In Britain most groundwater resources are held in bedrock aquifers and the unsaturated zone forms an important buffer between landfill disposal sites and the water table. The processes which control the transfer of (1) persistent organic compounds, (2) inorganic anions and (3) heavy metals, in the unsaturated zone have been evaluated using experimental lysimeters constructed in the Lower Greensand at a site near Uffington, Oxfordshire. Complementary monolith and column experiments were conducted.
Biodegradation is the primary mechanism for the attenuation of the inorganic anions and the organic species, but the migration of heavy metals is limited by interactions with the sediment. Sequential, selective chemical extraction indicates that the stability of the geochemical phases retaining heavy metals controls their rate of movement. Although the processes which regulate the partition of the heavy metals are complex, uptake by sesquioxides and clay minerals are particularly important.