Hydrogeological risk assessment for a proposed landfill

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Courtesy of ESI Ltd - Environment Specialists

Client: Waste Management Industry

Location: Landfill site, UK

Services: Desk study, site investigation and data review, 3D MODFLOW groundwater modelling and hydrogeological risk assessment.

Issues: Potential risk to groundwater and surface water, environment and human health.  Landfill containment design optimisation.

Summary:   ESI undertook a desk study and detailed site investigation in order to enable robust conceptualisation of the hydrogelogical regime and subsequent groundwater flow and hydrogeological risk modelling of this landfill site. Of particular importance was inclusion of leachate re-circulation in the model. Development of the modelled site design in close liaison with the landfill engineers enabled the necessary site performance to be achieved cost effectively. The client also benefited from ESI’s close consultation with the Environment Agency, facilitating approval of the preferred containment solution for the landfill site.

ESI lTD. was appointed to provide an assessment of the hydrogeological regime and to undertake risk modelling as an appropriate basis for completing a Regulation 15 risk assessment.

The project included a desk study and field work in order to collect sufficient data to complete the project.  Specifically, the field work included the detailed logging of seven groundwater monitoring wells and subsequently undertaking pumping tests in these wells.

A lake had formed in the base of the quarry which was being drained during the course of the project.  The opportunity was taken to undertake a lake drawdown test, with lake water levels, lake abstraction rates and groundwater levels being monitored over a three month period.

A 3D MODFLOW model was developed for the lake drawdown that represented the key aspects of the hydrogeological system.

Using the borehole pumping test data, lake drawdown test data and the MODFLOW model, a consistent picture of the hydrogeological regime was determined.

Working in close collaboration with the landfill design engineers, a risk model of the landfill containment system was developed.  Of particular importance was the inclusion of leachate re-circulation in the model as it is proposed to partially treat the leachate, in order to reduce ammonium concentrations.

The landfill design incorporates a Dense Asphaltic Concrete (DAC) basal liner and considerable effort was made to obtain suitable properties data for this material for use in the risk model.  In particular, the diffusion co-efficient for the DAC had to be reliably estimated as the advective velocity through this material is so low that diffusive fluxes become significant.

The risk model simulates an advection-dispersion-retardation-decay transport model, using the ESI Laplace transform solution method.  The analysis along the pathway takes account of the geometry of the pathway, but is essentially one dimensional, with a simple description of the physical parameters affecting the contaminant migration along the pathway.

The risk model incorporates uncertainty associated with the input parameters via the use of a stochastic method.  Iterative development of the site design in liaison with the landfill engineers ensured that the required site performance could be achieved in the most cost effective manner.  ESI provided detailed technical justification for model parameters and undertook close consultation with the Environment Agency to facilitate approval of the preferred containment solution.

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.

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