Hydrogeochemical Variability of the Acidic Springs in the Rio Tinto Headwaters

Abstract

Peña de Hierro, located in southwest Spain, encompasses the springs and headwaters for the Rio Tinto River that emerge above normal faults and has been mined for its rich sulfide ore since 2500 BC. The springs are typically characterized by an orange coloration, typical pH of ~2.33, and contain elevated concentrations of heavy metals that are produced by acid rock drainage (ARD). ARD is a natural phenomenon that results from chemolithoautotrophs metabolizing the sulfide ore. Mining has amplified the magnitude of the acidity and concentrations of heavy metals evidenced within sedimentary cores from the Huelva estuary. Acidity, redox state, hydrochemistry and isotopic analyses were examined for the purpose of characterizing the subsurface flows and determining the interconnectivity of the groundwaters. Previous studies have documented the geochemistry of the springs, dating a select few, yet many springs remain uncharacterized. Acidity presented spatial variability throughout the field area, caused by extensive sulfide interactions which generated and modified the pH. Redox exhibited a large range of values due to oxygen diffusivity though the fracture network. The surrounding geology is highly heterogeneous because of intensive deformation during the Variscan and Tertiary periods, and this heterogeneity is shown in the varied aqueous chemistry. Fractionation patterns observed in δ2H and δ18O values predominantly reflected enrichment by intensive evaporation and depletion in δ18O as a result of the proposed sulfatic-water model for Rio Tinto’s hydrogeology. The analysis illustrates minimal hydrologic interconnectivity, evidenced by the extensive physical and chemical contrasts within such a small proximity.

Coupled C, H, N, S and Fe biogeochemical cycles operating in the continental deep subsurface of the Iberian Pyrite Belt

“…In fact, we have identified the presence of genes coding for nitrate reduction in eight of the nine sequenced genomes isolated from the IPB subsurface. This result, together with the Fe 2+ oxidation capacity shown by some IPB isolates, strongly suggests the importance of this metabolic activity in the IPB subsurface, the origin of the high concentration of secondary Fe minerals and the presence of soluble Fe along the column as the source of the high concentration of iron detected in the Tinto basin (Allman et al, 2021;Fern andez-Remolar et al, 2018). Recently, the interaction of members of the Acidovorax genus with pyrite in the subsurface of the IPB and the correspondent Raman spectral change of this mineral associated with this interaction has been reported (Escudero et al, 2021).…”

“…It has been generally assumed that the extreme conditions found in the Río Tinto basin were the result of 5000 years of mining activity in the area (Alvarez & Nieto, 2015 ; Leblanc et al, 2000 ; van Geen et al, 1997 ). However, recent geophysical, hydrogeological, and other geological data suggest that the IPB subsurface acts as a huge underground reactor, in which sulfidic minerals are the main energy source and the metabolic reaction products drain to the river (Allman et al, 2021 ; Gómez‐Ortiz et al, 2014 ). Here, we report a detailed multidisciplinary analysis of the microbiology and microbially driven geochemical processes operating in the deep subsurface rock matrix of the IPB.…”

Inferred pyrite growth via the particle attachment pathway in the presence of trace metals

“…Prior to Na 2 S addition, trace metals (Co, Cu, Mo, Ni, Zn) were added to Set-1 and Set-2 experiments to obtain metal:Fe ratios of 1:10 5 and 1:10 2 , respectively, to determine their effects on pyrite formation. These ratios represent the broad range of environments (e.g., low temperature sediments, acid mine drainage, hydrothermal vents) in which natural pyrite can form (Von Damm et al, 1985;Shaw et al, 1990;Allman et al, 2021).…”

Settling of Iron and Aluminum Particles in Acid Solutions for Acid Drainage Remediation

“…Iron and aluminum precipitates are naturally present in high quantities in the sediments of riverine systems affected by acid drainage [15,16]. Ochre-colored sediments, characteristic of iron oxyhydrosulfates, and white precipitates, characteristic of aluminum oxyhydrosulfates, enriched in trace elements, form part of the fine sediment on stream beds of various sites affected by acid drainage [17][18][19][20][21][22][23]. The settling of these phases significantly reduces the pollutant flux in these water bodies [19,24], constituting a key process in these systems.…”