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• Introduction
• Site Setting
• Geology and Hydrogeology
• Investigations to date
• Current Understanding
  of the Site
• Strategy and Planning
• Regulatory Requirements
• Management Activities
  • Safety case
  • Best Practicable Means (BPM) Assessment
  • Groundwater Monitoring
  • Leak Detection
  • Site Procedures to Excavate and to Dispose of Soil
  • Phase 2 of the Sellafield Contaminated Land Management Programme
  • Data Information Management
  • External Stakeholders
• Supporting Information

The Sellafield site sits on sediments (known as superficial deposits or drift) deposited by a variety of glacial, river (alluvial), lake (lacustrine) and marine processes that have been altered and added to in the near-surface by activities at the site. 

The underlying bedrock is Triassic Sandstone, a red fine to medium grained rock with bedding planes dipping south west towards the coast and with occasional bands of finer grained mudstones. The depth to bedrock beneath the superficial deposits varies from 3m to 60m due to the presence of “channels” in the buried bedrock surface, probably representing pre-glacial valleys that were infilled with sediments during the Ice Age. The main buried channel or valley lies to the north west of the site, roughly associated with the route of the River Ehen. A side buried channel from this extends east to run under the Separation Area in the middle of the site.  (SCLS Geological report)

Examples of Sediments Found in a Borehole at Sellafield Clay Sand and gravel with cobbles Clay with cobbles

The superficial deposits largely comprise coarse grained sands and gravels with some patchy layers of clay.  Boreholes drilled into the sediments typically encounter groundwater 5 – 15 m below ground level, with some clay layers possibly causing the groundwater to be locally higher than would otherwise be expected.  Groundwater in the lower part of the superficial deposits appears to be in connection with groundwater in the bedrock.  The superficial deposits are classified by the Environment Agency as a minor aquifer (underground water resource) and the underlying Triassic Sandstone is classified as a major aquifer.  There is no known use made of these water resources in the vicinity of the Sellafield site but springs on the beach are fed by groundwater and groundwater may also flow into the lower reaches of the rivers. (SCLS hydrogeological report