Tracing Pb and Possible Correlated Cd Contamination in Soils by Using Lead Isotopic Compositions

Journal Pre-proof Tracing Pb and Possible Correlated Cd Contamination in Soils by Using Lead Isotopic Compositions Yi Huang, Shipeng Zhang, Ying Chen, Li Wang, Zhijie Long, Scott S. Hughes, Shijun Ni, Xin Cheng, Jinjin Wang, Ting Li, Rui Wang, Chao Liu

PII:

S0304-3894(19)31482-7

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121528

Reference:

HAZMAT 121528

To appear in:

Journal of Hazardous Materials

Received Date:

28 August 2019

Revised Date:

15 October 2019

Accepted Date:

22 October 2019

Please cite this article as: Huang Y, Zhang S, Chen Y, Wang L, Long Z, Hughes SS, Ni S, Cheng X, Wang J, Li T, Wang R, Liu C, Tracing Pb and Possible Correlated Cd Contamination in Soils by Using Lead Isotopic Compositions, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121528

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Tracing Pb and Possible Correlated Cd Contamination in Soils by Using Lead Isotopic Compositions Yi Huang*a,b, Shipeng Zhang b, Ying Chenb, Li Wangb, Zhijie Long b, Scott S. Hughesc, Shijun Nib, Xin Chengb, Jinjin Wangb, Ting Lib, Rui Wangb, Chao Liub a

State Key Laboratory of Geohazard Prevention and Geoenvironment

Protection, College of Environment, Chengdu University of Technology,

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Sichuan 610059, China; Department of Geochemistry, Chengdu University of Technology, China;

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Department of Geosciences, Idaho State University, ID, USA;

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*Correspondence: [email protected]; Highlights:

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Results of this study indicated that contaminants in soils located in the hills upslope of the slag dump were coal and derivative products, and that these soils are isotopically distinct from downslope deltaic soils. Contaminants in downslope soils were slag and derivative products from V processing. The study also demonstrated the use of Pb isotopic tracers in low-to-moderate contaminant levels to predict potential sources, and also indicate that Pb is a viable surrogate to trace potential Cd contamination in a large vanadium titano-magnetite mine region.

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Abstract Concentrations of Pb and Cd in topsoil from 24 locations along the Baguan River near a smelting dump in west Panzhihua were measured using ICP-MS to examine the spatial distributions of these toxic heavy metals. Twenty-one profile samples, 7 from each of 3 locations down to 80 cm, were also analyzed to establish background levels and Pb - Cd correlations. Lead isotopic ratios in all 45 samples and potential sources of soil contamination were determined using MC-ICP-MS. Contamination levels of Pb and Cd in soils from both sides of the river ranged from low to moderate, and the

concentrations of Pb and Cd exhibited highly correlated behavior. Results of an isotope-tracer technique determined the number of end-member contaminants and background compositions contributing to the compositions of topsoils. Results of a binary mixing model indicated that contaminants in upslope soils from relatively

higher elevations were coal and derivative products, and that these soils are isotopically distinct from downslope soils. Contaminants in downslope soils were slag and derivative products from V processing. Results demonstrate the use of Pb isotopic tracers in low-to-moderate contaminant levels to predict potential sources and Pb is a viable surrogate to trace potential Cd contamination in Panzhihua region. Keywords: Cadmium; Lead; Contaminant sources; Isotope tracing

1 Introduction

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Tracing the sources and determining the chemical behaviors of heavy metals in the

environment is crucial for understanding their natural cycles that may be affected by

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contamination from human activities. Isotope tracer techniques can be used to identify the sources of heavy metals in soils (e.g., Zhang et al., 2011; Schmitt, 2012; Kumar et

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al., 2013; Fekiacova et al., 2015; Ying et al., 2017; Sun et al., 2018; Kong et al., 2018; Hu et al., 2018; Wang et al., 2019). Lead isotope tracers were developed in the fields of economic geology and petrology for two primary reasons. Lead is naturally

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abundant in chalcophile ore deposits, and stable isotope ratios of lead (e.g., 208

Pb/206Pb) are not vulnerable to subsequent fractionation by geochemical processes

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(Yu et al., 2012). Therefore, research into tracing the chemical cycles of chalcophile heavy metals such as Pb, Cd, Zn, etc. focuses predominantly on Pb isotopes (Reimann et al., 2011; Yang et al., 2012; Walraven et al., 2014; Sojka et al., 2018; Zhao et al.,

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2019; He et al., 2019; Liu et al., 2019).

Application of Pb isotopes in environmental monitoring began in the late 1960s and became widespread in the 1980s, yielding significant outcomes. Source apportionment techniques for Pb isotopes were mainly used to trace the pollution sources of heavy metals in gasoline (Chow et al., 1972), airborne particulate matter

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(Munksgaard et al., 1998; Carolina et al., 2018; Chia-Te et al., 2019), soil (Cloquet et al., 2006; Hansmann and Köppel, 2000; Kaste., 2003; Kong et al., 2018a; Kong et al., 2018b; Kumar, 2013; Rabinowitz, 2005; Rabinowitz et al., 1972; Reimann et al., 2012; Camizuli et al., 2014; Brewer et al., 2016; Civitillo et al., 2016; Prathumratana et al., 2018), and lake sediment (Blais et al., 1996; Eades et al., 2002; Andrew et al., 2018; Finn et al., 2019). The aforementioned studies attest to the varied sources and transport mechanisms of Pb, Cd and other heavy metals. However, the sources of anthropogenic Pb and Cd as

pollutants in soils, and their subsequent mobility in soil profiles and aquifers, must be distinguishable from natural concentration gradients related to rock substrate or other parental material. The effects of aquifer transport and human activities, including agriculture, also factor into near-surface modifications of topsoil profiles (Kong et al., 2018b). The problem of separating natural and anthropogenic sources is likely more apparent in areas where multiple operations such mining, smelting and refining occur, or in regions where potential sources of pollution are not obvious (e.g., Hansmann and Köppel, 2000). Although recent studies indicate that anthropogenic Pb can be highly

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soluble (Kong et al., 2018b), some soil profiles and dispersal patterns around potential sources indicate that Pb is generally less mobile than Cd (and Zn) and more likely to

be concentrated in the upper few cm (e.g., Sterckeman et al., 2000, 2002; Kong et al.,

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2018a).

Combined Pb and Cd isotope tracers, as well as heavy element enrichment factors,

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have been used recently to trace pollution sources in soils associated with mining operations. For example, Cloquet et al. (2006), using Pb and Cd isotopes to trace

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pollution sources of heavy metals in soils, demonstrated that while Pb isotopes are appropriate to trace natural contaminant sources, fractionated Cd isotopic

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compositions may be used to identify contamination related to anthropogenic sources. Wen et al. (2015) determined metal concentrations as well as Pb and Cd isotope ratios in soils and effluents related to mining/smelting activities in a Pb–Zn mining district to distinguish the pollution sources. Dos Santos et al. (2017) applied both Pb isotopic

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compositions and heavy metal enrichment factors to distinguish natural and anthropogenic influences on the Pb concentrations in soils. An important outcome of these applications is the potential to use Pb isotopic tracers to evaluate contaminant sources of multiple heavy metals such as Cd and Pb if they can be correlated due to the lack of significant fractionation in the environment.

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Panzhihua, a city that lies at the confluence of the Jinshajiang and Yalong rivers

(both tributaries of the Yangtze River, the longest in Asia), is home to the largest titano-magnetite mining district in China. Over four decades of development have resulted in abnormally high levels of Cd in the western district of Panzhihua where the Baguan River flows into the Jinshajiang River (Huang et al., 2012). Essential environmental issues in Panzhihua are to trace pollution sources of Cd in soils and quantitatively discriminate between natural and artificial factors of contamination.

This study uses Pb isotope tracing in soils collected from hillsides (upslope, plus one far upstream) and lower elevations (downslope) to investigate potential mechanisms underlying their response to mining and other human activities, and to elucidate the occurrence and accumulation of Pb and Cd in the mining region. The findings of this study are expected to inform further research into environmental impacts and biological safety in the vast land area along the middle and lower reaches of the Yangtze River. 2 Sampling and Analysis Methodology

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2.1 Sampling 2.1.1 Soil sample collection

Surface soils were collected from 24 different sites within a broad region covering

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~12 km2 surrounding a slag disposal site north of the confluence of the Baguan and

Jinshajiang rivers in west Panzhihua (Fig. 1) according to their apparent homogeneity

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and representation of the local terrain. Locations were selected to acquire samples upslope (6 samples) and downslope (18 samples) of the slag dump, and ranging ~10

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m to 2 km from both sides of the river. Soils that cover slopes modified during human activities were considered to be the results of artificial accumulation and were

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avoided.

Natural soil in the study area is classified as the Xigeda type, which is loose, poorly graded silty sand with minor clay. Before sampling, vegetation and fallen leaves in the vicinity of each site were removed. Approximately 1 kg of soil was then collected

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(by wooden spoon) from the uppermost 5 cm of the surface and placed into a plastic bag. Twenty-one samples were additionally collected (45 total soil samples) at three of the locations (one upslope and two downslope) from subsurface profiles (BS03, BS14, and BS33 in Fig. 1) to determine background concentrations in relatively uncontaminated soils and to confirm geochemical correlation of Pb and Cd over a

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range in concentrations. Seven profile samples from each location were obtained at 10 cm intervals to a depth of 80 cm.

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Location map of soil surface samples, potential contaminant facilities,

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Fig. 1.

e.g., slag disposal pit, coal separation plant, burned coal disposal areas, and V-

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Ti-magnetite smelter (the V processing plant is located off map downslope ~11 km east of the river confluence), and physiography within the western district of Panzhihua. Subsurface background samples were collected from three 80 cm deep

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profiles BS03, BS14 and BS33.

2.1.2 End-member samples

Analytical end-members of potential contaminants required direct sampling of materials related to coal burning and industrial smelting and refining, which are major environmental sources of Pb. Previous studies largely attributed Pb in atmospheric

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dust fallout to these two sources (Cheng et al., 1998; Yang, 2004; Yu et al., 2009), both of which exist in Panzhihua, a typical mining city. Coal, V-Ti magnetite ore, smelting slag (each ~0.5 – 1kg), and dust fallout (~20g) were obtained to provide analytical end-members in the study region (Fig. 1). Four samples of industrial coal, including two pure representatives (BDM01 & 06) and two low-purity samples (BDG02 & 05) were acquired from a coal separation plant located north (upslope) of the slag dump (Fig. 1). Additional coal-related samples included burned coal powder

(FH2 and FH3), coal fly ash (HF08-3), and atmospheric dust (HD09) collected near the coal separation plant. Four slag samples (PGZ01 – 04) were obtained directly from the refined slag disposal pit. Additional smelter-related samples included smelter dust (PGF14, on map) collected directly within 20 m of the smelter and atmospheric smelter dust (PGD15, 21, 24, 28) collected in the vicinity. Samples related to vanadium processing included unprocessed V-Ti magnetite ore (ZJK34, LJK31 & 35) and dust fallout (FCD26, 26A, 27) from the vanadium production plant located downslope (outside map boundary) ~11 km east of the confluence of Baguan and collected from the tail-pipe of a local automobile for comparison.

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2.2 Analytical methods

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Jinshajiang rivers. One sample of exhaust residue (PGG, ~20g) was additionally

2.2.1 Analysis of Pb and Cd concentrations

Inductively coupled plasma – mass spectrometry (ICP-MS) was used to determine

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the concentrations of Pb and Cd in all samples. Reagent purification, glassware handling, and sample preparation followed accepted methods described by Zhang digestion.

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2.2.2 Analysis of Pb isotope ratios

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(2016), which involved pulverizing each sample to 200 mesh size prior to acid

Lead isotopic analyses were performed following the traditional procedure of Chow (1965) using a Thermo Fisher Neptune MC-ICP-MS instrument provided by the Applied Nuclear Technology in Geosciences Key Laboratory of Sichuan Province.

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Ratios of 206Pb/204Pb, 208Pb/204Pb, 208Pb/206Pb, and 206Pb/207Pb were obtained for all 45 soil and 24 end-member samples, and compiled as average values from multiple (usually 5) runs. Standard Pb reference material (NIST981), certified by the U. S. National Institute of Standards and Technology, was analyzed with every batch of 5 samples. These analyses were used to correct for mass fractionation (although

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minimal) and to serve as a measure of analytical precision. Throughout the experiment, the order of magnitude for Pb concentrations in blank solutions was 10−8, with NIST981 as the standard (measured 207Pb/206Pb ratio = 0.91449±0.00004), and

the analytical precision (2σ) of the 207Pb/206Pb ratio was better than 0.03%. 3 Analytical Results 3.1 Concentrations and correlations of Pb and Cd Table 1 summarizes the range and average concentrations of Pb and Cd in 45 surface

and profile soil samples collected from the study area. Both elements exhibit wide

ranges in concentration that attest to potential variation due to atmospheric contaminant transport and collection in soils. The average Pb concentration of 36.3 ± 16 µg/g appears somewhat higher, than the previously reported average value of 23.6 ± 6.2 µg/g in this region of Sichuan Province (Zhao et al., 1985). Although the previous average lies within the range of values in Table 1, the newer data suggests some contaminant transport and concentration in the Panzhihua region. Maximum values for Pb (83.2 µg/g) and Cd (0.84 µg/g) are significantly higher than reported average upper crust values of Pb = 17 ± 1 µg/g and Cd = 0.089 ± 0.027 µg/g

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(uncertainty depicts range in reported average values) compiled by Rudnick and Gao (2014). The maximum values are also higher than calculated global average mean

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concentrations of 32 ppm Pb and 0.53 ppm Cd in surface soils representing numerous analyses compiled by Kabata-Pendias and Pendias (2001). Although values higher

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than global averages will likely represent anthropogenic influences, their compilations further demonstrate wide ranges in average values of both elements in surface soils (Pb = 10–67 ppm, Cd = 0.06–1.1 ppm) from various regions. Average levels of Pb

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and Cd listed in Table 1 are relatively close to global averages, but the wide ranges in natural concentrations globally illustrates the need to determine background levels

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and element/element fractionation in any specific region in order to trace pollutant sources. This need is further demonstrated by peak values in this study being far below the peak values of mining-related Pb and Cd (and other heavy metals)

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measured in polluted soils and sediments (e.g., Pb >30,000 µg/g and Cd >100 µg/g) from other districts in China (Lei et al., 2010; Yang et al., 2010). Table 1. Summary of Pb and Cd concentrations (µg/g) in surface and subsurface profile soil samples from the study area. Minimum (µg/g)

Average (µg/g)

Standard Deviation

Coefficient of Variation (%)

0.84

0.14

0.30

0.16

0.53

83.2

19.4

36.3

16.0

0.44

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Cd Pb

Maximum (µg/g)

Several key points can be made related to the overall concentrations of Pb and Cd in topsoils collected downslope (D) and upslope (U) of the slag disposal pit (Table 2), with respect to all samples analyzed. All of the Pb and Cd concentrations in surface samples (0–5cm) are above their respective minimum values (Table 1), indicating that the lowest concentrations in the region lie below the surface. The maximum

concentrations of both elements are also found in the surface soils, further suggesting that Pb and Cd pollutants have not been transported significantly to deeper levels in the soil. Sixteen of the 24 topsoil samples (67%) have higher than average concentrations of Pb, and 14 samples (58%) have higher than average concentrations of Cd. All of the subsurface profile samples have Pb concentrations lower than average, and only one profile sample near the surface (10–15cm, BS14) has higher than average Cd concentration. Notably, except for BS03, the northernmost sample located several km upslope from the slag disposal pit (Fig. 1), all five upslope samples

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collected near the pit record values of both Pb and Cd above the averages in Table 1.

0.00017 0.014 0.00034 0.0003 0.0002 0.014 0.00036 0.00026 0.00042 0.00032 0.0074 0.00056 0.0006 0.00044 0.00017 0.0003 0.00048 0.00028 0.00052 0.0002 0.00022 0.00066 0.00048 0.00022

Pb/204Pb

±SD

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Pb/206Pb ±SD

40.48 40.33 40.12 40.41 40.34 39.75 40.13 40.12 40.10 39.98 40.16 40.28 40.32 40.44 40.41 40.54 40.22 40.65 40.43 40.66 40.61 41.61 40.35 40.62

0.0002 0.011 0.00064 0.00052 0.00026 0.026 0.0005 0.00052 0.00072 0.00058 0.0056 0.00054 0.001 0.00042 0.00026 0.0002 0.00048 0.00038 0.00048 0.00028 0.00028 0.0008 0.00066 0.00026

2.125 2.118 2.125 2.125 2.127 2.133 2.127 2.132 2.131 2.139 2.134 2.125 2.129 2.124 2.135 2.132 2.132 2.130 2.127 2.053 2.054 2.086 2.044 2.031

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Pb/207Pb ±SD

0.00009 1.181 0.0012 1.183 0.00031 1.172 0.00028 1.179 0.00018 1.177 0.0011 1.165 0.000184 1.172 0.00032 1.169 0.00034 1.168 0.00028 1.162 0.00084 1.170 0.000082 1.177 0.00042 1.175 0.00022 1.181 0.00014 1.170 0.000124 1.175 0.00013 1.171 0.000064 1.179 0.00018 1.179 0.00013 1.220 0.000082 1.219 0.00017 1.221 0.000056 1.220 0.000084 1.232

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19.10 19.04 18.88 19.02 18.98 18.68 18.87 18.82 18.82 18.69 18.82 18.95 18.94 19.06 18.93 19.01 18.86 19.08 19.01 19.81 19.78 19.95 19.74 20.01

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BS08 BS09 BS11 BS12 BS14 BS15 BS16 BS19 BS22 BS25 BS28 BS31 BS32 BS33 BS49 BS51 BS53 BS79 BS03 BS37 BS40 BS42 BS65 BS76

D D D D D D D D D D D D D D D D D D U U U U U U

Pb/204Pb ±SD

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Sample Type

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Table 2. Pb and Cd concentrations* and Pb isotopic compositions with analytical uncertainties (listed as standard deviation of ICP-MS signal counts) of topsoils in the study area. 0.000066 0.0018 0.0002 0.00014 0.00015 0.0012 0.000076 0.00019 0.00018 0.00019 0.00022 0.000094 0.00015 0.00018 0.0001 0.00017 0.000084 0.000068 0.00015 0.000062 0.000072 0.00011 0.000092 0.00011

Pb (µg/g) 21.13 21.31 30.31 30.92 26.50 45.41 68.85 51.51 71.08 62.34 43.63 39.78 38.32 29.90 57.63 45.29 46.63 28.54 28.26 59.18 43.15 39.05 83.18 73.61

*Analytical uncertainties for Pb and Cd concentrations are Pb = <1% and Cd = 5-10% of values listed, and reflected in the number of reported significant digits. SD: Standard Deviation 3.2 Pb isotope ratios

Cd (µg/g) 0.16 0.21 0.32 0.29 0.26 0.49 0.73 0.41 0.84 0.36 0.30 0.30 0.35 0.29 0.41 0.35 0.28 0.19 0.17 0.45 0.32 0.40 0.73 0.55

Measured Pb isotope ratios in topsoil samples (Table 2) and potential end-member contaminant samples (Table 3) represent potentially contaminated areas and pollutant sources in the study area (Fig. 1). Because 204Pb exists in relatively low concentrations (i.e., low isotopic abundance) in the environment, the measured 208

Pb/204Pb and 206Pb/204Pb ratios were subject to relatively higher analytical

uncertainties leading to lower analytical precision compared to the ratios of 208

Pb/206Pb and 206Pb/207Pb. Based on this rationale, data analysis and isotopic

modeling focused more on the ratios of 208Pb/206Pb and 206Pb/207Pb, which have the

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highest analytical precision; whereas, the two ratios with 204Pb yielded less precise Pb isotopic models, making it somewhat more difficult to identify specific pollution sources.

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Pb/204Pb 41.49 41.28 41.14 41.29 41.09 41.18 40.79 41.12 40.21 39.89 39.88 39.39 40.45 40.23 40.42 40.45 40.33 40.40 39.67 40.52 39.96 39.89 39.56 40.33

±SD 0.08 0.0032 0.0076 0.00084 0.0019 0.0018 0.0014 0.00106 0.0004 0.00044 0.0046 0.02 0.00062 0.00042 0.00011 0.000078 0.00046 0.00046 0.0004 0.0024 0.07 0.00026 0.013 0.00034

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Pb/206Pb 2.070 2.079 2.080 2.072 2.071 2.079 2.100 2.064 2.149 2.141 2.139 2.176 2.152 2.150 2.149 2.147 2.137 2.149 2.151 2.116 2.147 2.138 2.170 2.151

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±SD 0.082 0.003 0.008 0.00076 0.0009 0.0048 0.0014 0.0011 0.00021 0.00026 0.0028 0.015 0.00072 0.00034 0.00018 0.000052 0.00042 0.00046 0.00032 0.004 0.0004 0.00018 0.014 0.0003

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Pb/204Pb 20.05 19.86 19.77 19.93 19.84 19.81 19.42 19.92 18.71 18.63 18.65 18.10 18.80 18.71 18.81 18.84 18.87 18.80 18.44 19.15 18.61 18.66 18.24 18.75

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Type low grade coal low grade coal high grade coal high grade coal burned coal burned coal coal plant dust coal fly ash V dust fallout V dust fallout V dust fallout V-Ti-Mgt ore V-Ti-Mgt ore V-Ti-Mgt ore smelter slag smelter slag smelter slag smelter slag smelter atm dust smelter atm dust smelter atm dust smelter atm dust smelter dust exhaust residue

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Sample BDG02 BDG05 BDM01 BDM06 FH2 FH3 HD09 HF08-3 FCD26 FCD26A FCD27 ZJK34 LJK31 LJK35 PGZ01 PGZ02 PGZ03 PGZ04 PGD15 PGD21 PGD24 PGD28 PGF14 PGG01

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Table 3. Pb isotopic compositions of potential end member samples. Analytical uncertainties reported as standard deviation of ICP-MS signal counts. ±SD 0.0054 0.00088 0.0005 0.00012 0.0011 0.003 0.00019 0.00015 0.00019 0.00024 0.0019 0.004 0.00017 0.0002 0.000088 0.000042 0.00034 0.000032 0.000056 0.0024 0.11 0.00012 0.00074 0.00003

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Pb/207Pb 1.231 1.225 1.210 1.228 1.224 1.223 1.197 1.230 1.154 1.157 1.158 1.132 1.157 1.158 1.158 1.161 1.162 1.158 1.143 1.185 1.152 1.158 1.128 1.156

SD: Standard Deviation Noticeable differences in the Pb isotopic composition of each potential end member are observed among coal, smelting slag, and fallout dust (Table 3). Coal samples exhibit the highest 206Pb/207Pb and lowest 208Pb/206Pb ratios. Smelter slag and dust fallout have overall lower 206Pb/207Pb ratios and higher 208Pb/206Pb ratios compared to

±SD 0.0074 0.0016 0.00028 0.00015 0.0034 0.0064 0.000096 0.00022 0.00014 0.00017 0.000098 0.02 0.00012 0.00012 0.00012 0.00003 0.00026 0.00011 0.000062 0.013 0.00078 0.000048 0.0028 0.000028

those in coal, thus demonstrating a preliminary potential to identify pollutant endmember sources. Lead isotopic compositions of soils (Table 2) comprised mostly radiogenic Pb and exhibited considerable variation. Ranges in Pb isotopic ratios are 206Pb/204Pb = 18.681–20.007 (mean = 19.0704 ± 0.5988), 208Pb/204Pb = 39.7479–41.6087 (mean = 40.3883 ± 0.6014), 208Pb/206Pb = 2.0305–2.1394 (mean = 2.1183 ± 0.0363), and 206

Pb/207Pb = 1.1622–1.2416 (mean = 1.1827 ± 0.0341). Sturges et al. (1987)

indicated that natural Pb has a higher 206Pb/207Pb ratio (>1.2) than does artificial Pb

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(0.96–1.2) from industrial emissions. The lead isotope ratios of surface soil samples we collected are clearly within the range of deposits derived from human industrial

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factors; thus, these samples were apparently contaminated. The 206Pb/207Pb and 208

Pb/206Pb ratios of soil samples collected from the slag dump in west Panzhihua

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were roughly between the ratios of these isotopes in coal, smelting slag, and dust fallout.

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4 Discussion

4.1 Correlation of Pb and Cd Concentrations

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The relative concentrations of Pb and Cd and other heavy metals are known to become fractionated, especially in polluted areas that have experienced significant chemical processing due to soil cultivation and sediment reworking (e.g., Lei et al.,

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2010; Yang e al., 2010; Zhang et al., 2015; Liu et al., 2016). Moderately contaminated soils in the western district of Panzhihua indicate an apparent association of Cd with Pb, such that their concentrations maintain consistent proportions. Co-variation of Cd vs. Pb concentrations (Fig. 2) illustrates the possibility that these two elements have not been significantly fractionated in the study area and that values above background

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were likely derived from equivalent sources. The covariance (R2 = 0.7882), with an X-Y intercept near zero, strongly suggests that the soils have not been subjected to significant chemical reworking and that sites with higher Pb will also have higher Cd.

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Fig. 2. 2D co-variation diagram of Cd vs. Pb. Standard deviation depicted for average composition. This apparent association of Pb and Cd in soils from the study area is further

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depicted in their respective surface distribution. Chemical contour maps constructed from the analyses (Fig. 3) illustrates the distribution patterns of Pb and Cd in the study area. The similarity in chemical patterns demonstrates the feasibility of using Pb

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tracers in areas where Pb/Cd ratios are fairly constant to simultaneously trace the sources of both Pb and Cd contamination. We suggest that Pb isotopic tracers, used to determine potential end-member contaminant sources, will also yield the sources of Cd pollution. Correlated Pb and Cd values and their similarity in surface distribution further suggests that neither element was subject to post-depositional migration after

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being deposited (presumably by fallout), and that if Pb isotopes can trace Pb contaminant sources, then they can be used to trace the sources of Cd pollution.

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Fig. 3. Contour maps of Cd and Pb concentrations (µg/g) of soils in the study

4.2 Upslope vs. Downslope Soils

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area (after Chen, 2012). See Fig. 1 for map location.

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Lead isotopic ratios in soils from hills upslope of the slag pit are different overall from Pb isotopic ratios determined for downslope samples, which are located closer to the river confluence (Table 2). Co-variation of 208Pb/206Pb and Pb/207Pb vs. 1/Pb (Fig. 4) illustrate two trends that may be used to determine

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potential pollution end members at different contaminated sites. Downslope samples, and the upslope BS03 sample collected from alluvial soil, show only moderate variation in 208Pb/206Pb (R2 = 0.53). Upslope (and upslope) soils, although represented by much fewer samples, depict a somewhat stronger

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correlation (R2 = 0.71) and greater variation between 208Pb/206Pb ratios and Pb concentrations. On the contrary, upslope samples depict meager correlation of

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Pb/207Pb ratio to Pb concentration (R2 = 0.17), while downslope samples are

more moderately correlated (R2 = 0.67). Regardless of the degree of correlation

for each set, they indicate wide separation of isotopic signatures with respect to Pb concentrations. It is possible that the distinct differences in isotopic signatures between upslope and downslope samples may be interpreted to indicate contributions from

contaminant sources that are distinct from each other. Alternatively, because contaminant levels are fairly low in the region, they may be derived from completely different soil forming processes and thereby exhibit different intrinsic isotopic signatures. Downslope samples (as well as the upslope representative BS03) are notably detrital, derived as deltaic or alluvial sediment deposited broadly near the confluence of the Baguan and Jinshajiang rivers. Upslope samples occur on steep slopes above the river system, and were likely derived in situ from weathered parental rock, and thus not involved in transport

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and deposition. We suggest that the latter interpretation or a combination of the two scenarios is likely, such that the two types are related to isotopically different sources; however, upslope samples have also experienced

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contamination from coal processing. A third possibility is that we have not

identified an end-member contaminant that contributed to upslope soils although

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there is no way to confirm that without additional sampling and analysis.

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Fig. 4. Correlation of Pb isotopic ratios vs. 1/Pb in soil samples located in hills upslope of the slag disposal pit and samples from lower slopes downslope of the disposal pit. Note that BS03, alluvial soil collected near the river, plots with downslope alluvial samples from lower elevations.

4.3 Soil profile compositions

Soil samples from three profiles (BS03, BS14, and BS33) enable the determination of background composition as an end-member in mixing models. Concentrations of Pb in these profiles (Fig. 5) exhibit a noticeable decrease in the 30–40 cm depth range and increase slightly in the 40–50 cm range. The Pb concentrations are greater in the shallow layers of soil than in the deep layers and show little elemental migration. Concentrations of Cd show similar slight decrease with depth, except for a single sample obtained near the surface (BS14, 10–20 cm depth). Although these three locations yield some of the lowest concentrations (Pb < 30 µg/g; Cd < 0.3 µg/g) in the

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study area, making them suitable as background values, both Pb and Cd depict moderate enrichment in surface or near-surface soils. The profiles show that BS03, the farthest sample upslope, represents the least contaminated site (of these three)

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with respect to Cd and that both elements, with the exception of the anomalous Pb at

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40-50 cm, display the least overall variability with depth at that location.

Fig. 5. Concentrations of Pb and Cd, and Pb isotopic ratios (208Pb/206Pb & 206

Pb/207Pb) in samples from soil profiles at three locations (BS03, BS14 and BS33,

Fig. 1).

Isotopic ratios of 208Pb/206Pb and 206Pb/207Pb indicate slightly divergent trends (Fig. 5), with 208Pb/206Pb depicting an almost negligible decrease and 206Pb/207Pb showing a slight increase in with depth. The least variability in Pb isotopic ratios among the three profiles, as with elemental concentrations, occurs at the BS03 location. Moreover, the isotopic ratios in potential end-member components (Table 3) would plot on either side of the sample collected at the bottom of this profile. These relations suggest that the Pb isotopic signatures of the deepest profile samples, especially

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BS03, adequately represent the background composition of the least contaminated topsoil.

The causes for contamination of Pb and Cd in soil profiles, while not entirely evident

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in profile analyses, can be evaluated to shed light on potential mechanisms for the

variation of Pb isotopes. The increase in 206Pb/207Pb ratio with depth likely occurs as

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Pb from dust fallout or other external source having lower 206Pb/207Pb is mostly trapped in surface soils, with limited downward movement. Lead is also absorbed by

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clay minerals and combines with hydrogen sulfide and other chemical groups to form stable and complex compounds (Xiao et al., 2011).

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Lead-containing coal and smelting slag are both considered to be anthropogenic, and human-processed Pb is relatively more soluble and more likely to migrate. However, Pb-containing coal is “chemically” altered in the separation process to remove undesirable fractions of rock and soil; whereas, smelting slag generally undergoes

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only physical breakdown into smaller fragments. Thus, Pb in coal and coal dust, being somewhat more altered than Pb-containing smelting slag, would exhibit relatively greater chemical migration. This may be one factor that causes the 206Pb/207Pb ratio in coal to be higher than that in both smelting slag and the background soil. We suggest that much of the variation in Pb isotopes in the middle soil layers stemmed from the

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mobility of Pb derived from coal in surface soils, with a 206Pb/207Pb ratio slightly

higher than those in shallow and deep soil layers.

5 Characteristics of Pb isotopes and sources of Pb 5.1 Co-variation in Pb isotopic ratios Co-variation of 208Pb/206Pb and 206Pb/207Pb isotope ratios (Fig. 6) reflects the intrinsic composition of Pb in soils (background) and the compositional range in potential sources of contamination. The cluster of coal (BDM) and coal derivatives, a

potential end member contaminant, has a higher 206Pb/207Pb range of 1.210 – 1.228 compared with 1.158 – 1.162 in V-Ti-magnetite smelting slag (PGZ), which is also a potential end member contaminant. The respective ranges in 206Pb/207Pb in dust fallout from the smelting plant and the vanadium product factory (both variably mixed with atmospheric dust) are 1.128 – 1.185 and 1.132 – 1.158. Also, the lowest values in each range are found in nearly pure smelter dust and V ore (Table 3), and most 206

Pb/207Pb ratios in the non-coal sources cluster in the range 1.152 – 1.162.

The determination of end member contaminants is not straightforward. Isotopic

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signatures in Figure 6 suggest a strong linear correlation between smelting slag and soil collected downslope of the slag disposal pit. The plots of the 206Pb/207Pb and 208

Pb/206Pb ratios also indicate that downslope soils had a strong linear relationship

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with smelter dust (PGF14), although smelting slag possibly exerted a greater impact

on the soils because of its closer geographical position (Fig. 1). Soil samples collected

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from the slopes upslope of the pit differ with higher 206Pb/207Pb and lower 208Pb/206Pb ratios compared to smelting slag, but plot more closely with coal. Upslope soils, with

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a 206Pb/207Pb range being similar to that of coal, appear to have been considerably affected by coal processing. By contrast, downslope soils withstood more impact from

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smelting slag and other ore-related constituents, with their 206Pb/207Pb ratios lying on a coincident trend. Soils from downslope locations were apparently affected by variable influence on background composition due to ore-derived and coal-derived sources, but the influence on upslope soils is more complex.

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There seems to be little isotopic difference between vanadium ore, smelter slag and V processing emissions. Potential end members with relatively low 206Pb/207Pb ratios, including the slag disposal site, are too similar to distinguish individually, but they all derive ultimately from V mining operations. More significantly, coal has nearly the highest 206Pb/207Pb ratio, making it a unique pollution end member (and a likely

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contributor to PGD21, atmospheric dust collected near the smelter). However, the isotopic ratios of four upslope samples that plot off the main trend (Fig. 6) present significant complications to a potential contaminant model. For example, one upslope soil sample, BS37, was sampled close to the disposal pit (Fig. 1); yet, the isotopic signature is far removed from slag and plots very near the coal isotopic signature. This suggests that smelting slag from the dump possibly had much weaker effect on the Pb isotope ratio of that sample than its proximity to the slag pit would suggest.

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Fig. 6. Correlation of 208Pb/206Pb vs. 206Pb/207Pb in soil samples compared to three end members (1) BDM Coal, (2) PGZ smelter dust, and (3) natural

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backgrounds in the western district of Panzhihua. A fourth end-member is possibly depicted by BS76 representing different soils upslope of the slag disposal pit. Potential mixing trends are shown by dashed lines.

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Thus, for some soils with a relatively low 206Pb/207Pb ratio, smelting slag plots near the low threshold of the 206Pb/207Pb range, suggesting a close association as a pollution end member. In addition, with the strong 2D correlation of Pb and Cd (Fig. 2) depicting the lowest concentrations in subsurface samples, Pb signatures in all soils must be partly derived from the parental (background) soil layer. It is therefore

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appropriate to select the natural background as one compositional end member, and slag as another; however, the third mainly anthropogenic coal-related contaminant, while appropriate for downslope samples, is not unequivocally involved in the compositional variation in upslope soils (except BS42 collected near the coal separation plant). Higher 206Pb/204Pb and lower 208Pb/204Pb ratios relative to background suggest the influence of coal on upslope soil compositions; however, a separate linear trend of four soil samples and BDM01 coal (Fig. 6) reflects greater complexity than simple mixing. Very likely the soil represented by sample BS76,

collected from higher slopes, was derived from a different parent composition than the soil represented by downslope samples. 5.2 Pb isotope mixing models Recent studies to decipher the sources of contaminants have relied on theoretical mixing models based on potential sources and likely scenarios. A general equation for isotopes and trace elements by Yang et al. (1997) predicts outcomes based on the combination of n sources and the application of binary and ternary mixing models.

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They established a binary mixing model by solving the equation under four conditions (ratio/ratio, ratio/element, ratio-1/element, and element/element) and a ternary mixing model by applying the conditions of ratio/ratio/ratio and element/element/element

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(summarized by Li et al., 2004).

Similar mixing models have been developed for rock petrogenesis as well as the

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determination of pollutants. For example, Zhang et al. (2003) used a mixing model incorporating multiple sources and sinks to simulate the combined effects of

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assimilation and fractional crystallization of magma, thus predicting the migration and composition of trace elements and stable isotopes. Yuan et al. (2001) employed a Pb-

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isotope mixing model involving multiple sources and sinks to simulate the sources of Pb in the Aketishikan gold deposit in Xinjiang and identified mantle, upper mantle, and crust sources of Pb. More relevant to this study, Cloquet (2006) used a binary mixing model, with smelting and atmospheric fallout as end-members under the

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conditions of ratio/ratio, ratio-1/element, and element/element, to analyze the pollution sources of Pb in soils, finding that it stemmed from dust fallout emitted by smelting plants.

Based on these studies, we used both binary and ternary mixing models to investigate the sources of Pb in Panzhihua and, by association, to identify the sources

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of Cd (e.g., Zai et al., 2007). Using this method, the sources of Pb and Cd in soils above background levels were determined to be the same based on the element/element condition (i.e., uniformity of Pb/Cd ratio) proposed by Yang et al. (1997). Moreover, the number of pollution sources (either 2 or 3) at different contaminated sites was estimated under the ratio-1/element condition. If the pollution sources were estimated to be 2, then the contribution of each end member was calculated in percentage points under the ratio/ratio condition. If the pollution sources

were estimated to be 3, the contribution of each end member was calculated in percentage points under the ratio/ratio/ratio condition. 5.3 Estimating end member contributions A mixing model was used to estimate the contribution of each end member to the composition of surface soils (Bird, 2010) as follows: 20 X

20 X Pb Pb ) S  ( 20 X ) B Pb  100% XA  20 X Pb 20 X Pb Pb ( 20 X ) A  ( 20 X ) B Pb Pb

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( 20 X

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In this model, XA is the contribution rate and (20XPb/20XPb)S, (20X/20XPb)A, and

(20X/20XPb)B refer respectively to soil (e.g.206Pb/207Pb ratio in soil), the Pb isotope ratio of end member A, and the Pb isotope ratio of end member B. The model was used to

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calculate and average 208/206Pb, 208/204Pb, 206/207Pb, and 206/204Pb. The results showed that for the downslope soils of Baguan River, the contributing proportions were 9.5%

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for Pb-containing coal and 90.5% for Pb-containing smelting slag (or smelter dust), and for the upslope soils of the river, the contributing proportions were 85.2% for Pb-

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containing coal and 14.8% for Pb-containing smelting slag. These results illustrate how even moderately contaminated soils such as those in the study area can be traced to sources of potentially heavy pollution. Although the

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modeled contributions to upslope soils and its close association in Figure 6 indicate a possible coal-derived component, the mixing lines for those soils are consistent with these soils belonging to a completely different soil type relative to downslope soils. 6 Conclusions

The distribution of Pb and Cd across the western district of Panzhihua follow similar

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patterns owing to uniformity in Pb/Cd ratios in surface and subsurface samples. We interpret the strong correlation of Pb and Cd in soils of all types in the vicinity to indicate that values above background involve similar pollution sources. Soil profiles indicate decreases in Pb concentration with the soil depth, confirming only meager vertical transport of less soluble heavy metals. Concentrations of Pb and Cd in soil profiles further indicate that soils in west Panzhihua were moderately contaminated up to ~81 µg/g Pb and up to 0.85 µg/g Cd (values that are far lower than heavily contaminated soils in other mining districts). Mixing models using Pb isotopic ratios

indicate that much of the Pb in downslope deltaic soils, deposited in a broad region near the confluence of the two rivers, contains a significant component of slag or smelter dust derived from V-bearing titano-magnetite ore. Isotopic trends also demonstrate that upslope soils likely developed in situ from weathered parental rock with different intrinsic isotopic signatures compared to downslope soils. Mixing models, close isotopic association, and close proximity of upslope soils to the coal separation plant indicate a possible coal-derived component as contaminant; however, more detailed sampling and analyses are necessary to confirm the causes of isotopic

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difference between the two soil types. Our assessment confirms moderate anthropogenic Cd and Pb contamination, which originated from coal burning and ore smelting, in regions surrounding the slag disposal pit in west Panzhihua. Measures to

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reduce heavy metal contamination in soils in this slag dump should begin with conditions of different locations in the region.

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controlling both contamination sources and addressing the respective contamination

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Acknowledgements

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Conflict of interests The authors have declared that no conflict of interest exists.

This study was supported by the National Natural Scientific Foundation of China (41977289), and the National Key Research and Development Project

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(2018YFC0214001, 2018SZDZX0022). Many thanks to the local residents of West Panzhihua who allowed us to collect the samples for this study. This manuscript was significantly improved by comments from two anonymous reviewers; however, the authors take full responsibility for any misleading information presented herein.

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