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Surgical strike

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

When petrol retailer Rontec was planning to sell 7o Total petrol stations it asked environmental consultant SLR to undertake a major series of ground investigations earlier this year in a tight six week programme. 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.

“With 70 petrol stations requiring detailed intrusive site investigation 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.

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.

Main challenges

“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.”

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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