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Oil Spill, Newark on Trent

Site Overview

The property was a former public house with extensive cellars and dated from around 1680. In 2005 this was modernised and a substantial extension with a 2.0m deep prefabricated concrete ‘Thermonex’ cellar was added to the northern elevation, together with a yard and stable block.

During August 2012 hydrocarbon odours were noted in the cellar beneath the recent extension. The householder inspected the heating system and identified a split in the polyethylene Aboveground Storage Tank (AST), located up gradient and to the north of the house. It was estimated that approximately 2500 litres of kerosene heating oil was lost to ground during the incident.

 

Objective

The aim was to ensure that the householders could remain in the property without a risk to human health and to remove the risks posed by contaminated soils/groundwater without resorting to extensive demolition and excavation works.

The remediation strategy for the site comprised of a three stage approach. The first stage involved the installation of a Vapour Extraction Unit (VEU) to reduce vapour concentrations in the cellar. This was coupled with the excavation of 10.0 tonnes of contaminated material from the spill location and chemical oxidation treatment beneath the stable block. The second stage consisted of the installation of an EDM Ecotrac vacuum enhanced recovery system to remove free/dissolved phase product and soil vapour. The third stage utilised a four week programme of chemical oxidation treatments to remove residual soil and groundwater contamination.

 

Methodology

EDM mobilised to site within five hours of instruction to install a Vapour Extraction Unit (VEU) to reduce the Volatile Organic Compound (VOC) concentrations in the cellar. Prior to fitting the VEU the VOC concentrations in the cellar were up to 87.0ppm but after 30 minutes of air extraction with the VEU the VOC concentration fell to a maximum of 4.1ppm.

An initial site investigation identified significant kerosene contamination in near surface soils below the floor of the stable block, beneath and immediately adjacent to the AST and also indicated that the groundwater present at 2.1mBGL had been heavily impacted. To determine the extent of the groundwater contamination plume the scope of the investigation was expanded to include the advancement of fourteen boreholes to a depth of 6.0mBGL down gradient of the AST. A plume of free product was noted up to 25mm thick extending 10.0m from the AST to the rear external wall of the house and significant free product was found to be present in the drainage system around the cellar.

During the first stage works the AST base and contaminated soils in the vicinity of the source area were excavated to a maximum depth of 1.6mBGL; 10 tonnes of material was removed to an appropriately licensed landfill. To enable the stables to remain in use residual contamination beneath the floor of the stable block was treated with three chemical oxidant injection treatments over a two week period. In-situ chemical oxidation is a remediation method used for soil and/or groundwater remediation to reduce the contaminant concentrations to acceptable levels.

This is accomplished by injecting or introducing chemical oxidisers directly into the contaminated medium (soil/groundwater) to breakdown the contaminants in place. In terms of cost to remediated volume the method is more expensive than excavation and disposal but its use can provide major cost savings in circumstances where it offers an alternative to expensive demolition, excavation and reinstatement works.

The groundwater contaminant plume at 2.1mBGL was remediated during the second stage of works by dual phase extraction using two EDM Ecotrac telemetry controlled systems. These use down borehole skimmer pumps fitted with hyhobic filters to remove free product. Additional vacuum extraction pumps recover vapour and create a low pressure/groundwater gradient in the boreholes to promote free product migration towards the boreholes for recovery. Recovered product was collected in a storage tank for appropriate disposal and the extracted vapour was passed through a Granular Activated Carbon (GAC) filter to remove any VOCs.

This remediation strategy was chosen as the most cost effective option because it did not require demolition, excavation and reinstatement works. These works are both expensive and in close proximity to the house could have potentially caused structural damage. The EDM Ecotrac system has the advantage of being linked through telemetry to an EDM computer. This enables the units to be controlled remotely so that pumping and recharge periods can be adjusted by the operator to maximise recovery rates. The units continually monitor and send data on the product and vapour recovery rates, holding tank levels, filter capacities and will send an alert if there is a power loss or if servicing is required. This ensures that recovery rates are optimised and reduces site visits for monitoring and maintenance. The EDM Ecotrac dual phase recovery system was operated for a period of nineteen weeks. Initial free product recovery rates were over 100 litres/week but these fell exponentially over a period of twelve weeks until by week thirteen no free product was present. The system was then reconfigured to principally recover kerosene vapour over a further six week period. By week nineteen VOC borehole concentration levels had been reduced sufficiently to comply with EDM’s target values.

The third stage works were aimed to ensure the long term protection of the cellar, constructed at groundwater level, from product emanating from any remaining pockets of residual contamination. Eighteen injection probes were advanced to a depth of 3.0mBGL in the delineated area of the plume. These were used to deliver twelve chemical oxidation treatments over a four week period.

The excavation was validated with eight soil samples taken from the sides and base of the excavation. The groundwater remediation works were validated with three rounds of groundwater sampling. All samples were submitted for analysis to Jones Environmental Laboratory a UKAS accredited independent laboratory. Analysis confirmed that all soil and groundwater samples were within remediation target values. Following validation the boreholes and injection probes were decommissioned and the site was reinstated to its original condition to the satisfaction of the householder.

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