Sequential extraction procedure as a tool to investigate PTHE geochemistry and potential geoavailability of dam sediments (Almadén mining district, Spain)

•Reservoir sediments accumulate significant amounts of potential toxic metals.•BCR steps and indexes are complementary tools to estimate the environmental risk.•Environmental risk varies according to chemico-physical properties of sediments.

Sediments from Castilseras reservoir (Almadén) were collected to assess the potential environmental hazards associated with the availability of Potentially Toxic or Harmful Elements (PTHE) in this freshwater environment. PTHE concentrations in sediments were investigated using a four step (F1-F4) Sequential Extraction Procedure (SEP) and the results were evaluated using Principal Component Analysis (PCA) and different risk indexes and criteria. Considering the fractions results and the PCA treatment, two groups of elements can be differentiated in the easily leached by weak acids fraction F1 and these indicate the presence of sulfate and secondary precipitates of carbonates as the main sink for easily accessible metallic elements. In reducible fraction F2, chemical analysis revealed a significant relationship between the majority of the analyzed elements and the Fe oxy-hydroxides, with the exception of Co, Hg and Ni, which are preferentially associated with Mn oxy-hydroxides. In the oxidizable fraction (F3), a significant relationship between Cu, Fe, Hg and Zn (representative metals of the existing ore deposits) and sulfides/organic matter was observed. The environmental assessment based on the aforementioned criteria provides risk levels varying from moderate to high. The risk appears to be higher near the dam, where the higher PTHE levels in sediment are due to a selective accumulation of fine-grained particles.