Thatcham Tertiaries Operational Catchment

About

The Thatcham Tertiaries Groundwater Body (GWB) describes part of the Lambeth Group Strata (also known in the past as the Reading Beds) that lie around the edge of the Lower River Kennet valley. The Thatcham Tertiaries GWB is located around the axis of the western end of the London Basin syncline. The Thatcham Tertiaries GWB coincides with the eastern end of the River Kennet catchment, the southern end of the River Lambourn catchment and the southern side of the River Pang catchment. The Thatcham Tertiaries are a Secondary A Aquifer with water-bearing permeable strata that provide a low level of storage. Generally they consist of inter-bedded sands and clays that can also be laterally variable. Generally the lower units of the Lambeth Group are more sandy and can bear water. Since these sandy units sit directly over the Chalk aquifer, there can be hydraulic continuity between the Chalk and the base of the Lambeth Group. Given the close proximity of the Chalk any boreholes constructed in this area will usually be extended down into the Principal aquifer, therefore the Lambeth Group in this part of the country is little used for water abstraction (this contrasts with the London area where the Lambeth Group can provide significant volumes of water from the Thanet Sands). The main role for the water from the Thatcham Tertiaries in this area will be to contribute base flow to some of the local streams. In some locations particularly the Pang Valley, water will flow off the clay rich fields and make its way down into the sandy rich horizons of the Lambeth Group and into the Chalk via dolines or swallow holes. This can mean that pollutant transport has the potential to be rapid via these preferential pathways.