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The First Implementation of a Combined Electric Resistive Heating (ERH) and Multi Phase Extraction (MPE) Remedy at a Fractured Bedrock Site in Scotland, UK

Shortlisted for Best Application of Remediation Technologies (sponsored by NHBC)


Background: High resolution site characterization at a site in Scotland (UK) delineated a hot-spot area of residual soil TCE contamination (>200 mg/kg) within a clay dominated subsoil, which represents a historic remnant of the original disposal area at the site. A high concentration, predominantly TCE plume, has developed within the underlying sandstone aquifer and the uppermost mudstone aquitard. Measured groundwater concentrations of TCE within the core of the impacted zone vary between approximately 5% (55,000 µg/l) and 10% (110,000 µg/l) of TCE aqueous solubility, which is well above the generally accepted 1% (11,000 µg/l) rule of thumb for the suspected nearby presence of DNAPL.

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Insert Figure A – Hydrogeological CSM Cross Section Through Site.

The groundwater discharge volume from the source area, which is located on the former site car park area, is estimated at 0.41m3/d, with a dissolved phase TCE mass discharge of 41.5 g/d, which equates to approximately 15kg/yr. With residential properties bounding the downgradient area(s) of the site and impacts observed in an adjacent surface water stream (known locally as a Burn), remedial action was required with the principal objective of reducing the overall mass flux leaving the site by at least one order of magnitude.  

Remediation Objectives: Due to the complexities of this complex fractured bedrock setting, a detailed remedial alternatives evaluation was undertaken. This evaluation included the bespoke approach of inviting remediation contractors to review the CSM and produce their own remediation solutions (utilising their own specialisms) based on the principal remediation design basis developed by Geosyntec. A key part of the remediation design basis was to ensure: -

  • Soil concentrations reduced from 200-300mg/kg which was observed in the environmental characterisation phase of works to <1mg/kg; 

  • Remediation implementation recovers at least 90% of the estimated TCE mass within the aquifers and reduced which should resultantly reduce TCE source concentrations from ~200mg/l to less than 5mg/l; and 

  • Identified sensitive receptors (namely neighbouring residents and the Burn) which were located within 50m from the site where not adversely impacted from the site during remediation construction and implementation either by adverse soil, vapour, or groundwater impact change.

Remediation Design: The inclusion of preliminary remediation tendering as part of the remediation options appraisal confirmed to the project team that the implementation of a single “traditional” remediation treatment approach would not meet the fundamental remediation design basis. Electric Resistive Heating (ERH) was identified as the preferred remedial technology for this specific environmental setting. The remedy design was optimized to achieve the mass flux reduction remediation goals by targeting the treatment target zone where the highest proportion of residual mass resided, which occupied an area of ~900m2 and approximately ~8,000m3 of fractured bedrock. Additional hydraulic and vapor control, using a complementary peripheral MPE system, was included in the overall remediation design to protect to nearby sensitive receptors. Based on an overall mass flux remedial objective and an optimised target treatment zone, a multiple lines of evidence verification strategy was agreed with the regulatory authorities.       

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Insert Figure B – Treatment Target Zone Surface Infrastructure including peripheral MPE System.

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Insert Figure C – ERH / MPE Equipment Infrastructure

Final collaborative remediation design was able to optimise / reduce the remediation target volume by 20% before implementation resulting in a very significant cost saving to our client before the project even commenced. During the remediation planning phase understanding and agreeing the power supply requirement was a critical factor, appreciating our client’s sustainability goals Geosyntec was able to arrange the provision of certified “green energy” to implement the remediation; utilising Scottish wind energy.

 

Remediation Implementation: During the ERH subsurface installation works the peripheral MPE wells where advanced initially and connected to the remediation system in the first two weeks of site works. This allowed for localized mass which was released into the fractured bedrock aquifer to be recovered and avoided an increase in the mass flux leaving the site as part of the installation works which was vitally important with the project having mass flux derived remediation criteria. 

 

Upon ERH implementation subsurface temperatures relatively quickly increased and within approximately 2 months of operation subsurface temperatures were above the azeotropic boiling point of TCE (73°C) and by the end of the project on average greater than 95°C. This led to a secondary phase in overall TCE mass recovery where we infer that mass residing in not easily accessible fractures and the bedrock matrix was effectively recovered. The remediation process removed ~1,200kg of TCE from the targeted remediation treatment zone.

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Figure D – Cumulative TCE Mass Removal

As designed the remediation was successfully completed within the 20-week estimated timeframe. However, effective and efficient remediation optimisation driven by Geosyntec and the remediation contractor resulted in the only ~80% of the total power demand actually being required; once again providing significant cost savings to our client. The MPE and ERH systems recorded very high levels of operational uptime (>95%) with the MPE system boundary vapour/temperature required to mitigate risk to the surrounding sensitive receptors.

 

When these good ERH temperatures are compared to the peripheral groundwater monitoring dramatic differences could be observed with the boundary wells being observed to only altering by ~5°C and generally found to be under applied vacuum induced by the MPE system.  

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Insert Figure E – Left: ERH Temperature Profile from with TTZ. Right: Peripheral Monitoring Well Showing Relatively Minor Temperature Change.

Remediation Verification: To achieve the agreed mass flux remediation goal it was modelled that groundwater concentrations within the treatment target zone would need to be reduced by approximately one order of magnitude. Upon completion of the ERH thermal treatment MPE continued to operate for a period of two additional weeks to remove any residual steam / high concentrated vapour from within the treatment target zone. Following this period of final MPE operation groundwater quality verification was completed. The results showed that compared to the maximum TCE source zone concentration post remediation concentrations had be reduced by approximately two orders of magnitude, demonstrating (via multiple lines of evidence) that the bespoke remedial approach had successfully removed the source of TCE contaminant from these two fractured bedrock aquifers to the satisfaction of our client and the regulatory authorities.

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Insert Tables G: Top – Sandstone Aquifer Post Remediation TCE Verification. Bottom – Mudstone Aquifer Post Remediation TCE Verification.