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Fuelling the fast track for ground investigation

Doing 70 site investigations in six weeks sounds like a tall order, but that is what consultant SLR delivered for petrol station operator Rontec.

Ground investigations for potentially contaminated sites are usually undertaken through a phased approach with a desk study to identify potential source of contamination carried out ahead of a physical investigation. Testing samples may result in the need for a second phase of investigation before the results are reported to the client and remediation plans drawn up.

This investigation process takes time, so delivering such a programme in six weeks would be considered challenging and on that basis delivering the same programme for 70 sites in six weeks might sound impossible.

Nonetheless, that is exactly the task that petrol retailer Rontec asked environmental consultant SLR to undertake when it planned a major divestment operation earlier this year.

After Rontec acquired Total’s petrol station business, the company decided to sell off 70 of the sites, but there was a high risk that some would be contaminated and due diligence called for detailed information.

According to SLR technical director Dan Collins, Rontec determined which sites should be investigated before launch of the divestment process.

Rontec called for 70 sites to be investigated in six weeks

Rontec called for 70 sites to be investigated in six weeks

“We initially started work with Rontec when we were asked to assist with the due diligence for the sale of 18 Total sites to a supermarket chain,” he says. Rontec also wanted to divest some of its other properties and time was of the essence.

“With 70 petrol stations requiring detailed intrusive site investigation over such a short period we formed a team of 23 in-house specialists to carry out the work systematically at sites located as far apart as Milford Haven and South Shields,” he explained.

“The intrusive investigation of potentially contaminated sites is undertaken as a phased approach whereby the outcome of each phase dictates the approach of the next.

“What this means in practice is that work usually commences with a desk-based assessment of known information which is then used to develop a conceptual model of possible sources of contamination, pathways and receptors.

“This is in turn used to design the site investigation. For many brownfield sites, this means that the scope of the site investigation cannot be fully determined until the desk-based phase is complete.

“Given the scale of the Rontec divestment portfolio, a reasonable estimate of the scope of the investigations – and cost – needed to be supplied in advance of the works. Since all sites were petrol stations the most likely sources of contamination – the storage and dispensing of fuel – across the sites were effectively identical. This meant that a relatively generic scope of works could be proposed, which would be fine-tuned for individual sites following completion of the desk studies.”

Over a four week period, SLR’s team undertook three investigations a day. On each site a number of driven tube or rotary boreholes was drilled, soil and groundwater samples collected, field tests undertaken and monitoring wells installed. In some cases, sites had already been the subject of a recent site investigation. Where this was the case, existing monitoring wells were used to collect groundwater samples.

Careful planning was undertaken to ensure problems were minimise once work started on site

Careful planning was undertaken to ensure problems were minimise once work started on site

“One of the main challenges was to deliver a detailed initial site investigation in a single day while maintaining a high standard of health and safety provision,” says Collins. “On all intrusive site investigation projects there is, in particular, a risk of damage to, or from, below ground services. On active petrol stations, this risk is magnified and the consequences of rupturing fuel lines or below ground fuel tanks are severe.

“To limit risks to site operatives and the environment from damage to fuel infrastructure detailed service plans and records were obtained in advance of the works. These, coupled with historic maps and reference to hydrogeological data, were used to select provisional locations for boreholes. Typically, boreholes were located as close as possible to features such as below ground fuel tanks and pump islands and up and down gradient of these features with respect to anticipated groundwater flow direction. The presence of local human receptors was also taken into consideration, for instance, when residential properties were located within a short distance of the boundary of a site.

“On the day of the site investigation potential borehole locations were checked with cable avoidance tools and all boreholes were advanced with hand tools to a depth of 1.2m to provide an additional level of certainty that the selected locations were safe to drill.”

This approach paid off on the Rontec project since, several times, previously unknown services were encountered within service pits and would have been damaged had this policy not been in place.

Soil and groundwater samples were tested for a range of potential contaminants with testing involving hydrocarbons as a minimum due to the nature of the sites, but with other testing included based on the desk study.

In terms of the assessment of risks to human health, groundwater and local surface waters SLR derived generic assessment criteria that would be protective of the continued use of the sites as petrol stations.

Where these relatively conservative assessment criteria were exceeded by measured soil and water contaminant concentrations further, targeted, site investigation and/or detailed quantitative risk assessment was undertaken.

Following completion of the first 70 sites, two further portfolios of sites were offered for sale, bringing the total number of sites on which site investigations were undertaken to over 100.

In total the investigation resulted in the review of 10,000 historical maps, the drilling of 409 boreholes totalling 1.83km in length, excavation of 450m hand dug pits, installation of 1.7km of groundwater monitoring wells and testing of 759 soil samples and 420 groundwater samples.

“The result of the above process is that a good number of the 100 sites could be sold without the need for remediation because we were able to demonstrate that the sites were suitable for continued use as petrol stations using risk assessment,” says Collins. “Sites that did call for remediation or further site investigation were assessed on a site by site basis and suitable technologies selected to reduce costs.”

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