Optimisation of beryllium-7 gamma analysis following BCR sequential extraction

Area of Science:

• Environmental Geochemistry
• Radiochemistry
• Soil Science

Background:

• Cosmogenic beryllium-7 (7Be) is a valuable sediment tracer at catchment scales.
• Accurate application relies on understanding its geochemical associations in soil, particularly irreversible adsorption.
• Sequential extraction methods are used to determine 7Be's association with soil phases, but low activity can cause high counting uncertainties.

Purpose of the Study:

• To evaluate and compare three methods for preparing and quantifying 7Be extracted using the BCR three-step scheme.
• To reduce analytical uncertainties in the determination of 7Be geochemical associations in soil.
• To optimize a method for reliable sediment tracer studies.

Main Methods:

• Evaluation of three preparation and quantification methods for 7Be extracted via the BCR three-step scheme.
• Optimal method: triplicate BCR extraction, stable Be analysis of subsamples, coprecipitation of 7Be with metal oxyhydroxides, and thin source gamma analysis.
• Application to BCR extractions of natural 7Be in four agricultural soils, with analysis of stable Be and soil:solution ratios.

Main Results:

• The optimal method yielded good counting statistics (~10% 2σ) within a 24-hour analysis period for activities >40% of total.
• Total 7Be recoveries ranged from 84% to 112%, with high reproducibility of the extraction procedure (<1% RSD).
• Gamma counting uncertainties dominated the overall uncertainty; stable Be is a poor proxy for 7Be fallout at high concentrations.

Conclusions:

• A refined BCR extraction and quantification method significantly reduces analytical uncertainties for 7Be in soils.
• This optimized approach provides statistically useful sequential extraction profiles for reliable catchment-scale sediment tracing.
• Understanding soil:solution ratios is crucial, as it can affect the apparent distribution of Be fractions.