Quantifying and characterising metal concentrations in Derwent Estuary sediments using portable X-ray fluorescence spectrometry

Abstract

The Derwent Estuary is highly enriched in potentially toxic elements, such as Zn, Pb, Cu, As, Hg and Cd, owing to inputs from historical industrial activity adjacent to the river, predominantly prior to strict environmental protections introduced in the 1970s. Contaminants are now buried at shallow depths within the sediment profile, in one or two highly concentrated layers decreasing in concentration away from an electrolytic zinc refinery, regarded as the main source of the contaminants. Enriched metals (Zn, Pb, Cu, Cd and As) in the estuary were estimated from data collected from 37 sediment cores using a portable X-ray fluorescence spectrometer that was validated against inductively coupled plasma mass spectrometer analyses. The thickness of the metal and metalloid enriched layers ranges from 32.5 to 107.5 cm, with an average thickness of 63 cm. Sedimentation rates based on this layer and the time since the start of zinc processing are approximately 0.46 cm/year. Sedimentation rates based on the thickness since maximum metal and metalloid concentrations are between 0.17 and 1.64 cm/year. Based on these sedimentation rates, the average time it will take for surface sediments to return to background metal and metalloid concentrations, if left undisturbed, is approximately 123 years. KEY POINTS Metal contaminant concentrations in Derwent Estuary sediments have two clear peaks in concentrations at depth, where estuary sediments have been enriched at varying rates over the last 97 years. The contaminated layer varies in thickness throughout the estuary, but has an average thickness of 63 cm, a maximum thickness of 107.5 cm and a minimum thickness of 32.5 cm, from surface sediments to the bottom of contamination. All surface sediments were enriched in metals or metalloids, but the overall trend in metal concentrations is decreasing from a major enrichment peak to moderate concentrations at the sediment–water interface. The time required for surface sediments to return to acceptable concentrations as defined by the ISQG is estimated to be on average 123 ± 88 years for the whole estuary with different parts of the estuary returning to background at different times.