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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 s
1 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 kg
1 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 h
1). The average annual maximum ground mining may be considered to behave as a pseudokarstic and minimum temperatures in the studied area are 17C and aquifer, in contrast to conventional porous media flow (char- 8C, respectively, with an annual average of 13C over the last ff acterized by slower moving di use flow through the intergranu- 20 years. The potential evapotranspiration calculated by the lar pore spaces and/or fractures in the rock). Groundwater flow Thorntwaite expression is 691 mm a
1, and given that the in pseudokarstic aquifers is essentially characterized by multiple average annual precipitation is 966 mm a
1, the average yearly flow paths, extreme ranges of hydraulic conductivity, and a high effective precipitation in the area is 275 mm a
1. Finally, the degree of unpredictability. maximum flood flow to be calculated is 1144 l s
1. The weathering of As-bearing minerals in the spoil heap results in the transport of dissolved As into surface waters and underlying aquifers. Surface water flowing through the valley of Geology the San Tirso River is polluted by contact with leachable The study area is located in the northwestern margin of the materials in the spoil heap. Asturias Central Coal Basin, in a zone of intense tectonic Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018

232 J. Loredo et al. deformation including the La Peña and La Carrera faults of Wastes impoundment NW–SE trends (Fig. 1). It is included within the stratigraphical Surface waste piles resulting from mining and metallurgical unit known as La ‘Justa – Aramil’ (Luque 1985; Luque et al. operations were placed in the drainage basin of the San Tirso 1991), which consists of a thick series of shales with inter- River downstream of the shaft and the metallurgical plant. The calation of sandstones, conglomerates and limestones of Upper spoil heap extends over c. 20 000 m2, and the volume of wastes Carboniferous (Westphalian) age, and coal beds, which usually stocked is estimated at 150 000 m3, reaching in some areas up to have thicknesses of <50 cm. These carboniferous sediments are 10 m in thickness (Baldo 2000). Different sectors can be overlain discordantly by Upper Stephanian – Lower Permian distinguished in the spoil heap, corresponding to different types materials of the San Tirso formation, comprising calcareous of waste materials (waste rocks, low grade ore, and pyrometal- ff conglomerates, clay lutites, and tu aceous materials (from lurgical plant wastes). bottom to top in the series). Variations in physico-chemical parameters at the disposal site alter the stability of sulphides which are undergoing a vigorous weathering process, leading to the mobilization of ecotoxic Mineralization elements. This process is particularly enhanced due to the The Hg mineralization is distributed in six conglomeratic bodies humid climate and geomorphologic characteristics of the site, constituted by clasts of siliceous nature, located in carbonifer- which lies in a deep valley. Part of the adjacent land is expected ous sediments and associated to an intensely deformed zone to be contaminated, either directly as a result of the washing (Luque 1992). These sediments constitute a complex anticline effects of rainfall and subsequent chemical and mechanical structure of N–S trend, faulted at the hinge (Luque et al. 1991). dispersion, or indirectly through airborne dispersion. The The mineralized bodies show lenticular morphology corre- relative chemical stability of As and other potentially harmful sponding to fluvial deposits within the carboniferous series. elements such as Hg, Pb and Sb determines the extent of their Mercury generally appears in the form of cinnabar (HgS), anomalies found in soils. In the case of As, anomalously high though metacinnabar and native Hg are also occasionally found concentrations can be detected at a distance of 2 km down- (Luque 1985). Cinnabar is irregularly distributed both in veinlets stream from the mining site. Moreover, As concentrations inside the conglomeratic bodies and scattered inside the matrix higher than the local geochemical background have been of the conglomerate; it also appears in contact zones between detected in urban soils and dust particles in the town of Mieres, Carboniferous (sandstones and sandy lutites) and Permian its centre being located 3 km away from the spoil heap (Loredo materials (tuffaceous rocks) (Luque 1985). Other metallic min- 2000). erals which are present in the paragenesis of the ore deposit are pyrite (FeS ), melnikovite, sphalerite (ZnS), marcasite (FeS ), 2 2 MATERIALS AND METHODS pyrrhotite (FeS), chalcopyrite (CuFeS ), galena (PbS), stibnite 2 A multi-element geochemical study has been undertaken in (Sb2S3), realgar (AsS) and exceptionally native gold (García- Iglesias & Sancho 1974). waste materials from the spoil heap, adjacent soils, plants, The As is present in the ore deposit as realgar and As-rich stream sediments and surface and underground waters in the pyrite, which is very abundant in this deposit, as it was found by area of the old mining and metallurgical works. microprobe analysis. However, arsenopyrite (FeAsS) is rare in the mineralization. Smithsonite, hemimorphite, Cerussite, Waste materials goethite, malachite, jarosite, melanterite and gypsum are present Measurement of pH was made in-situ using a HANNA portable as secondary minerals formed by alteration of the ore before soil pH-meter. Samples of mining wastes were studied minera- mining. The gangue constituents are quartz, carbonates (anker- logically for gangue and ore. Host rock samples included in ite, calcite and dolomite) and clay minerals (kaolinite and mining wastes were studied by thin sections and standard dickite). The mineralization is of epigenetic origin, being petrographic techniques. derived from the circulation of low temperature hydrothermal Twelve representative samples of the different sectors of the solutions (Loredo et al. 1988) which circulated through the spoil heap were collected at a depth of 50 cm by manual auger fractures corresponding to the continental rifting during the drilling. Some trenches were dug to study the characteristics of Permian period (Martínez García 1981). waste materials at depth. In order to determine the vertical contaminant distribution, prospecting holes were drilled to a maximum depth of 6.5 m. In the laboratory, waste samples Mining were slowly dried at temperatures of 40C for 48 h. After Mining activity in the area goes back to the presence of the drying, samples were crushed and homogenized. They were Romans at the Iberian Peninsula, in the first and second sieved at 147 µm and quartering was carried out using an centuries, where their activities were restricted to mining the aluminium rifler. Representative subsamples of c. 1 g were sent upper mineralized levels. The modern extraction of Hg at the to a laboratory (ACME Analytical Laboratories, Vancouver, La Peña- El Terronal site had been made by drift mining and by Canada), where multielemental analysis of the soil pulp resulting means of an underground mine with exploitation in 12 levels. from an acid digestion of 2–2–2 HCl–HNO3–H2O for 1 h at The late 1960 s and early 1970 s was a time of great production. 95C was performed by Inductively Coupled Plasma–Emission During this time, production in excess of 500 flasks/month was Spectrometry (ICP-ES), and Cold Vapour Atomic Absorption achieved at the La Peña – El Terronal deposit, constituting the Spectrometry (CV-AAS) in the case of Hg. Element concen- second most productive Hg mine in Spain, and the eighth most trations in the hot aqua regia leachates are not total concen- productive Hg mine in the world. The ore was treated on-site trations, since not all minerals are decomposed during the at temperatures of c. 580C in roasting furnaces, where the digestion (Fletcher 1981). For the purposes of the study, sulphide ore was oxidized to produce SO2 and Hg vapour. The however, the results obtained for As and other metals in the fumes were passed through a system of condensers to precipi- aqua regia extraction are considered as total concentrations, due tate the Hg. With the important decline of the Hg price, the to the fact that hot aqua regia totally decomposes sulphides, mine eventually ceased working in 1974. which are the major primary sources of these elements, as they Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018

Arsenic mobilization from mine waste piles 233

Table 1. Summary of analytical results for spoil heap material, soils, sediment samples and groundwater samples collected in monitoring wells drilled on the spoil heap (all data for solids in mg kg
1 and in mg l
1 for water)

Element As Cu Hg Pb Sb Zn SPOIL (n=12) Min. 22 2.0 1.0 7.0 1.5 4.0 Max. 72,153 62 1,555 368 777 92 Mean 15,967 28 279 72 118 30 St. Dev. 26,898 17 555 111 247 29 SOIL (n=76) Min. 5.3 5.0 1.7 8.0 <2 12.0 Max. 9,116 151 1,585 187 27 472 Mean 708 24.7 114 49.8 3.1 110 St. Dev. 1,453 19.2 238 38.5 5.0 104 SEDIMENTS (n=3) Min. 652 22 12.5 23 0.45 74 Max. 806 38 25.5 27 0.75 90 Mean 729 30 19 25 0.6 82 St. Dev. 77.0 8.0 6.5 2.0 0.2 8.0 GROUNDWATER (n=5) Min. 0.02 0.0018 0.0003 <0.002 0.0003 0.021 Max. 3,190 0.136 0.0093 0.049 0.0196 0.589 Mean 640 0.343 0.0027 0.0064 0.0069 0.155 St. Dev. 1,430 0.573 0.0038 0.0083 0.009 0.245 n=No. samples. are the primary sources from which As-rich secondary minerals a hydrological cycle, upstream and downstream of the mining- are formed. Quality control was achieved by routine analysis of metallurgical works and spoil heap, with the aim to quantify duplicates and the use of certified reference standards. changes in As contents according to climate conditions. Groundwater was sampled in private wells close to the spoil Soils heap (which extract water from permeable carboniferous sedi- ff ments), monitoring wells drilled in the spoil heap and in the The study of soils a ected by mining and metallurgical works underground mine. The water samples were filtered and two covered an area of 360 000 m2 (a sampling grid of 144 squared  drops of ultra-pure HNO3 added for preservation; they were units of 50 50 m was used). Soil samples (76 in total) were placed in plastic bottles and stored refrigerated until analysed by taken at the nodes of the grid and sent for multielement analysis ICP-ES and CV-AAS. using laboratory procedures analogous to those used for waste materials. The soil horizons were identified by their character- istic appearance, and soil samples were collected from c. 20 cm RESULTS AND DISCUSSION depth. Sampling was carried out by hand using a shovel as close as possible to the points defined by the grid. pH and organic Spoil heap materials matter content determinations were also carried out. The microscopic study of wastes samples from the spoil heap shows abundant Fe sulphides (pyrite, marcasite and pyrrhotite) Herbaceous plants which are in general in an advanced stage of oxidation with Over an area of 6001000 m, different types of plants were hematite and Fe hydroxides being formed. Some cinnabar was sampled, including grass, fruit trees, eatable crops and thickets. also observed. Electron microprobe studies of these samples Samples were taken at a variable distance from the old mining confirm the high content of As in pyrites and pyrrhotites, and metallurgical works in the spring. For some types of plants, reaching in some cases concentrations up to 1.25%. The it was possible to collect bark material as well as foliage. Both presence of Fe sulphoarsenides has also been detected. The materials were carefully washed in ultrasonic baths and dried, presence of Au reaching values up to 0.321% confirm geo- then milled and analysed separately. Samples were digested chemical data reported by García-Iglesias & Sancho (1974). analogously to soil samples and a total of 32 elements have been Measurements of pH in waste materials on the El Terronal quantified using ICP-ES and CV-AAS. spoil heap show values quite different depending on the sector of the spoil heap (and, consequently, type of wastes), ranging Stream sediments from 3.1 to 8.6 units. Total As concentrations encountered in 12 samples regularly distributed along the spoil heap and Three samples were collected downstream the mining area,
covering all sectors range from 22 to 72 153 mg kg 1, with from the upper 10 cm of the sediments at the low energy
an average concentration of 15,967 mg kg 1. The leachable environment of San Tirso River, which flows through the metals, Pb and Zn, are present at average concentrations of 72 bottom of the valley where the old mine, smelter and the
and 31 mg kg 1, respectively; the average concentration of Sb spoil heap are located. Sample preparation and analysis was
is 118 mg kg 1. The correlation coefficient between pH and As analogous to that for the soils. concentration is negative, whereas for example, in the case of pH and Mn content, it is 0.81. Table 1summarises the concen- Surface and groundwaters trations determined for some of the elements present in Measurements of pH and conductivity were made in-situ. representative samples of spoil heap wastes, soils and ground- Surface waters of San Tirso river were sampled monthly during water. Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018

234 J. Loredo et al.

Fig. 3. Clustering of variables determined in soils (Linkage method: Ward).

Soils natural high affinity for As, particularly As(V), it could be Soils adjacent to the spoil heap and the mining/metallurgical expected that these minerals in soils or in spoil heap wastes installations show pH values between 3.4 and 5.9 units (average retain As when it is leached from the ore minerals stocked in the pH of 4.4 units). The organic components of soil have a high tailing piles. The soil survey data were used to plot approximate affinity for heavy metal cations, because of the presence of contours of elemental concentrations in surface soils over the ligands that form chelates with the metals, limiting their area. Arsenic distribution in soils at the site of abandoned mobility. Values ranging from 2.3% to 13.6% of total organic mining and metallurgical works is represented in Figure 2; matter were obtained. The portion of soil particles above 70 µm geochemical anomalies ascending up the slope of the valley are ranges between 76.0% and 93.2%. The analysis of the clay clearly associated with the spoil heap and the chimney. Other fraction by XRD shows the presence of illite, kaolinite and geochemical anomalies were found at the south of the soil illite-smectite. sampling grid. These might be associated with other Hg mine Local geochemical background values can be used to evaluate workings of small size in the area. This includes both the the content and distribution of metal and metalloid concen- presence of small spoil heaps and the dispersion caused by the trations found in the area. Total As concentrations in soils at ore transport to the metallurgical plant at El Terronal. Total Hg
1 concentrations in soils (Loredo et al. 1999) reflect similar sites away from the mined area range from 6 to 20 mg kg .
1 This value is similar to mean values of As for uncontaminated anomalies with values reaching up to 1585 mg kg . Spatial world soils of 5–10 mg kg
1 (Alloway 1995; Vinogradov 1959; distribution of As in soils is well correlated with Hg distribution, Bowen 1979). In comparison, chemical analysis of samples reflecting a high significant positive correlation between these from the mined areas and the spoil heap environment show As elements (r=0.88). The application of multivariate statistical concentrations ranging between 5.3 and 9,116 mg kg
1. The techniques to geochemical data gives a four-factor model, mean values for areas near analogous mineral deposits in the where the elements As, Hg and Al are grouped together in one world range from 400 to 900 mg kg
1 (Adriano 1986; Jialin & factor, probably related to the association of the first two Thornton 1985). Some Spanish regions (not in the case of elements to the clay fraction of the soil. Elements such as Co, Asturias) have established their own soil standards, but national Fe, Mn and Ni vary together, whereas heavy metals such as Ba, regulation in terms of limits for As and other harmful elements Cu, Pb and Zn appear clustered in another group. Finally, in soils has not yet been established, although it is currently elements related to the natural, high carbonated soil fraction being developed, based on risk considerations. However, it can (Ca, Mg and Sr) are also grouped together in another factor. be noted that most of the soil samples analysed in this study These groups of variables (elements) are shown in the dendro- widely surpass any known standards for As, such as the existing gram obtained by cluster analysis (Ward linkage method) of Spanish or Dutch guidelines (whose intervention level for a Figure 3. standard soil is set at 55 mg kg
1). The high As concentrations in soils are clearly associated Sediments with waste piles and chimneys used for the pyrometallurgy Total As concentrations in samples of stream sediments col- treatment of the ore. Since the Fe and Mn hydroxides have a lected on the San Tirso river, downstream of the spoil heap, Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018

Arsenic mobilization from mine waste piles 235

Table 2. Summary of chemical data from surface water monitoring. The sampling points – immediately upstream and downstream from the spoil heap – correspond to the same base stations

pH Total As (mg l
1) Sulphates (mg l
1) Upstr. Downstr. Upstr. Downstr. Upstr. Downstr. July 98 7.98 7.93 <0.005 5.5 470 410 August 98 7.95 8.02 <0.005 5.6 460 440 September 98 8.01 7.99 <0.005 4.1 430 410 November 98 8.03 8.00 <0.005 0.9 140 130 December 98 8.07 8.05 <0.005 1.9 170 160 January 99 8.16 7.95 <0.005 2.6 400 410 March 99 8.03 8.02 0.006 7.9 280 300 April 99 8.16 8.09 <0.005 3.1 420 360 May 99 8.00 8.02 <0.005 2.5 n/a n/a September 99 7.40 7.30 <0.005 2.5 n/a n/a October 99 7.50 7.30 0.001 1.7 n/a n/a November 99 n/a 7.30 n/a 2.4 n/a n/a December 99 7.45 8.20 0.003 3.4 n/a n/a n/a: not available. were high (729 mg kg
1, average value), whereas total Fe spoil heap shows As concentrations of 2.7 mg l
1 and a concentration was 3.01% (average value). These results indicate sulphate content of 380 mg l
1. Mercury concentrations up- that As was transported by suspended solids from the waste stream and downstream of the spoil heap, are always piles and in association with precipitated Fe oxides. Total <0.001 mg l
1 (detection limit), due to its low solubility. concentrations of heavy metals in bulk sediments have typical Variations in As content downstream of the mine workings values in agreement with those reported for other uncontami- are associated with seasonal changes and rain periods, having nated sediments (Forstner & Wittmann 1983; Salomons & values from 4.1 to 5.6 mg l
1 in dry months (July, August, Forstner 1984). September) and values from 0.9 to 1.1 mg l
1 in wet months (November, December, January) (Fig. 4, Table 2). These data suggest that As-enriched waters are related to leaching of Water quality As-rich minerals stocked in the tailing piles. These waters are Some chemical analyses of surface water collected both up- discharged from the tributary to the Caudal River, which is an stream and downstream of the site are given in the Table 2, important river in the region where salmon fishing is permitted. where it can be observed that As concentration ranges from The analysis of water in the Caudal River 20 m downstream of values <0.005 mg l
1 upstream of the spoil heap to values the confluence with San Tirso River shows an As content of ranging from 0.9 to 7.9 mg l
1 in samples collected down- 1.6 mg l
1; this content decreases to about 0.01 mg l
1 at a stream. The estimated mean of total As in stream waters at a distance of 150 m from that confluence. world scale is 0.004 mg l
1 (Reimann & Caritat 1998). The The analysis of water of boreholes at the site shows As analysis of water in San Tirso River 300 m downstream of the contents up to 3190 mg l
1 (Table 1). Analysis of mine water

Fig. 4. Relation between rainfall and As concentration in samples of San Tirso river immediately downstream of the spoil heap. Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018

236 J. Loredo et al.

Table 3. Summary of results and biological absorption coefficient (BAC) obtained from the analysis of some types of ground cover, vegetables and trees specimens

Arsenic (mg kg
1) Mercury (mg kg
1) n Min. Max. Mean BAC Min. Max. Mean BAC Grass 10 <2 53 16 0.053 0.2 4.8 1.6 0.048 Heather 7 <2 10 4.7 0.010 1.0 18 7.1 0.183 Bracken 4 3.0 8.0 5.5 0.008 4.0 0.5 1.4 0.014 Potato 3 3.0 3.0 3.0 n/a 0.5 0.6 0.5 n/a Cabbage 4 <2 4.0 3.0 n/a 0.3 0.3 0.3 n/a Eucalyptus 5 <2 13 3.2 0.013 0.2 1.2 0.6 0.012 Pear tree 4 <2 4.0 3.0 n/a 0.3 0.8 0.6 n/a n=No. samples n/a: not available.

flowing intermittently through the portal located between contents found in surface and groundwaters downstream of the Union and Peña shafts show an average pH of 3, conductivity mine and the spoil heap. The As and Hg anomalies in soils of 4 mS cm
1, 1.6 mg l
1 As, 0.049 mg l
1 Pb, 0.0005 mg l
1 adjacent to mining and pyrometallurgic installations are clearly Sb and 0.11 mg l
1 Zn. Groundwaters in private wells up- associated with tailings disposal and to condensation of vapours stream of the spoil heap, which are used for irrigation and as from the metallurgical plant. Arsenic and Hg concentrations in drinking supply for animals, show As contents ranging from soils decrease analogously as a function of distance from these 0.001 to 0.015 mg l
1. sources. In the absence of a Spanish regulation in terms of Canadian and United States drinking water limits for total As limits for As in soils, it can be noted that most of the soil are set at 0.025 mg l
1 and 0.01 mg l
1, respectively (Health samples at this site widely surpass any known standards for As, Canada 1996; USEPA 1998). These low regulatory concen- such as the Dutch ones. Herbaceous plants (grass) sampled in trations reflect the toxicity of As. The Spanish legislation different areas near old mining works show high As and Hg concerning mining/industrial effluents limits the total As con- contents. tent to 0.5 mg l
1, and 0.05 mg l
1 for surface waters destined Results of this study indicate that As in mining wastes is for the production of drinking water (BOE 1985). being transported, gradually contaminating the San Tirso River and finally the Caudal River, one of the most important fluvial courses of the region. Hydrogeological and hydrogeo- Impact on biota chemical studies suggest that groundwater flow at the site of The biogeochemical study of plants in the area of the spoil heap the underground mine is controlled by the dense network of and adjacent soils found that pasture (common grass) has high
galleries and that the acid drainage produced by circulation As contents ranging up to 53 mg kg 1 (average value of of water through the mine flows towards the Caudal River, 16.3 mg kg
1); the average concentration of Hg is 1.6 mg kg
1
where it is subjected to dilution. High As levels are observed with a maximum value of 4.84 mg kg 1. Some tree samples in surface waters downstream of the main mining works. have elevated contents in As and Hg, which were higher These values are especially high immediately downstream of in foliage than in bark material. Other crop species analysed, the spoil heap where they occasionally reach up to 7.9 mg l
1 such as potatoes, show concentrations of As in the order of
As in summer periods. The total flow of predicted polluted 3mgkg 1, especially near the tailing piles and other mine water from its circulation through the spoil heap is estimated workings. In addition to Hg and As, other elements with at 3465 m3 a
1; fortunately, the flow of San Tirso River contents above background levels in some parts of the studied represents only a small portion of the average flow of the area are Cu, Pb and Zn. Caudal river. Nevertheless, leaching of As-rich materials In order to investigate the influence of trace element levels in stocked in the spoil heap contributes an important As load to soils on uptake by plants, the biological absorption coefficient the Caudal river. (BAC) was used. The BAC is the ratio of trace element Thus, there is clearly an environmental impact in the valley of concentrations in plants to those in soils (Kabata-Pendias & San Tirso River, reflected by elevated concentrations of As in Pendias 1992). The BACs of As in plant species decrease in the soils and in surface and groundwaters. This project shows the following order: grass, eucalyptus, heather and bracken; for Hg need for a long term study and an environmental risk assess- the BACs decrease as follows: heather, grass, bracken and ment in the area. This need is critical as a highway is planned to eucalyptus (Table 3). This indicates that pasture (common cross the spoil heap in the near future, so waste stabilization and grass) accumulates As from soil to a greater degree than other remedial action must be undertaken accordingly. plant species in the area under study. According to these results, regular consumption of plants by the local population might The authors wish to thank Gwendy Hall, H. E. Jamieson and the become a health problem from long-term exposure in areas of anonymous reviewers for constructive comments on the manuscript old Hg mining works, as well as being a potential problem for which improved this paper. animals grazing in the area. In addition to the food chain, the direct oral intake of soil particles can play an important role, if REFERENCES dust from polluted soil is airborne and inhaled (Schöndorf et al. ADRIANO, D.C. 1986. Trace elements in the terrestrial environment. Springer-Verlag, 1999). New York. ALLOWAY, B.J. 1995. Heavy metals in soils 2nd edn. Chapman and Hall, New York. CONCLUSIONS ARAMBURU,Y.&ZULOAGA, F. De. 1899. Monografía de Asturias. Estableci- Mineralogical and geochemical data from waste materials miento Tipográfico de Adolfo Brid, Oviedo, Spain. ASTOLFI, E., MACCAGNO, A., FERNÁNDEZ, J.C.G., VACCARA,R.&STIMOLA,R. stocked in the spoil heap suggest that the weathering of As-rich 1981. Relation between arsenic in drinking water and skin cancer. Biological Fe sulphides (pyrite, marcasite) contribute to the high As Trace Element Research, 3, 133–143. Downloaded from http://geea.lyellcollection.org/ by Michael David Campbell on December 4, 2018

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