Temporal variability of beryllium-7 fallout in southwest UK.

Cosmogenic beryllium-7 has been widely employed as a sediment tracing tool and continued development of its use as a soil erosion tracer requires knowledge of fallout temporal dynamics. Data regarding beryllium-7 fallout in the UK are scarce and here the authors provide a record of beryllium-7 fallout in southwest England spanning a two-year period. A monthly fallout record was developed for Plymouth, UK using regular rainfall sampling to determine beryllium-7 rainfall activity concentration (Bq L(-1)) and deposition flux (Bq m(-2)). Data showed a general tendency for higher activity during the spring/summer months and lower activity in the autumn/winter months. Comparison with data for other UK sites (Chilton and Aberporth) for the same period found significant differences in (7)Be activity in rainwater and lower variability in Plymouth than Chilton and Aberporth. Total deposition was largely controlled by rainfall in Plymouth although regression coefficients suggested greater importance of other atmospheric controls at the Chilton and Aberporth sites. Use of a deposition proportion to rainfall proportion ratio identified periods when deposition was influenced by varying (7)Be activity in rainfall. Broad ranges in ratios were found for Chilton and Aberporth and this has implications for sediment tracer studies requiring estimates of (7)Be deposition flux across months or seasons.

Optimisation of beryllium-7 gamma analysis following BCR sequential extraction.

The application of cosmogenic (7)Be as a sediment tracer at the catchment-scale requires an understanding of its geochemical associations in soil to underpin the assumption of irreversible adsorption. Sequential extractions offer a readily accessible means of determining the associations of (7)Be with operationally defined soil phases. However, the subdivision of the low activity concentrations of fallout (7)Be in soils into geochemical fractions can introduce high gamma counting uncertainties. Extending analysis time significantly is not always an option for batches of samples, owing to the on-going decay of (7)Be (t(1/2)=53.3 days). Here, three different methods of preparing and quantifying (7)Be extracted using the optimised BCR three-step scheme have been evaluated and compared with a focus on reducing analytical uncertainties. The optimal method involved carrying out the BCR extraction in triplicate, sub-sampling each set of triplicates for stable Be analysis before combining each set and coprecipitating the (7)Be with metal oxyhydroxides to produce a thin source for gamma analysis. This method was applied to BCR extractions of natural (7)Be in four agricultural soils. The approach gave good counting statistics from a 24 h analysis period (~10% (2σ) where extract activity >40% of total activity) and generated statistically useful sequential extraction profiles. Total recoveries of (7)Be fell between 84 and 112%. The stable Be data demonstrated that the extraction procedure had a high reproducibility (<1% RSD), thus gamma counting uncertainties dominated the overall uncertainty. In addition, extractions of soil equilibrated with stable Be at a concentration below the Cation Exchange Capacity (CEC) of the soil demonstrated that doubling the soil:solution ratio to enhance the mass of soil used in a sequential extraction scheme affects the apparent distribution of approximately 10% of the total Be. At high concentration, stable Be was found to be a poor proxy for (7)Be fallout in sequential extractions.

Measurement of 237Np in environmental water samples by accelerator mass spectrometry.

Accelerator mass spectrometry (AMS) was used to measure 237Np in environmental water samples extracted from Irish Sea sediments. The samples were of limited volume (approximately 700 ml) and of low activity (0.06-0.79 mBq l-1; 2.30-30.3 pg l-1). AMS proved to have the required sensitivity for measuring these samples, and was in principle capable of measuring much smaller amounts, as low as 0.4 microBq (3.9 x 10(7) atoms). However, the background level in the procedural blanks showed that there was a systematic low level 237Np contamination of each sample, arising from the 239Np yield monitor used in the separations procedure, which effectively increased the detection limit of these analyses.