OR/17/001 Background natural radioactivity in soils
|Tye, A M, Milodowski, A E, and Smedley, P L. 2017. Distribution of natural radioactivity in the environment. British Geological Survey Internal Report, OR/17/001.|
The use of total element concentrations for radiological assessments of environmental media has been undertaken previously (EA, 2007). This was undertaken on a UK regional basis by combining the specific activities of 40K, 232Th and 238Th, these being 31.6 Bq g-1 K, 4.1 Bq mg-1 Th and 12.2 Bq mg-1 for U respectively, with total element concentrations.
Availability of data
Data concerning the National datasets for background radioactivity (U, Th, K) in soils include the G-BASE datasets, which only have partial coverage for UK soils. This relates to how the geochemical survey evolved over 30 years. However, G-BASE sediment and soil data were previously used in an assessment of radioactivity in environmental media (EA, 2007). However, a re-analysis of the National Soil Resource Institute Geochemical Atlas (originally published by McGrath & Loveland (1992) with a sampling resolution of one sample per 5 km2 grid) by BGS provides national coverage but at a slightly lower sampling density than G-BASE for England and Wales. This is known as the Advanced Soil Geochemistry Atlas and contains data and maps for total soil concentrations of U, Th, and K and is available on-line at: www.bgs.ac.uk/GBASE/advSoilAtlasEW.html
G-BASE soil geochemistry data and maps been obtained for Topsoils <2mm and subsoils (<250µm) can be can be found at: www.bgs.ac.uk/products/geochemistry/home.html?src=topNav
A recently completed high resolution survey of the South West of England, a major area of England with high natural background radioactivity, has top soil and sediments data and maps for U, Th and K can be found at: www.bgs.ac.uk/gbase/gBaseSW.html
At present Scotland has limited National coverage of Soil Geochemistry. The Geochemical Atlas for Scottish Topsoils (Paterson, 2011) has information regarding Potassium. However, proxies of soil parent materials through the use of stream sediment will give an indication of those geological formations that may result in high background radioactivity concentrations. Regional G-BASE data for stream sediments can be found at: www.bgs.ac.uk/data/maps/maps.cfc?method=listResults&mapName=&series=RGA&scale However, there are strong similarities between the geology of Scotland and Northern Ireland where a recent geochemical survey (TELLUS) has been undertaken with soil geochemistry being included. This can be used as proxy data. Datasets can be found at: www.gsi.ie/Tellus/
The mean and median values of Uranium in soils from England and Wales using data from the Advanced Soil Geochemistry Atlas are 2.4 and 2.3 mg kg-1 respectively. In soils, it occurs in common minerals such as Zircons, Apatite, monazite and carbonates but also binds strongly to humus and peat (Read et al. 1993). Soils formed from geological parent materials with high U, typically show the highest U concentrations. Within England and Wales, 90% of the data from the Advanced Soil Chemistry atlas was below 3.3 mg kg-1. Geological formations where elevated concentrations occurred include the (a) lower cretaceous non-marine sandstones and clays of the Weald, (b) the Devonian and Carboniferous terrane of Cornwall, including the Granites, (c) the Holocene peats of the Fen Basin, (d) the marine alluvium of the Lincolnshire coast, e) the soils above the Carboniferous and Permian limestone outcrop stretching from the Peak District to Newcastle and the (f) Silurian strata between Welshpool and Hereford. Typically concentrations of U in the upper 10th percentile have concentrations ranging from 3.3 to ~90 mg kg-1. In Scotland, concentrations of U in Caithness are considered to be 3.5 times greater than the UK average (Nicholson et al. 1990). Using TELLUS data from Northern Ireland as a proxy for Scotland because of the close similarity in geology, concentrations of U in soils derived from granites can exceed 100 mg kg-1, but the median concentration is 10 mg kg-1 and the 75th percentile value is <30 mg kg-1. However, for other geological formations found in NI that are similar to Scottish formations, the soils typically have a 95th percentile which is <5 mg kg-1. These include soils formed on Gabbro, Lithic Arenites (sandstones), Basalts, Andesites, Acid Volcanics, Psammite and Semi-pelites, conglomerates, sandstone, mudstone and limestone. In Scotland, concentrations of U in soils from Sutherland associated with the Helmsdale granite had concentrations of U between 1.1 and 16 mg kg-1 (Nicholson et al. 1990).
Enhanced concentrations of Uranium may occur not only as a result of contamination from industrial processes and mining activites, but through the widespread application of Fertiliser P. Recent work by Ahmed et al. (2014) demonstrated the effects of P fertilisation using paired depth profiles of arable and woodland soils in Nottinghamshire. In arable soils there was good agreement between Total P and Uranium to depths of about 50 cm, with U concentrations in the top soil of being ~ 1.6 mg kg-1 compared to 1.2 mg kg-1 in the adjoining woodland soils. Uranium in the arable soils was strongly correlated (r=-0.94) with Ca, suggesting that co-precipitation of U with poorly soluble Ca-Phosphates occurred (maybe because of liming). Sequential extractions were undertaken to understand the binding phases of U in the non-residual fraction of a sandy loam soil and showed that over 50% was bound to organic matter, sulphides or Fe/Mn oxides.
A more famous example of enhanced U concentrations in soils occurs at Broubster, Caithness in Scotland, and demonstrates the potential of peat to bind U. This occurs through the release of U from the Ciathness flag sequence of sandstone where the U resides in diagenetic apatiotes and U-Si-Ti phases. Weathering processes have released the U, with the 4000 yr old peat being the sink. The maximum concentration of U found in the peat has been recorded as 1200 ppm (Read et al. 1993).
Thorium is naturally associated with K, U and some rare earth elements such as Ce. It is similar to Uranium, as it is found in minerals such as thorite and monazite, is also present in Zircon, sphene and epidote. From the advanced soil atlas of E&W, the mean and median concentrations of Th in soils of England and Wales are 8 mg kg-1. The upper 10th percentile has a concentration range of 11–51 mg kg-1 and soils associated with the following geology fall within this range: soils associated with (a) the Silurian of mid and SW Wales, (b) the old red sandstone of the Welsh Borders, (c) the Devonian — Carboniferous sedimentary terrane in the south West but excluding the granite outcrops, and the (d) Jurassic outcrop of central England extending from Somerset to Lincolnshire with the ironstones and ferruginous sandstones having the highest concentrations and lastly e) the non-marine Lower Cretaceous sedimentary strata of the Weald.
Soils with the lowest concentrations include those associated with Chalk on the downs of southern England and East Anglia — these soils typically have concentrations <6 mg kg-1. In Scotland, concentrations of Th in soils from Sutherland associated with the Helmsdale granite had concentrations of Th between 2.3 and 32.4 mg kg-1 (Nicholson et al. 1990).
With the exception of industry, the distribution of Th within P fertilisers is considered to be the major route through which background concentrations of Th in soils may be enhanced. Ahmed et al. (2014) examined Th along with U in adjoining arable and woodland soils and found that Th distribution was similar in arable and woodland soils, although slight enrichment was found with depth. This was largely considered to be because Th in P fertilisers is often less than the concentrations found in soils whilst U is concentrations in fertilisers can be several hundred times higher than is found in soils. However, the majority of the non-residual fraction of Th in both the woodland and arable soil was held in the organic matter/sulphide fraction of a sequential extraction.
Potassium in the soils of England and Wales are in the range 0.03 to 1.9% up to the 90th percentile. The upper 10th percentile covers the range of 1.9–4.17% based on data from the Advanced Soil Geochemistry Atlas. The median concentration is 1.2%. Despite the ubiquitous use of K as a fertiliser on agricultural land, geological formations rich in clay minerals still notably possess identifiable elevations in K concentrations. Soils with K concentrations above 1.9% largely include those based on parent materials consisting of undifferentiated mudstones and siltstones such as (i) the Devonian strata in the south west of England, (ii) the Triassic age Mercia Mudstone group that extends from Somerset to Nottinghamshire, (iii) the lower Palaeozoic aged sediments of Mid Wales and (iv) Carboniferous sediment rocks in Northumberland and Berwick.
Estimates of background U, Th and K concentrations of soils associated with geology at UK nuclear sites
Using information from the different Geochemistry databases we compiled information from soils or sediments associated with the geology on which the UK nuclear establishments are built. These can be seen in Table 1. Different databases needed to be used, particularly for Scotland where a lack of soils data is available, for different parts of the country as a consequence of how the G-BASE sampling program developed over 30 years.
|Location||U (mg kg-1)||Th (mg kg-1)||K (%)||Source|
|Hartlepool||2.8||<9||1.7||G-BASE Sub soil|
|Heysham||2.8||<6||1.4||G-BASE Sub soil|
|Springfields||2.5||<6||1.5||G-BASE Sub soil|
|Capenhurst||2||7||2.2||G-BASE Sub soil|
|Donnington||2||8.4||1.7||G-BASE Top soil|
|Sizewell||1.2||2.2||0.71||G-BASE Top soil|
|Bradwell||2.8||7.6||1.6||G-BASE Top soil|
|Amersham||1.6||4||0.55||G-BASE Top soil|
|Culham||1||2.6||0.41||G-BASE Top soil|
|Harwell||1.5||8||1.0||G-BASE Top soil|
|Aldermaston||1.7||6.8||1.1||G-BASE Top soil|
|Berkley and Oldbury||3.1||12.4||2.3||G-BASE Top soil|
|Cardiff||2.4||6.5||1.1||G-BASE Top soil|
|Hinkley Point||<4||11||1.9–4.0||Advanced Geochem Atlas|
|Winfrith||<2||<6||<0.9||Advanced Geochem Atlas|
|Portsmouth||<3||<8||<1.5||Advanced Geochem Atlas|
|Dungeness||<3||<10||<1.5||Advanced Geochem Atlas|
|Devonport||4.1||15||3.2||Advanced Geochem Atlas|
- Environment Agency. 2007. Assessment of naturally-occurring radionuclides in England and Wales. Environment Agency Science Report, SC030283/SR, 78 pp.
- McGrath, S P, and Loveland, P J. 1992. The Soil Geochemical Atlas of England and Wales. London: Blackie
- Paterson, E. 2011. Geochemical Atlas for Scottish Topsoils. Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen.
- Read, D, et al. 1993. The migration of uranium into peat-rich soils at Broubster, Caithness, Scotland, Uk. Journal of Contaminant Hydrology, 13(1–4), 291–308.
- Nicholson, S, Long, S E, and McEwen, I. 1990. The levels of Uranium and and thorium in soils and vegetables from Cornwall and Sutherland. Report number AERE-R–13435. UKAEA Harwell. ISBN 07058-1625-7.
- Ahmed, H, Young, S D, Shaw, G. 2014. Factors affecting uranium and thorium fractionation and profile distribution in contrasting arable and woodland soils. Journal of Geochemical Exploration, 145, 98–105.