MTFI In-situ Remediation

REDS Group was commissioned to redevelop a Motor Transport Fuel Installation at a MOD site in South West England.

The project involved removing old underground fuel tanks and constructing a new above-ground tank and dispensing island. During works, an unrecorded underground tank with hydrocarbon contamination was discovered. The poorly backfilled vault and remaining infrastructure were removed, followed by remedial works to extract free-phase hydrocarbons and contaminated soils.

Project Scope

REDS Group was commissioned to redevelop a Motor Transport Fuel Installation (MTFI) facility at a MOD site in South West England. Conditional planning permission was granted for this redevelopment, involving the removal of former below ground fuel tanks and construction of a new above-ground fuel tank and dispensing island.

During redevelopment, a previously unrecorded former below ground tank installation was encountered, with residual hydrocarbon contamination. The tank vault had been backfilled with poorly compacted concrete and remnants of forecourt infrastructure, requiring removal to provide a suitable foundation for the new tanks. Initial remedial works removed free-phase hydrocarbons and contaminated soils.

Remediation

Geology comprised clay soils down to ~3.5m depth, overlying sand and gravel units. Groundwater contamination occurred in the deeper sand and gravel, with hydrocarbon plume migration identified to the south of the site. Due to the depth of contamination, further excavation was deemed unsustainable. A revised Detailed Quantitative Risk Assessment derived site-specific remedial targets to address residual groundwater contamination.

We designed a remedial strategy for the site based on in-situ treatment of residually impacted groundwater using a combination of In-Situ Chemical Oxidation (ISCO) and Oxygen Releasing Substrate (ORS).

Initial treatment of the residual hydrocarbon plume to the south of the new fuel installation was completed using In-Situ Chemical Oxidation (ISCO). We constructed temporary injection wells and additional sentinel monitoring wells at the site and then injected reagents into the underlying gravel aquifer using our chemical mixing and injection system via the installed treatment well network.

The reagent solution was injected into each location daily over the course of three days to ensure suitable volumes were introduced into the residual hydrocarbon plume. Each location was then injected with clean water to further distribute the reagent into the aquifer. The final phase of our remedial treatment was completed via follow-on treatment using ORS, which releases oxygen to accelerate bioremediation in the contaminant plume over 9 to 12 months.

Verification

Post-remedial verification monitoring confirmed effective natural attenuation, with hydrocarbon concentrations in groundwater reduced by over 95%, meeting site-specific remedial targets. All works were supervised by REDS Environmental Consultancy.