clean-up techniques has not been fully assessed. Central to this understanding are tremendous uncertainties that plague groundwater pollution and remediation problems. Dynamic and stochastic simulation models of water flow and solute transport in saturated and unsaturated zones, combined with carefully selected field experiments, are crucial tools for proper assessment and management of groundwater contamination, and need to be further improved.

5.3 Unsaturated Zone: Exchange with Surface Water and Coupled Flow

The zone of groundwater level fluctuations is characterized by frequent changes between saturated and unsaturated conditions. To describe and control the processes occurring within this zone, a major effort has been launched during the last two or three decades by scientists of different disciplines. Unfortunately, segmented disciplinary research has hindered up to now, an exhaustive experimental and theoretical understanding of the unsaturated zone. So, challenges for both basic and applied research remain, as does the urgency for progress inasmuch as the soil and groundwater resources are increasingly vulnerable to long-term pollution. Knowledge of the interactions, feedbacks, biochemical and hydrochemical processes is far from

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