Above Watertable Landfill (Groundwater Risk Assessment)

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Courtesy of ESI Ltd - Environment Specialists

Client: Waste Management Industry

Location: Co-disposal landfill site

Services: Hydrogeological risk assessment, landfill monitoring borehole installation, sample collection and data interpretation.

Issues: Change in waste stream and potential risk to groundwater, 

Summary: ESI provided site supervision services for the installation of monitoring boreholes, field testing and sample collection. Data analysis and Hydrogeological Risk Assessment (HRA) services were also provided.

The client benefited from regular communications between ESI and the Agency such that all parties were in agreement with each stage of the assessment. Risk assessment modelling of various different engineered barriers helped define the most effective containment system for the site ensuring adequate protection of the environment.

A waste disposal company contracted ESI to carry out a groundwater risk assessment for one of its co-disposal landfill sites.  The Agency had requested a review and update of the previous groundwater risk assessment due to a change in the waste stream following the introduction of the new landfill regulations in July 2004.  The co-disposal landfill was to accept only hazardous wastes after July 2004.  The hazardous landfill lies above a major aquifer. 

The project involved the supervision and logging of boreholes and the installation of groundwater monitoring wells, field testing, sample collection and the collation and analysis of all available data to aid the formulation of a hydrogeological conceptual model.  A Probabilistic Risk Assessment (Monte Carlo analysis) was carried out, using in-house stochastic modelling techniques which use the advection-dispersion-retardation-decay transport model. This modelling approach was chosen to provide both a robust and transparent assessment of risk using the source-pathway-receptor methodology.  A number of contaminants were modelled to represent the predicted potential risks to the identified receptors associated with the proposed landfill.  At the request of the Agency sensitivity analyses were also carried out including the permeability and thickness of the basal engineered barrier, source term decline and the hydraulic parameters of the major aquifer. 

Regular reporting of the results of the risk assessment kept both the client and EA in agreement with each stage of the assessment.  Following meetings with the EA and the client a number of engineering failure scenarios were simulated including the failure of the HDPE geomembrane and the failure of the leachate extraction system.  Modelling of the various engineered barriers helped define the most effective containment system for the waste ensuring adequate protection of the environment.

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