Arsenic Mobilization from Waste Piles of the El Terronal Mine, Asturias, Spain
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Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018 Arsenic mobilization from waste piles of the El Terronal mine, Asturias, Spain J. Loredo, A. Ordóñez, C. Baldo & J. García-Iglesias Departamento de Explotación y Prospección de Minas. Universidad de Oviedo, Escuela Técnica Superior de Ingenieros de Minas, c/ Independencia, 13. 33004 Oviedo, Asturias, Spain (e-mail: [email protected]) ABSTRACT: Arsenic is associated with Hg in the mercury deposits of the Asturias region of northern Spain. Until 1974, the mines of La Peña and El Terronal exploited an ore deposit located in the valley of the San Tirso River (Mieres district). At this site, a great volume of waste material from mining and metallurgical works have been stock-piled for years, constituting a spoil heap of about 20 000 m2 in area. Total As concentrations in representative samples of waste materials on the surface of the spoil heap reach a maximum value of 72 153 mg kg 1 and an average value of 15 967 mg kg 1; analysis of deeper samples (trenches on the spoil heap) exceed 99 999 mg kg 1. The total flow of polluted water from its circulation through the spoil heap is estimated at 3465 m3 a 1. Weathering processes facilitate the dissolution and leaching of As and heavy metals in the ore and waste material. Arsenic contents in surface waters (San Tirso River) immediately downstream of the spoil heap reach 7.9 mg l 1. The impact on the area, reflected by elevated concentrations of As in adjacent soils, biota and surface/underground waters, is discussed. KEYWORDS: Arsenic, mercury mining, spoil heaps, Asturias Interest in the study of As in the aquatic environment has the form of old industrial installations (shaft, mine buildings, increased in recent years. This is partly due to public health roasting furnaces, chimneys, etc.) and significant quantities of concerns of As contamination of drinking water supplies, but mining and metallurgical wastes stocked in piles on the surface, also to increasing concerns that As in drinking water may have without any preventive measures to impede the formation of long term deleterious health effects even at very low concen- leachates. trations. Cases of As contamination in groundwater leading to As a result of the mining works developed at this site, a great human As poisoning have been widely reported in the literature volume of wastes resulting from mining and metallurgical (e.g. Borgono & Greiber, 1971; Astolfi et al. 1981; Cebrian et al. activities has developed in the valley of the San Tirso River, a 1983; Lu 1990; Das et al. 1996; Southwick et al. 1983). Asturias tributary of Caudal River (one of the most important rivers in is a region located in northern Spain with abundant Hg deposits the region). San Tirso is a mountain river of variable flow and has been an important Hg producer on the global scale. depending on local rainfall; it flows through the spoil heap close Average annual production was c. 15 000 flasks; a flask of Hg to the mining and metallurgical installations. The instability of weighs 34.47 kg (SADEI 1968–1991). Arsenic in these deposits As-rich sulphide minerals existing in the mining and metallurgy is abundant as realgar (AsS) and As-rich pyrite. Transport of wastes, abandoned in a humid environment and affected by metals and metalloids from a mine site can occur by various surface waters, results in the production of As-rich leachates mechanisms, including mine drainage, runoff from waste piles, which are incorporated into the Caudal river. fugitive dust and deposition, and percolation into the ground- water system. As a consequence, dispersion of As by leaching from mining waste materials stock-piled in spoil heaps may OVERVIEW OF THE BEHAVIOUR OF ARSENIC IN pose an environmental risk for humans and ecosystems. WASTES The most important Asturian Hg mines are La Peña and El Arsenic mobility in soil-waste systems is determined by the Terronal (Mieres). Both mines exploited the same ore deposit. mineralogical form of As in the mine wastes, the type of soil, Although Hg extraction in La Peña Mine is known to date back the concentration of As, the concentration and type of com- to the Roman period (centuries I and II B.C.), it was in the 19th plexing ligands, both organic and inorganic, the pH, and the and 20th centuries that Hg mining began to be an important and redox status. As organic constituents of the waste matrix prosperous industry (Dory 1894; Aramburu & Zuloaga 1899; degrade, or as pH or redox conditions change, either through Gutiérrez Claverol & Luque Cabal, 1993). There has not been natural processes of weathering or human manipulation, the any mining activity at this site since 1974 when both mines were potential mobility of this metalloid will change as soils con- closed as a consequence of international health concerns over ditions change (McLean & Bledsoe 1992). Hg (Gutiérrez Claverol & Luque Cabal, 1993). At this site, In most As-contaminated mine sites, As is present the legacy of the historical mining activities remains today in as arsenopyrite or arsenical pyrite. However, in this case, Geochemistry: Exploration, Environment, Analysis, Vol. 3 2003, pp. 229–237 1467-7873/03/$15.00 2003 AEG/Geological Society of London Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018 230 J. Loredo et al. Fig. 1. Schematic geological map of the study area. arsenopyrite is very rare in the exploited ore. In most of sites desorption of arsenate from Fe-oxide surfaces becomes favour- where metallurgical operations (roasting) have been taken place, able as pH values become alkaline (Fuller & Davis 1989; As exists as As O , or as anionic As species leached from Dzombak & Morel 1990). 2 3 ff As2O3, oxidized to As(V), and then sorbed onto Fe-bearing Arsenic a ects almost every major bodily function and minerals in the soil. Arsenate forms (e.g. scorodite) commonly metabolic pathways and their toxic effects have long been occur as alteration products in areas with As mineralization and, known (Azcue & Nriagu 1994; Mertz 1981). The toxicity of As given the right conditions, these arsenates readily co-precipitate varies depending upon its chemical state: in general, inorganic with other supergene minerals such as jarosite. Arsenic may be forms are more toxic than organic complexes, and soluble present also as a sulphide (arsenopyrite or As-rich pyrite) and in forms more than insoluble. Trivalent As, or arsenite com- organometallic forms (Boyle et al. 1998), as well as the volatile pounds have been reported to be more toxic and more mobile compounds, arsine and its methyl derivatives. These As forms than the corresponding pentavalent As, or arsenate forms. The illustrate the various oxidation states that As commonly exhibits arsenite form is c. 60 times more toxic than arsenate (Ferguson (III, III, and V) and the resulting complexity of its chemistry & Anderson 1974). in the environment (USEPA 1997). Arsenic (V) exhibits anionic behaviour in the presence of water, and hence its aqueous CHARACTERISTICS OF THE AREA concentration increases with increasing pH, and it does not complex or precipitate with other anions. Location One important factor controlling the mobility and the The main spoil heap is located in the valley of San Tirso River, concentration of trace elements in spoil heap materials and soils 2 km NE of Mieres downtown (Fig. 2). The village of Mieres is the pH, because it affects all adsorption mechanisms and the with 30 000 inhabitants lies 20 km SE of Oviedo, capital city of complexing of metals in the soil solution. Arsenic does not Asturias. Although the area where the spoil heap is located is necessarily follow the pattern of being most mobile at low pH, sparsely populated, there is a school very close to its southern because it behaves as an anion in solution. Adsorption and margin (c. 50 m). desorption reactions between arsenate and Fe-oxide surfaces are particularly important controlling reactions because Fe oxides are widespread in the geologic environment as coat- Hydrogeology ings on other solids, and because arsenate adsorbs strongly San Tirso River drains a catchment of 6 524 946 m2, and its to Fe-oxide surfaces in acidic and near-neutral pH water average flow ranges from 30 to 300 l s1 depending on (Dzombak & Morel 1990; Waychunas et al. 1993). However, the seasonal conditions (Baldo 2000). Considering the whole Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018 Arsenic mobilization from mine waste piles 231 Fig. 2. Contours of total As over the spoil heap and adjacent soils (grid in meters; contour interval: 700 mg kg1 As). surface of the spoil heap (20 000 m2) as permeable with an Climate ffi infiltration coe cient of 0.7 (spoil heap without vegetal cover), In contrast to most areas of Spain, Asturias has a humid climate the calculated flow of water infiltrating the spoil heap (accord- 3 1 characterized by abundant precipitation during most of the year. ing to rainfall and evapotranspiration data) is c. 3465 m a . Average annual rainfall is 966 mm, and the annual relative This is the water flow that has access to the waste materials in average humidity is about 78%. Maximum daily precipitation, the spoil heap. Underlying the spoil heap are alluvial sediments calculated by adjustment of precipitation data to Gumbel (1–2 m thick) covering shales and sandstones of carboniferous distribution, is 71 mm, 82 mm, 90 mm and 98 mm for periods age. of 10, 25, 50 and 100 years, respectively (the ‘one hundred-year The groundwater flow system in mining wastes and under- 1 hour rainfall’ is 40 mm h1).