Master Thesis

Short-term, mid-term, and long-term effects of sea level rise on metal(loid) mobilization from the Río Tinto estuary sediments

Merle Käberich (10/2020-12/2021)

Support: Britta Planer-Friedrich, Carolin Kerl

Climate change-induced sea level rise is expected to cause flooding of low elevation estuaries, such as the Río Tinto estuary in Spain. However, it is unknown how the Río Tinto sediments, which are polluted by acid mine drainage, will react to seawater inundation. Therefore, we analyzed the short-term, mid-term, and long-term metal(loid) mobilization from two sampling sites: Tinto A with low soil pH is under river water influence and situated on the uttermost end of expected seawater inundation. Tinto B with high soil organic carbon content and reducing conditions is already under stronger tidal influence. While contamination with metal(loid)s at both sampling sites exceeded environmental standards, Tinto A contained more As (1181 mg/kg versus Tinto B 556 mg/kg) and Tinto B more cations (e.g. Zn 2006 mg/kg versus Tinto A 144 mg/kg). Arsenic was primarily mobilized by reductive dissolution of Fe (hydr)oxides and concentrations in pore water upon inundation were substantially higher for Tinto B than for Tinto A (up to 113 versus 12 µg/L). In contrast, cations were mobilized by competitive sorption, leading to higher mobilization from Tinto A compared to Tinto B (e.g. Zn up to 8 versus 0.6 mg/L). Surprisingly, metal(loid) mobilization – with exception of Cr – was independent of inundation water salinity but only governed by sediment characteristics. For most metal(loid)s, the sampling site with higher total contents released lower concentrations in our experiments, thus still retaining major contaminant loads. Due to the heterogeneity of Río Tinto sediments, an extrapolation of results to larger estuary areas is difficult. Nevertheless, seawater inundation in the future may change characteristics of upper estuary sediments, such as Tinto A, to become similar to Tinto B. Consequently, this could activate the mobilization potential of high As sediment loads and thereby pose serious environmental risks.