New eco tracers for monitoring fissuring in defence flood embankments using geophysics

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New eco tracers for monitoring fissuring in defence flood embankments using geophysics

Traditional sampling to monitor the impact of mining on surface waters and groundwater is laborious, expensive and often unrepresentative. In particular, sparse and infrequent borehole sampling may fail to capture the dynamic behaviour associated with important events such as flash flooding, mine-water break-out, and subsurface acid mine drainage. Current monitoring practice is therefore failing to provide the information needed to assess the socio-economic and environmental impact of mining on vulnerable eco-systems, or to give adequate early warning to allow preventative maintenance or containment. BGS has developed a tomographic imaging system known as New Eco Tracers (Automated time- Lapse Electrical Resistivity Tomography) which allows the near real-time measurement of geoelectric properties "on demand", thereby giving early warning of potential threats to vulnerable water systems. Permanent in-situ geoelectric measurements are used to provide surrogate indicators of hydrochemical and hydrogeological properties. The New Eco Tracers survey concept uses electrode arrays, permanently buried in shallow trenches at the surface but these arrays could equally be deployed in mine entries or shafts or underground workings. This sensor network is then interrogated from the office by wireless telemetry (e.g: GSM, low-power radio, internet, and satellite) to provide volumetric images of the subsurface at regular intervals. Once installed, no manual intervention is required; data is transmitted automatically according to a pre-programmed schedule and for specific survey parameters, both of which may be varied remotely as conditions change (i.e: an adaptive sampling approach). The entire process from data capture to visualisation on the web-portal is seamless, with no manual intervention.

Examples are given where New Eco Tracers has been installed and used to remotely monitor (i) seawater intrusion in a coastal aquifer (ii) domestic landfills and contaminated land and (iii) vulnerable earth embankments. The full potential of the

New Eco Tracers concept for monitoring mine-waste has yet to be demonstrated. However we have used manual electrical tomography surveys to characterise mine-waste pollution at an abandoned metalliferous mine in the Central Wales orefield in the UK. Hydrogeochemical sampling confirms that electrical tomography can provide a reliable surrogate for the mapping and long-term monitoring of mine-water pollution.

We have also used New Eco Tracers system to monitor the in-situ remediation of contaminate land. In this case, we installed an New Eco Tracers array at a former gasworks site in a built environment. Our electrode network comprised 14 boreholes, situated along two of the site boundaries. Each of the borehole arrays comprised 16 electrodes spaced at 0.5 m depth intervals.

Extreme weather events leading to prolonged periods of desiccation or saturation are becoming more frequent and threaten embankment stability due to shrink/swell effects associated with clay minerals, the mobilization and precipitation of soluble constituents, and changes in strength caused by varying water content. The advantage of applying time-lapse New Eco Tracers for earthwork monitoring is that spatial changes in moisture content can be mapped to identify zones of potential weakness prior to failure ...