From source to sink: Rare-earth elements trace the legacy of sulfuric dredge spoils on estuarine sediments

Abstract

Land disposal of dredged sulfide-rich coastal sediments generates secondary coastal acid sulfate soils (CASS), as previously reduced sulfide minerals oxidise to produce acidic drainage rich in Fe, SO42− and rare-earth elements (REEs). Few studies investigate both the source and the sink of REEs in the context of interpreting their mobilisation and potential use in tracing anthropogenic activity. Here we investigate REE signatures in estuarine sediments (and overlying surface waters) that have received acute, long-term (>15 years) acidic drainage from legacy sulfuric dredge spoils. It was found that the dredge spoil continues to act as a source of acidity (pH 3.5–5.5), Fe and REEs during development of CASS, and contains negligible acid volatile sulfide (AVS, a proxy for FeS) and relatively low concentrations of ΣREE (mean 44.5 mg/kg, range 4.1–362 mg/kg). In the receiving sediments, high AVS concentrations (mean 92.2 μmol/g, range 0.38–278 μmol/g) reflect elevated FeS content, likely due to high inputs of Fe and SO42− from the acidic drainage, and correspond with a high concentration of total S (mean 852 μmol/g, range 105–2209 μmol/g) and an accumulation of ΣREE (mean 670 mg/kg, range 19.9–1819 mg/kg). Importantly, where drain sediments that were previously enriched in highly reactive sulfidic minerals and trace elements and have become exposed to the atmosphere (e.g. Site 3) and partially oxidised, they provide a further source of acidification, remobilising the REEs to the downstream sediments. Interestingly, we also found a clear positive correlation between phosphorous and REEs both in the dredge spoil and sediment, suggesting phosphate minerals may act as a sink for REEs in CASS influenced drain sediments. This is further supported by strong positive gadolinium anomalies (1.1–1.6) and high calculated anthropogenic Gd values (12–38%), which may reflect the influence of phosphate fertiliser on this eutrophic system.