Environmental radioactivity

Our environmental radioactivity facility concentrates on:

  • sediment profiling of peat and lake deposits to support reconstruction of recent environmental history
  • radon measurements to understand the natural distribution and migration of this gas in the environment

Peat profiling

Peat profiling

Peat cores are used to reconstruct the accumulation of peat, through analysis of sample slices taken at intervals down a core. In ombrotrophic peat bogs, for which accumulated material is derived from the breakdown of plants, there is no mineral source of natural radioactivity within the layers of peat. Where the peat has not been disturbed by erosion, such as from rivers or human activity, it can accumulate a continuous record of atmospheric deposition of minerals to the land surface.

In order to make this archive relevant to the history of landscape evolution and records of human activity, it is important that the layers of peat can be dated. The age-dating provides a chronological context to the other measurements, such as organic markers or chemical pollutants. These data allow us to understand the processes of peat accumulation and look at the implications of peat erosion into local water courses, as well as providing records of Anthropocene activity.

The ability to date peat cores uses the natural deposition of a radiogenic isotope of lead (Pb) onto the earth's surface from atmospheric fallout. The isotope Pb-210 forms in the atmosphere due the decay of the naturally occurring radioactive gas radon (Rn, isotope Rn-222), and is deposited as a particulate.

When Pb-210-containing material is deposited onto the surface of peat, it is retained and gradually buried as organic matter continues to accumulate through time. The Pb-210 atoms in turn decay at a well-characterised rate (half-life), and because no further Pb-210 is added to the buried layers, the rate equation can be used to reconstruct the age of the slices of peat taken for measurement from the core. We measure Pb-210 using one of our gamma spectrometers.

In addition to Pb-210, anthropogenic-derived radioisotopes can be measured by gamma spectrometry and used to corroborate these models. For example, atmospheric bomb tests were associated with the release of the caesium isotope Cs-137, which peaked in the 1960s: the rate of decay of this isotope is well understood and can be used to assess the age of peat material.

Using the complementary techniques of Pb and Cs dating, the age of peat and rates of peat accumulation can be modelled from approximately the last 150 years. We are able to link this with other established techniques to reconstruct contaminant loading onto peat from atmospheric deposition, such as with stable Pb isotope dating and isotopic liability testing using Pb isotope ratios determined by ICP-MS.

Reference

Rothwell, J J, Taylor, K G, Chenery, S R N, Cundy, A B, Evans, M G, and Allott, T E H.  2010.  Storage and Behavior of As, Sb, Pb, and Cu in Ombrotrophic Peat Bogs under Contrasting Water Table Conditions.  Environmental Science & Technology, Vol. 44, 8497–8502.  10.1021/es101150w

Lake, river and estuary sediment profiling

The same principles described for peat can be applied to dating lake and large river sediments. However, there is an added complication, as Pb-210 is incorporated into these sediments not just from atmospheric sources, but also from deposition of water-borne sediment. These minerals are ultimately derived from the erosion of rocks in the river catchment, and will contain Pb-210 from the decay of naturally occurring uranium-bearing minerals in the sediment. These minerals represent a continuous source of replenishment of Pb-210 in the core, whilst the Pb-210 from atmospheric deposition is isolated as the sediment accumulates. This inherent mineral-derived Pb-210 activity needs to be taken into account when calculating a deposition age for a sample slice from the core and to reconstruct sedimentation rates via making measurements on a sequence of samples through the core.

Reconstruction of river and lake deposition or estuarine sediments is used to understand human–environment interactions as a result of population growth, urban expansion, climate change and pollution events.

Reference

Kemp, A C, Sommerfield, C K, Vane, C H, Horton, B P, Chenery, S, Anisfeld, S, and Nikitina, D.  2012   Use of lead isotopes for developing chronologies in recent salt-marsh sediments.  Quaternary Geochronology, Vol. 12, 40–49.

Radon in soil and water

Radon in water

Radon (Rn) gas is a natural decay product from both the uranium series (Rn-222) and thorium series (Rn-220). Most rocks will contain small concentrations of uranium and thorium, decay from which provides a well-known background level of Rn. Certain rocks can have relatively elevated concentrations of the Rn parent elements and hence can give rise to high concentrations of Rn.

Rn is soluble in water and can readily dissolve into ground water. This may then be released directly as a gas via fissures or dissolved in water by flow from aquifers. In turn this could lead to unusually high concentrations of Rn in some areas, potentially becoming a risk to health if trapped in unventilated buildings.

We measure Rn in water and soil using a combination of liquid scintillation counting, alpha counting and gamma spectrometry. Measurement of Rn has well established applications in developing natural tracer and hazard assessment methods, e.g. in soil profiles and domestic drinking water supplies. This is an important aspect of establishing and monitoring baselines as part of BGS research into baseline groundwater conditions in areas that have the potential for extraction of unconventional gas resources.

Contact

For more information please contact Charles Gowing or Simon Chenery.