INTERACTIONS BETWEEN DEEP AND SHALLOW GROUNDWATERS ON BASIN MARGINS – INSIGHTS FROM THE SELLAFIELD STUDY

Groundwaters in a coastal sandstone aquifer and underlying low permeability basement rocks were investigated at Sellafield in northwest England. Hydrogeological measurements were supported by geochemistry and geophysics, and groundwater flow concepts were quantified with numerical hydrodynamic models. The findings increase our knowledge about groundwater movements between basement and aquifer at the edge of a sedimentary basin.

There are two groundwater régimes in the basement rocks: relatively dense saline water originating from the deeper basinal part of the aquifer and gravity-driven fresh groundwaters originating as recharge in upland catchments. This is a contrast between geological fluids that are essentially stagnant and groundwaters that have been susceptible to the impacts of uplift, erosion and climate during the Tertiary and Quaternary periods.

Shallow groundwater in the aquifer is fed dominantly by direct recharge for which water quality depends on local factors at the surface. With increasing depth in the aquifer, groundwater generally becomes less mobile, older, and saline. Where the basement contact occurs at intermediate depths, the relative permeabilities and local structure control the transition between mobile and immobile water. External changes in climate and sea level have had a small impact on this interface over recent geological time. Where the aquifer is thicker and reaches greater depths, density may be the controlling factor in the groundwater column.

The overall picture is of groundwaters from different sources that are close to hydraulic equilibrium with present topography and climate. However basement groundwaters respond to these factors slower than the aquifer and are likely to contribute relict water to parts of the aquifer. This conceptual model has general significance for the evolution of groundwaters at basin margins and the present state of water quality, sedimentary diagenesis, and fracture mineralisation in these environments.