Marc Davies of WYG Environmental describes the assessment and remediation of a large UK chemical works which required various different remediation approaches.
WYG has been involved the assessment and remediation of a chemical works in the UK for the past nine years. The varying complexity of the contamination issues, together with some significant physical constraints at the site, has led to the application of a number of remediation techniques to address the impacts.
The site has been occupied for the production of chemicals since World War I, producing products for aircraft and later for the textile industry. The site grew throughout the 20th century, eventually occupying over 100 hectares, but began to decline in the 1980s. Around two-thirds of the site is now disused and demolished.
Chemicals used in the manufacturing processes, together with associated by-products, has resulted in impact to groundwater by a number of contaminants, predominantly trichloroethene (TCE) and its degradation products, benzene and the flame retardants tris 2-chloroethyl phosphate (TCEP) and tris chloroisopropyl phosphate (TCPP).
Assessment
Numerous stages of site investigation have been completed by WYG under strict health and safety controls, required as a result of the ongoing operational nature of the site. These works targeted suspected areas of impact, derived from desk study information, such as chemical storage areas and production areas, and recorded contamination impacts to groundwater and some soils.
The investigations were staged to allow a practical and efficient refinement of the identified impacts on this large site. Secondary stages were focused around delineation of source areas, plume investigation and pathway interrogation.
The main groundwater at the site, which is confined by a layer of alluvium, lies in a River Terrace Gravels (RTG) unit present from around 4m bgl to around 7m bgl. The groundwater is in continuity with a river which bounds a significant part of the site and is the principal risk driver.
The scale and complexity of the contaminant impacts identified required quantitative risk assessment up to Tier 4 to be undertaken. This, in addition to groundwater modelling, determined the remedial requirements. The unusual nature of some of the contaminants involved required extensive laboratory testing and site trials in order to determine effective remedial processes. This led to the development of a multi-faceted remedial strategy.
Remediation
The remediation approach at the site can be split into three areas:
Flame retardants' remediation by chemical oxidation
Flame retardant compounds, the chlorinated phosphates TCEP and TCPP, are rare contaminants with just four production facilities in the EU. These compounds were identified at significant concentrations in a plume of some 20,000m2 in the RTG unit, together with dispersed adsorbed Dense Non-Aqueous Phase Liquid (DNAPL).
The plume was just 60m from the river, and therefore active remediation was required. Flame retardants are recalcitrant contaminants that are neither very volatile nor biodegradable. The rarity of these contaminants and resultant lack of successful remedial projects to research meant that options were not plentiful.
The options were further restricted by the presence of a myriad of live below-ground services which effectively ruled out any excavation. Following a technology screen, in-situ chemical oxidation was considered to be the only potentially suitable method, and laboratory-based treatability trials followed by field-based pilot trials determined that Fenton's reagent made the most suitable oxidant.
Active remediation started on site with contractor QDS in October 2007, using chemical oxidation supplemented with pumped mass recovery. The programme is ongoing and initial results have provided excellent feedback on the treatment system on what is believed to be the first flame retardants remediation project in Europe.
Monitored natural attenuation
Dissolved phase contaminant plumes were identified in five areas of the site, associated with historical storage activities, predominantly of TCE and its breakdown products cis-1-2-dichloroethene (cDCE) and vinyl chloride (VC), and benzene, and in one instance both.
At remediation options stage, these plumes were considered potentially suitable for monitored natural attenuation (MNA). It was first necessary to confirm the absence of NAPL in order to verify the assumptions made in the risk assessment.
In accordance with Environment Agency guidance this was followed by the collection of primary, secondary and tertiary lines of evidence and subsequent modelling to simulate the natural attenuation process at the site, particularly as the contaminants concerned are degraded by different processes.
This confirmed that MNA was indeed a suitable remediation solution and the programme began at the end of 2005 for a period of between three and five years. The programme is subject to ongoing monitoring and evaluation. The data collected to date indicates that the solution is effective.
TCE DNAPL remediation
In one area of the site, the presence of DNAPL arising from a TCE source has been identified. The volume and density of the DNAPL source necessitates active remediation, and remediation options are currently being considered.
Pilot trials using electron donors to enhance anaerobic bio-degradation of the dissolved phase have been undertaken. In-situ and ex-situ techniques are currently under evaluation, possibly in combination with MNA to address the residual dissolved phase impacts.
Closure
One of the key aspects in progressing this project successfully has been a sound investigation data set and a robust risk assessment completed in accordance with significant laboratory and field trials, to enable a practical and sustainable strategy to be implemented.
Marc Davies, WYG Regional Director
