Rare earth elements as tracers of sediment contamination by fertilizer industries in Southern Brazil, Patos Lagoon Estuary

Abstract

The content of rare earth elements (REE) and their distribution in sediment cores collected from the Patos Lagoon estuary from southern Brazil were investigated. Sediment cores (ca. 44 cm) were obtained from two locations within the estuary, including a relatively unimpacted site (PI) that has not been strongly affected by human activities and a well-known polluted area within Coroa do Boi Bay (CB), located in the industrialized central part of the estuary, to capture possible features that may help to identify contamination by the local fertilizer manufacturing industry. Sediments from both coring locations are enriched in the light REEs (LREE) over the heavy REEs (HREE) when normalized to the North American Shale Composite (NASC). The PI core was collected in the mid-estuary, an area where salinity values were lower than 10 practical salinity units and where river water and seawater mixing occurs. The features observed in PI sediments likely reflect preferential removal of LREEs from the water column owing to salt-induced coagulation of river-borne colloids. Sediments samples collected from the impacted CB location have substantially higher REE contents than PI sediments. Furthermore the relatively constant REE content and measures of REE fractionation (i.e., ΣLREE/ΣHREE ratios, NASC normalized patterns, NASC normalized La/Yb ratios) with depth in sediments from the CB site suggests that the impact of fertilizer manufacturing on these sediments is a long-term phenomenon. Using sedimentation rates determined for Coroa do Boi Bay, we show that anthropogenic contamination of the CB core sediments by total urban, industrial, and port emissions of REEs can be traced to the early 1900's and was largely continuous throughout the 20th century.