Setting
The site is located near a marina in a beautiful setting on the saline River Colne Estuary. It is the subject of a redevelopment proposal to a mixed end-use, comprising of houses with private gardens, apartment and commercial properties along the waterfront and associated amenity space, including a playground, construction of a restored wet dock and jetty. The client commissioned Knight Environmental to carry out a site assessment and prepare a remedial strategy for the proposed redevelopment of the site. Knight Environmental were then successful in tendering for carrying out the soil stabilisation remediation works.

Site Assessment
Knight Environmental conducted site investigation works to assess areas of the site not investigated previously, and to update the risk assessment to be in line with current guidance. The investigation confirmed much of the anticipated geological, hydrological and hydrogeological setting; which was principally Made Ground over granular Terrace Deposits, over more cohesive Alluvium. In addition it also provided an indication of the contamination distribution within the Made Ground. The Made Ground was the result of previous in-filling with uncontrolled construction and demolition wastes. Potentially significant risks to human health and the River Colne were assessed from the organic and inorganic contaminants in the Made Ground. The quantitative risk assessments carried out by Knight Environmental effectively evaluated the level of risk to human health and the River Colne, and developed site specific target levels for remediation.

Development of the Remedial Strategy
The site is only accessible via an unadopted unpaved residential road, accessible to major roads only by route through a residential estate near the centre of Wivenhoe. Discussions with the client and the regulatory authorities highlighted the importance of developing a remedial strategy that would minimise traffic through the village and the associated effects on the village residents. This highlighted the importance of developing a sustainable solution that would keep the contaminated soil on the site.

The site-specific target levels for remediation were developed were used to assess the viability of two potential remediation options: off-site disposal (dig and dump) and ex-situ chemical and physical soil stabilisation. These were the only two options reviewed since they would achieve effective remediation of both organic and inorganic contamination using only one remediation process. The pros and cons of each strategy were discussed with the client and soil stabilisation was chosen as the preferred strategy.

Treatment Works
The treatment plant comprises of a treatment pit, a 30 tonne excavator with Allu bucket, water source and mixing area. Water, bentonite and chemicals were mixed and then pumped into the treatment pit. The Allu bucket mixed the soil with specified amounts of the chemical/bentonite mixture and cement. The treated material is then stockpiled and left to cure for between 7 and 28 days before backfilling on-site. The works were carried out effectively and ahead of schedule, resulting in an approximate saving of £200,000.00 when compared to rates for dig and dump. There was also a significant saving to the client when a new area of contamination was found. If landfilled, this material would have classified as hazardous waste requiring pre-treatment before disposal. However, since the soil stabilisation was been carried out, the additional contaminated materials were incorporated into the treatment works at minimal cost to the Client.

Upon completion of the works, a validation completion report detailing the remedial works undertaken was submitted. The report concluded the potential risk to controlled waters following the soil stabilisation works was not significant. The report gave a description of the outstanding remediation required to protect human health via the incorporation of a cover layer. It also highlighted the importance that the importation of fill to the site should be regulated under an exemption from waste management licensing and all imported soils would need to be validated.

Additional Works
Knight Environmental will also have an active role in the validation of the cover layer and are currently managing a long-term gas monitoring programme for the site. Some areas of the site are at significant risk from high concentrations of methane and carbon dioxide, measured at levels consistent with landfill. By carrying out the detailed soil-gas monitoring programme, we will obtain sufficient data to carry out a risk assessment for the gassing regime at the site, and prescribe gas protection measures to be incorporated into the building design. This is just one example of how Knight Environmental provide an excellent service to our Clients for all aspects of contaminated land.