Capping of Former Waste Management Sites, Castleford, by NewFields UK
Category 1 - Best Project Preparatory Work and Category 6 - Best Project Closure/Verification
The former Hickson and Welch Site was a chemical manufacturing facility, located approximately one-kilometre northeast of Castleford town centre that operated between 1915 and 2006 producing a wide range of chemical products including dyestuffs, pesticides, herbicides, timber treatment products and pharmaceutical intermediaries. Operations included a main factory area where chemicals were produced along with three waste management areas where chemicals were stored, and periodically various operational wastes were disposed.
These three waste management areas are known as Landfill 227, Lowfields and the Green Lane Landfill, all of which are located to the east of the main factory site, along the bank of the River Aire, and cover an area of around eight hectares.
The three waste management areas comprise a mixture of made ground, general waste, building rubble, effluent cake products (from wastewater treatment) and other process related media (e.g. iron oxide catalysts). Landfill 227 was constructed with an underlying puddle clay liner to prevent leachate movement, but the other two areas predate the legal requirements for preventative measures. All three sites overlie a geological sequence of alluvial clays, overlying an alluvial gravel layer, which lies directly on a bedrock of Coal Measures mudstone. The alluvial gravel contains the local groundwater aquifer that is hydrologically connected to the River Aire, with groundwater flow predominantly towards the River. The alluvial clays provide a lower permeability horizon that slows the downward migration of percolating waters from the surface into the gravel aquifer.
The holders for the permits associated with the three waste management areas no longer owned the land. The landowners, a development company, were keen to redevelop the main factory area and needed the waste management areas closed and made safe in order to do so. In 2012, the Environment Agency confirmed that further works needed to be undertaken to meet the necessary standards required for the waste licenses to be surrendered. At this time, however, the existing remediation plans for the areas were deemed either unfeasible or presented significant uncertainties, with estimated costs that were not commercially viable.
The three waste management areas have been subject to many stages of assessment by several entities from the early 1990s until present. These previous works produced large amounts of environmental data (in the tens of thousands of data points), covering differing aspects of the site that spread across numerous reports, excel files, maps and lab reports. Despite the wealth of available data, little consensus existed between stakeholders and authorities with regards to the Conceptual Site Model (CSM) of the sites and the remediation strategy.
NewFields integrated all data into our bespoke environmental database and GIS allowing a comprehensive review of the site data. This allowed the development of an accurate and detailed CSM for the sites and the identification of data gaps including the lack of a clear definition of receptors and compliance points. The CSM for the site showed that without preventative measures, there was a completed risk pathway between the waste material (source) and the River Aire (receptor). Furthermore, there was also a high probability of exceeding the Surface Water Environmental Quality Standard (SWEQS) for a few chemicals over time. There was also a direct risk pathway between the exposed contaminated materials on the surface of each area and anyone walking on the site. Although contamination had not been detected in surface water samples collected from the River Aire, the potential risks to the river were a concern to the Environment Agency who indicated the need for preventative measures.
NewFields performed a hydrogeological risk assessment that recognised the concentrations of substances in soil from the made ground and alluvium within the site and the sensitivity of receptors outside the site. The Monte Carlo (probabilistic) simulation packages LandSim (Environment Agency, 2003) and ConSim (Environment Agency, 2007) were used to model the impact from the site under two scenarios: without any intervention and with an intervention designed to reduce the infiltration of precipitation and the associated leaching of chemicals from the waste material. The loading of substances in the River Aire were also calculated.
The agreed compliance points for the sites were the boundaries of the areas, particularly the downgradient boundary along the River Aire. Any intervention and remediation strategy would be deemed successful if the groundwater at the point of entry into the River Aire was below SWEQS with a dilution factor applied. The results of the hydrological risk assessment indicated that reducing the amount of contamination entering the groundwater aquifer would create environmental betterment such that, given the proportion of dilution available in the River Aire, no substances at 95%ile (worst case) or the 50%ile (most likely) would exceed the SWEQS.
A human health risk assessment was also performed which identified that direct contact with the contaminated soils was the only pathway to end site users, thus any remediation strategy would need to mitigate against this.
The objective of the overall remedial scheme was to control the risks to the River Aire and make the site suitable for future re-use, whilst reducing the overall cost of the intervention. Following agreement on the compliance points, NewFields undertook a review of possible remedial alternatives. This included a review of the operational background and uses of each area, available site investigation data, hydrogeologic conditions and groundwater modelling, surface water quality criteria, and the available remedial alternatives. From an initial list of possible alternatives, a short list was selected for further detailed costing based on relative scoring for practicality, effectiveness, durability, efficiency, sustainability and cost. The short list alternatives for soil remediation included capping, off-site disposal, in-situ thermal desorption (six phase heating), ex-situ thermal desorption, biological treatment, soil washing, and stabilization. The short list alternatives for groundwater remediation were in-ground barrier, hydraulic barrier, permeable reactive barrier, pump and treat and monitored natural attenuation.
Remedial alternatives in the short lists were further evaluated, using a qualitative scoring system, based on a list of key criteria including likelihood of improvements to ground conditions, long-term sustainability, stakeholder and regulatory opinions, health and safety, overall cost, implementation time, and perceived short term impacts (odour, nuisance risks), with weighting applied to criteria that were critical to the success of the closure and licence surrender of the sites. The analysis concluded that a containment cover system (cap), with surface water management (drainage system) was the preferred remediation method. The cost of this system was estimated to be around 60% lower than previously suggested remediation plans, while achieving the same requirements.
In order to achieve the intended environmental benefits, the capping system had to comply with the following requirements:
Reduce vertical infiltration of rainwater through the impacted ground into the groundwater aquifer, thus removing the contamination pathway and reducing the impact on the River Aire.
Collect all surface water and storm water run-off from the caps and direct it towards the River Aire. Discharge to the River Aire at a ‘greenfield run-off rate’.
Provide a barrier between contaminated land and future site visitors.
The designed caps comprised a series of geosynthetic liners including a geosynthetic clay liner (GCL) to provide a low permeability barrier, geo-composite liners to provide protection and encourage further drainage and, on Landfill 227, a welded LLDPE (Linear low-density polyethylene) liner to provide an additional barrier to percolating waters. These were sandwiched between blinding layers and restoration soils of locally sourced, clean clay material and topped with a layer of topsoil to provide a growing substrate for revegetation.
The surface water drainage was carefully designed and modelled to provide capacity and correct flow direction of collected run-off waters. Risers were used at the discharge points to the River Aire to control discharge into the river during low flow periods. The risers also allow larger volumes of water to flow into the river during, and immediately following, flood events.
The location of these caps was within an EA designated flood zone 3 (1% or greater chance of flooding from the river). This presented one of the more challenging aspects of this project, to successfully cap these areas in a way that would not impact the flood storage capacity of the site and avoid flooding to areas offsite. These issues were worked in close coordination with the Wakefield Council drainage officer. Following intensive flood and hydrological modelling, swales, and retention ponds, were added to the design. The dimensions and locations of these served the dual-purpose of managing run-off and to off-set the loss in flood storage within the capped area. Planning Approval for the three sites was spread over two separate applications, and both applications were approved by the local council.
Construction consisted of two phases: Phase 1 involved the capping of Landfill 227 and associated drainage structures while Phase 2 involved the capping of Lowfields and Green Lane landfills and associated drainage structures. Both phases involved the careful reshaping of the landform prior to capping to create an optimum topology for surface water run-off, before installing liners and covering it with restoration soils. This required working around various utilities/services, including an above-ground, high-pressure gas main. Extra care was taken to ensure that no damage was done to these services, including using proximity alarms on heavy plant machinery. Construction Quality Assurance (CQA) was completed by NewFields, with third-party oversight and review, and both phases of construction were verified and approved by Local Council Authorities.
Figure 3: Photo taken during the Phase 1 of the construction
Since the end of construction, groundwater and surface water samples have been collected on a six-month basis. The purpose of the continuous sample collection prior to, during and after construction is to provide documentation of any changes to contaminant levels. Compliance monitoring wells along the bank of the River Aire have shown a significant reduction in all COCs to levels below the required Site-Specific Assessment Criteria (SWEQS with dilution) and some below SWEQS (without dilution). There have been no detections of any COCs in surface water samples. Additionally, leachate levels within a dedicated leachate monitoring well in Landfill 227 indicate that leachate production has stopped. Compliance monitoring will continue through long-term closure. When considering flood-storage, the as-built caps and drainage features have provided a net gain of 249m3 of floodwater storage within flood zone 3. At present, the site is undergoing revegetation and the landowner is considering using the area as a solar panel farm as well as outdoor activity space.
The site has long history of industrial activity. Former waste disposal areas posed a risk to the adjacent River Aire and prevented the future use of the site. Furthermore, the permit holder sought to surrender existing waste management licenses associated with these sites. Over many years, several site investigations and assessments were carried out but ultimately failed to obtain agreement on the way forward between the relevant stakeholders, which resulted in an impasse. The conversion of multiple SI data sets into a structured database and GIS allowed the development of a more accurate Conceptual Site Model, clear identification of receptors and compliance points that allowed a better understanding of the key issues. Once this was communicated to the stakeholders, agreement on the proper remedial alternative to meet the requirements of the project’s objectives was obtained.
The selected remediation method was implemented at a fraction of the cost of other possible alternatives while meeting the risk criteria required of the project. Compliance verification sampling shows the site currently meets the project criteria at the compliance points. The site may now be returned to beneficial use within the landowners planned future development of the area. This is a rare occurrence in which hazardous waste landfills, in proximity to a major river, have been successfully capped and the risks mitigated. NewFields were able to achieve this through effective management and use of environmental data, developing a clear CSM, and identifying the key exit criteria that stakeholders needed to agree the way forward.