Controls on the distribution of 7Be for an improved understanding of sediment and associated contaminant storage

Purpose: Beryllium-7 has been widely applied as sediment tracer in river basins, however, factors influencing its geochemistry in this context have rarely been addressed. We investigated sediment-water interactions controlling the distribution of ⁷Be in channel sediments for improved storage time and particulate contaminant dynamic assessments in a catchment affected by legacy and contemporary mining pollution. Methods: The investigation was undertaken in the River Fal, UK, to assess sediment storage times and particulate contaminant dynamics. Samples were collected and analysed for ⁷Be, ¹³⁷Cs, ²¹⁰Pb and ²³⁸U by gamma spectrometry, aqua regia extractable P, As, Fe, Cu, Pb, Sn and Zn by inductively coupled plasma (ICP) spectrometry and Total Organic Carbon (TOC) by CHN analyser. Particle size distribution (PSD) and the 1 mol L^{− 1} HNO₃ extractable Fe and Mn fractions were analysed by laser diffraction and ICP, respectively, to assess relationships with the radionuclide activities. Dissolved and the 1 mol L^{− 1} HCl extractable ⁹Be was quantified to determine its distribution coefficients (K_d). Results: Beryllium-7 was significantly correlated with the specific surface area (SSA), TOC and the Fe and Mn extractable fractions. A strong inverse correlation between the ⁹Be K_d and suspended sediment concentration (SSC) demonstrated the particle concentration effect. Suspended sediments (SS) showed higher SSA than the channel bar sediments (CBS). Storage times and the proportion of recently deposited sediments (% RDS) were estimated using different methods for addressing particle size effects, which were not statistically different. Conclusions: Occurrence of particulate contaminants was related to two dominant sources and processes: (1) in the upstream site, co-precipitation with Fe-oxides and Organic Matter (OM) enrichment; and (2) in the lower site, reworking from historic polymetallic mining during high flow events with wider relevance to legacy pollution challenges in mine-impacted rivers.