Site Investigation, Risk Assessment & Remediation of DNAPL at a Site in Grangemouth

May 20, 2019

TerraConsult were commissioned to carry out a site investigation and the remediation of a 2 ha site in Grangemouth, which is being redeveloped as a new waste to energy facility. The investigations identified extensive contamination by petroleum hydrocarbons, including Dense Non-Aqueous Phase Liquids (DNAPL’s). TerraConsult prepared and implemented a remediation strategy, verified the works and secured regulatory approval.

 

Site Investigations

 

The site characterisation investigations commenced in 2017 on disused land within an active chemical works site. This identified an area as being heavily contaminated by petroleum hydrocarbons, VOCs and SVOCs including DNAPL, within the underlying Tidal Flat Deposits. A second phase of investigations in 2018 further characterised the site, enabling a detailed quantitative risk assessment (DQRA) to be undertaken and a remediation strategy to be produced.

 

Remediation

Following regulatory approval of the remediation strategy TerraConsult were contracted to carry out the remediation works, which commenced in January 2019. This included the drilling of 355 remediation wells and a further 12 groundwater verification monitoring boreholes, all completed in three weeks. The remediation wells were drilled to a maximum depth of 8 mbgl and installed within the underlying Tidal Flat Deposits.  

 

On completion of the drilling works, Dual Phase Vapour Extraction (DPVE) was undertaken comprising the extraction of 84,000L of groundwater and vapours from the remediation wells. Vapours were treated with a GAC filter system and the liquids disposed of off-site.  Extraction was carried out over a nine week period after which 800 doses of 25L Chemox solution were injected into the remediation wells.

 

Verification of the remediation works comprised baseline and post completion groundwater monitoring, and the calculation of source mass removal through groundwater/vapour extraction and chemical destruction by Chemox. The remediation works resulted in the betterment of the groundwater conditions as demonstrated by two methodologies, mass removal and achievement of remedial targets.

 

  • A total of approximately 230 kg of liquid phase and approximately 40 kg of vapour phase contaminants were removed, with additional destruction by Chemox (noting the Chemox treatment process will continue to work for approximately 3 months after final injection).

  • Significant reduction in the concentration of contaminants in the groundwater with an average percentage reduction of the groundwater concentrations ranging from 38.7 % (aromatic C10-C12) to 98.7 % (aniline).

  • Remediation target concentrations for mean concentrations of benzene and chlorobenzene were achieved within the final verification groundwater sampling.

 

TerraConsult were able to meet the remediation targets in spite of difficult conditions, including the DNAPL being within very low permeability stratum and the demanding/short programme required by the client.  The period from commencing installation of 355 remedial wells to gain regulatory sign off was only 17.5 weeks. The works were completed on programme. 

 

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