An Airborne Gamma Ray Survey of Parts of SW Scotland in February 1993. Final Report

Field: Earth and Planetary Sciences (Physical Sciences)

An airborne gamma ray survey was conducted for the Scottish Office Environment Department of coastal and inland parts of SW Scotland to define existing background levels, to locate features worthy of further attention, and to demonstrate the emergency response capabilities of radiometric methods. Coastal areas were surveyed with 500 m line spacing. Inland areas were specified to 2 km line spacing, however it was possible to achieve 1 km line spacing in the majority of the inland zone. Fieldwork was conducted between the 1st and 16th February 1993. A total of over 17,000 gamma ray spectra were recorded, using a 16 litre NaI spectrometer mounted in a helicopter flying at 50-75m ground clearance and 120kph. A total area of 3650 km2 was surveyed in 41.6 flying hours, from roughly 4370 line kilometres. The data were reduced in the field using standard SURRC procedures for background subtraction, stripping of spectral interferences, altitude correction, and calibration. Preliminary maps of the distribution of 137Cs, 40K, 214Bi, 208Tl, and estimated ground level gamma dose rate were produced during the fieldwork period using working calibration values derived from previous surveys. A set of core samples was collected from Wigtown Merse, Longbridgemuir and Caerlaverock merse for calibration purposes, and aerial observations were performed at these sites. Further soil sampling and ground level in-situ gamma spectrometry was performed in the summer of 1993 to investigate the applicability of the calibration to a range of upland soil types and topographical environments. These locations received peak deposition from the Chernobyl accident, are vulnerable to wet deposition, and are difficult to monitor rapidly using ground based methods. A total of 76 soil cores, subdivided into 168 separate samples was thus collected for high resolution gamma spectrometry in the laboratory. This was conducted from April to November 1993. For the terrestrial sites the aerial survey estimates based on the working calibration, were in good agreement with both in-situ gamma spectrometry and the results of core analysis. This validates the preliminary maps in these contexts, and confirms that a general calibration is sufficient for fallout mapping under emergency response conditions. On coastal salt marsh sites (merse), where aged deposits of Sellafield derived activity have accumulated, subsurface activity profiles for 137Cs and 241Am and the presence of superficial levels of 134Cs were observed from the soil cores. Similar features have been observed in previous surveys. In these cases the effects of source burial must be taken into account to avoid underestimation of activity levels by both ground-based and aerial gamma spectrometry. A separate set of detailed maps for the principal merse sites was therefore prepared using a calibration factor derived from the soil cores from this context. There are prospects for developing spectral analysis procedures to account for source depth in aerial surveys. Source burial on the merse also has implications for sampling techniques, and for dose rate measurement, which would merit further consideration. The radiometric maps show clearly the distributions of each individual nuclide and indicate the contribution which individual localised features make to the overall gamma ray dose rate. Naturally occurring nuclides reflect the underlying geological and geomorphological contexts of the landscape. The main granite intrusions, most notably at Cairnsmore of Fleet, the Loch Doon Granodiorite, Glencairn of Carsphairn, the Dalbeattie granite, and Criffel Pluton are readily visible in 40K, 214Bi and 208Tl maps, and control their local radiation environments. A number of areas of enhanced 214Bi, which may reflect radon potential, were noted. A transient radon associated 214Bi signal was observed on the west of the Wigtown peninsular during the survey. Examination of spectral data in the vicinity of Dundrennan has confirmed that there is no evidence of widespread terrestrial contamination arising from the use of depleted uranium projectiles on the range. The 137Cs map indicates the environmental distribution of this nuclide in considerable detail. Levels of 137Cs range from approximately 2 kBq m-2, a level consistent with global weapons’ testing fallout, from 2-40kBq m-2 on terrestrial sites affected by deposition from the Chernobyl accident, and from 40 kBq m-2 to over 200 kBq m-2 on tide washed pastures which have accumulated marine sediments from the Irish Sea. All three levels are represented within the survey zone, in a manner which is consistent with the findings of previous aerial surveys in adjacent areas, and with ground based studies. The main Chernobyl deposition in Dumfries and Galloway appears to have occurred between an area just east of the Nith, and Glenluce. The northern limit has not yet been defined, and there may be grounds for considering extension of the northern and particularly eastern limits of the inland survey zone. Within the survey zone the deposition pattern is complex, including both upland and lowland components. The plume trajectories for deposition inferred from these observations are oriented northwards rather than in the NW directions predicted by meteorological derived estimates. This may explain the contradiction between results from the Central Highlands and the estimated fallout patterns. The data presented here both add to previous knowledge, and serve as a baseline against which any future changes can be measured. The survey provides systematic coverage of the sedimentary and terrestrial coastal system for the first time, and has identified a number of merse sites which have accumulated radioactivity from past marine discharges from Sellafield, and which are not routinely monitored under existing Scottish Office arrangements. Some of these locations are extensive and fall within SSI’s; furthermore they are key sites for studying future deposition trends. It would seem prudent to review radiological assessments in the light of this work to ensure that the patterns of occupancy and sensitive ecologies of the merse are taken fully into account. The emergency response potential of aerial radiometrics has been clearly demonstrated in this project. It provides the only practical means of providing comprehensive environmental measurements of remote and upland landscapes on a short time scale, with an effective area sampling density some 106-107 times greater than soil sampling. Results are compatible with ground based approaches, and could focus ground based efforts effectively under emergency conditions. Modern approaches to data recording and analysis are able to produce maps during the survey period. National baseline mapping in Scotland, at 1 km resolution, would require less than 800 flying hours; 20 km line spacing would take roughly 40 hours of flight time. A long term programme of high resolution national baseline mapping, coupled to an emergency response standby arrangement, would provide an extremely cost effective way of preserving the capability developed since the Chernobyl accident, while producing high quality environmental data for research purpose