ESI Consulting Ltd

Sub-watertable landfill (regulation 15 risk assessment)


Courtesy of ESI Consulting Ltd

Client: Waste Management Industry

Location: UK

Services: Site investigation, data collation and interpretation, Controlled water and ecological risk assessment services, Expert witness.

Issues: Sub-water table landfill site. Potential risk to groundwater.

Summary: ESI produced a controlled waters risk assessment model for this sub-water table landfill site, which enabled the most cost effective, environmentally protective, containment design to be selected. Our client therefore benefited from significant cost savings, whilst precluding any risk to controlled water receptors.

A major waste disposal company wished to redevelop the remaining void of a large landfill site on the principle of hydraulic containment.  The site had undergone extensive mineral extraction and was at that time operated on the principle of dilute and attenuate.

The client wished to adopt an engineered containment design extending below the recovered water table.  This required a Regulation 15 groundwater risk assessment to be undertaken for consideration by the Environment Agency (EA) as part of an application to modify the existing waste management licence for the site.

ESI was appointed to carry out a Probabilistic Risk Assessment (Monte Carlo analysis) using in-house stochastic modelling techniques. The modelling approach was chosen to provide a robust and transparent assessment of risk using the source-pathway-receptor methodology.

The in-house software allows a wider range of design scenarios to be explicitly simulated and uses an advection-dispersion-retardation-decay transport model, representing the current state of the art in contaminant transport modelling.

The project included a synthesis of available data to construct a hydrogeological conceptual model for the site. Where data was not readily available a substantial field investigation programme was undertaken by ESI. The agreed contaminants of concern and migration pathways were represented in the model as the basis for prediction of the risks associated with the proposed landfill development and a number of agreed alternative failure scenarios were simulated.  The figure above illustrates the schematic conceptual model for one configuration for engineering the site below the water table in the Hythe Beds.

Regular reporting of the results of the risk assessment kept both the client and EA in touch with each stage of the assessment. Working in close liaison with the landfill engineer an iterative, modelling-based approach to the development of the containment design ensured that the most cost effective design was chosen consistent with ensuring adequate protection of the environment. This allowed very significant cost savings to be achieved by our client.

ESI provided expert witness at the subsequent waste management licensing appeal hearings.  The inspector found that “the methodology adopted for risk assessment had taken note of the Agency’s recommended approach and that is had been intelligently and appropriately adapted to reflect the sub-water table setting within the probabilistic modelling,” and the appeal was won.

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