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Steps in soil pollution by the toxic spill of a pyrite mine (Aznalcóllar, Spain) Martín, F.; Simón, M.; Ortiz, I.; García, I.; Fernández, J.; Fernández, E.; Dorronsoro, C.; Aguilar, J. Department of Soil Science, University of Granada (Spain). Introduction On 25 April 1998, th e walls of two contig uous pon ds containing the ore-processing residu es from a pyrite mine located in Azn alcóllar (southwestern Sp ain) broke op en (F igur e 1), and to xic water an d tailings were spilled into the Agrio an d G uadiamar River basin, affecting some 40 km 2 . The tailings spread in a down -river directio n an d stop ped at 40 km from the point o f the spill. Th e p olluted w ater continued some 10 km more and reached th e Gu ad alqu ivir River, affecting the National Park of Do ñana (pro claimed by UNESCO in 1994 as part of World Heritag e). Nevertheless, a retention dam was rap idly con stru cted, min imizin g the damag e of th e toxic wastes in the wild life reserve. The aim of th is work is to describe the steps in soil po llution over time. Materials and methods On 4 May 1998, nine days after the spill, seven sectors in the affected area were studied along the basins of the Agrio and Guadiamar Rivers, analysing tailings, polluted water and contaminated as well as uncontaminated soils: near the mine (M), at the point of the spill; Soberbina (SO ), at 5.5 km from the spill; Puente de las Doblas (D), at 12 km; Aznalcázar (A), at 21 km; Quema (Q), at 29 km; Los Pobres (LP), at 34 km and Pescante (P), at 36 km (Figure 2). In each sector, a square plot was laid out (25 m x 25 m). At each corner and in the centre of the plot, samples were taken of tailings as well as of the soil at 0-10 cm and at 10-30 cm in depth. In order to monitor the contamination over time, each plot was sampled on 3 more dates: 20 May, 4 June and 22 July 1998. However, in two sectors (D and LP), the tailings were removed before 4 June. In Quema, two plots with tailings (250 m 2 ) were left untouched for scientific study and an additional sample was taken on 19 July 1999 (450 days after the spill). Field descriptions of the soils were based on procedures of the Soil Survey Staff (1951). In all soils, physical, chemical and physico-chemical properties were determined (Table 1): particle-size, pH, bulk density, electric conductivity, total carbon, organic carbon, equivalent carbonate content, cation-exchange capacity (CEC), exchangeable bases, total iron (Fe t ), ir on oxides (Fe d ), iro n-oxide amorpho us forms (F e o ) and total sulphur. A satu rated extract o f the tailings was prepared and the sulphates were precipitated as BaSO 4 . Samples of the tailings and soils, very finely ground (< 0.05), were digested in strong acids (HNO 3 + HF + HCl). In each digested sample and saturated extract of the tailings, Cu, Zn, Cd, As, Pb, Sb, Bi and Tl content were measured. To provide a quantitative assessment of the soil structure, a structural-development in dex (SDI) was for mulated, usin g the equation : SDI= Size x Grade, where values o f the gr ad e are g iven in Table 1, and the size of the structure take the follow ing values: fine=10, medium=7, coarse=5, very co ar se= 3. Results and discussion First Step l Toxic water and tailings penetrated the soils (Figure 3). l The principal pollutants were Zn, Pb, Cu, As, Sb, Bi, Cd, and Tl (Simón et al., 1999). l Because the water from the toxic spill contained no Bi, the total Bi contamination of the soils must have come from the tailings. l The quantity of tailings that penetrated the soil in each sector (Z) can be calculated by the equation: Z (g kg -1 ) = (CS Bi - UCS Bi ) 10 3 / T Bi , where T Bi is the Bi concentration in the tailings and CS Bi and UCS Bi are the Bi concentration in the contaminated and uncontaminated soils, respectively, all expressed in mg kg -1 (Figure 3). l The range of the total contamination of each element was extremely broad, as penetration of the tailings depended on soil characteristics (Figure 4). l 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. l The quantity of tailings that penetrated each soil generally decreased with depth. l The pollution tended to acidify the soils, although this trend was not strongly evident apparently due to the buffering effect of the CaCO 3 in most of the soils. References Nordstrom, D.K. (1982). Aq ueous pyrite oxid ation and consequen t formation of seco ndary iron minerals. In: Kitrick, J.A.; Fanding, D .S. and Hossner, L. R . editors. Acid sulphate weatherin g. Madison, WI. S oi l Sc. So c. Am.:37-56. Regowski, A.S.; Pionke, H.B. and Broyan, J.G. (1977). Modeling the impact of strip mining and reclamation processes on quality and quantity of water in mined areas: a review. J. E nviron . Qual ., 6:237-244. Simón, M .; O rtiz, I.; García, I.; Fernánd ez, E.; Fernández, J.; Dorronso ro, C. and Aguilar, J. (1999). Pollu tion of soils by the to xic spill of a pyrite mine (Aznalcóllar, Spain). The Science of the T otal Environment , 242:105-115. Stu mm, W.Y. and Morgan , J.J. (1981). Aquatic Chemistry: An introdu ction emp hasizing chemical equ ilibria in natural waters. John Wiley & Son s, NY. 218 pp. Second Step l Drying and consequent aeration of the tailings that remained on the surface of the the soils rapidly oxidized sulphides to sulphates, lowered the pH and solubilized part of the formely insoluble pollutants (Fig.7). l These processes were more pronounced in the middle and lower sectors of the basin, where the particle size was finer, the sulphur content higher and the bulk density less. l The soluble elements infiltrated the soils with the rainwater, swiftly augmenting the soil pollution (Figure 6). l Given that no rain fell for a long time after the spill, the 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 (Figure 8). l The mobility rates of the elements in the tailings increased with time and those in the soils diminished . l The pollutants tended to concentrate in the first 10 cm of the soils without seriously contaminating the groundwaters, at least in the carbonate soils. l The total concentration of each element was directly related to the square root of the time elapsed after the spill (Figure 9). l This results underscore the urgency of removing the tailings from the soil surfaces. 0 2 4 6 8 10 M SO D A Q P LP SECTORS Tl (mg / kg) Tl-ox Tl-w Tl-t 0 100 200 300 400 M SO D A Q P LP SECTORS Cu (mg /kg) Cu-ox Cu-w Cu-t 0 1000 2000 3000 4000 M SO D A Q P LP SECTORS Zn (mg / kg) Zn-ox Zn-w Zn-t Figure 5. Contamination in the seven study sectors (0-10 cm) due to the tailings (t), the polluted water (w) and the oxidation process (ox) on 22 July. Figur e 2. Map of the zone aff ected by the spill,showing the situ ation of th e seven st udy sectors. Mine Soberbina Las Doblas Aznalcázar Los Pobres Pescante Quema 2 km Toxic spill Tailings completely saturated by water. Reductive conditions. Stable sulfide particles First Step Figur e 9. Relation between the concentr ation of Zn and th e sq uare ro ot of th et ime elapsed after t he spill. Zn (mg/kg) 0 500 1000 1500 2000 2500 -100 0 100 200 300 400 500 Time (days) Transformed Fit to Sqrt Zn = 400.627 + 96.682 Sqrt Time ( days ) r 2 = 0.92487 Figure 3. Penetration of the toxic water and tailings into the soils at the beginning of the spill. Figur e 4. Pen etration of th e tailin gs accor ding to the soil char act eristics. Quema Poorly-developed structure Clean contact Small penetration Pescante Very well- developed structure Wide cracks Strong penetration Figure 5. Detail of asoil aggregate. Second Step Parcial draining, oxidation and dissolution Figure 7. Partial soiubilization of the formely insoluble pollutants. May 4th May 20th Figure 8. White salty crust formed three weeks after the spill. Figure 1. Breaking of the walls of the ponds and toxic spill inthe Guadiamar basin. __ ___ __ __ ___ __ __ __ ___ (%)_ ___ __ __ ___ __ __ ___ __ __ ___ _ Sec tors pH Ca CO3 Org an i c Ca rbo n Gra ve l San d Sil t Cl ay Fe d Struc t ure typ e/ si ze /g rad e SDI M (T) M (0 -10 ) M (1 0-3 0 ) SO (T) SO (0-1 0 ) SO (10 -3 0) D (T) D (0-1 0 ) D (10 -3 0) A (T) A (0-1 0) A (10 -30 ) Q (T) Q (0 -10 ) Q (1 0-3 0) P (T) P (0-1 0) P (10 -30 ) LP(T) L P(0 -10 ) L P(1 0-3 0 ) 5. 0 7. 6 7. 9 4. 4 7. 3 7. 4 4. 9 7. 6 7. 4 4. 8 7. 7 7. 8 4. 3 7. 9 8. 1 5. 1 7. 2 7. 8 4. 9 7. 8 7. 8 0. 0 2. 9 2. 5 0. 0 0. 3 0. 0 0. 0 1 8.6 1 9.6 0. 0 1 4.8 1 4.6 0. 0 9. 4 7. 0 0. 0 1 4.3 1 4.7 0. 0 1 5.3 1 5.5 0. 50 0. 50 0. 43 0. 30 0. 80 0. 65 0. 21 0. 93 0. 61 0. 23 1. 65 1. 34 0. 22 1. 36 0. 92 0. 22 1. 02 0. 61 0. 24 0. 98 0. 46 0. 0 1 2.3 9. 6 0. 0 0. 8 1. 2 0. 0 2 9.7 4 2.8 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 0. 0 2. 1 1. 9 0. 0 6. 9 5. 8 9. 6 3 9.6 5 1.3 9. 6 6 8.0 7 1.2 2. 0 2 2.5 1 6.1 0. 6 1. 9 1. 7 1. 2 2 1.9 1 9.4 1. 8 7. 7 6. 3 1. 4 1. 1 0. 9 7 2.9 2 2.0 1 6.6 7 5.9 1 8.5 1 6.6 8 9.0 2 1.4 2 2.2 8 3.0 5 2.5 5 4.8 8 1.9 3 7.7 4 5.1 8 1.5 2 7.1 2 8.2 8 2.2 2 9.7 3 1.0 1 7.5 2 6.1 2 2.5 1 4.5 1 2.7 1 1.0 9. 0 2 6.4 1 8.9 1 6.4 4 5.6 4 3.5 1 6.9 4 0.4 3 5.5 1 6.7 6 3.1 6 3.6 1 6.4 6 2.3 6 2.3 - 1. 29 1. 15 - 0. 89 1. 00 - 0. 96 1. 12 - 0. 81 0. 92 - 1. 15 1. 44 - 1. 10 1. 18 - 0. 84 0. 88 pl / vc /3 sb k / c / 2 sb k / vc /1 pl / vc /3 sg /- / 0 sg /- / 0 pl / vc /3 sb k / m / 2 sb k / m / 2 pl / vc /3 a bk / vc /1 m / - /0 pl / vc /3 sb k / m / 1 sb k / c / 1 pl / vc /3 abk/ f /3 a bk / vc /1 pl / vc /3 a bk / vc /1 m / - /0 9 10 3 9 0 0 9 14 14 9 3 0 9 7 5 9 30 3 9 3 0 ______________________________________________________________________ M= Mi ne , SO= Sob e rb i na , D= Pu en te d e l as Dob l as, A= Azn a l cáza r,Q = Qu e ma , P=Pe sc an te a nd L P= L os Po bre s. Struc t ure type: pl = pl a ty, a bk = a ng u lar bl o ck y, sb k= su b an gu l ar b l o ck y, m=ma si v e, sg = si n g l e g ra i n. Struc t ure size: f = fi ne,m = medi u m, c= co a rse ,vc = ve ry co a rse . Struc t u re g rad e: 0 = stru ctu re l es s, 1= weak ,2 = mo d era te, 3= stro ng . Table 1. Analytical data, structu re and st ructur e-developmen t in dex (SDI) of th e tailings ( T) and cont am inated soils (0-10 and 10- 30 cm in depth) b ysect ors. 0 2 4 6 8 10 12 14 M SO D A Q P LP SECTORS Cd ( mg / kg) Cd- ox Cd-w Cd-t 0 100 200 300 400 500 600 700 M SO D A Q P LP SECTORS As ( mg / kg) As- ox As-w As-t 0 200 400 600 800 1000 1200 1400 1600 1800 2000 M SO D A Q P LP SECTORS Pb ( mg / kg) Pb- ox Pb-w P b -t
