Assessment of agricultural soil contamination by potentially toxic metals dispersed from improperly disposed tailings, Kombat mine, Namibia

    Assessment of agricultural soil contamination by potentially toxic metals dispersed from improperly disposed tailings, Kombat mine, Namibia Marta Mileusni´c, Benjamin Siyowi Mapani, Akalemwa Fred Kamona, Stanko Ruˇziˇci´c, Isaac Mapaure, Percy Maruwa Chimwamurombe PII: DOI: Reference:

S0375-6742(14)00011-9 doi: 10.1016/j.gexplo.2014.01.009 GEXPLO 5286

To appear in:

Journal of Geochemical Exploration

Received date: Accepted date:

17 July 2013 10 January 2014

Please cite this article as: Mileusni´c, Marta, Mapani, Benjamin Siyowi, Kamona, Akalemwa Fred, Ruˇziˇci´c, Stanko, Mapaure, Isaac, Chimwamurombe, Percy Maruwa, Assessment of agricultural soil contamination by potentially toxic metals dispersed from improperly disposed tailings, Kombat mine, Namibia, Journal of Geochemical Exploration (2014), doi: 10.1016/j.gexplo.2014.01.009

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ACCEPTED MANUSCRIPT Assessment of agricultural soil contamination by potentially toxic metals dispersed from improperly disposed tailings, Kombat mine, Namibia Authors:

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Marta Mileusnića, Benjamin Siyowi Mapanib, Akalemwa Fred Kamonab, Stanko Ružičića,

University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Department

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a

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Isaac Mapaurec, Percy Maruwa Chimwamurombec

of Mineralogy, Petrology and Mineral Resources, Pierottijeva 6, HR-10000 Zagreb, Croatia b

University of Namibia, Faculty of Science, Department of Geology, Private Bag 13301,

University of Namibia, Faculty of Science, Department of Biological Sciences

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c

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Windhoek, Namibia

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Corresponding author: [email protected]; +385 1 5535 797

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[email protected]

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[email protected]; [email protected]; [email protected]; [email protected];

Abstract. The Kombat tailings dam, surrounded by agricultural lands, has been exposed to

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water and wind erosion over a long period of time. The objectives of this research were: (1) to characterize the tailings and the surrounding agricultural soils with respect to the mineral and trace element composition; (2) to determine the degree of soil pollution using soil contamination indicators; (3) to assess the environmental risk of polluted agricultural soil; and (4) to identify dominant type (mechanical and/or chemical) and dominant agent (water and/or wind) of metal dispersion from the tailings. A sequential extraction procedure was used to determine binding mechanisms involved in the retention of metals in tailings and soils under the influence of tailings, which indicate the trace metals bioavailability, the threat to groundwater pollution, as well as the dominant type of dispersion. Among seven analysed elements, copper and lead showed significantly high concentrations in tailings, especially in dry season (up to 9086 mg/kg and 5589 mg/kg, respectively). As a consequence, adjacent arable soils have high concentrations of Cu and Pb (up to 150 mg/kg and 164 mg/kg, respectively). Enrichment factors for lead and copper reveal severe contamination, while

ACCEPTED MANUSCRIPT geoaccumulation indices indicate moderate to strong contamination by both elements. The combined pollution index points out high contamination. The main binding phase for Cu and Pb is the reducible fraction (oxides, hydroxides, oxyhydroxides). Similar metal distributions

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in the sequential extraction fractions of tailings and soils support the assumption that wind

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and water disperse tailings predominantly by mechanical transport to the surrounding agricultural soil. Although agricultural soils are contaminated with Pb and Cu, these metals

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are relatively strongly bound to the soils and are of medium risk for their mobilisation after The Risk Assessment Code (up to 20 % for Cu and up to 36 % for Pb). Though rehabilitation of tailings dam, as well as limitation of certain crop use on polluted agricultural land, is

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recommended.

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environmental impact; copper mining

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Keywords: soil pollution, potentially toxic metals, tailings, sequential extraction;

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1. Introduction

Mining and associated activities have a negative impact on the environment both during the mining operations and for years after mine closure. One of the main concerns is tailings; mine waste that is generally toxic. The composition of tailings is directly dependent on the

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composition of the ore and gangue as well as the process of mineral extraction used on the ore. The bulk quantity of tailings is very fine ground barren rock with significant quantities of metals found in the host ore, and substantial amounts of added compounds used in the extraction process (Lottermoser, 2007). Improperly disposed tailings are prone to wind and water erosion. The physical and chemical characteristics of tailings favour metal mobilisation to the environment. Local communities in Otavi Mounatainland (north central Namibia) have been particularly exposed to the detrimental effects of mining and ore processing on the environment, agriculture and public health concerns because of widespread mining operations (Tsumeb, Kombat and Berg Aukas mining districts), economic pressures and weak environmental awareness. Mapani et al. (2010) found severe trace metal contamination of the surface soils, as well as high accumulation of trace elements in crops, south and east of the abandoned

ACCEPTED MANUSCRIPT mining district Berg Aukas, and delineated a zone of high risk to human health. An environmental exposure assessment, which showed anomalous lead and arsenic concentration in urine and blood of inhabitants, was performed in the Tsumeb area with almost century long

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mining and smelting activities (Hahn, et al. 2008). The presence of highly soluble phases in

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slags might be responsible for the significant release of toxic elements in the Tsumeb area, through their rapid dissolution during thunderstorm events occurring between October and

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March (Ettler et al., 2009).

Extensive mining and ore processing activities in the Kombat area, which lasted for 46 years, left significant amounts of mining waste in the form of abandoned, improperly disposed

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tailings that was identified as a main source of contamination (Kříbek and Kamona, 2006). The tailings dam is located within a farming area, where wheat and corn are cultivated,

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whereas livestock is kept for the beef industry and accounts for a major source of local farmers’ income.

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The subject of investigation in this paper is agricultural soil around Kombat tailings dam. The

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objectives of this research were: (1) to characterize the contamination in tailings and the surrounding agricultural soils; (2) to determine the degree of soil pollution; (3) to assess the

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environmental risk of polluted agricultural soil; and (4) to identify dominant type (mechanical and/or chemical) and dominant agent (water and/or wind) of metal dispersion from the tailings. The results are to be used for decisions about future land use, soil remediation and

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tailings reclamation methods.

A pseudo total concentration of metals in soil, determined after aqua regia digestion, is a commonly used procedure for estimation of the maximum element availability to plants (Vercoutere et al., 1995). Residual elements that are not released by aqua regia digestion are mostly bound to silicate minerals and are considered unimportant for estimating the mobility and behaviour of the elements (Niskavaara et al., 1997). According to Marr et al. (1995), aqua regia digestion might give close results for the maximum levels of polluting metals such as Cd, Cu, Pb, and Zn in soils, which are the main elements of interest in this study. In order to evaluate the main binding processes by which metals are retained in the soil and tailings, it is useful to obtain information on how the metals are distributed among the geochemical phases (binding agents) present. Sequential extraction is a useful technique for determining chemical forms of metals in different media such as soils, sediments and different anthropogenic material. Different schemes of sequential extraction techniques have been

ACCEPTED MANUSCRIPT widely applied to soils contaminated by mining operations (e.g. Durn et al.; 1999; Garcia et al., 2008; Kaasalainen and Yli-Halla, 2003), as well as to tailings (e.g. Dold, 2003; Favas et al., 2011, Martínez-Martínez et al., 2013). These techniques are mostly designed to

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differentiate between metals bound in exchangeable and adsorbed, acid soluble (carbonates),

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reducible (oxides, hydroxides and oxyhydroxides), oxidisable (sulphides and organic phases) and residual fraction. There is some uncertainty concerning incomplete selectivity of reagents

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and an extent of post-extraction redistribution of leached metals among un-dissolved solid phases (Gomez-Ariza et al., 1999). Nevertheless, the results obtained from sequential extraction can be used at least qualitatively to understand binding mechanisms involved in the

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retention of metals in tailings and soils under the influence of tailings, which indicate the trace metals bioavailability, the threat to groundwater pollution, as well as the dominant type of

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dispersion. In this study, a combination of the Tessier extraction scheme (Tessier et al., 1979) and procedure optimised for carbonate-rich sediments (Sulkovski and Hirner, 2006) was used.

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2. The study area

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The mining town of Kombat (Otjozondjupa Region, Grootfontein District) (Figs. 1 and 2) is located at the piedmont of Otavi Mountainland (northern Namibia), at latitude 1943´S and

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longitude 1743´E, and has an altitude of about 1,600 meters above mean sea level.

2.1. Geological setting

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The Otavi Mountains in northern Namibia are the easternmost exposure of the Neoproterozoic Otavi Platform, which represents a sedimentary sequence of platform carbonates deposited along the whole western and southern passive margin of the Congo Craton. Sedimentary rocks of the Otavi Mountains host numerous ore deposits, e.g. Tsumeb, Kombat, Berg Aukas and Khusib Springs (Fig. 1) that developed during the Pan-African tectonic activity in the southern core of the Damara Orogenic Belt. Details of the stratigraphy and ore geology of the Otavi Mountainland (OML) are given in Miller (1983, 2008), Frimmel et al. (1996), Hoffmann et al. (1996, 2004), Kamona and Günzel (2007) and Boni et al. (2007). The stratabound, syntectonic Kombat Cu-Pb (Ag) mineralization forms subvertical, irregular and lens-like ore bodies, located in the contact zone between phyllite and underlying dolomites on the southern flanks of the Otavi Mountains. Bornite, with minor tennantite, and chalcopyrite, galena, sphalerite and chalcocite are by far the most common primary minerals. Secondary mineralization is composed of malachite, chalcocite, azurite, arsenates, cerussite,

ACCEPTED MANUSCRIPT chrysocola, cuprite and native copper. Details of the structural evolution and ore characteristics of the Kombat ore deposits are given in Innes and Chaplin (1986), Deane

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(1995) and Changara (2009).

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2.2. Geographical setting

Otavi Mountainland (OML) stretches over wide region of 5000 sq km with an elevation of

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1300-1900 m asl. Carbonates are the dominant bedrock in the study area. The name given to the broader area (OML and northern part of Outjo District) of extensive dolomite and limestone formations is Karstveld. Karstveld landscape is characterised by numerous karsts

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phenomena (scarps, sinkholes, caves and stream captures). The Karstveld groundwater system consists of a number of distinct water bodies including subterranean cave lakes, cenotes, and

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hemi-cenotes. In the Kombat area, besides carbonates, phyllites and metavolcanic rocks can also be found as bedrock. All watercourses are dry for the most part of the year. Geographical characteristics of the area are taken from the Digital Atlas of Namibia (Directorate of

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Environmental Affairs, 2002). Soils can be classified as calcic regosols, calcic cambisols and

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pelitic vertisols according to the FAO (1997) classification (Kříbek and Kamona, 2006). Only a 0-5 cm thick layer of rock debris covers hilly parts of the mapped regions. Steep slopes are

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usually covered by calcic regosols foothills and slightly inclined carbonate plains are dominated by calcic cambisols and flatlands by pelitic vertisols. The vegetation consists of deciduous woodland savanna (Mendelsohn et al., 2002) with a relatively high diversity of

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scattered tall trees within a dense understory of smaller trees and shrubs.

2.3. Climate

The Otjozondjupa Region has a dry semi-arid climate of subtropic latitudes (BSh) according to the Köppen–Geiger climate classification (Kottek et al., 2006) as there is less rainfall than evapotranspiration. Kombat is located within the summer rainfall zone (October to April). The mountainous topography of the Otavi Mountainland, which rises more than 800 m above the surrounding plains, exerts an orographic effect by intercepting moisture, surface heating of bare rock surfaces and through convection. The Mountainland therefore forms an outlier of greater rainfall within the arid interior of Namibia, with the secondary effect of ensuring some precipitation during even the driest years (Sletten et al., 2013). Mean rainfall at the nearby

ACCEPTED MANUSCRIPT Grootfontein meteorological station, is 532 ± 176.6 mm per annum (as determined over 65 years) with most rainfall expected from January to March, though the rainfall amount may vary by more than 600 mm between individual years. Other rainfall stations in the Otavi

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Mountainland such as Tsumeb (501 mm/yr), Otavi (519 mm/yr), and Ghaub (587 mm/yr)

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indicate the variability that may occur. Evapotranspiration is high with a potential evaporation of 2900 mm/y and maximum evaporation rates from October to December, leading to a

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negative theoretical water balance for most of the year.

The mean annual temperature for a 17-year period at Grootfontein is 21.0 ± 3.3 °C, though there is a marked seasonality in the air temperature with a mean austral summer (November to

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January) temperature of 24 °C and a mean winter temperature (May-July) of 16 °C.

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The wind flow fields for Grootfontain meteorological station are dominated by stable winds from the south-easterly quadrant. These winds have an average wind speed between 3 and 5 m/s and they occur 40 % of the time. During summer (January) and autumn (April), north-

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easterly winds are more frequent. Winter months are characterized by the highest frequency

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of easterly winds.

2.4. Mining history and current land use Kombat had been a major mining town from 1962 until 2008 when the mine was closed.

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During this period, 24,550,280 t of copper ore was crushed, grinded and processed by flotation at Kombat, which resulted in approximately 300 million tones of tailings (Kříbek and Kamona, 2006), that were heaped within farming lands, circa 3 km to the south east from the mine entrance (Fig. 2). The tailings dam spreads over 150,000 m2 (15 ha) and has a height of 20 m in average. The main land use in Kombat at the moment is agriculture that is oriented mostly on livestock production and some cash crops such as maize and wheat. Density of cattle is 5-12 animals/km2, livestock (stocking) density is 20-39 kg/ha and carrying capacity of the land is 40-49 kg/ha (Directorate of Environmental Affairs, 2002). Grain, vegetables and fruits are also produced. Rainwater and mine water are used for both drinking and irrigation.

3. Materials and methods

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3.1. Sampling Sampling design in this study was based on the results of regional environmental geochemical

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mapping which had been undertaken in 2005 (Kříbek and Kamona, 2006). Soil sampling in

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the frame of regional geochemical mapping was done at 1x1 km interval grid. Results of that mapping indicated that soils in the Kombat area are strongly contaminated with As, Cu, Pb,

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Hg and S in the neighbourhood of mine works and the tailings dam (especially agricultural land west and northwest from the dam).

The comprehensive assessment of the pollution from the tailings dam into the surrounding

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area presented in this study was done after assuming that the main agents of material transfer from tailings into surrounding areas are wind (Fig. 3) and rain water which comes as run-off

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from the tailings dam in episodes of heavy downpours (Fig. 4). As presented on Fig. 2 by wind rose and described in Climate section, south-easterly winds prevail. Although runoff from the tailings affects land in all directions, gully, which was formed by water (Figs. 2 and

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4), intersects tailings dam (striking southeast – northwest) and causes preferential water flow

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with dominant direction to the west and to the south. In this study, there were three types of samples collected, tailings (14 samples), designated as

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T-samples; soil samples from areas in the windward direction from the tailings assumed as polluted (13 samples), designated as P-samples; and soil control samples (20 samples), obtained about 5 kilometres southern from the tailings dam, which were indicated as C-

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samples and assumed to have background concentration of analysed elements. Sampling areas are shown in Fig. 2 while accurate GPS position of each sampling point in Apendix 1. Soil material at each location was collected in a square of 10 x 10 m, with four subsamples taken from the corners, and single subsample from the centre, where the GPS readings for each sample were taken, and mixed and homogenized together to form composite material. Each subsample weighted about half kilogram. Samples were taken from the uppermost 4 cm of the soil profile with a plastic blade. Organic litter and coarse carbonate pebbles are removed from the soil samples. Tailings samples were also sampled in a 10 x 10 m square. All sample locations were sampled twice, once in the wet season (20-25. March 2006; designated with A), and secondly in the dry season (19-20. August 2006; designated with B). Salt crusts were sampled at some locations in dry season (5 samples).

3.2. Sample preparation

ACCEPTED MANUSCRIPT Approximately 0.5 kg of samples were air-dried, ground and passed through 0.2 mm standard plastic sieve of the British Geological Survey. The size fraction of < 0.2 mm was further

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milled and homogenized in an agate ball mill for chemical and mineralogical analysis.

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3.3. Mineralogical analysis

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Mineral composition of all samples (Appendix 1) was determined by means of powder X-ray diffraction (XRD) using a Philips diffractometer PW1710 (CuKα radiation - λ=1.54056 Å) equipped with a graphite monochromator, proportional counter, and automatic divergence slit.

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Scan settings were 2–70° 2θ, 0.02° step size, 1 s count time per step while generator settings were 40 kv, 35 mA. Identification of phases was achieved using the commercial package

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PANalytical X'Pert HighScore Version 1.0d and the International Center for Diffraction Database PDF2 (Newtown Square, PA, USA).

Unfortunately, X-ray diffraction, because of a high detection limit (cca. 5%), is not an ideal

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technique for mineralogical analysis of such kind of a mixture, as minerals of interest, such as

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sulphides and secondary oxyhydroxides represent minor proportion in bulk samples. Hence, conventional technique such electron microprobe analysis or even better newly developed

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analytical techniques such synchrotron-based X-ray absorption spectroscopy should be used to collect information about minerals that are present in low abundance (Jamieson, 2011). In the absence of such sophisticated techniques, selective dissolution is useful for the study of

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the secondary minerals in tailings to understand their importance in the element cycling pathway in mine waste environment (Dold, 2003). 3.4. The pH analysis

The soil and tailings pH values were determined in suspension of selected original samples (Appendix 1) in 1M potassium chloride suspension after 24 hours using the PHM 201 pHmeter with pHC 2085 pH electrode and the T 201 temperature compensator. Calibrations were carried out using standard IUPAC buffers.

3.5. Chemical analyses Pseudo total concentration of trace element (Cu, Pb, Zn, Cr, Ni, Co, Cd, Mo) were obtained for all samples (Appendix 1) after aqua regia (HNO3 :HCl, 1 : 3) digestion of 2 g of sample at 80ºC for 2 h, by flame atomic absorption spectrometry (AAnalyst 700, Perkin Elmer). The accuracy of analysis was controlled by analysis of USGS geological reference soil standard

ACCEPTED MANUSCRIPT GXR-2 in the analysed sample batches. The accuracy of analysis expressed as the ratio between measured (aqua regia extracted) and recommended values of elements in the standard samples (total acid digestion) was in the range ±5-20%. Precision based on aqua

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regia digestion of selected replicate samples was below 10%.

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Different binding sites of metals in selected soil and tailing samples (Apendix 1) were analysed using sequential extraction. Selection is based on the type of sample, sampling

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season and metal concentrations, whereby a series of single reagents is used to extract operationally-defined phases (the selectivity depends on such factors as chemicals employed, the time and nature of contact, and the sample to volume ratio) in a defined sequence from 1 g

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of the sample. As Kombat soils and tailings have a high content of carbonates and are highly contaminated with copper and lead, to avoid possible problems with the selectivity and re-

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adsorption, a combination of the Tessier extraction scheme (Tessier et al., 1979) and procedure optimised for carbonate-rich sediments (Sulkovski and Hirner, 2006) was used to give four fractions as shown in Table 1: exchangeable, acid (bound to carbonates), reducible

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(bound to iron and manganese oxides, hydroxides and oxyhydroxides) and oxidisable (bound

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to organic matter and/or sulphides. The residual fraction is calculated as the difference between aqua regia extracted metal concentration and sum of these two sequential extraction

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steps.

The sequential extraction was conducted in centrifuge tubes (polyethylene, 50 ml) and stoppered with a screw cap. Between each successive extraction step, the liquid phase was

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separated by centrifugation at 3000 rpm for 10 min. The residue was washed with 10 ml of deionised water (after steps 1, 2 and 4) and with 10 ml 25 % acetic acid (after step 3), homogenized with ultrasound and centrifuged for 10 min at 3000 rpm. This second supernatant was added to the first supernatant. Nitric acid (1 %) was added to dilute the supernatant to prevent precipitation of metals. The residue of the sample was used for the successive extraction steps. It has to be emphasized that the pH of the solution was measured after each step, and the second step was repeated for each sample as the pH changed for more than 0.5 caused mainly by carbonate dissolution (Sulkovski and Hirner, 2006). Thus, results for the second step are sums of concentration of two acetic acid extractions. All the reagents used in the extraction procedures were of analytical grade. The resulting solutions were analysed by flame atomic absorption spectrometry (AAnalyst 700, Perkin Elmer). 3.6. Degree of soil contamination and risk assessment

ACCEPTED MANUSCRIPT Metal concentrations in analysed soils and tailings were compared with Canadian Soil Quality Guidelines (CCME, 1999, 2007). They are defined as the concentrations recommended to provide a healthy, functioning ecosystem capable of sustaining the existing and likely future

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uses of the site by ecological receptors and humans. Canadian soil quality guidelines can be

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used as the basis for soil contamination assessment of four types of land uses: agricultural, residential/parkland, commercial, and industrial. Canadian Soil Quality Guidelines (CCME,

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1999, 2007) for analysed elements expressed in mg/kg are shown in Table 2. Metal concentrations in tailings are compared with industrial land use, while those in soil with agricultural land use.

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The degree of contamination is expressed as: (1) enrichment factor (Lăcătuşu, 1998), (2) geoaccumulation index (Müller, 1969), and (3) combined pollution index / coefficient of

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industrial pollution (eg. Kříbek and Nyambe, 2002, Abrahim and Parker , 2008). All of these soil contamination indicators are calculated with respect to local background, i.e. median

and lead concentrations are used.

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values of metal concentrations in control soil samples. For combined pollution index, copper

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Enrichment factor (EF) is the ratio of metal concentration in the sample and median of the background concentrations. Geoaccumulation Index (GI) is a natural logarithm of the ratio of

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concentration of metal in a sample divided by median of the background concentrations. Sometimes it is the median concentration multiplied by a correction factor of 1.5. The factor of 1.5 suggested Loska et al. (1997) for the correction of metal fluctuations in the

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environment and small anthropogenic influences. In this study, the factor is not used as local background was applied in the Combined Pollution Index (CPI) / Coefficient of Industrial Pollution (CIP) is the sum or enrichment factors for analysed contaminants divided by the number of enrichment factors included in the calculation. Table 3 shows the classification of contamination based on these indicators. Metal concentrations in aqua regia extracts cannot be the sole indicator of the environmental risk caused by polluted soil and tailings, since they can be tightly bound in a mineral lattice of minerals and although present in significant amounts do not affect the environment. One approach to environmental risk assessment is the classification of samples to Risk Assessment Code (RAC), introduced by Perin et al . (1985), which many authors primarily used for metal contamination of marine sediments.

ACCEPTED MANUSCRIPT 3.7. Results and discussion

4.1. Characterisation of Kombat tailings

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Mineral content of all tailing samples is dominated by carbonates (calcite and dolomite),

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primary gangue minerals. Other primary non-sulphide minerals present in small amounts in all analysed samples are quartz and micaceous material. Gypsum, a secondary mineral

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precipitate, commonly produced by the weathering of mine waste material and mineralized rock and by evaporation (Nordstrom, 2011), was detected in several samples. Evaporation is an important mechanism in the formation of mineral salts. The surface precipitates are

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commonly referred to as “efflorescences”. Beside gypsum, there are indications that rozenite, another sulphate which efflorescences, is present in the Kombat tailings. Among secondary

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oxides, there are indications of cuprite presence.

The reaction of the Kombat tailings is circumneutral (pH values ranges from 7.0 to 7.7) as a result of abundant carbonate minerals which play an extremely important role in acid

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buffering reactions (Lottermoser, 2007) neutralizing acid generated from sulphide oxidation.

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Calcite, compared to dolomite, has a rapid rate of reaction and is more easily dissolved. This is particularly prevalent in an open mine waste environment where it is exposed to a carbon

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dioxide gas phase. (Sherlock et al, 1995). Concentrations of seven trace elements (Cu, Pb, Zn, Cd, Cr, Ni, Co), as well as manganese and iron were analysed in tailings samples. Main statistical parameters of their concentrations

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are listed in Table 4. Concentrations of Cr, Ni and Co are very low and are similar for samples from both seasons as well as for efflorescence salts (median values: 16 mg/kg; 2.2-4.1 mg/kg; 4.1-5.7 mg/kg, respectively). On the other hand, concentrations of Cd and Zn are elevated and significantly higher in dry season (median values: 1.5-4.1 mg/kg and 51-111 mg/kg, respectively). Concentrations of copper and lead are very high (median values: 1255-2489 mg/kg and 811-2901 mg/kg, respectively) and vary significantly even between samples of the same season (eg. 641-9086 mg/kg Cu in the dry season). The median of Pb and Cu concentrations are higher (1.4 and 1.3 times, respectively) for dry season period (Table 4, Figs. 5 and 6). In both types of samples (regular tailing samples from dry season and salt crust samples) concentrations vary significantly but are in similar ranges (Table 4). It is probable that all samples from dry season have significant amount of secondary salts. Significant quantities of trace elements, including metals and metalloids, can coprecipitate with secondary salts. Due to a large surface area of secondary salts, trace elements can be adsorbed

ACCEPTED MANUSCRIPT as well. Thus, secondary salts can temporarily, during dry season, immobilize potentially toxic elements. The problem arises during the wet season when mineral efflorescence tends to be soluble and release their stored metals (Lottermoser, 2007). Although the high amount of

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carbonate minerals conditions neutralized drainage, some potentially toxic elements, such as

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arsenic, which can be abundant in sulphide mine tailings, may still be present in relatively

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high concentration in the waters draining such tailings (Jamieson, 2011). Pb, Cu and Zn concentrations in tailings are in agreement with abundance of their minerals in the Kombat ore. Cu and Pb ore minerals far outweigh sphalerite (Zn ore) in the Kombat ore

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deposit (Innes and Chaplin, 1986).

While concentrations of Cd, Cr, Ni, Zn and Co in tailings do not exceed Canadian soil quality

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guidelines for industrial land use (Tables 2 and 4), the concentration of copper in all analysed samples is significantly higher (5 to 100 times) than the guideline value of 91 mg/kg and that of lead exceeds the guideline value of 600 mg/ kg in all but one sample, with the highest value

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being 9 times higher.

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The distribution of Pb in sequential fraction of the Kombat tailings is very similar for all analysed samples. The dominant fraction is reducible fraction (oxides, hydroxides,

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oxyhydroxides) with approximately 60% of the sum of lead concentrations in four analysed fractions. The second most important fraction is carbonate fraction with approximately 30% of total lead. Exchangeable fraction has about 10% of lead, while oxidisable fraction (in the

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case of tailings mainly sulphides) has only around 1% of lead. The distribution of Cu, unlike Pb, in sequential fractions of tailings is variable. Nevertheless, the two most important fractions are reducible fraction (oxides, hydroxides, oxyhydroxides) followed by oxidisable fraction (mainly sulphides). Reducible fraction contain about 30-60 % of lead, oxidisable fraction about 10-40%, carbonates 10-20% and exchangeable fraction about 20%. The high variability of sequential extraction data for tailings could be caused by poor selectivity of applied sequential extraction analysis. To increase the selectivity and the accuracy of geochemical data interpretation in tailings studies, sequential extraction adapted to specific mineralogy should be used (Dold, 2003).

ACCEPTED MANUSCRIPT 4.2. Characterisation of soils The mineral content of soil samples is dominated by quartz and different amounts of carbonate minerals. Other minerals present are: K-feldspar, plagioclase and clay minerals.

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and the total carbonate amount in these samples is higher.

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Polluted soil samples in contrast to control samples contain dolomite in addition to calcite,

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Soil pH values range from 8.1 to 8.6. According to USDA NRCS (2013) this soil is classified as moderately to strongly alkaline. Soil pH is in agreement with the results of mineralogical analysis that confirmed the presence of carbonates. The presence of carbonates, and related

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pH, is probably controlled not only by the bedrock lithology but also by the carbonate-rich tailings, which are dispersed by wind (dust fall-out) and water to the agricultural land. The

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support for this presumption is the higher amount of total carbonates, as well as presence of dolomite in polluted samples, compared to control samples. Concentrations of seven trace elements (Cu, Pb, Zn, Cd, Cr, Ni, Co), as well as manganese

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and iron are analysed in soil samples (Table 4). Control samples data represent background

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values of analysed elements in the soil of the Kombat area. Comparing to background values, agricultural soil westward from tailings contain elevated concentrations (Table 4, Figs. 5 and

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6) of copper, lead, zinc and manganese (approximately 7-8 times higher for Cu and Pb; and 2 times higher for Zn and Mn). Concentrations of these elements differ slightly between seasons and their medians are a little bit elevated for samples collected in wet season (Table 4). There

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is a possibility that secondary salts in tailings are dissolved by rain water during wet season and transported to the agricultural land by torrents that form due to heavy downpours. Concentration of copper and lead in all arable soil samples adjacent to tailings dam exceed Canadian soil quality guidelines for agricultural land use (Canadian Council of Ministers of the Environment, 2007). Guideline values of 63 mg/kg and 70 mg/kg for copper and lead, respectively, are exceeded by up to 2.5 times. The importance of the soil fractions as binding sites for copper and lead, determined by sequential extraction analysis of polluted soil samples, follow this sequence (Figs. 7 and 8): Cu: CRFE (53-78%) >> RES (2-28%) > OR/SULF (5-13%)  EXC (1-14%)  CARB (3-9%) Pb: CRFE (58-82%) >> CARB (4-22%) > RES (2-13%)  OR/SULF (1-6%)> EXC (0-18%)

ACCEPTED MANUSCRIPT 4.3. Degree of arable soil contamination and environmental risk assessment Soil contamination indicators are calculated with respect to local background, ie. median value of metal concentrations in control soil samples from both seasons (18 mg/kg Cu and 16

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mg/kg Pb). Median, mean and range values of soil contamination indicators are presented in

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Table 5. Correction factor of 1.5, for the fluctuations of a given metal in the environment and

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very small anthropogenic influences, proposed by Loska et al. (1997) is not applied as the local background is used. Only copper and lead contaminated the arable soil (Table 5). Enrichment factors for lead (4.4-10.2) and copper (4.1-8.4) reveal the following degrees of soil contamination: severe to very severe, and severe, respectively. Geoaccumulation indices

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for lead (2.2-3.4) and cooper (2.0-3.1) indicate mainly moderate to strong contamination by both elements. The combined pollution index, calculated on the basis of copper and lead,

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varies from 4.3 to 9.3 and indicating high to very high contamination. The distribution of metals in different soil phases offers an indication of their availability, which in turn reflects the risk associated with the mobilisation of metals. The fractions most

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influenced by human activity and changes in environmental conditions include the

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exchangeable and carbonate-bound fractions, which are considered to be weakly retained and may equilibrate with the aqueous phase, hence becoming more rapidly bioavailable (Rath et

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al., 2009). The Risk Assessment Code (RAC) gives an indication of the possible risk by applying a scale to the percentage of metals present in the exchangeable and carbonate fractions (Perin et al, 1985). Accordingly, if this value is < 1% there is no risk, 1-10%

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indicates low risk, 11-30% medium risk, 31-50% high risk, and > 50% very high risk. In the Kombat agricultural soil, the percentage of copper in the first two soil fractions is lower than 20% for all analysed samples in both seasons indicating low risk to medium risk. Risk assessment codes for lead instead indicate low risk in samples collected in dry season (about 5%) and medium risk to high risk (22-36%) in samples collected in wet season.

4.4. Dominant type and dominant agent of metal dispersion Due to long periods of drought during winter months and short heavy downpours during summer, wind prevails over water as the agent of material transfer from tailings into soil. Accordingly, mechanical transport of the Kombat tailings dam prevails over chemical transport. Similar metal distribution (Figs. 7 and 8) in sequential extraction fractions of tailings and soils (the most significant binding site is reducible fraction) are in agreement with

ACCEPTED MANUSCRIPT the assumption that wind and short term but strong water torrents are dispersing tailings mechanically to the surroundings agricultural soil in the Kombat area. The presence of dolomite and higher amount of total carbonates in polluted soil compared to control soil could

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have the same origin. The high pH values of tailings promote metal precipitation and prevent

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chemical weathering on a large scale followed by chemical dispersion in wet season. However, some potentially toxic elements, such as arsenic which is associated with lead and

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copper in sulphide ore minerals of the Kombat ore deposit, may be mobilised and chemically dispersed by water. Hence, there is a need for a speciation study of arsenic in the soil of Kombat area.

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4.5. Recommendation for Kombat tailings dam rehabilitation and land use of surrounded

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area

There is an environmental and health risk caused by instable and exposed tailings of the abandoned Kombat mine. Whilst heavy rain can cause collapsing, even a slight breeze can

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freely spread loosely deposited fine-grained toxic materials in the surrounding area and pose

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arable soil contamination as well as health risk in residential area. Hence, rehabilitation of the tailings dam would be desirable. An effective strategy to isolate tailings is capping with

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impermeable layer of solid, inert material such as different mineral barriers, which would prevent percolation of water through tailings (Lottermoser, 2007). Covering tailings with soil and planting vegetation can follow capping or it can be independently. Phytostabilisation is a form of phytoremediation that uses plants for long-term stabilisation and containment of

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tailings, by sequestering pollutants in soil near the roots. Vegetation would reduce wind and water erosion, as well as immobilise metals by adsorption, accumulation, and by providing a zone around the roots where the metals can precipitate and stabilise. Hence, pollutants become less bioavailable and livestock, wildlife, and human exposure is reduced. It is important: (1) to establish initial plant communities using local species that are tolerant of drought, poor soil texture, the lack of organic material and nutrients, as well as potentially toxic metals, and (2) to establish soil invertebrate and microbial communities to ensure natural organic decomposition, essential to the rebuilding of soil. No definite experiments have been done to establish phytoaccumulators of heavy metals in the Kombat area, however, there are plants that were dominant right on the edge of the tailings dam which could potentially be explored as stabilisers. In the past, the mine management planted Prosopis sp. which was not very successful in stabilisation of soil. This plant could be used in combination with Dichrostachys cinerea, Acacia karroo, Grewia flavescens, Commicarpus pentadrus,

ACCEPTED MANUSCRIPT Chloris virgata and Ziziphus mucronata, as well as herbaceous Commicarpus pentandys which grows very well on the tailing dam (Dumba, 2013). As the arable soil adjacent to the tailings dam is polluted with regards to copper and lead, this

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area should be avoided for agriculture, or its use limited. It is necessary to avoid crops, such

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as sweet potatoes, cabbage, and Irish potatoes, that accumulate potentially toxic metals in

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edible parts of the plant as well as leafy vegetable as their leaves could be covered by tailings dust. Metals are mostly accumulated in roots, so root vegetables should be avoided. Kříbek et al. (2012) found that cassava, which grew on the soil polluted by copper mining activities in the area of Copperbelt in Zambia, do not accumulate metals and arsenic. Mapani et al. (2010)

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suggested cultivation of maize in the polluted area of Zn-Pb- V Berg Aukas mine

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5. Conclusion

Among seven analysed elements, copper and lead showed significantly high concentrations in tailings, especially in dry season (up to 9086 mg/kg and 5589 mg/kg, respectively). As a

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consequence of improper tailing protection from erosion, adjacent arable soil west of the

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tailings dam have high Cu and Pb concentrations (up to 150 mg/kg and 164 mg/kg, respectively), exceeding Canadian guideline values for agricultural land use (63 mg/kg and 70

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mg/kg, respectively). Enrichment factors for lead and copper reveal severe contamination, while geoaccumulation indicate moderate to strong contamination by both elements. The combined pollution index point out high contamination. The main binding phase for Cu and

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Pb is the reducible fraction (oxides, hydroxides, oxyhydroxides). Similar metal distributions in the sequential extraction fractions of tailings and soils support the assumption that wind and water disperse tailings predominantly by mechanical transport to the surrounding agricultural soil. The first two most soluble sequential fractions of soil do not have significant amounts of these two elements of concern resulting in a medium risk from copper and lead after Risk Assessment Code (up to 20 % for Cu and up to 36 % for Pb). Even in the case of mobilisation, moderately to strongly alkaline soil would cause re-precipitation. Nevertheless, it is recommended to limit the use of certain crops that accumulate Pb in and Cu in edible parts of the plant (eg. root vegetables), as well as leafy vegetables because their leaves could be covered by tailings dust, in order to reduce potential health risks. Given the fact that carbonate minerals dominate in tailings, the drainage is neutralized and lead and copper are rendered immobile. Mechanical weathering of the Kombat tailings dam

ACCEPTED MANUSCRIPT prevails over chemical weathering, and wind is the dominant agent for metal transport to the adjacent agricultural soil. The rehabilitation of the tailings dam would be desirable. An effective strategy to isolate tailings is capping with impermeable layer of solid, inert material

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and/or phytostabilisation, i.e. covering tailings with soil and planting vegetation.

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Acknowledgements

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This study was carried out within the framework of the IGCP/SIDA Project No. 594, Assessment of the impact of mining and mineral processing on the environment and human health in Africa. Fieldwork was financed by Department of Geology, Faculty of Science,

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University of Namibia. Laboratory work was sponsored by Department of Mineralogy, Petrology and Petroleum Engineering of Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Croatia. We are grateful to Mendine Thomas and Johanna

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Kaluwapa who assisted with field work logistics, as well as to Michaela Hruškova who just before our laboratory work adjusted a new AAS device. The authors sincerely thank Prof.

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Bohdan Kribek, Managing Guest Editor of the Journal of Geochemical Exploration for his

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invitation to submit this contribution. We appreciate the critical reviews, constructive comments and valuable remarks from reviewers that greatly improved the initial manuscript.

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We also thank Miss Danielle Cartwright for help in reviewing the English of the paper.

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Changara, L., 2009. The lithological and structural controls on mineralisation at Kombat Mines, Otavi Mountainland, northern Namibia. PhD thesis, University of Namibia, 480. Deane, J.G., 1995. The structural evolution of the Kombat deposits, Otavi Mountainland,

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347–365.

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Mendelsohn, J., Jarvis, A., Roberts, C., Robertson, T., 2002. The Atlas of Namibia. a Portrait

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Miller, R. McG., 1983. The Pan-African Damara orogen of South West Africa/Namibia. In. Miller, R. McG. Ed., Evolution of the Damara Orogen of South West Africa/Namibia.

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Geological Society of South Africa, Pietermaritzburg, pp. 431–515 Spec. Publ. 11. Miller, R. McG., 2008. Geology of Namibia. Volume 2 : Neoproterozoic to Lower

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Palaeozoic. Ministry of Mines and Energy, Geological Survey, Windhoek. Müller, G., 1969. Index of geoaccumulation in sediments of the Rhine River. Geojournal 2:

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108–118.

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European arctic Finland, Norway, and Russia. Environmental Pollution 96, 261–274. Nordstrom; D.K. 2011. Mine Waters. Acidic to Circumneutral. Elements, Vol. 7/6, 393–398.

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Perin, G., Craboledda, L., Lucchese, M, Cirillo, R., Dotta, L., Zanette, M.L., Orio, A., 1985. Heavy metal speciation in the sediments of Northern Adriatic Sea-a new approach for environmental toxicity determination, in: Lekkas, T.D. (Ed.), Heavy Metal in the Environment. CEP Consultants: Edinburg, pp 454-456. Rath, P., Panda, U.C., Bhatta, D., Sahu, K.C., 2009. Use of sequential leaching, mineralogy, morphology and multivariate statistical technique for quantifying metal pollution in highly polluted aquatic sediments—A case study: Brahmani and Nandira Rivers, India. Journal of Hazardous Materials 163/2–3, 632-644. Sherlock, E.J., Lawrence, R.W. Poulin, R., 1995. On the neutralization of acid rock drainage by carbonate and silicate minerals. Environmental Geology 25, 43-54. Sletten, H.R., Railsback, L.B., Liang, F., Brook, G.A., Marais, E., Hardt, B.F., Cheng, H., Edwards, R.L., 2013. A petrographic and geochemical record of climate change over the last

ACCEPTED MANUSCRIPT 4600 years from a northern Namibia stalagmite, with evidence of abruptly wetter climate at the beginning of southern Africa's Iron Age. Palaeogeography, Palaeoclimatology, Palaeoecology 376, 149–162.

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with high carbonate content. Applied Geochemistry 21/1, 16-28.

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Sulkowski, M., Hirner, A.V., 2006. Element fractionation by sequential extraction in a soil

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Tessier, A., Campbell, P. G. C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 51, 844–851. USDA NRCS 2013. National soil survey handbook, title 430-VI. Part 618.47. Reaction pH.

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Soil Properties and Qualities.Available online at http.//soils.usda.gov/technical/handbook/.

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Accessed [04/11/2013].

Vercoutere, K., Fortunati, U., Muntau, H., Griepink, B., Maier, E.A., 1995. The certified reference materials CRM 142 R light sandy soil, CRM 143 R sewage sludge amended soil

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and CRM145 R sewage sludge for quality control in monitoring environmental and soil

Table caption Table 1

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pollution. Fresenius' Journal of Analytical Chemistry 352, 197–202.

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Sequential extraction procedure applied in this study (combination of Tessier et al., 1979 and Sulkowski and Hirner, 2006) Table 2 Canadian Soil Quality Guidelines (CCME, 1999, 2007) for analysed elements expressed in mg/kg. Table 3 Degree of contamination by metals according to soil indices. EF - enrichment factor (Lăcătuşu, 1998); Igeo - geoaccumulation index (Müller, 1969); CPI - combined pollution index (Abrahim and Parker , 2008). Table 4

ACCEPTED MANUSCRIPT Main statistical parameters of aqua regia extractable concentration of analysed elements expressed in mg/kg (T-tailings; SP-polluted arable soil; SC-control soil; wet-sampled in wet season; dry-sampled in dry season; BDL-bellow detection limit). Remark: Cadmium in all soil

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samples were bellow detection limit.

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Table 5

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Enrichment factors, geo-accumulation indices and combined pollution indices of arable soil (EF: enrichment factor. GI: geo-accumulation index. CPI: combined pollution index).

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Figure Caption

Fig. 1. Location Map of the Otavi Mountainland and its simplified lithology and stratigraphy

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with position of Kombat mine.

Fig. 2. Sketch of the Kombat area showing the locations of mine shafts (triangles), settlement, tailings dam and tailings impoundment, three different sampling areas (T – tailings; P –

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polluted soil; C – control soil), as well as wind rose. Fig. 3. Wind erosion of the tailings dam during winter (dust storm).

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Fig. 4. Water erosion of the tailings dam during summer (gullies). Fig. 5. Box and Whiskers plots for Cu concentrations presented separately for all sample types and both seasons (T wet –tailings samples from wet season; T dry –tailings samples

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from dry season; T salt – salt samplings from tailings dam; PS wet – polluted arable soil samples from wet season; PS dry – polluted arable soil samples from dry season; CS wet – control soil samples from wet season; CS dry – polluted arable soil samples from dry season). Fig. 6. Box and Whiskers plots for Cu concentrations (T wet –tailings samples from wet season; T dry –tailings samples from dry season; T salt – salt samplings from tailings dam; PS wet – polluted arable soil samples from wet season; PS dry – polluted arable soil samples from dry season; CS wet – control soil samples from wet season; CS dry – polluted arable soil samples from dry season). Fig. 7. Distribution of copper in sequential fractions of the representative tailings, polluted soil and control soil samples. Legend: EXC – adsorbed/exchangeable fraction; CARB – carbonate fraction, FEMN – reducible fraction (iron and manganese oxides, hydroxides and

ACCEPTED MANUSCRIPT oxyhydroxides); OR/SUL – oxidisable fraction (organic matter/sulphides); RES – residual fraction. Fig. 8. Distribution of lead in sequential fractions of the representative tailings, polluted soil

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and control soil samples. Legend: EXC – adsorbed/exchangeable fraction; CARB – carbonate

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fraction, FEMN – reducible fraction (iron and manganese oxides, hydroxides and

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oxyhydroxides); OR/SUL – oxidisable fraction (organic matter/sulphides); RES – residual

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fraction.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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T

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Figure 8

ACCEPTED MANUSCRIPT Table 1

Label

Reagent/concentration/volum e

1.

Exchangeable

EXC

Ammonium acetate (NH4C2H3O2) 1mol/l 12 ml

Acid

CARB

Acetic acid (CH3COOH)

3.

Reducible

FEMN

pH 2.9 16 h agitation at 20C

pH 7 (regulated with 25 % HAc)

NH2OHxHCl

3 h water bath at 90C Shaking every 20 min

TE CE P OR/SUL

AC

Oxidisable

1 h agitation at 20C

Hydroxylamine hydrochloride

1 mol/l

4.

pH 7

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37 ml

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0.11 mol/l

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2.

Conditions

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Fraction

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Step

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Sequential extraction procedure applied in this study (combination of Tessier et al., 1974 and Sulkowski and Hirner, 2006)

12 ml

cooling at room temperature

Acetic acid (CH3COOH)

1 min ultrasound homogenizer

25 % 10 ml Hydrogen peroxide (H2O2)

1 h at 20C, occasional shaking

8.8 mol/l

2 h water bath at 85C

10 ml + 10 ml

pH 2 (regulated with HNO3)

Ammonium acetate

16 h agitation at 20C

NH4C2H3O2 (1mol/l)

ACCEPTED MANUSCRIPT Table 2 Canadian Soil Quality Guidelines (CCME, 1999, 2007) for analysed elements expressed in

Industrial soil

Cd

1.4

22

Cr

64

87

Co

40

300

Cu

63

91

Ni

50

50

Pb

70

600

Zn

200

360

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Element Agricultural soil

T

mg/kg.

ACCEPTED MANUSCRIPT Table 3 Degree of contamination by metals according to soil indices. EF - enrichment factor (Lăcătuşu, 1998); Igeo - geoaccumulation index (Müller, 1969); CPI - combined pollution

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Degree of contamination Slight Moderate Severe Very severe Excessive Uncontaminated to moderate Moderate Moderate to strong Strong Strong to very strong Very strong Nil to very low Low Moderate High Very high Extremely high Ultra high

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Index Value EF 1.1 – 2 2.1 – 4 4.1 – 8 8.1 – 16 > 16 Igeo 0-1 1-2 2-3 3-4 4-5 >5 CPI < 1.5 1.5-2 2-4 4-8 8-16 16-32 > 32

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index (Abrahim and Parker , 2008).

ACCEPTED MANUSCRIPT Table 4

Ni

Skewness 2.43

Kurtosis 6.11

7

2515

1378

641

9086

2954.17

117.46

1116.57

2.44

6.16

T-salt

5

2580

2489

554

6030

SP-wet

7

111

115

74

150

2096.86

81.26

937.75

1.39

2.40

25.02

22.47

9.46

0.05

-0.02

SP-dry

6

108

109

76

138

22.36

20.69

9.13

-0.10

-0.67

SC-wet

10

18

18

12

24

3.85

21.38

1.22

-0.08

-1.07

SC-dry

9

17

16

13

2.90

17.40

0.97

-0.07

-1.88

T-wet

7

918

811

596

T-dry

7

1670

1029

710

1428

359.28

39.12

135.80

0.93

-1.11

5007

1515.26

90.75

572.71

2.35

5.73

T-salt

5

3015

2901

SP-wet

7

113

116

SP-dry

6

119

124

SC-wet

10

18

15

SC-dry

9

19

19

T-wet

7

45

T-dry

7

104

T-salt

5

91

SP-wet

T-wet

20

IP

T-dry

388

5589

2546.32

84.46

1138.75

0.05

-2.98

71

164

33.07

29.19

12.50

0.33

-0.81

85

156

27.72

23.36

11.32

-0.13

-1.18

10

34

7.29

40.84

2.30

1.25

1.08

11

30

5.59

29.64

1.86

0.98

1.11

51

33

54

9.15

20.13

3.46

-0.71

-1.64

111

49

157

34.75

33.37

13.13

-0.19

0.38

102

51

123

30.09

33.20

13.45

-0.51

-1.93

7

45

51

33

54

9.15

20.13

3.46

-0.71

-1.64

6

45

44

31

62

12.51

28.01

5.11

0.24

-1.59

SC-wet

10

29

27

22

36

4.41

15.38

1.40

0.42

-0.55

SC-dry

9

27

27

22

33

3.61

13.27

1.20

0.49

-0.59

7

2.3

1.5

BDL

5.4

2.12

93.43

0.86

0.75

-1.36

T-dry

7

4.2

4.1

BDL

7.6

2.64

62.59

1.08

-0.26

-0.42

T-salt

5

3.5

3.5

BDL

5.7

2.01

57.53

1.01

-0.01

-2.71

T-wet

7

16

16

14

17

1.23

7.74

0.47

-1.28

2.49

T-dry

7

15

16

10

19

3.02

20.09

1.14

-0.59

-0.22

T-salt

5

15

16

10

19

3.66

24.64

1.64

-0.35

-0.93

SP-wet

7

26

30

14

33

7.93

30.54

3.00

-0.91

-1.16

SP-dry

6

20

20

16

26

3.33

16.94

1.36

1.13

1.83

SC-wet

10

35

36

25

46

7.34

21.12

2.32

-0.06

-1.41

SC-dry

9

25

25

16

35

5.60

22.80

1.87

0.40

0.73

T-wet

7

4.8

4.1

2.0

7.7

2.17

45.19

0.82

0.39

-1.18

T-dry

7

2.6

2.2

0.2

7.9

2.48

94.56

0.94

1.96

4.60

T-salt

5

2.3

2.3

1.2

3.4

0.81

35.56

0.36

0.12

-0.27

SP-wet

7

9.2

9.3

7.6

11

1.26

13.60

0.47

0.13

-1.52

SP-dry

6

9.4

9.3

7.9

11

1.13

11.96

0.46

0.13

-0.82

SC-wet

10

17

19

10

23

4.71

28.21

1.49

-0.35

-1.62

SC-dry

9

11

9.2

8.1

16

2.87

26.69

0.96

1.01

-0.42

AC

Cr

Standard error 983.57

SC R

Standard Deviation 2602.29

NU

Max 7820

SP-dry

Cd

Coeficient of Variation 127.63

MA

Min 429

D

Zn

Median 1255

TE

Pb

T-wet

Mean 2039

CE P

Cu

N 7

T

Main statistical parameters of aqua regia extractable concentration of analysed elements expressed in mg/kg (T-tailings; SP-polluted farmland soil; SC-control soil; wet – sampled in wet season; dry – sampled in dry season; BDL – bellow detection limit). Remark: Cadmium in all soil samples were bellow detection limit.

2.5

6.7

1.74

40.64

0.66

0.55

-1.52

T-dry

7

5.8

5.6

4.6

8.4

1.26

21.62

0.48

1.52

3.01

T-salt

5

5.6

5.7

4.1

6.7

0.96

17.10

0.43

-1.00

2.12

SP-wet

7

5.2

5.2

4.4

6.1

0.53

10.27

0.20

0.20

0.53

SP-dry

6

5.1

5.2

3.7

6.3

0.86

16.81

0.35

-0.68

1.51

SC-wet

10

4.9

4.9

3.4

6.4

0.90

18.28

0.28

0.01

-0.07

SC-dry

9

4.6

4.6

2.4

6.4

1.21

26.45

0.40

-0.30

0.04

T-wet

7

1568

1538

1416

1754

111.10

7.09

41.99

0.68

0.30

T-dry

7

1485

1483

1359

1650

91.16

6.14

34.46

0.69

1.47

T-salt

5

1387

1361

1290

1492

92.68

6.68

41.45

0.31

-2.87

SP-wet

7

798

868

515

1163

217.20

27.20

82.09

0.42

0.02

SP-dry

6

760

697

411

1362

325.33

42.83

132.82

1.48

2.98

SC-wet

10

370

367

243

575

107.00

28.91

33.84

0.59

-0.34

SC-dry

9

353

353

253

563

101.79

33.93

1.19

1.12

T-wet

7

10302

10510

5440

16790

4714.62

45.77

1781.96

0.37

-1.68

T-dry

7

7604

5838

3295

16483

4587.86

60.34

1734.05

1.32

1.84

T-salt

5

8394

8255

2403

NU

28.85

15280

4882.85

58.17

2183.68

0.35

-0.05

SP-wet

7

15022

14425

12988

20070

2489.30

16.57

940.87

1.68

2.95

SP-dry

6

13873

13549

11743

17230

1988.01

14.33

811.60

0.95

0.72

SC-wet

10

15772

15288

11748

21228

2763.10

17.52

873.77

0.51

0.66

SC-dry

9

14943

15025

7450

23580

4463.45

29.87

1487.82

0.41

1.55

SC R

IP

T

4.1

D

4.3

TE

Fe

7

CE P

Mn

T-wet

AC

Co

MA

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Table 5 Enrichment factors, geo-accumulation indices and combined pollution indices of farmland

Igeo

Cu Pb

CE P

TE

D

Cu+Pb

AC

CPI

Max 8.4 7.7 10.2 9.8 3.1 2.9 3.4 3.3 9.3 8.7

Class Severe Severe Severe Severe Moderate to strong Moderate to strong Moderate to strong Moderate to strong High High

IP

Min 4.1 4.2 4.4 5.3 2.0 2.1 2.2 2.4 4.3 4.9

SC R

Pb

Median 6.4 6.0 7.2 7.7 2.7 2.6 2.9 3.0 6.8 7.0

NU

Cu

Mean 6.2 6.0 7.1 7.4 2.6 2.6 2.8 2.9 6.6 6.7

MA

EF

Season Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry

T

soil (EF - enrichment factor, Igeo - geoaccumulation index, CPI - combined pollution index).

ACCEPTED MANUSCRIPT Appendix 1 SAMPLE ANALYSES

T

DRY normal

salt

A

B

S

S 19 43.579

S 19 43.592

S 19 43.579

E 17 42.775

E 17 42.771

E 17 42.775

S 19 43.772

S 19 43.771

E 17 42.753

E 17 42.750

S 19 43.772

S 19 43.786

S 19 43.786

E 17 42.914

E 17 42.854

E 17 42.854

S 19 43.777

S 19 43.778

S 19 43.778

E 17 42.075

E 17 43.065

E 17 43.065

S 19 43.595

S 19 43.587

D

TE

E 17 42.122

E 17 43.124

S 19 43.580

S 19 43.576

E 17 42.978

E 17 42.976

AC

T6

CE P

TAILINGS

T4

+

AB

AB

A

+

ABS

ABS

-

+

ABS

ABS

-

+

-

AB

AB

AB

+

-

AB

AB

-

+

AB

AB

S

+

X

AB

AB

AB

+

X

AB

AB

-

-

X

AB

AB

-

-

X

A

A

-

-

S 19 43.625

S 19 43.626

S 19 43.626

E 17 42.862

E 17 42.865

E 17 42.865

S 19 43.545

S 19 43.544

E 17 42.690

E 17 42.693

S 19 43.550

S 19 43.555

E 17 42.615

E 17 42.405

S 19 43.553

S 19 43.552

E 17 42.401

E 17 42.273

T7

POLLUTED SOILS

P1

P2

P3

pH

AB

-

T3

AAS (SEA)

ABS

MA

T2

AAS (aqua regia)

ABS

NU

T1

T5

XRD

normal

IP

WET

SC R

Location

sample type

SEASON

S 19 43.566 P4

E 17 42.294

ACCEPTED MANUSCRIPT

E 17 42.305

S 19 44.110

S 19 44.108

E 17 42.444

E 17 42.453

S 19 44.111

S 19 44.117

E 17 42.583

E 17 42.579

P8

P9

X

AB

AB

-

-

X

AB

AB

A

-

X

AB

AB

AB

+

A

A

A

-

A

A

-

-

X

A

A

A

+

X

AB

AB

-

+

X

AB

AB

-

+

X

AB

AB

-

-

X

AB

AB

-

-

X

A

A

-

-

X

AB

AB

-

-

X

AB

AB

B

-

X

B

B

B

-

T

E 17 42.303

S 19 46.639 C1

-

X

S 19 46.654 C2

-

X

E 17 43.309 S 19 46.859

E 17 42.322

E 17 42.320

S 19 46.849

S 19 46.849

MA

S 19 46.859 C3

TE

S 19 46.845

E 17 42.041

E 17 42.046

S 19 46.838

S 19 46.835

E 17 41.892

E 17 41.897

S 19 46.511

S 19 46.511

E 17 41.893

E 17 41.894

S 19 46.506

S 19 46.508

E 17 42.063

E 17 42.518

S 19 46.507

S 19 46.502

E 17 42.216

E 17 42.046

S 19 46.502

S 19 46.507

E 17 42.368

E 17 42.374

CE P

AC

CONTROL SOILS

E 17 42.187

S 19 46.844 C5

C6

D

C4 E 17 42.189

NU

E 17 43.468

IP

S 19 44.114

SC R

S 19 44.108 P7

C7

C8

C9

C10

S 19 45.991 C11

E 17 42.951

ACCEPTED MANUSCRIPT S 19 46.859 C12

-

X

B

B

AC

CE P

TE

D

MA

NU

SC R

IP

T

E 17 42.368

-

-

ACCEPTED MANUSCRIPT Highlights

AC

CE P

TE

D

MA

NU

SC R

IP

T

Arable soil adjacent to Kombat tailings are highly polluted by Pb and Cu. Environmental risk of Pb and Cu in soil is medium. It is recommended to limit the use of certain crops in order to reduce health risk. Wind prevails over water as the agent of material transfer from tailings into soil.