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Introduction

A large petrol station located along one of the busiest motorways in northern Italy was undergoing extensive reconstruction, including the demolition of existing buildings, removal of above- and below-ground structures; as well as remediation. The existing underground fuel tanks (USTs) were removed and the contaminated soil surrounding the USTs was excavated to a depth of 5m BGL.

The geology consists of a highly permeable soil comprising sand and gravel to approx. 40m BGL. The average groundwater level is at approximately 4m to 6m BGL, where an extensive contaminant smear zone was observed.

The main excavation was approx. 1,100m2 and 5m deep, with the extent and depth limited by the proximity of the motorway (as well as the presence of groundwater).

The soil at the bottom of the excavation was highly impacted with Total Petroleum Hydrocarbons (TPHs) and BTEX compounds, with concentrations of up to 10,000mg/kg. The groundwater inside the excavation was highly contaminated, with evidence of Light Non Aqueous Phase Liquid (LNAPL) and average concentrations in the range of 500,000μg/L.

Phase I – Enhanced Desorption and Extraction

• Before application, the excavation was divided in seven segments, leaving some pathways for excavator passage. For each segment, a specific PetroCleanze application dose was defined.
• PetroCleanze was applied in each sub-area through the dig and mixed directly with the remaining impacted soil. Due to the high contaminant mass present in some of the areas, LNAPL appeared on the water in less than one hour after the first application (the photos shown overleaf were taken 30 minutes after application).
• The LNAPL was subsequently removed by selective pumping using a vacuum tanker.
• Repeated soil mixing and desorbed LNAPL removal took place over the following days across the whole area (for a maximum of 3 cycles in the most contaminated areas).
• The repeated soil-mixing allowed for ongoing desorption and removal of the contamination without the need to add more PetroCleanze. This process was continued for all the sub-areas until no more recoverable free phase was present.
• Finally, a short period of de-watering was completed to remove part of the residual dissolved phase contamination, with water samples taken before and after.

Phase II – Enhanced Aerobic Natural Attenuation

Following the initial excavation and enhanced desorption treatment, an ORC Advanced application was completed. This targeted the residual dissolved phase contamination. The ORC Advanced provided a controlled release of dissolved oxygen in order to stimulate and maintain enhanced aerobic biological degradation of the groundwater contamination for 9-12 months from the single application. This meant that the excavation could be immediately back filled and the construction continued. The remediation of the dissolved phase contamination therefore continued in situ, with no further operational costs and no disruption to the build programme.

To optimise the application process onsite, a pelletised version of ORC Advanced was chosen. This is because they can be applied directly in the excavation without the need for pre-mixing with water. The Pellets also minimise dust formation during application. The pit application was cost-effective, as it did not require additional equipment to be used on site and the application and mixing was safe, quick and straightforward.

Conclusion

• The site was remediated on time without disruption to the construction program through the use of enhanced desorption and removal, followed by in situ enhanced aerobic biological degradation.