Delgado, and 4 co-authors, reported in 2012 that rare earth elements (REE) were analyzed in surface sediments from the Guadiana Estuary (SW Iberian Pyrite Belt). NASC (North American Shale Composite) normalized REE patterns show clearly convex curvatures in middle-REE (MREE) with respect to light- and heavy-REE, indicating acid-mixing processes between fluvial waters affected by acid mine drainage (AMD) and seawater. However, REE distributions in the mouth (closer to the coastal area) show slightly LREE-enriched and flat patterns, indicating saline mixing processes typical of the coastal zone.
NASC-normalized ratios (La/Gd and La/Yb) do not discriminate between both mixing processes in the estuary. Instead, a new parameter (EMREE) has been applied to measure the curvature in the MREE segment. The values of EMREE > 0 are indicative of acid signatures and their spatial distribution reveal the existence of two decantation zones from flocculation processes related to drought periods and flood events. Studying REE fractionation through the EMREE may serve as a good proxy for AMD-pollution in estuarine environments in relation to the traditional methods
The authors concluded that Guadiana river receives acid mine drainage (AMD) from oxidation of sulfide-rich wastes of abandoned mining districts from the Iberian Pyrite Belt (IPB; SW Iberian Peninsula). Metal pollution is considerably diluted and attenuated in the river, which flows into an estuary historically considered non-polluted. However, in the current study the authors demonstrates herein that the Guadiana estuary is being certainly affected by current mining activities from the IPB through studying the concentration variations of rare earth elements (REE) in the surface sediments.
Flocculation reactions by neutralization occurring within the estuary, i.e. precipitation of iron and aluminum oxy-hydroxides, preferentially remove MREE from solution. Consequently, NASC-normalized patterns corresponding to the sediments from the main and secondary tidal channels show convex MREE-signatures. A new convexity metric (EMREE parameter) reported in previous studies was used to assess the significance of the curvature effect in the MREE whole segment, and hence, quantify the effects of AMD pollution. This index was a more sensitive tool to recognize convex MREE signatures than other NASC-normalized ratios where a single element represents to a set such as (La/Gd)NASC.
Spatial EMREE distribution revealed two zones where decantation of fine-sized particles from flocculation by acid mixing processes occurs in the estuary. The first upstream zone is larger and represents flocculation during long drought periods, while the second downstream zone is smaller and represents flocculation during short but intense rainfall events.
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