Site specific in-situ bioremediation

Response Bioremediation Contracting (a division of Response Environmental Services Ltd) was commissioned in November 1999 by A P Geotechnics Ltd, the independent consultant appointed by Michael Shanly Homes, to provide as site-specific bioremediation strategy to remediate an area of petroleum contaminated ground as a development site in High Wycombe.

The site investigations conducted by AP Geotechnics revealed the nature and extent of the contamination on the site. The main contaminant identified was 1,1,1 Trichloroethane with concentrations recorded at 212.66mg/kg to 2.9 metres depth. Investigations identified a variable sequence of strata, typically 0.7 metres of fill material over 9.3 metres of stiff brown, silty clay with some gravel over chalk.

In light of the proposed residential end use, the sensitivity of the location and the major chalk aquifer beneath the site, the remediation strategy had to be both responsive and sustainable.

Consultation on strategy
Following consultation with the local authority, NHBC and the Environment Agency, including the National Groundwater and Contaminated Land Centre, both the remediation strategy and the remediation target values of 10mg/kg for total VOCs and 5mg/kg for 1,1,1 Trichloroethane, as principal contaminant, were confirmed.

Response Bioremediation Contracting reports that it responded to the requirements of the project by engineering a site-specific and risk-based, in-situ bioremediation strategy which utilised Precision Injection Technology, incorporating Response Bio-Gel. The material is stated to be a non-toxic, non-corrosive and organic food grade substance found in most foods and used commonly within the pharmaceutical industry. The bio-gel itself is used as a carrier, an insulator and an initial source of nourishment for the bacteria used in the bioremediation of soils and water that contains elevated concentrations of petroleum hydrocarbon contamination.

Natural bacteria
Site-specific Response Bio-Gel contains the optimum composite of non-pathogenic, naturally occurring bacteria, found naturally in soils (including facultative anaerobes), nutrients and dissolved oxygen, to maximise the interface of bacteria and contaminant.

This promotes the development/colonisation of biomass and therefore facilitates bioremediation. As part of the colonisation process the bio-gel is consumed and converted into carbon dioxide and water. Response Bio-Gel is specifically engineered to respond to the site-specific ground conditions and contaminant properties, including risks pertaining to contaminant mobilisation, both laterally and vertically.

Accordingly, the composition of Response Bio-Gel and the technology selected to deliver the product to the contamination zone is engineered to suit each project.

The in-situ bioremediation works were executed in November 1999. Response Bio-Gel was injected manually into the target area of contamination utilising 19mm diameter injection lances under positive pressure in a regular grid pattern with injection points at 0.75 metre spacings. The bacteria within the Response Bio-Gel follow the contaminant front in all directions throughout the contaminated material.

In accordance with the agreed bioremediation strategy Response undertook a programme of treatment method validation. Soil samples were retrieved using a window sampler at week 6 and 11 after the bioremediation injection process. All samples were entered into a laboratory chain of custody and despatched to a NAMAS Accredited laboratory (Environmental Analysis Ltd) for chemical analysis. During the course of the works visual and olfactory evidence of TPH contamination was encountered. This was sampled and despatched for analysis accordingly.

Response Bioremediation Contracting reports that the chemical analysis results confirmed that the in-situ bioremediation treatment process has effectively reduced the elevated concentrations of VOCs and TPH as the site within an 11 week period. These results also confirmed that the remediation target values of 10mg/kg for Total VOCs and 5mg/kg for 1,1,1 Trichloroethane as principal VOC contaminant have been achieved, with maximum concentrations being recorded at 1.54mg/kg and <0.02mg/kg, respectively. The chemical analysis results also confirmed that the TPH contamination has been remediated to <5mg/kg, states Response.