ESI, now part of Stantec, were appointed to provide technical support on a range of groundwater related issues linked to the Lee Tunnel development.
As the tunnel was to be constructed through Chalk, the Environment Agency required evidence that no polluting materials would be introduced to the aquifer. An assessment was required on the materials proposed for sealing the tunnel (grout) and shafts (grout, bentonite and concrete).
The client provided samples of the proposed materials for leaching tests to BS12457-2. Results were compared to UK drinking water standards and showed no hazardous substances present within the materials.
A number of non-hazardous pollutants were present at concentrations above the drinking water standards. Simple dilution calculations were undertaken to demonstrate concentrations would be reduced below the drinking water standards within a short distance of the tunnel and shafts.
Based on data acquired from previous projects, existing groundwater quality data was compiled into an EQuIS database for use when designing the shaft construction works.
The client raised concerns that discharge into the Northern Outfall Sewer from shaft de-watering might introduce unacceptable concentration levels of ammoniacal nitrogen into the Thames Tideway, potentially impacting the operation of the desalination plant at the site. We provided valuable assistance in the negotiations between the client and the Thames Water.
Following concerns raised by the Environment Agency that dewatering shafts may draw saline water into the Chalk Aquifer if a fissure linked the shaft to the Thames Tideway, we developed a scoping groundwater model.
Calculations were undertaken to assess the likely impact on groundwater quality adjacent to the fissure following groundwater abstraction from the shaft. It was concluded that the impact would be very small, particularly as the aquifer was already saline based on its proximity to the Thames Tideway.
A pumping test was required at the shaft site to complete the design of the shaft construction method. Concerns had been raised that contamination present within the shallow groundwater might be mobilised by the pumping test.
Using existing groundwater quality data a risk assessment for the pumping test was undertaken. This concluded that although there was significant contamination in the shallow groundwater, this was unlikely to be mobilised by the pumping test and there was no significant risk to the Chalk groundwater. On the basis of these findings the pumping test proceeded with no impact.
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