ESI Ltd - Environment Specialists

Modelling to assist risk assessment and remedial strategy development for a petrol spill


Courtesy of Courtesy of ESI Ltd - Environment Specialists

Client: Confidential: Petroleum Retailer

Location: Luton, UK

Services: Groundwater modelling, contaminant transport modelling, remediation strategy development

Summary: Groundwater modelling demonstrated that hydraulic containment of a contaminant plume could effectively protect a public water supply in the medium term, allowing full abstraction rates to be restored while source removal was undertaken.

ESI was commissioned by a leading petrol retailer to undertake a groundwater modelling project on a petrol spill in the Chalk Aquifer north of London.  The petrol station suffered a loss of 40,000 litres of unleaded fuel from an underground storage tank over a period of 3 weeks.

The site was located about 250m upstream of three Public Water Supply (PWS) abstraction boreholes, within the Environment Agency’s Source Protection Zone 1 for the supply.  There were significant concerns that hydrocarbon migration to the supply source could result in exceedance of drinking water standards in the raw water supply, and lead to costly treatment or source relocation being required.

It was recognised that solving the hydrocarbon problem required an approach that took into account the variation in risk, to the PWS and aquifer, with time.  In the short term, there was an immediate perceived risk of breakthrough of hydrocarbon contamination at the PWS, and predictive assessment was therefore required regarding the likely development of the plume in the short term.  In the medium term there was a requirement to manage the risk to the PWS by controlling transport of contaminant, by interrupting the path between source and receptor.  At the time this was undertaken by means of a pump and treat system at a network of boreholes.

Groundwater modelling was undertaken to optimise the remedial strategy and provide supporting evidence to demonstrate that the system could meet its immediate and longer term objectives.  The final groundwater flow model consisted of a three layer model, which was constructed using Groundwater Vistas which is a user interface for the MODFLOW suite of codes (ESI, 2001). 

Regular reporting of the results of the risk assessment kept both the client and the regulators in agreement with each stage of the assessment.  Following meetings with the EA and the client a number of pumping scenarios were simulated including the failure of a number of the abstraction boreholes. This helped define the most effective containment system for the plume ensuring adequate protection of the PWS. 

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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