In-situ Stabilisation and Solidification/ In-situ Chemical Oxidation of Coal Tar Contaminated Soils 

Background

Oracle Environmental Experts were initially asked to provide a remedial options assessment (ROA) for a section of a former gasworks site that was being redeveloped for housing.

The site was contaminated with coal tar residues in soil and shallow groundwater associated with buried structures and pipework on the site. The ROA concluded that in-situ stabilisation / in-situ chemical oxidation (ISS/ISCO) was the most cost-effective and time-efficient solution for the client.

Oracle were then instructed to provide a remediation strategy and to complete the remediation works on the site.  

Preparation Works

Investigation of the site indicated that approximately 3000 tonnes of coal-tar-contaminated soil required treatment, primarily between 1 and 3m depth along the route of a disused pipe. Oracle conducted bench trials to establish which mix of reagents would be suitable for field-scale application of the remediation technology, with 16 different mixes of coal-tar-contaminated soils and remediation reagents produced and left to cure. Based on the bench-trial results, 3 reagent mixes were taken forward for 64-day dynamic tank testing.

Ultimately the most cost-effective mix of reagents was a mix of Sodium Persulphate (Klozur® SP from Evonik); Ordinary Portland Cement (OPC); and Ground Granulated Blast Furnace Slag (GGBS). The ISCO/ISS process works with the sodium persulphate treating the more soluble (mobile) fraction of the contamination, which is rapidly removed via chemical oxidation, and with the remaining heavier (less mobile) contaminant fraction stabilised in situ through the application of the OPC and GGBS, which together form a monolith with the consistency of silty clay.