A statistical approach to investigating enhancement of polonium-210 in the Eastern Irish Sea arising from discharges from a former phosphate processing plant

Abstract

Since the cessation of phosphoric acid production (in 1992) and subsequent closure and decommissioning (2004) of the Rhodia Consumer Specialties Limited plant in Whitehaven, the concentration levels of polonium-210 (210Po) in local marine materials have declined towards a level more typical of natural background. However, enhanced concentrations of 210Po and lead-210 (210Pb), due to this historic industrial activity (plant discharges and ingrowth of 210Po from 210Pb), have been observed in fish and shellfish samples collected from this area over the last 20 years. The results of this monitoring, and assessments of the dose from these radionuclides, to high-rate aquatic food consumers are published annually in the Radioactivity in Food and the Environment (RIFE) report series. The RIFE assessment uses a simple approach to determine whether and by how much activity is enhanced above the normal background.As a potential tool to improve the assessment of enhanced concentrations of 210Po in routine dose assessments, a formal statistical test, where the null hypothesis is that the Whitehaven area is contaminated with 210Po, was applied to sample data. This statistical, modified “green”, test has been used in assessments of chemicals by the OSPAR commission. It involves comparison of the reported environmental concentrations of 210Po in a given aquatic species against its corresponding Background Assessment Concentration (BAC), which is based upon environmental samples collected from regions assumed to be not enhanced by industrial sources of 210Po, over the period for which regular monitoring data are available (1990–2010). Unlike RIFE, these BAC values take account of the variability of the natural background level. As an example, for 2010 data, crab, lobster, mussels and winkles passed the modified “green” test (i.e. the null hypothesis is rejected) and as such are deemed not to be enhanced. Since the cessation of phosphoric acid production in 1992, the modified “green” test pass rate for crustaceans is ∼53% and ∼64% for molluscs. Results of dose calculations are made (i) using the RIFE approach and (ii) with the application of the modified “green” test, where samples passing the modified “green” test are assumed to have background levels and hence zero enhancement of 210Po. Applying the modified “green” test reduces the dose on average by 44% over the period of this study (1990–2010).