Michael Boyd and Stuart Martin of WYG Environmental describe how studies are underway to see whether, in the case of a former laundry in Belfast, the most viable and cost-effective remediation solution is in fact to let nature do the work.
WYG Environmental is currently undertaking a Monitored Natural Attenuation (MNA) feasibility study for chlorinated solvents in a major aquifer in Northern Ireland. MNA is defined by the Environment Agency as "monitoring of groundwater to confirm whether natural attenuation processes are acting at a sufficient rate to ensure that the wider environment is unaffected and that remedial objectives will be achieved within a reasonable timescale". This is the first time that a significant MNA study of this type has been undertaken in Northern Ireland.
The site
The site, a former laundry which had been in operation for over 120 years used perchloroethylene (PCE) within the dry cleaning process up until approximately 18 years ago and stored diesel on the site for refuelling vehicles. The geological sequence across the site is glacial till comprising silty clay with sand, silt and gravel horizons, overlying Sherwood Sandstone which is a locally important aquifer.
Site investigation and characterisation
A series of site investigations have been undertaken to fully characterise the site. Two distinct groundwater bodies were identified; one within the glacial till, which was in hydraulic connection with a weathered horizon of sandstone and the second, a main groundwater body within the sandstone aquifer.
Two principal receptors for contamination were identified; a river running adjacent to the site, which was proven to be hydraulically connected with the groundwater within the glacial till and a groundwater abstraction well within the sandstone which is located c. 600m from the site.
The investigation included the installation of 30 shallow wells on site to assess the impacts in the glacial till deposits and the installation of a further 14 deep multi-level wells, located both on and off site and nested at discrete intervals of 10m, 25m, 50m and 70m, the aim of which was to assess potential impacts to the sandstone aquifer.
An initial conceptual site model and generic risk assessment developed for the site indicated significant hydrocarbon and minor solvent impacts within the shallow aquifer and solvent impacts with minor hydrocarbons within the deeper sandstone aquifer. The sources were deemed to pose a potential risk to the adjacent river, abstraction well and future site users.
Remediation
A pump and treat (activated carbon) system was employed to remediate the shallow groundwater, which was run by MEL Ltd for nine months and treated a total volume of 700m3. A Detailed Controlled Waters Quantitative Risk Assessment (DQRA) was undertaken to determine Site Specific Target Levels (SSTLs) to ensure that the remedial works met the set objectives of mitigating the risks identified.
MNA sampling and results
A double valve pump system was installed at each multi-level borehole location, with each piezometer having a dedicated pump and custom-designed head works. The system utilises an electronic air controller to ensure optimum low flow rates can be achieved during sampling, thus reducing the risk for volatilisation of the solvents and ensuring that representative samples are collected.
A key indicator parameter used to support the feasibility of MNA is the available carbon content within the aquifer. The carbon content provides the ‘fuel' required for the reduction of PCE to its daughter products TCE (trichloroethene), cDCE (cis-1,2-dichloroethene) and eventually VC (vinyl chloride).
The Sherwood Sandstone aquifer provides favourable iron reducing and oxidising conditions down-gradient of the site, permitting the reduction of cDCE to VC. The detailed on and off-site monitoring indicated that the chlorinated solvent plume is relatively stable, with contaminant levels actively reducing over the last five years, and confirmed that no significant off-site migration was taking place.
A programme of monitoring is currently underway with quarterly monitoring being undertaken for the first two years, the aim being to demonstrate reducing trends in contaminants and add further evidence that MNA is a viable solution.
Contaminant distribution modelling is being undertaken using a custom GIS database, which enables direct comparison of plume dimensions and concentrations over time. The groundwater DQRA is being continually modelled and updated using the RAM and BIOCHLOR risk assessment models.
Conclusions
The results are very positive with supporting lines of evidence for natural attenuation, i.e., depleted oxygen, increased Total Organic Carbon (TOC) and presence of daughter products within the centre of the plume.
Monitoring undertaken over the last five years has shown a reduction in PCE levels of up to two orders of magnitude; supporting MNA as a sustainable, viable and cost effective remediation solution. Continued monitoring of the system will be undertaken to confirm that the remedial target objectives for MNA can be achieved within reasonable time frames.
Michael Boyd, Director
Stuart Martin, Senior Consultant
