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Total and dissolved rare earth elements (REE) were studied in a river reach affected by several inputs of acid mine drainage. The first four acidic discharges were of lesser importance compared to the last (Agrio River, coming from the Rio Tinto mines), which transported high loads of Fe and Al (2.1 and 4.2 ton/day, respectively) together with REE (16.4 kg/day) and other trace elements. In the acid mine drainage (AMD) sources, practically all the REE were dissolved and the North American Shale Composite (NASC)-normalized patterns showed an enrichment in medium REE, although some differences exist between the patterns of each source. The pH values in the river reach upstream of the confluence with the Agrio River ranged between 8.01 and 7.03, and most of the Fe and Al from the AMD sources precipitated. Downstream of this acidic discharge, the pH decreased to 2.98. Upstream of the first AMD input, the dissolved and total concentrations of ΣREE were very low ( 7, concentrations of ΣREE increased (up to 25 μg/L), mainly transported by the particulate phase. In the reach downstream of the Agrio River with acidic conditions, REE behaved conservatively. The REE NASC-normalized patterns of the river samples resemble that of the AMD sources, although an enrichment of heavy REE in the dissolved phase is observed linked to complexation by carbonates. Cerium and particularly La also show higher dissolved percentages, similar to HREE, which must be due to the lower affinity of these elements to be sorbed onto Fe oxyhydroxides. Variations in Ce and Eu anomalies are observed along the river reach as a consequence of the different AMD inputs. However, the values of the Ce anomaly are higher in total samples than in dissolved samples. Speciation results indicate that these differences are not caused by differences in oxidation states but by slight differences in the hydrogeochemical behaviour of Ce with respect to La and Nd.

Compared with pit lakes originated by coal mining, little is known about in-lake neutralization processes in pit lakes from sulfide ore mining in hard rock substrates, which are typically very deep and acidic and receive low carbon (C) inputs and groundwater flows. Physicochemical processes in water and sediments from a pit lake (San Telmo, 130 m deep) in the Iberian Pyrite Belt were investigated. San Telmo is a meromictic and highly acidic (pH 2.8) pit lake due to pH buffering by precipitation of Fe(III) minerals (schwertmannite and jarosite). The sediments have a low abundance of C (below 0.60%) and iron minerals (below 0.12%) compared to most coal-mining pit lakes. In San Telmo sediments, iron reduction and sulfide oxidation may be thermodynamically favored due to low pH values in pore waters (below 3.8) and the presence of reactive iron. Although schwertmannite is the main ferric mineral precipitating in the water column, mineralogical analyses reveal a low abundance of schwertmannite in the sediment. This may be due to the preferential use of this mineral in reduction reactions mediated by bacteria, together with a low replenishment rate of the schwertmannite pool in the sediment. The transformation of reactive iron (schwertmannite and jarosite) into goethite may limit sulfate reduction, promoting acidic conditions in the sediment. As long as the acid mine drainage continues to discharge into the lake, the pH buffering exerted by ferric minerals in the sediments will limit the neutralization of the pH by sulfate reduction.

The Tinto River estuary is one of the most contaminated coastal systems in the world due to the high amounts of pollutants this acidic river transports. The tidal influence and the fluvial discharges control the water mixing, which include a salt-induced process (that is typical of all estuaries) and an acid neutralization process (pH-induced mixing). These processes affect the geochemical behavior of uranium, which has a non-conservative pattern. Despite the high concentrations of dissolved uranium in the Tinto River, 234U/238U shows that uranium stems mostly from seawater (between 80 and 100%). Riverine uranium is adsorbed onto both Fe and Al precipitates when the acidic river water is mixed with seawater. Distribution coefficients (Kd) show that dissolved and particulate uranium are controlled by adsorption–desorption processes and the formation of carbonate complexes, both depending on pH. At low pH, uranium tends to be dissolved, and the adsorption by suspended particles is low. As the pH increases, the adsorption processes onto Fe and Al particles are more intense, increasing particulate uranium which reaches a maximum at pH 5.5, where the uranium solubility minimum occurs. At higher pH values, dissolved uranium increases by carbonate complexation.