Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques

GEXPLO-05371; No of Pages 11 Journal of Geochemical Exploration xxx (2013) xxx–xxx

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Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques J.A. Acosta a,⁎, P. Martínez-Pagán b, S. Martínez-Martínez a, A. Faz a, R. Zornoza a, D.M. Carmona a,c a b c

Sustainable Use, Management and Reclamation of Soil and Water Research Group, Technical University of Cartagena (UPCT), Paseo Alfonso XIII, 52, 30203 Cartagena, Murcia, Spain Departamento de Ingeniería Minera, Geológica y Cartográfica, Technical University of Cartagena (UPCT), Paseo Alfonso XIII, 52, 30203 Cartagena, Murcia, Spain Environmental Research Group, Universidad Pontificia Bolivariana, Circular 1a No. 70-01, Medellín, Colombia

a r t i c l e

i n f o

Article history: Received 1 October 2013 Revised 4 April 2014 Accepted 10 April 2014 Available online xxxx Keywords: ERI method Reclamation works Tailing pond Heavy metal

a b s t r a c t This study aims to evaluate the environmental risk of three reclaimed mining ponds using geophysics and geochemical techniques. The reclamation works were based in the use of some materials for covering the tailing layer in order to reduce its erosion and transport. Samples from dumped materials and tailing layers were analyzed for their properties and total, diethylene triamine pentaacetic (DTPA) extractable and water soluble metals. Electrical resistivity imaging (ERI) method helped to identify erosion processes and the thickness of dumped materials and their contact with tailing layers. Properties within dumped material promoted the colonization of natural plants on these tailings. However, total Pb, Cd, and Zn concentrations exceed the maximum admissible concentration allowed by legislation. Risk of mobility of these metals from the surface layer through plant uptake and leaching or runoff water has been significantly reduced. The concentration of some metals in dumped materials was higher than that reported in tailing layers: their immobilization is therefore recommended. According to ERI method, two ponds experienced erosion by water action. Efforts need to be focused to re-cover these areas to reduce the erosion and metal mobility from the tailing layers. The bedrock did not present any discontinuity that could allow the transport of heavy metals to deeper horizons. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Depleted mining areas which have undergone intensive mining activity present a challenge to local governments to overcome their potential hazards. The main impact of the abandoned mine areas is the presence of the unconfined tailing ponds which have very high concentrations of metals (Rodriguez et al., 2009). The layers of the unconfined ponds can be eroded by wind or by water runoff (Razo et al., 2004; Zanuzzi et al., 2009) and the metals transported to long distances (Chaoyang et al., 2009; Navarro et al., 2008). In addition, water soluble metals can be transported to adjacent soils or leached to subsoil and even groundwaters, processes that are enhanced by the leachates from sulphide oxidation, known as acid mine drainage (AMD), which are extremely acidic (Sánchez-Andre et al., 2014). Reclamation is necessary to reduce the damage to the environment presented by these tailings. A variety of techniques can be used for decontamination and remediation, including in situ or ex situ processes

⁎ Corresponding author. Tel.: +34 968 197587; fax: +34 968 325435. E-mail address: [email protected] (J.A. Acosta).

(soil-washing, physical separation, phytoremediation, leaching, etc.) (Masscheleyn et al., 1999; Meagher, 2000; Rojo et al., 2014) or those processes used to immobilize the contaminants to minimize their release into the environment (Kabas et al., 2013; Lee et al., 2014). Even some conventional remedial approaches to metal-contaminated soils usually involve removal and replacement of soil with clean materials (Brown et al., 2005), although this is not considered to be the most economically or environmentally sound solution (Alvarenga et al., 2008). The choice of technique will hinge on a number of factors related to the anticipated future use of the areas targeted for remediation – whether for industrial activities or for public use – and on technical as well as economic and legislative considerations (Ciccu et al., 2003). In particular, where decontamination proves to be economically unsustainable due to the lack of information about the actual nature of the material, or difficulties are created by the large volumes of the materials to be handled. Immobilization techniques based on the addition of suitable substances are generally acceptable, if accompanied by an environmental impact study. At La Sierra Minera Cartagena–La Unión (SE Spain), with N40 unconfined tailing ponds, both public administrators and land-owners in charge of this former mining district were concerned with these tailing ponds, as they are located near densely populated areas with residential,

http://dx.doi.org/10.1016/j.gexplo.2014.04.005 0375-6742/© 2013 Elsevier B.V. All rights reserved.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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J.A. Acosta et al. / Journal of Geochemical Exploration xxx (2013) xxx–xxx

industrial and commercial developments. So, many years' works were carried out to reduce the erosion processes and the impact on the landscape in some of the tailing ponds. This work comprised the dumping of material on the tailing ponds to create a soil cover to encourage the growth of vegetation. These filling materials had different origins including waste debris from demolition works, artificial cobbles, gravel, and sands extracted from nearby quarries. Although, the works achieved their objectives (reducing the landscape impact and mitigating the erosion of the ponds), limited attention was given to the effect about the behavior of the metals after application of these materials. In fact, limited study had been carried out to assess the effectiveness of these materials in the reduction of the impact on the environment or safety of the tailing ponds, regarding the heavy metal mobility and bio-availability. Several techniques are available which allow the evaluation of the risk of metal mobility, including geophysics and geochemical techniques. Geophysical techniques are complementary, non-invasive, inexpensive and fast-working tools to manage a wide range of engineering and environmental problems (Gómez-Ortiz et al., 2010; Hanna and Pfeiffer, 2007; Hatherly, 2013; Hutchinson, 2005; Martínez-Pagán et al., 2009). The ERI method, which is a group of geophysical techniques, can be used to check the thickness and extension of different materials, to highlight any erosion action in the soil, and to distinguish preferential pathways of water or acid mine drainage. In combination with this method, the concentration of metals in different phases of the soil (water soluble, DTPA extractable, and total concentration) can also be utilized together with geochemical techniques, which allow the assessment of soil pollution and the risk of metal mobility (Conesa et al., 2007; Parkpian et al., 2002). The main objectives of this study were to: 1) determine the degree of contamination of heavy metals in both surface and subsurface layers

of the ponds, and 2) evaluate the long-term effect of different materials used in former reclamation action on heavy metal mobility in the surface of the selected three tailing ponds by using geophysical and geochemical techniques. 2. Material and methods 2.1. Study area The mine tailing areas are located in the eastern side of the Cordillera Bética, and are part of a wide volcano-tectonic and metallogenetic belt that extends from Cabo de Gata to the Sierra de Cartagena (Fig. 1a). The climate is typically Mediterranean with average monthly temperatures of 9.3 °C in January and 24.4 °C in July. Mean annual precipitation is ~275 mm, mostly in spring and autumn while potential evapotranspiration reaches to 900 mm per year. Strong torrential rain occurs in a very short period of time, initiating both erosion processes and intensive relief change. Three tailing ponds were selected, Las Lajas, La Encontrada and El Beal (Fig. 1). The extension of each is 18,300 m2, 160,000 m2 and 85,000 m2, and their estimated volumes are 110,000 m3, 1,132,000 m3 and 1,809,000 m3, respectively. The reclamation works were carried out from around 1970 to 1980 and consisted of covering the surfaces with different materials. The material used in these former reclamation actions was a mixture of waste debris from demolished buildings, artificial cobbles, gravel, sands extracted from nearby quarries, and natural soil from nearby areas. The Las Lajas tailing pond was covered principally by waste debris from demolition works, sand, and natural soil from nearby areas. No reforestation was carried out, although the pond was colonized by vegetation compromising Piptatherum miliaceum, Hyparrhenia hirta,

Fig. 1. (a) Location map of the study area, (b) Location of ERI profiles at La Encontrada tailing pond, (c) Location of ERI profiles at Las Lajas tailing pond, and (d) location of ERI profiles at Llano del Beal tailing pond.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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Medicago minima, Helichrysum decumbens, Phagnalon saxatile, Thymelaea hirsuta, Dittrichia viscosa, Atractylis humilis, Asparagus horridus and Atriplex halimus covering about 70–80% of the surface of the pond. The La Encontrada tailing pond was also mainly covered by natural soil areas. In this pond reforestation compromises Acacia sp, Pinus halepensis, Cupressus sempervirens and Eucalyptus globulus. There is also a natural occurrence of shrub species such as P. miliaceum, H. hirta, Stipa, D. viscosa, Paronychia suffruticosa, H. decumbens, T. hirsuta, Thymus hyemalis, Sonchus tenerrimus, Lavandula multifida and P. saxatile. Vegetation covers approximately 40–50% of the pond surface. The El Beal tailing pond was covered by a heterogeneous material composed of cobbles, gravel, and sand which was extracted from nearby quarries, all of them poorly sorted. The vegetation in the pond is composed of P. miliaceum, Hyparrhenia sinaica and H. decumbens, which covers 10–20% of the surface. 2.2. Geophysical: electrical resistivity imaging (ERI) method A total of nine profiles of ERI were used from the three different tailing ponds, comprising three resistivity profiles on the La Encontrada, three profiles on Las Lajas and three resistivity profiles on El Beal. The ERI method is a combination of the electrical sounding and profiling methods in a single process (2-D resistivity imaging) to investigate complex geological structures with strong lateral resistivity changes (Sumanovac and Weisser, 2001; Tejero et al., 2002). This combination provides detailed information both laterally and vertically along the profile and is the most frequently applied technique in environmental studies (Seidel and Lange, 2007). Apparent resistivity measurements were obtained using a computer-controlled multielectrode system with a large number of electrodes (36, 54, or 72) laid out in a profile at constant intervals. Altogether 102 different electrode arrays were used (Szalai and Szarka, 2008) with different horizontal and vertical resolutions, penetration depths and signal-to-noise ratios. The datasets were acquired using the Wenner–Schlumberger electrode array, because it has both high signal-to-noise ratio and good vertical resolution. The latter is especially useful in determining the soil cover thickness and its extension on the tailing ponds (Gómez-Ortiz et al., 2007; Martínez-Pagán et al., 2009). We used a Syscal R1 Switch 72 resistivity meter (IRIS Instruments, 2001). Each stainless steel electrode was 30-cm long and connected to the resistivity meter through takeout clips for the galvanic coupling of the electrodes to the ground. The coordinates of each end electrode placement were recorded using a GPS unit, enabling us to correctly site the ERI profiles on the map and make static corrections as necessary. Electrical measurements of the profiles were analyzed in a two-stage process. Firstly, we used PROSYS II software (IRIS Instruments, France) for initial data processing to remove anomalous values caused by environmental electrical noise and to make the necessary correction for topographic effects. The corrected data was then processed by RES2DINV software (Loke, 2000; 2004) and interpreted. The RES2DINV software runs an inversion process which enabled us to obtain a 2D distribution of electrical resistivity related to the physical properties of the subsurface, named as an inverted resistivity image or 2-D resistivity section (Martínez-Pagán et al., 2009). This inversion process is based upon the smoothness-constrained least-squares method (deGroot-Hedlin and Constable, 1990; Loke and Barker, 1996). The root-mean-squared (RMS) error for each electrical section was obtained as a parameter showing the accuracy of the match between the measured and predicted apparent resistivity values (Ernstson and Kirsch, 2006). 2.3. Geochemistry: sampling and analytical methods Surface and subsurface samples were collected from the three ponds. The surface sampling was carried out according to a regular

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sampling grid with a distance between samples of 40 m, 100 m and 75 m for the Las Lajas, La Encontrada and El Beal respectively using Geographic Information System (GIS). Aerial photos were used to design the grid to ensure that sampling was representative of all the surface area of each pond. Samples were taken from the natural soil materials that were used to cover the ponds in the earlier reclamation (0–15 cm). Sub-surface samples were taken from a drilling carried out in the center of each pond. The depth of the drilling was 13 m, 15 m and 15 m for the Las Lajas, La Encontrada and El Beal, respectively. Samples were taken from every meter of the holes. The collected samples were air-dried in the lab, passed through a 2-mm sieve, homogenized, and stored in plastic bags at room temperature prior to laboratory analyses. Several analyses were carried out: pH and electrical conductivity were measured in a 1:1 and 1:5 deionized water/soil ratio solution, respectively, according to Soil Survey Staff (2004). Particle size analysis was carried out using the F.A.O.-I.S.R.I.C. (2006) after the combination of pipette Robinson and sieving. For the quantification of the total metals, a subset of each sample was ground, and an acid digestion (nitric– perchloric) was carried out (Risser and Baker, 1990) (210 °C for 1:30 h and the addition of 0.1 N HCl). Soluble metals were determined using a 1:5 soil–deionized water ratio (Ernst, 1996). DTPA was used in the ratio of 1:2 soil–extractant for soil extractable metals (Linsay and Norvell, 1978; Norvell, 1984). Measurements of metals were conducted using atomic absorption spectrophotometer (AAnalyst 800, Perkin Elmer). Reference soil SO− 4 from the Canadian Certified Reference Materials Project (Bowman et al., 1979) and blank reagent were used as the quality control samples during the analysis. The recovery of metals in the analysis was within b4% for Cd, b2% for Cu, b 3% for Zn and b 1% for Pb.

2.4. Variation of metal concentrations between dumped material and subsurface layers We estimated the variation of total, DTPA extractable and water soluble metal concentrations between tailing layers (subsurface samples) and dumped materials (surface samples) using the following ratio (Rx):

Rx ¼ ðXdm =Xtl Þ

where, Xdm was the mean concentration (mg kg−1) of the metal in the dumped materials and Xtl was the mean concentration (mg kg−1) of metal in the tailing layers. The value of Rx N1 indicates that the concentration of metal in dumped material is higher than that in the tailing layers. Values b 1 mean that the concentration of metal in dumped material is lower than in the tailing layers. Thus, values ~1 indicate a similar concentration in both.

2.5. Statistical analyses Prior to statistical analysis, the data set distribution was evaluated using Kolmogorov–Smirnov method; when the distribution was not normal, the data was log-transformed (Romic and Romic, 2002) before statistical treatment. Descriptive statistics (mean, error, maximum and minimum) of metals and soil properties were performed applying the Excel for Windows software package. We used the T-test to determine the existence of significant differences among properties and metals in the dumped materials and tailing layers. Relationships between dumped material and tailing layers were studied using Pearson correlations. Statistical analyses were performed with SPSS for Windows, Version 15.0 software.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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J.A. Acosta et al. / Journal of Geochemical Exploration xxx (2013) xxx–xxx

Table 1 Properties of the surface and subsurface samples from the ponds.a EC (dS m−1)

pH

Las Lajas

La Encontrada

El Beal

a

Mean Error Minimum Maximum Mean Error Minimum Maximum Mean Error Minimum Maximum

Clay (%)

Silt (%)

Sand (%)

Surface

Drill

Surface

Drill

Surface

Drill

Surface

Drill

Surface

Drill

7.66a 0.07 7.12 8.07 7.96a 0.04 7.66 8.17 7.84a 0.12 7.04 8.64

6.40b 0.27 3.94 7.67 7.40b 0.26 4.05 7.96 7.37a 0.31 4.12 8.21

1.34a 0.22 0.31 2.42 0.36a 0.13 0.11 2.49 1.25a 0.25 0.19 2.88

3.39b 0.21 1.85 4.54 2.51b 0.16 1.31 3.27 2.60b 0.12 1.87 3.38

10.24a 1.18 3.80 17.10 20.93a 1.60 9.93 30.28 7.61a 1.15 0.19 16.94

11.75a 1.49 3.67 22.99 8.22b 0.57 5.23 12.16 3.00b 0.53 0.64 6.64

30.51a 1.76 21.50 46.90 26.11a 1.85 14.62 41.51 47.54a 5.78 27.06 95.56

53.91b 5.39 26.65 83.35 27.67a 2.69 11.35 45.93 40.34a 3.45 17.25 54.49

59.24a 2.52 40.80 74.60 52.96a 2.49 32.48 72.71 44.86a 5.33 1.00 69.95

34.33b 5.90 1.00 65.73 64.11b 3.17 41.91 81.89 56.66b 3.85 40.98 81.96

Within the rows, different letters (a and b) indicate significant differences (P b 0.05) between means among dumped material and tailing layers after a T-test.

3. Results and discussion 3.1. Soil properties of the dumped material and subsurface layers of the ponds Table 1 shows the characteristics of the dumped materials and tailing layers of forming the Las Lajas, La Encontrada and El Beal ponds. pH follows the same distribution pattern in the three ponds; slightly higher in the surface than in the subsurface, where significant differences were observed. Based on pHw mean values, dumped materials can be classified between neutral and alkaline (7.66–7.96), and from slightly acid to neutral (6.4–7.37) in tailing layers (Soil Survey Division Staff, 1993). However, some tailing samples are extremely acid promoting metal mobility (Pérez-Esteban et al., 2014). The high pH value of the dumped material promotes metal precipitation thus decreasing their mobility, which is common in soil where climates are semiarid (Yaalon, 1997). According to the ANOVA test, electrical conductivity is higher in the tailing layers than in dumped materials for the three ponds. The samples from the surface were considered non-saline (0.36–1.24 dS m−1), while the tailing layers were very slightly saline (2.51–3.39 dS m−1) according to the classification proposed by Soil Survey Division Staff (1993). It is known that EC is pH related: when the pH decreases the tailing matrix causes the formation of more salts (Conesa et al., 2007; Wong, 2003).

Dumped materials in the La Encontrada and El Beal had a lower sand and a higher clay contents than the tailing layers, while silt percentages were constant (Table 1). Dumped material in the Las Lajas had a higher sand and lower silt percentages than the tailing layers, while the clay content was constant. This may result in the surface layer having a low water retention rate. Many factors such as poor physical structure, low water and nutrient holding capacity, acidity and alkaline reactions, water supply, toxic materials, salinity, poor stability, and high surface temperature are known to affect plant establishment on mine tailings (Asensio et al., 2013; Barrutia et al., 2011). 3.2. Total metal pollution in the tailing ponds 3.2.1. Dumped materials In terms of total metal content, there were marked differences among the dumped materials in Zn, Cd, Cu, and Pb (Table 2). High and positive correlations between total metals in dumped materials and tailing layers (r = 0.98 for Zn; r = 0.84 for Cd and r = 0.68 for Cu) indicated that total metal contents on the surface depend on the metal content in the tailing layers, except for Pb (r = 0.20). The highest mean levels reached for Cd and Zn were 31.5 mg kg−1 and 12,772 mg kg−1,

Table 2 Total metal concentrations on surface and subsurface samples.a Zn (mg kg−1)

Las Lajas

La Encontrada

El Beal

Mean Error Minimum Maximum Mean Error Minimum Maximum Mean Error Minimum Maximum

Pb (mg kg−1)

b c d

Cd (mg kg−1)

Surface

Drill

Surface

Drill

Surface

Drill

Surface

Drill

3760a 609 760 9438 3861a 475 2166 9813 12772a 1233 5108 22,312

5250b 555 887 10,658 4599b 739 1740 10,361 8312b 1340 846 17,909

5267a 860 750 13,058 5578a 1131 2840 21,272 4650a 475 2080 9346

4857b 1607 1228 25,275 2137b 140 1216 2888 2080b 196 1051 4052

114a 13.9 53.1 219 160a 30.3 38.0 465 72.3a 5.79 37.8 114

146b 23.6 61.9 383 128a 15.7 31.8 257 80.6b 6.80 39.9 129

7.15a 1.29 1.94 17.9 17.4a 1.39 9.06 30.5 31.5a 3.78 9.52 64.1

12.5b 1.91 2.31 27 21.5b 1.07 14.6 28.5 21.8b 3.74 0.67 46.3

Guidelines of maximum allowed concentration of heavy metals (mg kg−1) Zn Spainb 150–450 Netherlandc 1000 Denmarkd 720 a

Cu (mg kg−1)

Pb 50–300 400 530

Cu 50–210 500 190

Cd 1–3 5 12

Within the rows, different letters (a and b) indicate significant differences (P b 0.05) between means among dumped material and tailing layers after a T-test. BOE (1990), allowed level in agricultural soil, interval: soil pH b7 and pH N7. Ministerial Decree (1999), intervention values. Ministry of Housing (1994), intervention values.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

J.A. Acosta et al. / Journal of Geochemical Exploration xxx (2013) xxx–xxx

in the El Beal, while the highest mean values for Pb and Cu were 5578 mg kg−1 and 160 mg kg−1 in the La Encontrada tailing pond. Mean concentration of Pb, Zn and Cd in the dumped materials from the three ponds studied, except for Cd in the Las Lajas, exceeds the maximum concentrations admissible by Spanish, Dutch, and Danish legislation (BOE, 1990; Ministerial Decree, 1999; Ministry of Housing, 1994). Copper levels, however, were below the abovementioned threshold levels. Under Spanish, Dutch, and Danish legislation concentrations of Zn were higher than permissible by 8, 4, and 5 times for the Las Lajas and La Encontrada, and 28, 13, and 18 times in the El Beal. Pb concentration was exceeded by 17, 13, and 10 times in the Las Lajas, by 18, 14, and 10 times in the La Encontrada and by 15, 11, and 2 times in the El Beal. The Cd concentration was more than 10, 6, and 2 times higher for the El Beal. Under Spanish and Dutch legislation Cd concentration was 6 and 3 times higher in the La Encontrada while it was twice the level permissible by Spanish law in the Las Lajas. According to the current legislation, the dumped materials are polluted by Cd, Zn and Pb and new remediation action is necessary to reduce their total metal concentrations and to improve environmental conditions in these areas. 3.2.2. Tailing layers As can be seen in Table 2, El Beal had the highest values of Zn and Cd, 8312 mg kg−1 and 21.8 mg kg−1. The highest Pb and Cu concentrations were found in Las Lajas, 4857 mg kg−1 and 146 mg kg−1. Despite the high concentrations found for Pb, Zn and Cd, these were lower than those reported in the tailing spill from the Aznalcollar accident (8091 mg Pb kg−1, 8832 mg Zn kg−1 and 33 mg Cd kg−1) (LopezPamo et al., 1999), except for Zn in the El Beal pond. Mean concentration of Pb, Zn and Cd in the three ponds studied exceeds the maximum admissible concentrations in the Spanish, Dutch and Danish legislation (BOE, 1990; Ministerial Decree, 1999; Ministry of Housing, 1994). The Cu levels, however, were below the threshold levels (Table 2). Metal concentrations in all the tailing ponds were higher than that permissible by the Spanish, Dutch and Danish laws respectively: Cd concentrations were 7, 4, and 2 times higher for the La Encontrada and El Beal, and 4 and 2 times (Spanish and Dutch) for Las Lajas. Pb concentrations exceeded the limit by 7, 5 and 4 times for both La Encontrada and El Beal and by 16, 12 and 9 for the Las Lajas. Zn content was more than 10, 5 and 6 times higher for the La Encontrada, 18, 8 and 11 times higher for El Beal and 10, 5 and 6 times for the Las Lajas. 3.3. DTPA extractable and water soluble metals in the tailing ponds Table 3 shows the concentration of metals extracted with DTPA. These concentrations and their percentage with respect to total

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contents differed between the ponds in both dumped materials and tailing layers for all the metals studied. No relationship was found between the DTPA-extractable concentration in the dumped material and tailing layers, which indicated that the available metal content depends on the physico-chemical properties of the materials (Du Laing et al., 2002; Tack and Verloo, 1995; Ure and Davidson, 2001). In the dumped materials, an average of 1.6, 1.7 and 3.4% of total Zn was DTPA extractable for the Las Lajas, La Encontrada and El Beal ponds. For Pb and Cd these percentages were 5.6, 5.5 and 3.1% and 6.1, 2.5 and 7.4% respectively. The percentages of DTPA-extractable Cu were lower than the abovementioned metals (1.9, 0.5 and 0.9%). These suggest that there is a risk of excess Pb and Cu uptake by plants in the Las Lajas, while for Zn and Cd this risk is higher in the El Beal pond. Absorption by plants is greatest for Cd followed by Pb N Zn N Cu. However, Pb was shown to have the highest concentrations of DTPA extractable metal. All metals in the three ponds, except for Cd in the Las Lajas, had a total concentration of soluble fraction below 1% (Table 4). The highest percentages of Cu and Cd were found in the Las Lajas, about 0.04 and 1.15%, respectively. However, the highest percentages of water-soluble Zn and Pb were recorded for the La Encontrada, about 0.05 and 0.04%. The concentration of Zn and Pb was low; below 3 mg kg−1 for Zn and Pb and b0.2 mg kg−1 for Cu and Cd in the three ponds. Therefore, the risk of mobility of metals by leaching or runoff from the dumped material is not considered to be an environmental problem.

3.4. Electrical resistivity imaging (ERI) 3.4.1. The La Encontrada tailing pond The ERI electrical cross sections (Figs. 2–4) of the ERI profiles is indicated by black vertical lines where the profile number is labeled as P1 for ERI profile 1, P2 for ERI profile 2, and so on. Fig. 1b shows the layout of three ERI profiles that were carried out on the La Encontrada tailing pond. We laid out two ERI profiles with NW–SE orientation (profiles 1 and 3) and a third ERI profile of approximate W–E orientation (profile 2). As can be seen in Fig. 1b, all of these profiles were situated on the east-side of the tailing pond since that region displayed an outcropping of small cracks through the soil cover, indicating a potential subsidence area. Furthermore, a rocky mount composed of calcareous material is situated at the east of the tailing pond, and using the ERI method, we were able to detect the geological contact between tailing and bedrock. ERI profile 1 was 355 m long, with an electrode spacing of 5 m (Fig. 1b). The maximum penetration depth was ~60 m. A RMS error of 3.9% was obtained after 5 iterations. This profile was laid out to maximum length in order to cover the whole length of the tailing pond. Additionally, a 355 m long profile enabled us to obtain electrical information about the bedrock and its extension (Fig. 2a).

Table 3 DTPA extractable metal concentrations on surface and subsurface samples.a Zn (mg kg−1)

Las Lajas

La Encontrada

El Beal

a

Mean Error Minimum Maximum Mean Error Minimum Maximum Mean Error Minimum Maximum

Pb (mg kg−1)

Cu (mg kg−1)

Cd (mg kg−1)

Surface

Drill

Surface

Drill

Surface

Drill

Surface

Drill

59.5a 7.64 6.66 119 65.3a 12.1 9.24 163 428a 47.4 95.6 701

163b 13.1 33.4 214 65.3a 17.2 11.0 218 130b 39.8 2.91 648

294a 63.0 39.0 797 304a 61.8 118 976 145a 26.6 3.46 346

236a 49.9 1.01 532 276a 89.6 36.5 1182 358b 52.0 42.1 645

2.22a 0.44 0.20 6.26 0.74a 0.08 0.28 1.56 0.72a 0.11 0.33 1.88

6.07b 1.00 1.13 13.9 28.9b 5.93 16.8 90.9 8.39b 1.54108 0.36 18.68

0.44a 0.08 0.10 1.15 0.44a 0.06 0.22 1.08 2.33a 0.32 0.75 5.21

2.32b 0.69 0.37 10.8 1.00b 0.25 0.34 3.60 1.58b 0.25 0.04 3.70

Within the rows, different letters (a and b) indicate significant differences (P b 0.05) between means among dumped material and tailing layers after a T-test.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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J.A. Acosta et al. / Journal of Geochemical Exploration xxx (2013) xxx–xxx

Table 4 Water soluble metal concentrations on surface and subsurface samples.a Zn (mg kg−1)

Las Lajas

La Encontrada

El Beal

a

Mean Error Minimum Maximum Mean Error Minimum Maximum Mean Error Minimum Maximum

Pb (mg kg−1)

Cu (mg kg−1)

Cd (mg kg−1)

Surface

Drill

Surface

Drill

Surface

Drill

Surface

Drill

0.13a 0.05 nd 0.77 1.86a 0.31 0.17 5.01 6.21a 2.19 0.04 37.38

21.73b 6.03 0.21 64.22 3.66b 2.07 0.04 23.42 1.74b 1.16 nd 17.85

0.30a 0.06 0.05 1.01 2.41a 0.37 1.09 6.50 0.26a 0.10 nd 1.81

3.34b 1.28 0.15 16.25 2.53a 0.51 1.02 7.93 0.36a 0.20 nd 2.62

0.05a 0.01 nd 0.16 nd nd nd nd nd nd nd nd

0.52b 0.49 nd 6.87 0.41 0.04 0.24 0.58 0.02 0.02 nd 0.23

0.08a 0.02 nd 0.20 0.17a 0.03 0.01 0.42 0.07a 0.03 0.01 0.51

0.58b 0.21 0.11 3.24 0.99b 0.08 0.68 1.72 0.49b 0.10 nd 1.25

Within the rows, different letters (a and b) indicate significant differences (P b 0.05) between means among dumped material and tailing layers after a T-test. nd: not detected.

The 2-D resistivity section shows three main electrical layers well indentified on each electrical section. The upper layer had electrical resistivity values ranging from nearly 13 Ω m to slightly above 65 Ω m.

This upper layer corresponds to filled material composed of soil whose thickness was not uniform along the ERI profile, ranging from 0.3 m to 5–6 m. In fact ERI profile 1 highlights some areas with a lack

Fig. 2. Electrical resistivity section with the final model and interpretation at La Encontrada tailing pond of: (a) ERI profile 1, (b) ERI profile 2, and (c) ERI profile 3.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

J.A. Acosta et al. / Journal of Geochemical Exploration xxx (2013) xxx–xxx

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Fig. 3. Electrical resistivity section with the final model and interpretation at Las Lajas tailing pond of: (a) ERI profile 1 and (b) ERI profile 2.

of soil cover as a consequence of erosion and transportation action, as can be seen from position x = 275 m to x = 300 m. The area presents significant subsidence (Fig. 2a).

The middle electrical layer is associated with tailing material with resistivity values below 13 Ω m. The tailing layer presents two points, at the positions x = 150 m and x = 300 m, where it reaches the

Fig. 4. Electrical resistivity section with the final model and interpretation at Llano del Beal tailing pond of: (a) ERI profile 1 and (b) ERI profile 2.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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J.A. Acosta et al. / Journal of Geochemical Exploration xxx (2013) xxx–xxx

maximum thickness and these points are, surprisingly, situated on a fault and a preferential flow pathway (Fig. 2a). Indeed the occurrence of these geological fractures favors heavy metal washing through them by rain-water which percolates through the tailing. Because of the high electrical resistivity contrast between tailing and bedrock it is feasible to delineate both geological contact tailing-bedrock and any fault occurrence. In Fig. 2a this geological contact has been drawn by a dashed black line. The fault detected by ERI profile 1 is named “El Bosque” as mentioned in different geological studies. The bedrock constitutes Triassic bluish limestone and its typical electrical resistivity values (Telford et al., 1990) are in close agreement with those found by ERI profile 1 since these are above 150 Ω m. The ERI of profile 2 is 175 m long (Fig. 1c). The maximum penetration depth reached was ~ 30 m. A RMS error of 6.1% was obtained after 5 iterations (Fig. 2b). This profile was laid out to focus on the subsidence zone where soil cover is almost nonexistent, due to different erosion factors (poor vegetation settlement, water runoff, wind, micro-cracks, etc.). The ERI electrical section depicts this uncovered area characterized by electrical resistivity values below 13 Ω m from the position x = 45 m to x = 110 m. These values are associated with tailing so the ERI method corroborates the lack of soil cover on this site. The ERI of profile 2 at position x = 80 m in the tailing layer shows the maximum cover to be about 15 m deep in the subsistence area. According to mining terminology the La Encontrada tailing pond occurs on a hill-side and is well marked in Fig. 1b, as it shows the limestone dipped to west and characterized by electrical resistivity values above 150 Ω m. On the left-hand side of the electrical section we can see some soil cover of about 5 m with electrical resistivity values ranging between 15 Ω m and 30 Ω m. This soil cover layer is overlying the tailing layer which becomes thicker to the west in accordance with a hill-side pond typology such as the La Encontrada tailing pond (Fig. 2b). The soil cover depth was corroborated by a drill-hole previously made for the geophysical field work. The total length of this drill-hole was about 15 m: it did not reach the rocky substratum (Fig. 2b). Fig. 2c shows the electrical section of ERI profile 3. At a length of 265 m, it was the farthest one from the limestone hill (Fig. 1b). The maximum penetration depth reached was nearly 55 m. A RMS error of 4.3% was obtained after 5 iterations. On the electrical section the shallowest resistive layer, described previously, is associated with the soil cover. It has moderate electrical resistivity values ranging from 8 Ω m to about ~100 Ω m. This ERI profile displays areas where slight runoff action occurs on the surface from the position x = 50 m to x = 115 m and from the position x = 205 m to x = 265 m. They are characterized by electrical resistivity ranging between 10 Ω m and 30 Ω m. From the position x = 0 m to x = 50 m the electrical resistivity values of this shallow layer are slightly more resistive than the latest one (up to 100 Ω m). From the position x = 115 m to x = 205 m another area of well compacted soil cover was found with electrical resistivity values reaching to 100 Ω m. On the abovementioned electrical section the tailing becomes thicker than the tailing on the other electrical sections. This is consistent with the limestone layer dipping slightly to the west. The limestone bedrock layer is also well identified on the electrical section by the highest electrical resistivity values (more than 150 Ω m) (Fig. 2c). 3.4.2. The Las Lajas tailing pond Fig. 3a shows the electrical section obtained by ERI profile 3 of the Las Lajas tailing pond (Fig. 1c). The section was about 175 m long and SE–NW direction. The maximum penetration depth reached was nearly 35 m. A RMS error of 22.7% was obtained after 3 iterations. Unlike the tailing pond described before, this one is covered by a thin layer of soil that favored growing grasses. Waste debris from building demolition was found in the some areas. The shallowest electrical layer associated with waste debris material is roughly uniform ranging from 0.3 m to about 3 m thick. Waste material was poorly sorted and not compacted; it presented a high occurrence of air-filled voids, being consistent with

the measured electrical resistivity values (12 Ω m to 70 Ω m). These electrical resistivity values were more than those obtained for the La Encontrada tailing pond soil cover. The waste debris material is mainly from position x = 17 to x = 120 m. In contrast the thin grass layer is mainly situated between positions x = 130 m and x = 165 m and with lower electrical resistivity values. The shallowest electrical layer is overlaying the most electrical conductive layer, associated with tailing material, with electrical resistivity values below 10 Ω m. The tailing thickness is at maximum (up to 11 m thick) from position x = 85 m to x = 110 m (Fig. 3a). The bedrock layer is lying under the tailing which consists mainly of Paleozoic schist. The bedrock layer has the highest electrical resistivity value which was above 150 Ω m. According to the ERI profile 1 this layer of bedrock has a wavy tailing-bedrock contact due to its location close to a creek. The electrical section obtained from the ERI profile 2 is shown in Fig. 3b. This profile is also 175 m long and was laid out almost parallel to ERI profile 1, running in a SE–NW direction. The maximum penetration depth reached was nearly 35 m. But in this case a RMS error of 12.8% was obtained after 4 iterations. It is interesting to note the close electrical resistivity equivalence between the electrical sections of the ERI profiles 1 and 2. The distance of inter-profiles was about 35–40 m and this was considered a sufficient distance to identify important lateral geologic changes. In this tailing pond, a drill-hole had been made following the geophysics works, so we placed it on the ERI profile 2. Fig. 3b shows the drill-hole position and its geologic log. The depth of this drill-hole was about 15 m with a waste-debris thickness of nearly 3 m: the Paleozoic schist layer occurs at the bottom. This electrical section depicts the same electrical layers as described above, characterized by electrical resistivity values ranging from 12 Ω m to 70 Ω m. The tailing layer was characterized by electrical resistivity values below 10 Ω m, and the bedrock layer with electrical resistivity values above 150 Ω m. Neither electrical resistivity sections exhibit any electrical patterns connected to geological features in the bedrock such as faults and cracks, which could become a potential risk in carrying heavy metals to deeper depths and hence to contaminate aquifers. 3.4.3. The El Llano del Beal tailing pond Fig. 4a shows the electrical resistivity section of the ERI profile 1. This profile was setup following a diagonal direction to the north part of the pond. The orientation of profile 1 is NE–SW (Fig. 1c). This profile has a length of 175 m in which the electrodes were placed every 5 m. The maximum investigation depth was 32 m. The inverted resistivity image was obtained with a RMS error of 5.9% after 5 iterations. This tailing pond was covered by a heterogeneous material composed of cobbles, gravel, and sand, all poorly sorted. The ERI profile 1 has highlighted a shallow layer with a very irregular thickness of about 2–4 m, where the thickness depends on the tailing pond area. We used the geological information from an earlier drill-hole to delineate the contact between the filling and the tailing layers (Fig. 1d). It should be pointed out that the filling material was extracted, from small hills near the tailing pond and then dumped directly on the tailing pond without sorting, to support vegetation growth. Since this layer has a very heterogeneous and poorly sorted particle size, it contains a high percentage of air-filled voids. These voids are the main reason for an important electrical resistivity anomaly in the shallowest layer. This electrical resistivity value ranges between 50 Ω m and almost 100 Ω m. According to the ERI profile 1 the pond area under this profile does not present any region uncovered by erosion action. As can be seen on the electrical section (Fig. 4a), the shallowest electrical layer is continuous along the ERI profile 1. The electrical section displays a decreasing electrical resistivity from the shallowest layer indicating this material as transition (Fig. 4a). Contrasting electrical resistivity exists between filling and tailings and as a consequence the electrical resistivity section presents a transition between the cover and the tailing layer, possibly as a result of a mixing of these materials. This diffuse electrical transition alone makes it

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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difficult to establish the actual contact zone, but the drill-hole was useful in establishing a reliable depth of the tailing pond (Fig. 4b). The tailing electrical resistivity values were below 10 Ω m; however, the uppermost zone of the tailing layer shows a diffuse electrical transition, where electrical resistivity values start decreasing from 45 Ω m to below 10 Ω m. The lowermost zone depicts the usual range of electrical resistivity values between Paleozoic bedrock and tailing (above 13 Ω m). The contact between bedrock and tailing is represented by a dashed black line on the electrical resistivity section. We used geological information from outcropped material to properly delineate the contact since there was a lack of deep drill-holes near the ERI profiles. This contact has a depth ranging from 20 m to nearly 32 m and displays a wavy pattern without any fault, crack or preferential fluid pathway to a deeper horizon. Fig. 4b shows the electrical resistivity section of the ERI profile 2 which was in a diagonal direction to the north part of the pond oriented E–W (Fig. 1c). This profile was 265 m long in which the electrodes were placed every 5 m. The maximum depth was 45 m. The inverted resistivity image was obtained with a RMS error of 19.3% after 5 iterations. This relatively high error was due to electrical resistivity data from position x = 0 m to x = 30 m as a result of the slope (edge effect). The shallow depth had electrical resistivity values ranging between 50 Ω m and N100 Ω m and were more resistive to the right-hand side of the electrical section, due to a thicker soil cover (nearly 5 m) and with more air-filled voids. As is usual, the tailing layer is clearly recognizable by the electrical resistivity values ranging from 50 Ω m to b8 Ω m. Its thickness varies slightly along the ERI profile 2 ranging between a depth of 25 and 30 m. Evidence of the reliability of the electrical data is its similarity to the measured data. The ERI profile 2 electrical section common or cross point is positioned at x = 100 m where the tailing thickness is about 25 m depth. On the ERI profile 1 this cross point is positioned at x = 75 m. The lower electrical resistivity values beyond 150 Ω m are associated with the Paleozoic schist formation. This layer shows a lateral uniform distribution of the electrical resistivity values with a wavy boundary and hence there is no evidence of a fault which would suggest a potential risk of metals reaching deeper aquifers. 3.5. Effectiveness of dumped materials in the reduction of environmental risk The results corresponding to the ratios calculated for total, DTPAextractable and water soluble metal concentrations are shown in Figs. 5–7. Total concentrations of the dumped material used to cover the Las Lajas pond of Zn, Cu and Cd were 28, 22 and 43% lower than those reported in the tailing layers. However, the Pb concentration was 8% higher (Fig. 5). The material used in the La Encontrada was effective in the reduction of Zn and Cd with concentrations 13 and 19% lower than tailing layers. On the other hand the concentration of Pb and Cu

Fig. 5. Variation of total metal concentrations in dumped material and tailing layers.

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Fig. 6. Variation of DTPA extractable metal concentrations in dumped material and tailing layers.

was higher by 160 and 25% respectively (Fig. 5). The dumped material in the El Beal has only been effective in the total concentration of Cu, its concentration being 10% lower than in the subsurface layers. The total concentrations of Pb, Zn and Cd were 54, 124, and 44% higher on the dumped material than on subsurface layers. These results suggest that the metals present in the dumped materials come from a source other than the deeper layers of the ponds. If the current metal concentrations in the surface are due to mixing with the tailing layers, the maximum concentration in the dumped material should be the same as the concentration reported in these layers and not higher. Supporting this hypothesis, geophysical analysis has shown that no significant mix has taken place between the dumped material and tailing layers. Therefore, we think that after covering the ponds with these materials, the mining activities carried out in the surroundings have constantly contaminated by the new surface layer. Some authors have reported total concentrations of up to 12,000 mg kg−1 for Zn and 9000 mg kg−1 for Pb in other tailing ponds from the La Union–Cartagena Mining district (Acosta et al., 2011; Conesa et al., 2006; Garcia et al., 2005). In addition, Meza-Figueroa et al. (2009) concluded that climatic effects such as heavy wind and rainfall can have a significant impact on the dispersion of metals in semi-arid areas. The dumped material in the Las Lajas presents a DTPA extractable concentration of Zn, Cu and Cd much lower than in the layers of the pond (Fig. 6). In addition, the percentages of the total concentration extractable by DTPA are also lower in the dumped materials, decreasing from 3.1 to 1.6 for Zn, from 4.2 to 2% for Cu and from 18.5 to 6.1 for Cd. Therefore, the risk of mobility of these metals through uptake by plants has been significantly reduced. However, concentration and percentage of Pb were slightly higher in the dumped material.

Fig. 7. Variation of water soluble metal concentrations in dumped material and tailing layers.

Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005

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In the case of La Encontrada, the contents of DTPA-extractable Pb and Zn in the dumped materials were similar to the other layers of the pond; although the percentage of Pb was lower, decreasing from 12.9 to 5.5% of the total concentration. For Cu and Cd, both percentages and concentration were lower, so the risk of mobility of these metals was less. The materials used to cover the El Beal pond had lower percentages and concentrations of DTPA extractable Pb and Cu, however, these were higher for Zn and Cd, increasing the risk of mobility of these metals in terms of plant uptake. The percentages of the total concentration of metals extractable by DTPA reported in this study in both dumped material and tailing layers are lower relative to those found for other Pb–Zn mine tailings. By comparison, Ye et al. (2002) calculated percentages of 9.1, 33.9, and 32.3% for Pb, Zn and Cu, while Vega et al. (2004) calculated 19.3, 55.5, and 6.5%, respectively. The percentages observed in this study reveal a relatively smaller labile metal pool and suggest that the ponds examined are less weathered than those examined by Vega et al. (2004) and Ye et al. (2002). We conclude that a smaller proportion of the total metal pool is bioavailable in the three ponds studied. In the dumped materials the concentrations and percentages of water-soluble metals are lower than those reported in the tailing layers for the three ponds, with the exception of Zn in the La Encontrada. Therefore, the risk of mobility by leaching or runoff water is very low. The pond with least risk was the Las Lajas, where the concentration of all water soluble metals has been reduced by 90%. 4. Conclusions High pH, low salinity and balanced texture have likely promoted the natural plant colonization of the three tailing ponds studied. However, total Pb, Cd and Zn concentrations reported in these ponds exceed the maximum admissible concentrations determined by legislation, and consequently, new remediation actions are needed. The total Zn, Cd, and Cu concentrations in the dumped materials depend on the metal content in the tailings. However, Pb, Zn and Cd in the El Beal, Pb and Cu in the La Encontrada, and Pb in Las Lajas were higher in the dumped material than in tailing layers, suggesting that a source other than the deeper layers of the ponds contributed to the total metal levels. We believe that the mining activities carried out in the surrounding areas after these reclamation works have contaminated the new surface layers by the wind transport of metals into suspended particles. While the total metal concentrations have not been reduced below admissible concentrations in the dumped materials, the DTPAextractable metal concentrations and their percentages were much lower in dumped material than in tailing layers. For example: Zn, Cu and Cd concentrations and percentages are lower in the dumped material in the Las Lajas, the DTPA-extractable Cu and Cd were lower in the La Encontrada, and the materials used to cover the El Beal pond had lower percentages and concentrations of DTPA extractable Pb and Cu. Therefore, the risk of mobility of these metals through uptake by plants is very low. Based on our results we recommend that the roots of the vegetation used for reforesting the tailing ponds, where dumped materials exist, are shorter than the thickness of the dumped materials themselves. Water soluble concentrations and percentages are significantly lower in the dumped materials than in tailing layers in most of the metals and ponds, except Zn in the El Beal. Therefore, the risk of mobility of metals by leaching or movement in the runoff water is not considered to create an environmental risk. According to the ERI method, the La Encontrada and Las Lajas tailing ponds displayed areas uncovered by the dumped materials (removed by erosion and water runoff action) so the tailing material is almost exposed, highlighted by the lack of total vegetation cover. Therefore, efforts must be focused on the recovering of these areas to reduce erosion and metal mobility from the tailing layers. The nature of the bedrock does not present any discontinuity that could cause the transport of heavy

metals to deeper horizons. The Llano del Beal tailing pond showed the best condition in the soil cover: the ERI method showed no evidence of either any uncovered area on the surface, from runoff or erosion, or any discontinuity in the bedrock creating conditions which could cause the transport of heavy metal to deep aquifers. This study has shown that former reclamation works were effective in reducing the erosion of tailing layers and metal mobility; however action is needed to cover those areas now exposed. In addition, action is needed to reduce the bio-availability of some metals (e.g. Cd and Zn in the El Beal) by immobilization. Acknowledgments This work has been supported by the project no: CP-IP 213968-2 IRIS, funded by the European Union FP7. J.A. Acosta acknowledges a contract from Fundación Seneca of Comunidad Autónoma de Murcia (Spain). R. Zornoza acknowledges a “Juan de la Cierva” contract from the Ministry of Science and Innovation of the Government of Spain. 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Please cite this article as: Acosta, J.A., et al., Assessment of environmental risk of reclaimed mining ponds using geophysics and geochemical techniques, J. Geochem. Explor. (2013), http://dx.doi.org/10.1016/j.gexplo.2014.04.005