Case study using a multidisciplinary approach – ground investigation, risk assessment, groundwater lowering and site remediation at
Wymondham manufactured gas plant
G.W. Farrant, P. Horswill, T.E. Pang and R.A. Merlane

MWH successfully carried out ground investigations, environmental and human health risk assessment and remediation design for a former manufactured gas plant at Wymondham, UK. Working with National Grid Property Holdings a multidisciplinary approach was devised allowing remediation of two gasholders under artesian conditions.

MWH determined that the constituents of concern were PAHs, TPHs, metals, asbestos and phenols located in the made ground above a glacial till and the base of the gasholders dug into the glacial till. The glacial till comprised clay overlying the Chalk aquifer, which is exploited regionally for public
water supply. The glacial till provides a barrier against contaminant migration to the aquifer. This is confirmed by the fact that piezometric levels in the Chalk are near or above ground level. Ground investigations indicated however that the lower part of the glacial till is extensively fissured
and effectively in hydraulic continuity with the Chalk aquifer. This could in adverse circumstances allow contaminant pathways to develop. Geotechnical calculations indicated that if the gasholders were emptied the Factors of Safety against hydraulic uplift would be unacceptably low, allowing the
potential for fissure opening and excavation flooding. MWH’s water resources expertise was utilised to provide a design for groundwater lowering involving a single abstraction borehole in the Chalk. Stringent discharge consent requirements were met by pumping borehole development water into
tanks and discharging to foul sewer during off peak periods. Once consent requirements were met, continuous discharge was made to the near-by River Tiffey allowing effective groundwater lowering and subsequent excavation of the gasholders and successful remediation of the site.

Key words: containment, ecological risk, integrated approaches, public health, risk assessment, sediment management, site redevelopment, subsurface contamination, sustainable brownfield regeneration

Land Contamination & Reclamation, 14 (2), 382-387

DOI 10.2462/09670513.740

© 2007 EPP Publications Ltd

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