David Grouse of MEL describes how a combination of pump and treat, chemical oxidation and installation of an HRC cordon successfully treated contaminated groundwater at a
Ballymena site.
In March 2007 MEL (UK) Ltd were commissioned to review an initial generic quantitative risk assessment (GQRA) carried out by White Young Green, Belfast. The purpose of this review was to develop a robust strategy for the remediation of the contaminated groundwater on the site.
The assessment has been undertaken based on the redevelopment of the site for a residential end use. The initial Land Contamination Risk Assessment comprised a preliminary risk assessment to establish potential pollutant linkages and was based on an intrusive site investigation, to further investigate the linkages identified.
The site occupies an area of approximately 2.7ha and comprises a former textile dye works located near the centre of Ballymena, Co. Antrim. The majority of the site is covered with hard standing and buildings with a car park and an area of soft standing in the south of the site.
Groundwater contamination
As a result of the initial investigations carried the following contaminant levels were identified:
The areas of contamination may generally be considered as two plumes, of which the northern plume (A) covers an approximate area of 2,000m2 and the southern plume (B) an area of 1,800m2. Groundwater flow is generally north to south.
The areas of contamination were further delineated as shown in Figure 1.
1. Primary hotspot of northern plume
2. Secondary hotspot of northern plume
3. Area of more southerly plume B
Remediation strategy
Following a review of technologies MEL determined that the most robust approach for this project would be the combination of proven, tried and tested technology to deal with the grossly contaminated areas in each of the areas 1, 2 and 3. The principal strategy employed was in three parts:
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Install and operate an aggressive pump and treat system to reduce the high levels of contamination present within the groundwater.
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When this approach was demonstrated to be no longer cost effective the groundwater would be treated using in-situ chemical oxidation to remove the residual levels from the groundwater.
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In area 3, which represents the most direct line of groundwater flow to the adjacent river, there were low levels of Vinyl Chloride present. It was decided to install an effective cordon in this area using Hydrogen Release Compound (HRC), supplied by Regenesis, thus providing a long-term barrier to further travel of contaminants due to groundwater fluctuations and providing additional comfort in the overall remediation strategy.
This strategy was supplemented by detailed 3D site characterisation of the groundwater levels and flows by White Young Green, which defined the underlying strata and indicated that the contamination was in effect retained in a ‘bowl' in the northern area of the site and that seasonal fluctuations in groundwater level resulted in a possible ‘flushing' of some contaminants into the area. In view of the above we implemented the following remediation strategy in each area:
Areas 1 and 2
Stage 1 - Install a network of 27 abstraction wells, each 100mm, to facilitate the extraction of groundwater for treatment above ground. A pump and treat system was applied which extracted the impacted groundwater in the area where it is most highly contaminated and removed the contaminants by way of air stripping technology. The main components of the pump and treat system were:
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Surface-mounted groundwater abstraction pumps linked to a manifold to regulate the rate of abstraction from each of the wells
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Oil Water Separator (OWS) to remove any free phase product/oil (FPP) from the abstracted groundwater
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Telemetry equipment to allow for the remote monitoring of system parameters during commissioning and process works
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An air stripper to remove VOCs from the liquid phase into the vapour phase
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Granular Activated Carbon (GAC) filters to remove VOCs exhausted from the air stripper from the vapour stream prior to discharge to atmosphere
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GAC filters to remove any residual VOCs, or other organic contamination, from the treated water stream which had passed through the air stripper prior to discharge to sewer.
The system was operated 24 hours a day, seven days a week, for 24 weeks and monitoring was carried out every four weeks to ascertain the reduction in levels across the site and to target the abstraction to the worst impacted wells.
Figure 2 shows how effective the pump and treat system was in reducing the contamination levels present. However the rate of reduction did slow as anticipated and therefore at 24 weeks, having stopped pumping for four weeks, we recorded rebound of approximately 15% in the remaining 12 of the original 27 locations still above the SSTL.
Stage 2 - As set out in the original strategy the second phase consisted of direct push injection of sodium permanganate solution directly into the impacted groundwater. This was installed over a 3m depth of groundwater at 1m intervals and was injected on 3m centres to saturate the reduced plume area. This was repeated following collection of new data showing residual levels above the SSTL and the result was achievement of the SSTLs across the site.
Area 3
A network of nine monitoring wells were installed in area 3 (see Figure 1 cordon) to plot the reduction in the contaminant levels and to provide evidence of the ongoing ‘barrier effect' created by the injection of the HRC compounds in this area.
The initial concentration of contaminants in area 3 was such that in-situ remediation was implemented as the principal means of remediation. The design of the in situ groundwater remediation in area 3 was undertaken in conjunction with Regenesis and the methodology selected was in-situ anaerobic degradation. The reagent was injected across area 3 on a grid with nominal centres of 4.5m x 4.5m.
Combined success
It is clear on this occasion that no single remediation techniques would have achieved the level of reduction within the same timescale and for the same cost. This site required three differing technologies, this is not uncommon and it is becoming a framework for effective remediation of complex sites and where cost benefits and operational efficiencies can benefit the client and provide more comfort to regulators.
However, as in this case, the combination of chosen technologies has to complement each other and the limitation of each technology must be clearly understood.
Early discussion with the stakeholders, in particular the Northern Ireland Environment Agency, planning services and the client, was crucial in the process of agreement of the strategy and of the desired long-term approach to the site with regards to long-term monitoring and provision of supporting data.
David Grouse, MEL (UK) Ltd.
