Mapping the contaminants

Electrical Resistivity Tomography (ERT) is a new and rapidly evolving technology for the non-invasive mapping of the shallow subsurface and anomalous ground conditions. High resolution 3D volumetric images or tomograms can be obtained which allow irregular or spatially complex features to be visualised more precisely than would be possible with traditional geophysical survey methods. The emergence of this technique owes much to recent advances in high-density, computer-controlled data acquisition and mathematical inversion theory. Large volumes of ground can be scanned down to depths of tens of metres. These reconstructed resistivity images can be used to infer the spatial geometry of specific target features, including waste and leachate distribution, plumes, and buried landfill geometry.

This technology has been tested as part of a BGS — Environment Agency collaborative project to examine leachate composition and migration from the closed Thriplow landfill in Cambridgeshire. This landfill lies within chalk bedrock, a major aquifer in the UK. The study was to design to assess the pollution risk from these ‘dilute and disperse’ landfills.

The advantages of ERT are:

• The technique is non-invasive. This reduces the risk associated with boreholes of opening up new pathways or changing the hydraulic model.

• As landfills are highly heterogeneous, borehole sampling may only reflect localised conditions. In contrast, ERT gives a full 3D volumetric image of the subsurface that allows the spatial continuity of large-scale hydraulic processes to be assessed and visualised.

• These tomograms can help to optimise the siting of sampling boreholes (eg: for leachate level monitoring, targeting leachate concentrations and flow-paths, and for locating dry re-injection points) thus reducing drilling costs.

• ERT can be used for 4D monitoring to detect changes in leachate distribution with time caused by changes in seasonal hydraulic loading or in-situ hydrochemical reactions.

Imaging system

BGS has developed an innovative 3D imaging system that measures the electrical resistivity of the subsurface under PC control. This allows the acquisition of unprecedented data densities — a prerequisite for accurate tomographic reconstruction. The spatial resolution is determined primarily by the depth of investigation required and the inter-electrode spacing used in the survey. In theory, the technique is not limited by scale. Surveys can be designed for any resolution (eg: centimetric, metric, decametric) and areal coverage (eg: 1-50 ha). For most landfills, a typical dipole spacing might be 5-10m.

Most waste fluids or leachates are highly conductive (ie have low resistivity) due to their elevated ion concentrations, whereas the uncontaminated chalk and dry waste has higher resistivity values. In figure 3, the low resistivity material is shown as red/orange and the chalk and dry waste appears blue/purple. The image shows that the landfill comprised a number of discrete pits within the underlying chalk and that the deepest of these act as leachate drainage sinks. Conventional groundwater chemistry has confirmed the presence of elevated leachate levels (eg: >1000mg/l chloride) in the chalk bedrock, immediately below these pits. The inferred plume migrating down-gradient to the NW, had a direct bearing on where to site further boreholes and what source terms to use in the contaminant transport model. Subsequent drilling confirmed the presence of the plume.

Another promising potential application that has emerged from this work is the use of ‘differential’ or 4D tomographic imaging, in which 3D images are taken of the subsurface at regular time intervals to detect dynamic or temporal changes in leachate release. These changes are often related to seasonal changes in the hydraulic loading or gradient and are more readily detected by differential time-lapse tomography. Static spatial variations (eg: in geology, porosity or solid waste) are effectively removed by subtracting sequential images. The difference image is only sensitive to dynamic effects such as changes in saturation or pore-fluid conductivity. BGS has been awarded a major grant by Onyx Environmental Trust to develop time-lapse imaging technology over the next three years.

Landfill sites

It is likely that local authorities will be inspecting a significant number of closed landfill sites to comply with their duties under Part IIA of the Environmental Protection Act 1990. The Agency may also be inspecting some closed landfill sites that are candidate special sites. One objective of such inspections will be to establish whether significant pollutant linkages exist that cause or have the potential to cause pollution of controlled waters. The research carried out at Thriplow demonstrates that the location of leachate or groundwater monitoring boreholes can be a ‘hit or miss’ exercise. ERT has great potential in helping both operators and regulators to identify significant pollutant linkages associated with complex contaminated sites such as landfills.

The article is based on an article written for the EA ‘Underground’ in-house magazine (January 2002) and is published with the permission of the director of the British Geological Survey, (a component body of the Natural Environment Research Council) and the chairman of the Environment Agency.