Client: Waste Management Industry
Location: Hydraulically contained landfill site, UK
Services: Data review, hydrogeological risk assessment, hydraulic containment.
Issues: Potential risk to groundwater and surface water, environment and human health. Leachate level control and trigger level compliance.
Summary: ESI carried out a review of the modelling undertaken for this hydraulically contained site and updated the model as required. ESI also undertook calculations to derive the minimum head difference required between leachate and external groundwater. The results were used to check / revise interim leachate control and trigger levels and from these updated compliance levels were proposed.
This landfill site had been operated since the late 1980s and was issued with a Pollution Prevention Control (PPC) Permit in 2005. The hydrogeological risk assessment (HRA) for the site, submitted as part of the PPC permit application in October 2004, assumed that all the cells at the site were, and would remain, in hydraulic containment. Thus the site was permitted on the basis that leachate levels would remain below surrounding groundwater levels.
Supporting information to the HRA included an assessment for the site using the Environment Agency’s methodology to calculate the contaminant fluxes from hydraulically contained landfills . A range of liner hydraulic conductivities and head differentials was modelled. The conclusions showed that the diffusive flux out of the landfill would not result in detectable concentrations of List I substances in groundwater.
ESI subsequently reviewed the modelling undertaken in the HRA. The review showed that, on the basis of additional site-specific information collected since the original HRA (e.g. leachate quality and hydraulic conductivity), the parameterisation of the hydraulic containment model required updating.
ESI then undertook calculations to derive the minimum head difference between leachate and external groundwater. This head difference is needed to provide a sufficient inward advective flux to balance the diffusive flux of contaminants out of the landfill. The results were used to check, and where necessary revise, the interim leachate control and trigger levels that had been developed for specific cells. Separate calculations were undertaken for groups of cells due to the differing construction and the possible implications for contaminant fluxes.
The calculations predicted the concentrations at the outer edge of the cell liner. The results of the calculations based on maximum measured contaminant concentrations showed that a head differential of at least 0.1 m was needed to generate the necessary inward advective flux in Cells 1 to 6 and that a head differential of at least 1.10 m was required for Cells 7 to 16A.
A number of conservative assumptions were included in the calculations. Sensitivity analysis also showed that when less conservative values were used, this had only minor effects on the head differentials required.
Updated trigger (compliance) levels were proposed, based on the calculated head differential needed to maintain hydraulic containment. Control levels were also given, allowing for an additional 1 m safety margin.
ESI is recognised as a UK centre of excellence for groundwater modelling, and is contracted by both public and private organisations to carry out similar work.