The Impact of Acid Mine Drainage on the Water Quality of the Odiel River (Huelva, Spain): Evolution of Precipitate Mineralogy and Aqueous Geochemistry Along the Concepción-Tintillo Segment

The Odiel river emerges in Sierra de Aracena (N Huelva, Spain) as a clean, circumneutral river which shows abundant fish and fluvial microfauna. At 20km from the riverhead and along a 7km-long reach, this river receives four small discharges of acid mine water emanating from several abandoned mines of the Iberian Pyrite Belt (namely, Concepción, San Platón, Esperanza and La Poderosa-El Soldado). During two field studies performed in October 2003 and May 2004, it has been observed that these acidic waters (with flow rate of 0.2–8.5L/s and pH 2.3–2.8) transfer to the Odiel river significant amounts of acidity and dissolved metals (specially Fe, Al, Mn, Cu, Zn, Cd, Co and Ni) and sulphate. Despite this mine-related pollution, the pH of the river remains near-neutral (pH = 7–8, flow rate = 220–1,000L/s), as the alkalinity of the river (108–155mg/L CaCO₃ eq.) neutralizes the acidity and causes the precipitation of dissolved Fe and Al in the form of ochreous to whitish minerals (ferrihydrite, Al-oxyhydroxides). These poorly crystallized minerals retain, by sorption, large amounts of trace metals (specially Cu and Zn). Subsequently, the Odiel river converges with the acidic Tintillo river (pH = 2.5–2.8, flow rate = 48–240L/s), which drains a vast mining area occupied by large waste-rock piles and tailings impoundments around Corta Atalaya (Riotinto mines). At this confluence, all the alkalinity is totally consumed and the pH drastically decreases to around 3. The mineral paragenesis of the ochreous precipitates is then dominated by schwertmannite, which shows a very limited sorption capacity under such acidic conditions. Consequently, metal concentrations are sharply increased from near-zero to tens of mg/L (e.g., 18mg/L Fe, 76mg/L Al, 14mg/L Mn, 10mg/L Cu, and 20mg/L Zn in May 2004). The buffering capacity of the Fe(III) hydrolysis stabilizes the pH of the Odiel river around 3± 0.5 along the rest of its course to the Huelva estuary, and the water quality of the river is thus irreversibly damaged.