Project: Former Tannery, Kinghorn
Timescale: 12 months
Groundwater in shallow bedrock at this former tannery was contaminated with chlorinated solvents, mainly PCE and its breakdown products. Remedial targets were relatively low due to proximity of sensitive receptors. Natural monitored attenuation had been ineffective, so an alternative approach was required to enable the site to be re-developed for housing and to meet our client’s tight timescale for remediation.
Available site investigation data was limited, so ERS proposed extra boreholes to delineate the plume and gather information on the weathered bedrock horizon. Boreholes were drilled and installed in a manner which allowed subsequent re-use as injection wells in-situ remediation, thereby minimising costs. Low-flow groundwater monitoring and sampling from the new boreholes showed that concentrations of chlorinated solvents in groundwater were significantly higher than anticipated and suggested the possibility of residual DNAPL onsite.
The supplementary investigation was concluded in March 2020, just before lockdown associated with the onset of the Covid-19 pandemic. The subsequent closure of construction sites in Scotland impacted the remediation phase and reduced the overall timescale by 20%.
Remediation works resumed in July 2020, once construction sites were allowed to reopen and suitable Covid-19 measure had been put in place. To address the potential DNAPL source remaining in the made ground, the full extent of the former tannery’s degreaser shed was excavated to check for relict tanks or contamination hotspots. This identified olfactory evidence of solvent contamination around the southern foundation. The foundations and impacted soils were set aside and, due to the low volume, removed off-site. Unimpacted soils were re-used as backfill.
ERS proposed a combination of in-situ chemical reduction (ISCR) and enhanced reductive dechlorination (ERD). Our in-house quantitative polymerase chain reaction (qPCR) testing had shown a low abundance of chlorinated solvent degrading bacteria on the site. Therefore, a PCE degrading mixed microbial culture was proposed alongside the chemical remediation reagents.
Injection wells were installed on a grid pattern to cover the source and plume area. Reagent injection was undertaken by ERS’ in-house team and equipment, using inflatable packers and injection pumps.
Post-injection monitoring commenced at the end of August 2020 and took place at monthly intervals. The monitoring programme included low-flow sampling, in-situ parameters and laboratory analysis including qPCR.
Assessment of contaminant removal showed that the remediation had reduced the total mass of contaminants of concern by 96% by January 2021. Redox conditions were shown to be supportive of further reductive dechlorination, suitable for the continued degradation to ethene. Continued qPCR analysis also confirmed the presence of chlorinated solvent degraders capable of full contaminant breakdown. Therefore further reduction was expected.
A works verification report was submitted and accepted by the regulators in February 2021, within the client’s original timescale despite the delays due to the Covid-19 pandemic. The injection wells were subsequently decommissioned, and the client commenced development of the site in April 2021.