Chemical reduction provided the answer for groundwater contamination at a former automotive engineering site.
Ian Viney reports.
The UK's fist large scale chromium reduction project has achieved dramatic results on a 4.5ha contaminated site in Salisbury which is being redeveloped for housing.
The former AE Goetze automotive engineering works operated from the 1940s until 1998. Manufacturing involved a range of potentially contaminative activities, including storage and use of hydrocarbons, degreasants and plating products.
WSP Remediation, a subsidiary of WSP Environmental, worked with developer Taylor Woodrow Developments to clean up the site using an innovative chemical reduction approach.
The project not only met but exceeded remediation targets with a dramatic reduction of hexavalent chromium in groundwater, enabling 75% of the area to be released for redevelopment while remediation continued. The works saved more than £750,000 compared with conventional approaches.
Initial site investigation work identified soil contamination with total petroleum hydrocarbons (TPH) up to 6973mg/kg, trichloroethylene (TCE) up to 52.2mg/kg and chromium up to 30,960mg/kg, of which hexavalent chromium (Cr (VI)) comprised up to 9600mg/kg.
Additionally, groundwater within the area of the former chrome plating works was shown to be heavily impacted with Cr (VI) up to 280,000µg/l in the shallow gravel and chalk aquifers.
Quantitative risk assessment
A human health and controlled waters risk assessment found the arsenic, chromium TCE and TPH within soils was a potential risk.
After liaison with the regulatory authorities, WSP designed a remedial strategy to ensure that soils within 2m of the final ground surface did not contain concentrations in excess of agreed criteria for human health.
To mitigate risk to groundwater, soils impacted with Cr (VI) in excess of the site-specific target criteria of 4.1mg/kg were to be excavated to groundwater level. A groundwater remediation target of 345µg/l was agreed with the authorities.
Large scale pump and treat was considered but rejected as expensive and slow - it would have required the abstraction and treatment of about 400,000m 3 of groundwater. Also, it can be ineffective in chalk aquifers because of incomplete contaminant desorption leading to rebound in dual porosity systems.
A permeable reactive barrier (PRB), where a section of the aquifer material is removed and replaced with a permeable material such as granular iron, was also considered.
This was excluded because Taylor Woodrow wanted a complete cleanup solution without monitoring and maintenance liabilities.
WSP Remediation provided an all risk, fixed price design and construct solution for Taylor Woodrow and also acted as contractor.
About 18,000t of TPH, chromium, TCE and arsenic impacted soils in the vicinity of the former plating shop were disposed off site. This included 5000t of bright yellow chromium impacted soils (Figure 1) excavated to the depth of standing water (typically 3.5m). Site workers treated perched ground ater (Figure 2) with reducing agents (molasses as well as ferrous sulphate, a chemical reductant) in the base of the excavation beneath the former plating shop.
Initially pump and treat for the valley gravel aquifer was done as part of an overall mass reduction. Water treatment consisted of a settlement tank/oil water separator linked to a granular activated carbon unit (as a precautionary measure to address any hydrocarbon contamination) with final passage via an ion exchange resin to strip chromium contamination.
The most innovative and complex part of the project was the insitu reduction of chromium in the chalk aquifer. Cr (VI) is both toxic and carcinogenic and is also highly soluble and mobile.
The project used a chemically driven and biologically facilitated process to convert Cr (VI) to Cr (III) (also known as Cr (OH)3) under conditions of low redox potential. This was in the presence of ferrous iron to limit the toxicity and mobility of the dissolved chromium and mitigate the environmental risk.
To achieve this reduction, a 1% solution of molasses was added upgradient of the identified chromium plume, via a system of dosing wells.
Monitoring over the two-month dosing period illustrated that levels of dissolved oxygen and redox potential declined significantly.
The final phase of insitu treatment was the addition of ferrous sulphate to the molasses solution, to ensure reduction of Cr (VI) to Cr (III). Figure 3 is a schematic representation of the dosing system.
The outcome Analytical results confirm that the works have successfully reduced on site concentrations of Cr (VI) to below detection limits (2µg/l), in less than 12 months. The Cr (VI) to Cr (III) reaction is non-reversible under foreseeable conditions in the aquifer, providing a permanent solution.
The Environment Agency has now signed off and approved the remediation works after a period of monitoring.
Conclusion This first large scale chromium reduction project in the UK was signed off last December. It successfully reduced hexavalent chromium concentrations in a chalk aquifer, saving significant costs over a pump and treat alternative. It removed an off-site liability, enabling the residential redevelopment of a challenging brownfield site.
Ian Viney is managing director of WSP Remediation.