Modelling contamination flow and ground gas risk assessment

In a major development NHS Lothian are using a former car park to bring together the Royal Hospital for Sick Children with the Department of Clinical Neurosciences and the Child and Adolescent Mental Health Service.

During the project Brookfield Multiplex, the primary contractor, acquired the site of a former petrol station to create new access routes to the site. This land will be converted to green space on completion of the building works. CGL was commissioned to reassess existing data on soil and groundwater pollutants at the site with regard to the risks to human health, the underlying aquifer and local water courses.

CGL carried out a DQRA to model contaminant flows from the site using the Environment Agency’s Remedial Targets Worksheet. This used the prescribed concentration limits for the nearby water course, the Burdiehouse Burn, and a virtual down gradient receptor, to calculate the maximum permissible contaminant levels at the site (i.e. remedial targets). The model takes into account the soil structure and composition, as well as degradation and attenuation rates for the pollutants.

Using the model CGL was able to show that contamination levels were within acceptable limits, with respect to the risk to controlled waters, and only a capping layer was required to act as a barrier to the underlying soils to protect human health. CGL liaised with Edinburgh Council and SEPA to ensure they accepted the DQRA and the proposed remediation strategy.

Elsewhere on the site CGL carried out a ground gas risk assessment of the area to be used as a 3750 m² basement within the new hospital buildings. CGL was able to show that the risk from ground gas migration was low and therefore gas protection measures were not required, making a significant saving on the anticipated construction cost.