Pollution of soils by the toxic spill of a pyrite mine (Aznalcollar, Spain)
M. Simón, , I. Ortiz, I. García, E. Fernández,
J. Fernández, C. Dorronsoro and J. Aguilar
Departamento de Edafología y Química Agrícola,
Facultad de Ciencias, Universidad de Granada, C/Fuentenueva s/n, Campus Universidad,
18071 Granada, Spain
1. Introduction
2. Methods
3. Results and discussion
3.1. Characteristics of the tailings and the contaminated soils
3.2. Elemental contents of the tailings, water and contaminated
and uncontaminated soils
3.3. Pollution of soils by water and tailings from the toxic spill
3.4. Statistical study
3.5. Pollution of the soils 10 days after the spill and its evolution
over time
4. Conclusion
References
Pyrite mines have been worked for centuries in the province of Seville (southern
Spain), especially in Aznalcollar mining district, on the eastern edge of the
Iberian pyrite belt (Carvalho, 1976). The origin of this belt can be found in
the volcanic sediments produced by submarine volcanic intrusions, with alternating
acidic and basic episodes that emerged during the Hercinian period, from pre-existing
pre-Cambrian and Palaeozoic sedimentary materials, which precipitated finally
as metallic sulphides on the marine bed. The mineral phase, located mainly in
the upper part of the layer formed during the acidic episodes, consists of different
sulphides ( Almodovar et al., 1998): pyrite (83.1%), sphalerite (5.4%), galena
(2.1%), chalcopyrite (1.4%), arsenopyrite (0.9%) and non-productive materials
(7.1%).
In these mines, the processing of the ore consists of grinding, treating of
the particles with SO2(g) and slaked lime and finally separation by differential
floating of Cu, Pb and Zn at different pH values. The residues from this process
are composed mainly of pyrite with minor proportions of other sulphides. The
most abundant elements present in this residue are: Pb (0.8–1.1%), Zn
(0.5–0.8%) As (0.2–0.5%) and Cu (0.1–0.2%). These residues
are stored in very large walled ponds (approx. 1.4 km2). On 25 April 1998 the
walls of two contiguous ponds broke open, and approximately 36Å~105 m3
of polluted water (solution phase) and 9Å~105 m3 of toxic tailings (solid
phase) spilled into the Agrio and Guadiamar River basin (Fig.
1). The tailings spread some 40 km in a down-river direction, stopping at
Puente Don Simón. Meanwhile, the polluted water reached the Guadalquivir
River, affecting the National Park of Doñana (proclaimed by UNESCO in
1994 as part of World Heritage).
Nevertheless, the low dry-season flow of the Agrio and Guadiamar Rivers made the rapid construction of a retention dam possible at Lucio del Cangrejo, some 50 km downstream of the ponds, minimizing the damage of the toxic wastes in the wildlife reserve. The total surface area of the zone affected by the toxic spill was approximately 55 km2: 12.3 km2 of the eucalyptus plantation, 11.9 km2 cereals and sunflower, 9.9 km2 pasture, 5.4 km2 rice, 4.9 km2 marshland, 3.0 km2 fruit and olive trees, 2.9 km2 riverbeds, 2.2 km2 cotton, 0.8 km2 riparian vegetation, 0.8 km2 gravel bed, 0.5 km2 meadow and 0.4 km2 field crops.
Prior to the spill, mining activity at Aznalcollar had already acidified and increased the heavy-metal content of the waters of the Agrio and Guadiamar Rivers (Arambarri et al., 1984). The use of this water for irrigation has raised the heavy-metal content in the soils of the zone, and the contents in Cu and Zn have registered values higher ( Ramos et al., 1994) than those found in uncontaminated areas of the world ( Allaway; McGrath and Holmgren).
In the present study, the heavy metals and other associated elements in the
polluted water, toxic tailings and contaminated soil were evaluated in comparison
with uncontaminated soil in the area of the spill. The aim was to identify and
quantify the pollutants involved, to relate the pollution to soil characteristics,
and ultimately assess the extent of the damage caused to the soils of this area.
Seven sectors were studied along the basins of the Agrio and Guadiamar Rivers
(Fig. 1): near the mine (M), Soberbina
(SO), Puente de las Doblas (D), Aznalcazar (A), Quema (Q), Pescante (P) and
Los Pobres (LP). In each sector, two square plots were laid out (25 mÅ~25
m), one on the contaminated and the other on uncontaminated soil. At each corner
and in the centre of the plots, samples were taken of tailings (in contaminated
plots) as well as of soil at 0–10 cm and at 10–30 cm in depth (in
contaminated and uncontaminated plots). All samples, categorized according to
origin (tailings and the two soil depths), were air dried and screened (2 mm
screen size). Next, 250 g of each sample category from the five sampling points
per plot, were mixed and homogenized for laboratory analysis. In addition, isolated
pools of water from the toxic spill were also sampled and analysed. All samples
were taken on 4 and 5 May 1998.
The soils studied, contaminated and uncontaminated, were classified into two
categories according to the Soil Survey Staff (1997): Typic Xerofluvent (M,
SO, D and Q) and Typic Xerorthent ( A, P and LP). Field descriptions of soils
were based on procedures of the Soil Survey Staff (1951). To provide a quantitative
assessment of the soil structure, we formulated a structural-development index
(SDI) using the equation: SDI=SizeÅ~Grade, where values of the grade are
given in Table 1, and the size of
the structure take the following values: fine=10, medium=7, coarse=5, very coarse=3.
Particle-size distribution was determined by the pipette method after elimination
of organic matter with H2O2 and dispersion with sodium hexametaphosphate (Loveland
and Whalley, 1991). The pH was measured potentiometrically in a 1:2.5 soil–water
suspension. CaCO3 equivalent was determined by method of Bascomb (1961). Total
carbon was analysed by dry combustion with a LECO instrument. Organic carbon
was determined by the difference between total carbon and inorganic carbon from
CaCO3. Iron oxides (Fed) were extracted with citrate–dithionite (Holmgren,
1967) and measured by atomic absorption spectroscopy. In the field, tailing
samples were taken in hermetically sealed bags and their moisture content was
determined by weight difference after drying the samples at 110°C. A saturated
extract of the tailings was prepared ( US Salinity Laboratory Staff, 1954).
Samples of the tailings and soils, after being very finely ground (<0.05
mm), were digested in strong acids (HNO3+HF+HCI). In each digested sample, water
sample and saturated extract of the tailings, 24 elements were measured by ICP-MS
with a PE SCIEX ELAN-5000A spectrometer. For the statistical analysis, the StatView
4.02 program was used.
3.1. Characteristics of the tailings and the contaminated
soils
In the tailings the particle-size class (Table
1) was silty loam, the silt content surpassed 70%, and the structure was
platy. In addition, carbonate was absent, pH consistently acid, and the organic
content was extremely low.
The soils in general were relatively homogeneous with respect to certain properties.
All soils were neutral or slightly alkaline (pH between 7.2 and 8.1) and with
relatively narrow ranges of organic-carbon content (0.5–1.7%) and Fed
(0.8–1.5%). By contrast, there was a broader range of CaCO3 equivalent
(0–20%), gravel (0–43%) and particle size of the fine-earth fraction
(<2 mm): from clay loam (P and LP) to sandy loam (SO).
3.2. Elemental contents of the tailings, water and contaminated
and uncontaminated soils
Of the 24 elements analysed (Table 2),
the content in the tailings of Mn, V, Cr, Ni, Be, Y, Th and Sc was similar to
or lower than that of the uncontaminated soils. These elements have been excluded
as contaminants in this toxic spillage. Ba and Co in the tailings increased
two to fourfold with respect to uncontaminated soils, while Sn and Hg increased
eight to 12-fold; nevertheless, the content of the contaminated soils proved
very similar to that of the uncontaminated soils, and therefore could not be
clearly related to soil pollution. Mo and In in the tailings increased 25 to
35-fold with respect to the uncontaminated soils; Zn, Cu and Cd 50–90-fold;
and Tl, As, Bi, Pb, and Sb more than 90-fold (Sb 400-fold). Despite the magnitude
of their relative increases, Mo and In reached only minor median values in the
contaminated soils (0.4 and 0.1 mg kg-1, respectively); similarly, Se, absent
altogether from uncontaminated soils, was present in only one of the seven sectors
studied (P), with a low content (3.4 mg kg-1). In short, these results indicate
that the principal pollutants, in descending order of median concentrations
in the contaminated soils, were: Zn, Pb, Cu, As, Sb, Cd, Bi and Tl. The standard
deviation of these pollutants (Table
3) increased considerably in the contaminated soils with respect to uncontaminated
soils, often surpassing the mean value. This greater dispersion of the datapoints
indicates that the contamination had highly irregular effects on the soils of
the different sectors.
The elemental contents of the polluted water (Table 4) indicated their potential toxicity. Although the original quantitative chemical composition may have changed by the time of sampling (10 days after the spill), the median concentration in Mn, Ni, Co, Cd and Cu clearly exceeded the maximum allowed in water to be used for irrigation (Crook and Bastian, 1992). By contrast, other elements such as Cr, Mo, Sn and Bi were not detected. The standard deviation of the different elements was relatively low, indicating a certain homogeneity in the chemical composition of this polluted water.
3.3. Pollution of soils by water and tailings from the toxic spill
Because the water from the toxic spill contained no Bi, the total Bi contamination
of the soils must have come from the tailings. Thus, the quantity of tailings
that penetrated the soil in each sector (Z) can be calculated by the equation:
where TBi is the Bi concentration in the tailings and CSBi and UCSBi are the
Bi concentration in the contaminated and uncontaminated soils, respectively,
all expressed in mg kg-1.
Eq. (1) presupposes that in each sector the Bi concentration of the uncontaminated soil was the same as the contaminated soil before the spill, an assumption that may introduce an error. In any case, the results ( Table 5) indicate that the penetration of the tailings into the soil was highly irregular, varying considerably from one sector to another according to the characteristics of each soil, especially structure. In general, the penetration of the tailings diminished with soil depth, as structure size increased and grade decreased. The only exception was the sector Puente de las Doblas (D), where the soil had a substantial gravel content which increased in depth (Table 1), allowing greater vertical percolation, as well as lateral infiltration of the spill from the bank of Guadiamar River where the layer richest in gravel reaches the surface. Of all sectors, the greatest penetration of the tailings occurred in the upper 10 cm of Pescante (P) as the result of cotton cultivation, in which ploughing of the original clay loam formed an artificial unaccommodated, strong (grade 3), fine angular blocky structure (0.5–2.0 cm diameter), opening a great number of interpedal voids that allowed the tailings easy entry. Below 10 cm in depth, changed to accommodated, weak (grade 1), very coarse angular blocky structure (10–15 cm diameter), and thus the number interpedal voids diminished, thereby reducing penetration of the tailings. In the other sectors, not having been recently ploughed, less penetration resulted from coarser structure with weaker grade. The least penetration occurred in the sectors Soberbina (SO) and Aznalcazar (A); in the former the soil was structureless sandy loam and in the latter nearly structureless (angular blocks larger than 20 cm diameter) silty clay, with practically no gravel at either site.
From the quantity of tailings that penetrated the soil (Z, in g kg-1) and from the concentration of each element (i) in the tailings (Ti, in mg kg-1), estimates can be made of the quantity of each element (i) (Si) which entered the soil from the solid phase of the spill Eq. (2).
In addition, because the total pollution of the soils was caused both by water
and the tailings from the toxic spill, the quantity of each element (i) that
entered as part of the solution phase (Wi) can be estimated by the equation:
where (CSi-UCSi, in mg kg-1) is the total contamination of each soil for each
element (i).
The results (Table 6) indicate that
the range of total contamination for each element was very broad. The percentage
of this total belonging to the solid phase of the spill clearly varies from
one element to another. In the case of Cu, Zn and Cd, only 20% of the median
total contamination of the first 10 cm of the soil penetrated as part of the
solid phase of the spill, and thus the remaining 80% must have penetrated as
part of the solution phase of the spill. In any case, the range of these percentages
was extremely broad, as a consequence of the markedly different quantities of
tailings that penetrated each soil (Table
5). As and Sb entered the soil primarily through the solid phase (median
of 95%) and to a far lesser degree through the solution phase (5%). Meanwhile,
Pb and Tl registered intermediate values, penetration through the solid phase
reaching 75 and 85%, respectively. In addition, because the quantity of tailings
that penetrated each soil generally decreased with depth, the total contamination,
as well as the percentage of each element that entered the soil through the
solid phase, tended to diminish between 10 and 30 cm in depth.
Principal component analysis was performed with Si and Wi values, structural-development
index, and the analytical characteristics of the soils (Table
1). Three factors explain more than 80% of the variance ( Table
7).
Factor 1 represents soil pollution from the toxic spill, and thus includes the concentration of all of the elements that penetrated the soil in the solid phase, as well as the concentration of Sb, Tl and Pb that penetrated in the solution phase. The inclusion in this factor of SDI with high load shows structure to be the soil characteristic which best determined the penetration of the tailings, this penetration being greater the smaller the structure size and higher the grade of development. The negative sign of the pH indicates that the pollution tended to acidify the soil, although this trend is not strongly evident (low load) apparently due to the buffering effect of the CaCO3 in most of the soils.
Factors 2 and 3 are interpreted as heavy-metal sorption in the soil. Factor
2, which includes the contents in WZn, WCd, CaCO3, organic carbon and finest-particle
soil (silt+clay), all with relatively high loads, shows that the sorption of
Zn and Cd was caused by the formation of insoluble precipitates such as carbonates,
as well as organic complexation and cation exchange (Alloway, 1995). The increases
of the pH tended to favour these sorption processes, although the effect was
not pronounced (low load) probably because of the above-noted narrow range of
the pH values of the soils. Factor 3, including WAs and Fed with high loads
and WCu with low load, reveals that the specific adsorption of As and to a lesser
extent Cu was due primarily to the iron oxides (Alloway, 1995).
A correlation matrix was made with Wi and pH values as well as the CaCO3, clay,
silt, Fed and organic-carbon contents of the soils. The absolute values of those
correlation coefficients, equal or greater than 0.7 (Table
8), show that organic-carbon and CaCO3 content determined the accumulation
in the soil of Zn dissolved in the solution phase, and the CaCO3 content the
accumulation of Cd. The range of the different parameters should be taken into
account when considering these correlations. For example, the sorption of the
heavy metals by organic matter (Riffaldi and Levi-Minzi, 1975) and clays ( Farrah
and Pickering, 1977) increases at higher pH values; nevertheless, this effect
was not appreciable within the narrow pH interval (7.2–8.1) of the soils
studied.
3.5. Pollution of the soils 10 days after the spill and its evolution over time
The concentrations in the soil of the elements considered to be pollutants (Table
9) indicates that, in only one of the seven sectors studied (P), all elements
exceed the maximum permitted concentrations, for agricultural soils, set by
Canada (Sheppard et al., 1992), Belgium ( Stringer, 1990) and Holland (NMHPPE,
1991, in Alloway, 1995), as well as the ecotoxicological level set by Van Den
Berg et al. (1993). In D and LP, only Zn and As exceeded the aforementioned
maximum values allowed, and in A only Zn. In the other sectors, no element exceeded
these maximum values. Nevertheless, in the not-too-distant future, these latter
elements could exceed these maximums, given that, as the tailings dry and aerate,
a complex process oxidizes the sulphides to sulphates ( Nordstrom, 1982), lowers
of the pH ( Stumm and Morgan, 1981) and solubilizes part of the formerly insoluble
pollutants ( Rogowski and Caruccio).
This oxidation process and its rate could be appreciated by 4 May 1998. On this date, the tailings differed in moisture content as a consequence of their different thicknesses, the thinner areas drying more rapidly than the thicker ones. The concentrations of water soluble SO42-, Cd2+ and Pb2+ in the tailings on this date, measured in saturated extracts, increased logarithmically with declining moisture (Fig. 2). Given that between 25 April (date of the spill) and 4 May (date of sampling) no rain fell, these solubilized elements, remained in the solution phase of the tailings and, with evaporation, rose by capillary action to the surface, forming a white salty crust. It would be expected that subsequent rains would dissolve the soluble salts, which would then infiltrate the soil, raising the pollution level. This process, and its rapidity (in scarcely 10 days, the driest tailings multiplied their content in soluble Pb and Cd by 10-fold with respect to the wetless tailings), underscores the urgency of removing the tailings from the soil surfaces in these types of spills.
The present report identifies the pollutants, their concentrations in the soil
at the moment of the study, the primary mode by which each pollutant entered
the soil, and the rapid oxidation of the tailings that makes the soils susceptible
to a future increase in pollution. This indicates the necessity of monitoring
the concentrations using frequent sampling. A monitoring programme, with sampling
conducted every 20 days, is currently under way, and will be the subject of
future reports.
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