Airborne and ground-based radiometric investigation of colliery spoil near Shirebrook, the English Midlands

As a result of the UK’s industrial past, spoil heaps are widespread through our environment and encompass the waste products from a variety of different industrial processes. Two high-resolution airborne geophysical surveys, conducted over parts of the UK in 1998 and 1999, identified four radiometric anomalies in the East Shirebrook region corresponding to four areas of colliery spoil. These areas which are composed of shales, siltstones and sandstones extracted during mining mixed with left over coal displayed elevated potassium, uranium and thorium, relative to their surroundings. Ground follow-up work was undertaken at Shirebrook and Warsop Vale, the two disused collieries from the four radiometric anomalies identified by the airborne surveys in the East Shirebrook area. Active extraction ceased at these mines in the late 1980s and early 1990s and both have been undergoing reclamation and re-landscaping. The ground survey entailed ground-based continuous gamma spectrometry traverses over accessible parts of each site, ground-based static gamma spectrometry measurements over transects of the sites, surface soil sampling at these transect points for laboratory XRF analysis and short core sampling at Warsop Vale for gamma spectrometry analysis on a core logger. The ground survey confirmed, and more accurately delineated the anomalies observed by the airborne surveys. The ground-based continuous gamma spectrometry highlighted the marked difference between the radiometric signature of the spoil heaps and neighbouring agricultural land. A high degree of spatial variation was apparent in the ground-based gamma spectrometry data and highlighted that the spoil itself is not a uniform mass. Ground survey static gamma spectrometry results and XRF analysis of surface soil samples correlate significantly. Comparison of these surface soil samples (0- 15 cm) obtained at both Shirebrook and Warsop Vale to regional Geochemical Baseline Survey of the Environment (G-BASE) surface soil samples obtained up to 1 km from the spoil tips showed that thorium was the element most elevated above background. The radiometric data from the two airborne surveys correlate at the 99.95 % level for uranium, thorium and potassium. The values from the Geological Survey of Finland (GTK) airborne survey, flown in 1999 are slightly higher however, and this is probably the result of using the Finnish calibration range with very different rock types to those found near Shirebrook. The two surveys showed very similar patterns of radioactivity. They both distinguish well between areas of spoil and adjacent agricultural land. The higher spatial resolution (closer line spacing) of the GTK survey delineated the anomalies with a higher degree of detail. The GTK survey flying height was also lower than that of the High Resolution Airborne Resource and Environmental Survey Phase 1 (HiRES-1) airborne survey flown in 1998, thus reducing the sampling area, or ‘footprint’ from which the gamma radiation was averaged. This too increased the level of detail visible in the GTK survey results. The extent of the spoil tips, as indicated by the radiometric data, does not always coincide exactly with the mapped boundaries of the spoil on Ordnance Survey 1: 50 000 or 1: 25 000 maps. This suggests that, in the absence of up-to-date maps, or where spoil extends beyond the mapped boundary, the radiometric data may be used to accurately define the current extent of the spoil. This may usefully be linked to EM data that show the extent of conductive (and potentially contaminated) groundwater (e.g. Beamish, 2002b, Klinck et al, 2004). Although reclamation of the two former collieries has been successful in terms of the appearance of the site and in the return of wildlife to the areas, it appears that the radiometric anomaly still exists post-reclamation. However, the uranium concentrations encountered are well below Generalised Derived Limits and the maximum external annual gamma dose is less than 50 micro Sv. There does not appear to be a significant radiological hazard to members of the public from an external dose perspective