M. Simón

Pollution of soils by the toxic spill of a pyrite mine (Aznalcollar, Spain)

On 25 April 1998 the retention walls broke open in a pond containing the residues from a pyrite mine of Aznalcollar (southern Spain), spilling some 45 x 10(5) m3 of polluted water and toxic tailings into the Agrio and Guadiamar River basin, affecting some 55 km2. On 5 May, seven sectors in the affected area were studied, analysing tailings, polluted water, and contaminated as well as uncontaminated soils. The principal pollutants were: Zn, Pb, Cu, As, Sb, Bi, Cd and Tl. The range of total contamination of each element was extremely broad, as penetration of the tailings depended on the soil characteristics. Most of the Cu, Zn and Cd penetrated the soil in the solution phase of the spill, while the other elements penetrated mostly as part of the solid phase. Zn exceeded the maximum concentrations permitted by the international community in four of the seven sectors studied, As in three, and the other elements only in one sector. Drying and consequent aeration of the tailings rapidly oxidized sulphides to sulphates, lowered the pH and solubilized the pollutants. Therefore, future rains could aggravate the pollution problem, if the tailings are not quickly removed.

Effectiveness of amendments on the spread and phytotoxicity of contaminants in metal–arsenic polluted soil

A metal-arsenic polluted soil from sulphide-mine waste was treated, in all possible combinations, with two different amounts of marble sludge (98% CaCO3), compost (41% organic carbon), and Byferrox (70% Fe). Lixiviate and pore water from each treated and untreated soil were analysed, and lettuce-seed bioassays were performed. None of the treatments decreased the electrical conductivity of lixiviates or the concentrations of all pollutants found in both solutions. Marble sludge and compost increased the pH values and decreased the zinc, cadmium, copper, and lead concentrations in both solutions while increasing the arsenic concentrations in the lixiviates. Byferrox did not alter the physicochemical parameters or the concentrations of zinc, cadmium, copper, or lead in either solution but significantly decreased the arsenic concentrations in pore water. Compared with the Byferrox treatment, the mixture of marble sludge and Byferrox decreased redox potential values, increasing the arsenic concentrations in both solutions and the electrical conductivity of the pore water. All lixiviates were highly phytotoxic and seeds did not germinate. Pore-water phytotoxicity was related to electrical conductivity values and heavy-metal concentrations. The combination of marble sludge and compost was most effective at diminishing toxicity in lettuce. The soils treated with Byferrox, alone or mixed with marble sludge or compost, were the most phytotoxic.

Use of liming in the remediation of soils polluted by sulphide oxidation: A leaching-column study

Pure CaCO(3) in ascending quantities was added to a soil to study the effect of liming after contamination by an acidic solution from the oxidation of pyrite tailings. The samples were placed in percolation columns, and soils and leachates were monitored. In the soil samples, the mineralogy, pH, CaCO(3), iron, and total content in Cu, Zn, As, Cd, and Pb were determined. The presence of CaCO(3) in the soils considerably limited their acidification, favouring the precipitation of Cu, Zn, and Cd, and promoting precipitation of iron and SO(4)(2-) ions in the form of iron hydroxysulphates and gypsum. The iron hydroxysulphates tended to retain the less mobile elements (As, and Pb) near the top of the soil. The more mobile elements (Zn, Cd and Cu) precipitated in deeper layers, directly related mainly to the CaCO(3) added and to pH. The CaCO(3) clearly did not reduce Zn and Cd toxicity effectively enough, given that the concentrations of both elements were above the toxic level in all leachates. The amounts of liming needs to be properly controlled, as excessively high pH limits As fixing, at the same time as the effectiveness of CaCO(3) is limited by coating precipitation, reducing its capacity to react with the acidic solution.

Soil evolution over the Quaternary period in a Mediterranean climate (SE Spain)

Palaeosols in the Granada Basin (SE Spain) have been studied in two different situations: surface soils on geomorphically stable surfaces since the Early Pleistocene with younger pedogenic overprinting and buried soils on unstable surfaces from the Middle–Late Pleistocene on which successive erosional–depositional episodes have alternated with pedogenic episodes. For each soil clay and iron accumulation indices, the Fet+Alt/Sit ratio, clay mineralogy and micromorphological features were used to estimate the degree of soil development. From the Early to the early Late Pleistocene, the main pedogenic processes were the leaching of carbonates, weathering, illuviation and rubification, which resulted in Bt horizons with red colours, clay texture, clay coatings and kaolinite neoformation. The degree of weathering and the development of these Bt horizons varied over time, and the soils that formed on the surfaces from the Early Pleistocene show strongest weathering and development. However, after their formation, there were periods in which they were partially truncated and recalcified, resulting in polygenetic soils. The different degrees of development of the buried soils during the last 474,000 years indicate that the wettest warm period was stage 7 and the driest, stage 5. Stages 9 and 11 must have had climates with intermediate wetness. Since the clay accumulation and iron oxide accumulation indices, the differences in Fet+Alt/Sit ratio between Bt and C horizons, the extent of kaolinite neoformation and the micromorphological features of the soils formed during stage 7 are all similar to the surface soils that formed on Early Pleistocene deposits, these features cannot be used to date surfaces older than 242,000 BP. By contrast, the soils that formed during stage 7 and later periods show different extents of development and thus can be used for the approximate dating of landforms. D 2002 Elsevier Science B.V. All rights reserved.

Ambient trace element background concentrations in soils and their use in risk assessment.

The definition of ambient background concentrations (ABCs) is used in this study to assess the potential environmental risk of trace elements in soils and parent materials from Granada, Spain. Two different layers of soil (0-20 and 20-40 cm) and parent material samples were collected at 93 sites. From cumulative frequency distribution curves, ABCs for As, Co, Cu, Cr, Ni, Pb and Zn were estimated at 3.5-20, 7-23, 13-25.6, 29-66, 7-20, 15-36, and 5.5-76 mg kg(-1), respectively. Tukey box-plots were used to discriminate different concentration classes and identify potentially contaminated sites. Weakly-weathered soils (Entisols) over carbonate materials showed the lowest background contents, the most developed soils (Alfisols) over metamorphic rocks the highest ones. Outliers were mainly found near a former iron mine where arsenic concentrations were by far exceeding the corresponding regional ABC. These soils were however, not toxic to Escherichia coli and Vibrio fischeri. The prediction of site-specific ABCs together with bioavailability and toxicity assessment is a valuable tool for giving further insight into the risk of trace elements in soils.

Thallium Behavior in Soils Polluted by Pyrite Tailings (Aznalcóllar, Spain)

Thallium content and chemical speciation was studied at 91 sites contaminated by water and tailings spilled from the settling pond of a pyrite mine into the Agrio and Guadiamar rivers in Aznalcóllar (Spain). The contamination was highly heterogeneous, with 15% of the affected area seriously contaminated, 55% moderately contaminated and 30% uncontaminated. The total Tl content in the surface horizon increased with respect to the background level, more than 4-fold in the uppermost 10 cm of the soils, and clearly decreased with depth without contaminating either the subsoil or groundwater. Most of the Tl (approximately 75%) was in non-extractable forms, either as a component of the particles in the tailings or adsorbed to crystalline oxides. The remaining Tl was held on, or occluded in, amorphous or poorly crystallized oxides. In acidic soils, the adsorption of Tl was dominated by iron oxides (Feo) and, in neutral-alkaline soils, by aluminium oxides (Alo). A relatively high amount of the Tl adsorbed by amorphous oxides in the uppermost 10 cm of the soils was extracted with acetic acid, and was presumably bio-available (mean values approximately 15% of the Tlo). The EDTA is a strong extractant of inorganic forms of aluminium and, consequently, the quantity of Tl extracted by EDTA in neutral-alkaline soil (mean values more than 10% of the total Tl) could be higher than the truly bio-available fraction. Approximately 1% of the total Tl was extracted with calcium chloride, but only in the neutral-alkaline soil was the extraction significantly related to the cation-exchange capacity and, thus, adsorbed by the negative charges of the clay and organic matter. The Tl soluble in water (< 0.1%) declined with the pH in the neutral-alkaline soils, and was unrelated to any soil property in the acid soils. Thus, the behavior of Tl is determined by climatic conditions, soils properties and time.

Physico-chemical properties of the soil-saturation extracts: estimation from electrical conductivity

With the aim of estimating certain physico-chemical properties of soil-saturation extracts from electrical conductivity, we prepared saturation extracts from 67 soil samples having different quantitative and qualitative salt contents. The solution of each saturation extract was vacuum . pumped, and used to determine the electrical conductivity EC , electrical conductivity calculated .EC , ionic composition and total soluble salts. The analyses for kinds and magnitudes of ion c pairing, for the ionic strength and for the calculation of activity coefficients of each solution were processed using a special computer program. From these simple and reliable measurements of . electrical conductivity EC and EC , a large number of useful physico-chemical properties of c soil-saturation extracts can be estimated: total soluble salts, total soluble salts corrected for neutral ion pairs, intensity of the relaxation and electrophoresis phenomena, ionic strength and activity coefficient of the ions. q 1999 Elsevier Science B.V. All rights reserved.

Soil-landscape evolution on a Mediterranean high mountain

Abstract During the Quaternary, Sierra Nevada (southern Spain) was affected by several glacial–periglacial episodes, decreasing progressively in intensity with time. These cold episodes alternated with the ones characterized by landscape stability and soil formation. Twelve soils that were formed on pre-glacial surfaces and glacial deposits of various ages were studied in four sectors: Laguna Seca, Lanjaron Valley, Mulhacen Valley, and Corral del Veleta. The soil parent material in all of these sectors was mica schist. Organic matter content, cation exchange capacity (CEC), exchangeable bases, saturation degree, and pH, were found to be not correlated with the age of the surface. The macromorphological differentiation of the soils, their clay and Fed contents, and especially, the clay and iron oxide accumulation indices, increased with the relative age of the surface. In contrast, the Feo content and the Feo:Fed ratio diminished with the age of the surface. During the last few pedogenic episodes, soils were formed by strong leaching and acidification, but with very little chemical weathering. All of these features suggest that climatic shifts during the pedogenic episodes towards conditions less favourable to soil development, in addition to vegetation changes and disturbances on the soil surfaces, which started by periglacial processes, account for the soil evolution of the Sierra Nevada better than time as the only variable factor. The soils did not develop monotonically with time and the sequence of surfaces cannot be regarded in a strict sense as a chronosequence. The soils developed over a series of stepped pedogenic episodes; in each episode, especially in the oldest, the properties of the soils related to weathering could have approached or reached a steady state. Nevertheless, when these properties are plotted against time, a continued increase in soil development results.

Characteristics of the organic matter of mediterranean high-mountain soils

Abstract The organic matter from the A horizons of five soils developed under a shrubby cover and under pine groves in a Mediterranean high-mountain area has been studied. The characteristics of humic acids (HA) are compared with those reported by other researchers in a variety of environmental conditions. The organic matter from pine groves has a higher C/N ratio, a greater content in fresh or littletransformed organic matter, and a lower humic extract than that of shrublands. In general, the humic acids show a low humification index, with greater relative proportion of aliphatic chains and less condensed aromatic-type structure as the pH and base saturation diminish. The climatic extremes which typify these ecosystems, very cold winters and very dry summers, together with the acidity and low base saturation of the soils, seem to be the determining causes of the low-humification index. Felbecks hypothesis of alteration of plant components appears to be the principal process controlling the humic-acid evolution of the organic matter in these soils.

Impact of unconfined sulphur-mine waste on a semi-arid environment (Almería, SE Spain)

Thirty-five soil samples were taken from unconfined mine waste, stream sediments, and surfaces unoccupied by mining and presumably unaffected by it, in a sulphur-mining zone surrounded by carbonate material and characterized by a semi-arid climate with short torrential storms. These samples were analysed and the results compared to estimate the spread of pollution in the landscape and to assess potential environmental risk. The mean concentrations of S, Zn, Cd, Pb, and As in mine waste were between 3.5-fold (As) and 50-fold (S) greater than unaffected soils. Oxidation of S led to a sharp drop in pH, strong weathering of minerals, and solubilisation of the constituent elements, forming a toxic acidic mine drainage with highly concentrated pollutants that were discharged into the drainage channels. Successive acid mine drainage into the soil on the valley floor spreads acidification and pollution downstream. The high carbonate content in surrounding soils played an important role in the increase of the pH and precipitation of S, Pb, and Al of the affected soils. Meanwhile, high mobility of Zn, Cd and As under basic conditions and a low Fe concentration explain the broad spread of these elements, as high concentrations were detected in soil more than 2000 m from the source. Only the soil solutions from near the waste dump (first 500 m) were highly phytotoxic, and moderately phytotoxic from 500 to 1500 m away. The concentration of pollutants in the leachates was clearly higher than in soil solutions, even in the soils located over 2000 m from the source, implying that the size of the polluted area will increase with time.