Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain)

GEXPLO-05457; No of Pages 8 Journal of Geochemical Exploration xxx (2014) xxx–xxx

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Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain) J.J. Ramos-Miras a,⁎, P. Díaz-Férnandez b, A. SanJosé-Wery b, J.A. Rodríguez-Martin c, N. Roca d, J. Bech d, L. Roca-Perez e, R. Boluda e, C. Gil a a

Dept. Agronomía, área Edafología y Química Agrícola, Escuela Politécnica Superior, Universidad de Almería, Ctra. de Sacramento s/n, 04120 La Cañada (Almería), Spain Dept. Desarrollo Sostenible Facultad de Ciencias y Artes, Universidad Católica de Ávila, Canteros s/n, 05005, Ávila, Spain I.N.I.A., Departamento de Medio Ambiente, Ctra. de La Coruña 75, 28040 Madrid, Spain d Laboratorio suelos, Facultad Biología, Universidad de Barcelona, Avda Diagonal 645, 08028 Barcelona, Spain e Dept. Biologia Vegetal, Facultat de Farmàcia, Universitat de València, Av. Vicent Andrés i Estellés s/n, 46100, Burjassot, València, Spain b c

a r t i c l e

i n f o

Article history: Received 17 February 2014 Accepted 10 September 2014 Available online xxxx Keywords: Parent material Anthropogenic activities Baseline concentrations

a b s t r a c t The total, water soluble and extractable concentrations with EDTA of As from topsoils from the Amblés Valley (Ávila, Spain) were determined. The geochemical baseline concentrations of total As were established, and the relationships between the concentration of the different As forms and soil properties were investigated. Total As content in soils was related with parent material, whereas anthropogenic activities affected its mobility. Iron, aluminium, clay content, soil organic matter and soil pH were the main controlling factors for As soil concentrations. The geochemical baseline concentrations obtained (mg kg−1) were 7.3–35 in soils on granite parent material and 2.2–6.8 in soils on alluvium–colluvium parent material for total As. The baseline concentration values for the water soluble and EDTA extractable As forms were also established. For water soluble As, the baseline concentrations (mg kg−1) were 0.06 for natural soils and 0.37 for agricultural and industrial soils, respectively, and the baseline concentrations for As extractable with EDTA were 0.39 mg kg−1. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Arsenic is a trace element whose distribution and concentration in the environment are a serious issue as millions of individuals are affected by As toxicity worldwide (Bhattacharya et al., 2007). In the environment, As can originate from both natural and anthropogenic sources (Gonzaga et al., 2006), while the contamination scale ranges from local to regional. Arsenic has been detected in groundwater in several countries around the world, including Spain, with concentration levels above the drinking water guideline value set at 10 μg l−1 by the World Health Organization Guidelines for drinking-water quality (WHO, 1996). Soil contamination by As can cause loss of vegetation cover and water contamination, and As can enter animal and human food chains (Meharg and Hartley-Whitaker, 2002; Tu and Ma, 2002; Zhang et al., 2006). Although water is the main form of As intake in humans, consuming As-contaminated crops, vegetables or animals is also an important form of human toxicosis (García-Sánchez et al., 2010). In Spain, the presence of As has been related mainly with mining activities because oxidation of sulphur minerals releases As into the environment. Arsenic has been detected in aquifers and soils in southern parts of the Spanish Autonomous Community of Castilla-León in central ⁎ Corresponding author. Tel.: +34 950 01 5057; fax: +34 950 01 5319. E-mail address: [email protected] (J.J. Ramos-Miras).

Spain, with levels of up to 50 μg l−1 in water and over 50 mg kg−1 in soil (García-Sánchez and Álvarez-Ayuso, 2003; García-Sánchez et al., 2005, 2010; García-Villanova et al., 2005; Moyano et al., 2009; Sahún et al., 2004). In Bangladesh and Argentina, the presence of As in groundwater has been associated with the geochemical characteristics of soil parent material (Bhattacharya et al., 2007; Smedley and Kinniburgh, 2002). In the Spanish province of Ávila, risk of human contact with As has increased substantially in the last two decades because residential and urban areas have extended into former agricultural land and groundwater is used by the new urban populations. To the north of this province, “La Moraña” is the most documented area because it forms part of the “Los Arenales” aquifer, which extends through the aforementioned limiting provinces. Despite recent research into As levels in soils or waters in the province of Ávila being limited, local public opinion in this province has raised the alarm that As is present in waters and soils through different human activities; e.g., intense irrigation farming, wastewater disposal, agricultural practices, use of agrochemicals, etc. Nevertheless, there is no empirical evidence to justify this relationship to date. Some authors have associated the presence of As with the use, production and commercialisation of As-based compounds, and the hypothesis of a natural As origin prevails, which indicates that new research is needed to acquire more data to assess this aspect. Thus studying the origin of As in soil is an aspect that has attracted much interest in recent years, and not only in Spain, but elsewhere.

http://dx.doi.org/10.1016/j.gexplo.2014.09.003 0375-6742/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003

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For this reason, the present work focused on studying As distribution in topsoil horizons in the high Adaja river basin in the Amblés Valley (Ávila, Spain). The principal soil use in this area is conventional farming. The total, water soluble and extractable concentrations with EDTA As forms were analysed, and their origin and the causes that might favour their higher or lower concentrations are discussed. This work aimed to: (i) determine total contents and the extractable concentrations with EDTA and water soluble forms of As in Amblés Valley soils (ii) assess the origin of As in these soils; (iii) establish baseline concentrations, and (iv) establish the relationships between As contents and soil properties. 2. Material and methods 2.1. Study area and sampling The Amblés Valley is situated in central Ávila (Spain), and the high Adaja river basin is where practically the entire valley lies, covering an area of 740 km2 (Sánchez Muñoz, 2002). The city of Ávila is located in this valley. The geology of the valley is composed of granite rocks from the Palaeozoic (granite rocks) and by sedimentary materials in depressed regions from the Quaternary (alluvium–colluvium sedimentary materials). The main types of soil in the study area are Lithosols in the cumbers zones, Cambisols on slopes, and Regosols, Luvisols and

Fluvisols in the valley. Forty soil samples were collected from the upper 25 cm of the soil profile with a manual auger and were transferred to polyethylene bags to be transported to the laboratory (Rodríguez Martín et al., 2013). The topsoil horizon samples from the Amblés Valley were grouped firstly according to land use as follows: 15 natural soils (NS) considered control soils, 14 agricultural soils (AGS), and 11 soils affected by dumping through industrial and urban activities (IUS); and secondly into two groups according to soil parent material: 18 soils on granite bedrock (SG) and 22 soils on alluvium– colluvium material (SA). Fig. 1 includes the location of the soil samples. Of the 40 selected topsoils, in a 10 m2 randomly square five different subsamples were taken per site. Then each subsample was taken from a depth of between 0 and 20 cm. Finally these subsamples were mixed to form a compound sample of the soils. This procedure was followed for all the selected sample sites. 2.2. Analytical methods All the soil samples were air-dried, sieved with a 2-mm grid sieve and stored in hermetically sealed polyethylene bags until analysed. A standard soil analysis for each soil sample was carried out. The sand (2–0.05 mm) and clay (b2 μm) fractions were determined by the pipette method (sedimentation). Soil organic matter content (SOM) was determined by the method of Tyurin (1951). Cation exchange capacity

Fig. 1. Lithology of the study area and locations of sampling points.

Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003

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Table 1 Soil properties and statistics (n = 40).

MINV AM MAXV STD MEDIAN

Sand g kg−1

Clay g kg−1

SOM g kg−1

N g kg−1

CEC cmolc kg−1

SEC %

pH H2O

EC dS m−1 25 °C

P mg kg−1

Al g kg−1

Fe g kg−1

Mn g kg−1

391 699 903 134 746

11 213 503 113 185

0.3 6.1 34.5 7.6 4.3

0.3 1.7 5.9 1.0 1.2

2.2 11.2 34.2 6.4 9.6

13.0 70.8 100.0 28.8 74.3

4.5 6.3 7.8 0.8 6.3

0.02 0.50 10.0 1.10 0.20

5 118 471 44 64

11 85 115 45 94

3.1 33 395 10 23

0.2 0.7 1.2 0.3 0.7

SOM—soil organic matter, N—total nitrogen, CEC—cation exchange capacity, SEC—saturation of the exchangeable complex, EC—electric conductivity, P—available phosphorous, Al—total soil aluminium, Fe—total soil iron, Mn—total soil manganese, MINV—minimum value, AM—mean value, MAXV—maximum value, STD—standard deviation.

(CEC) was determined by saturation with sodium acetate solution (pH = 7.0), followed by the replacement of sodium with ammonium acetate (pH = 8.2) and determination by FAAS. The soil reaction (pH) was measured in a 1:2.5 soil–water suspension and electrical conductivity (EC) was determined in the saturation soil paste extract (MAPA, 1994). Available phosphorous (P), expressed as P2O5, was determined by the method proposed by Olsen et al. (1954). The total content of iron (Fe), aluminium (Al) and manganese (Mn) was determined by X-ray fluorescence spectrometer (WD-XRF). To determine total As concentration (AsT), acid digestion was done using HClO4, HCl and HNO3 (Totland et al., 1992). To validate the analytical method, two reference materials were employed: JSO-1 and JSO-2 (the Geological Service, Japan). Recoveries of 98 ± 4% were obtained. The EDTA extractable fraction of As (AsE) was determined following the method described by Quevauviller et al. (1996); 5 g of soil, sieved through a 2-mm sieve, was weighed, added in a polypropylene flask with 50 ml of EDTA, pH 7 (1:10 p/v ratio), and then, the flasks were placed on a rotary shaker for 1 h at 20 °C. Then it was centrifuged in a Sigma 4K10 at 3000 g for 10 min. The supernatant was filtered through a Whatman 40 paper, and was kept in polypropylene flasks at 4 °C until analysed. The water soluble fraction of As (AsS) was extracted with water in a 1:10 soil:deionisated water ratio shaken on an end-overend shaker for 1 h at 30 rev/min and then centrifuged for 10 min at 3000 g. Finally, AsS was determined in the supernatant. All the As determinations were made with a quadruple mass spectrometer using a Thermo plasma source model (Q-ICP-MS), X Series 2, equipped with an Xt interface, a screened burner and a concentric quartz nebulizer. Limits of determination (LOD) were established at 0.01 mg kg−1. The As total, the water soluble and extractable concentrations with EDTA in soils were presented as mg kg−1 of dry matter. The extractable fraction percentage (APE) and the water soluble fraction percentage (SPS) were calculated as the percentage of EDTA extracted As and water extracted As in relation to total As, respectively (Andreu and Boluda, 1995; Gil et al., 2004, 2010; Gimeno-García et al., 1996; Ramos-Miras et al., 2011). According to Massas et al. (2010), and Tarvainen and Kallio (2002), these APE and SPS values can provide a basis to compare the potential mobility and toxicity of trace elements in soils, and should distinguish between anthropogenic contamination and natural origin. According to Tarvainen and Kallio (2002), levels of the available fraction index below 5% indicate non-contaminated soils.

2.3. Statistical analysis All the statistical analyses were performed using the SPSS 15.0 software for Windows. The mean (AM), ranges (MINV–MAXV), standard deviation (STD) and the median are presented. A Spearman's correlation analysis was done to establish the relationships among the As concentrations (total, extractable with EDTA and water soluble) and soil properties. A Kruskall–Wallis test was used to test any significant differences in the As levels between the different soil groups. A Bonferroni test was carried out for the post hoc comparisons between pairs of soil groups. The confidence interval for the Student's t-test was calculated at α = 0.05.

2.4. Baseline concentrations The geochemical baseline concentration (GBC) represents the natural level of a non-contaminated element in soil (Bech et al., 2005). To estimate the GBC, we employed the “standard threshold method” (STM) described by Fleischhauer and Korte (1990). To segregate multiple populations, a log-normal population of samples was assumed and Table 2 Total content (AsT), extractable with EDTA (AsE) and water soluble arsenic (AsS) concentrations in Amblés valley soils on granite bedrock (SG) and alluvium–colluvium materials (SA). APE and SPS are the percentages of extractable and water soluble forms, respectively. Soil code

AsT mg kg−1

AsE mg kg−1

AsS mg kg−1

APE %

SPS %

SG1 SA2 SA3 SA4 SG5 SA6 SA7 SG8 SA9 SG10 SA11 SA12 SA13 SG14 SA15 SA16 SG17 SG18 SG19 SG20 SA21 SA22 SG23 SG24 SG25 SG26 SA27 SA28 SA29 SG30 GS31 SA32 SA33 SG34 SG35 SA36 GS37 SA38 SA39 SA40 MINV AM MAXV MEDIAN STD

5.27 5.93 6.77 5.89 35.11 8.00 7.30 16.43 7.82 15.60 7.56 12.45 9.10 9.24 14.85 26.82 8.29 1.94 125.10 4.51 7.53 4.68 7.44 5.91 6.99 20.26 2.86 25.71 8.34 13.41 2.20 3.97 4.65 22.84 7.79 8.48 20.85 12.11 3.19 6.80 1.94 13.25 125.1 7.81 19.60

0.23 0.23 0.29 0.11 0.40 0.17 0.32 0.58 0.38 0.23 0.09 0.15 0.20 0.25 0.74 0.95 0.13 0.28 0.73 0.24 0.62 0.22 0.10 0.20 0.07 0.51 nd 0.85 1.00 0.06 Nd 0.45 Nd 2.51 0.09 0.15 0.12 0.93 Nd 0.03 0.03 0.41 2.51 0.24 0.46

0.01 0.08 0.25 0.05 0.15 0.06 0.19 0.27 0.17 0.02 0.10 0.06 0.06 0.05 0.84 0.47 0.02 0.12 0.32 0.21 0.12 0.07 0.04 0.02 0.02 0.63 Nd 0.45 0.84 0.62 Nd 0.07 Nd 0.17 0.06 0.15 0.19 0.42 Nd 0.19 0.01 0.21 0.84 0.14 0.23

4.36 3.88 4.28 1.87 1.14 2.13 4.38 3.53 4.86 1.47 1.19 1.20 2.20 2.71 4.98 3.54 1.57 14.43 0.58 5.32 8.23 4.70 1.34 3.38 1.00 2.52 nd 3.31 11.99 0.45 Nd 11.34 Nd 10.99 1.16 1.77 0.58 7.68 Nd 0.44 0.44 3.90 14.43 3.01 3.55

0.19 1.35 3.69 0.85 0.43 0.75 2.60 1.64 2.17 0.13 1.32 0.48 0.66 0.54 5.66 1.75 0.24 6.19 0.26 4.66 1.59 1.50 0.54 0.34 0.29 3.11 nd 1.75 10.07 4.62 Nd 1.76 Nd 0.74 0.77 1.77 0.91 3.47 Nd 2.79 0.13 1.99 10.07 1.42 2.11

MINV — minimum value, AM — mean value, MAXV — maximum value, Median. STD — standard deviation, and nd — not detected. Bold values are the mean and range values all soils studied.

Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003

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Table 3 Arsenic concentrations in different Spanish soils (mg kg−1). As forms

In this work

García-Sánchez et al. (2010)

Moyano et al. (2009)

Álvarez-Ayuso et al. (2008)

De Miguel et al. (2007)

Díez et al. (2007)

García-Sánchez and Álvarez-Ayuso (2003)

Navas and Machín (2002)

AsT AsE AsS

1.9–125 0.03–2.5 0.01–0.84

5.5–150 na 0.004–0.107

18.70 ± 7.71 na 0.21 ± 0.21

17–21 na na

4.3–16 na na

0.5–116 na na

0.2–59 na na

11.8 ± 10.9 na na

AM ± STD; MINV–MAXV; na, data not available; AsT, total content; AsE, extractable EDTA; AsS, soluble content.

a probability plot was drawn. If the data corresponded to a single population, the graph would form a straight line, and the different populations in this graph can be interpreted with distinct straight-line segments. Where two populations overlapped, an inflection point form was drawn on the plot, this being the “threshold point”, and this point represented the separation point between two populations. In mathematical terms, the chosen threshold removed the high and low data of the selected population until the skew of the distribution became minimal (Fleischhauer and Korte, 1990; Rodríguez Martín et al., 2009). This method allows the identification and separation of populations for subsequent statistical analyses (Tack et al., 2005) The baseline concentration (BL) measurement represents the variation of natural concentration of a given element in soil with no human influence (Chen et al., 1999; Ramos-Miras et al., 2011). The STM was used to establish BL for extractable-EDTA and water soluble-As. The baseline As concentrations were calculated from the segregated data set using the median values. 3. Results and discussion The main soil characteristics in Amblés Valley soils are provided in Table 1. These soils had a significant sand content (699 g kg−1), and soil pH ranged from 4.5 to 7.8, with 70% saturation of the exchangeable complex and a mean CEC of 11 cmolc kg−1. The soil organic matter (SOM) and nitrogen values fell within a wide range (0.3 to 34.5 g kg−1 and 0.3 to 5.9 g kg−1, respectively). These SOM and N levels are due to

Table 4 Influence of parent material (SA, alluvium–colluvium material; SG, granite bedrock) (A) and land use (NS, natural soils; AGS, agricultural soils; IUS, industrial–urban soils) (B) on soil properties.

the cold climate prevailing in the area, which lowers the organic matter mineralisation rate. The available P levels were high (5–471 mg kg−1, with a mean value of 118 mg kg−1), mainly because of the phosphate fertilisers used in the study area. The recorded electrical conductivity of the soil water extract (EC) was low. Table 2 shows the total content, water soluble and extractable As levels, as well as the As extractable and water soluble fraction percentage in Amblés soils. The AsT levels obtained in this study were in the same order as those reported by other authors for Spanish soils (Table 3), although the values reported by de Miguel et al. (2007) in urban soils were lower. The results also reveal that the AsS levels were in the same order as those reported by Moyano et al. (2009) in soils in the vicinity of the Amblés valley with similar parent materials. Nevertheless, García-Sánchez et al. (2010) indicated lower AsS levels in natural soils in the province of Salamanca than those found in the present study. Several authors (e.g. Gimeno-García et al., 1996; Ramos-Miras et al., 2011) have suggested that the available fraction index is an indicator of element mobility in soil. Although mobility of the chemical element in soil depends on the soil characteristics to which the pollutants are added, as well as to the chemical element itself, some authors (e.g. Massas et al., 2010; Tarvainen and Kallio, 2002) have suggested that the available fraction index is also an appropriate indicator of recent soil contamination history because elements added to soils as pollutants are less adsorbed by soil components, so these metals are more mobile and could pass to other environmental compartments. The APE and SPS values obtained herein were below 5%, except for samples S15, S18, S29, S32 and S34 (Table 2). According to the criteria of Tarvainen and Kallio (2002), our results showed that soils were not contaminated, and these values do not entail toxicological risks. A similar result was reported by Romaguera et al. (2008), who found that the percentage of the As

(A) Soil properties

SA (n = 18)

Clay (g kg−1) SOM (g kg−1) CEC (cmolc kg−1) pH P (mg kg−1) Al (g kg−1) Fe (g kg−1) Mn (g kg−1)

223 40 9.9 6.5 137 77 19 0.6

± ± ± ± ± ± ± ±

SG (n = 22)

114a 49a 6.1a 0.8a 114a 19ª 8ª 0.2a

209 79 14.1 5.8 47 92 32 0.9

± ± ± ± ± ± ± ±

113a 87b 6.2b 0.6b 81b 22b 7b 0.3b

Ftest

P-value

0.510 5.797 8.997 14.775 17.780 5.472 16.670 6.485

0.475 0.016 0.003 0.000 0.000 0.019 0.000 0.011

(B) Soil properties

NS (n = 15)

AGS (n = 14)

IUS (n = 11)

Ftest

P-value

233 ± 134ab 178 ± 82a 284 ± 109b 8.747 0.033 Clay (g kg−1) SOM (g kg−1) 96 ± 99a 37 ± 29b 17 ± 5b 23.0027 0.000 −1 a b ab CEC (cmolc kg ) 129 ± 63 8.4 ± 6.8 11.6 ± 2.04 21.511 0.000 pH 6.1 ± 7a 6.3 ± 0.8ab 6.8 ± 0.7b 7.836 0.019 P (mg kg−1) 74 ± 75a 177 ± 122b 69 ± 10.7a 17.330 0.000 Al (g kg−1) 86 ± 21ª 74 ± 26a 93 ± 20ª 5.382 0.068 Fe (g kg−1) 29. ± 7a 21 ± 11ª 21 ± 8a 5.973 0.051 Mn (g kg−1) 0.8 ± 0.5a 0.6 ± 0.5ª 0.6 ± 0.6a 3.680 0.159 A different letter indicates statistically significant differences (P b 0.05) after Krustall– Wallis and Bonferoni's post-hoc tests. SOM—soil organic matter, CEC—cationic exchange capacity, P—available phosphorous, Al—total soil aluminium, Fe—total soil iron, Mn—total soil manganese. AM ± STD.

Table 5 Influence of parent material (SA, alluvium–colluvium material; SG, granite bedrock) (A) and land use (NS, natural soils; AGS, agricultural soils; IUS, industrial–urban soils) (B) on soil As forms in the Amblés valley. (A) As-forms

SA (n = 18)

AsT (mg kg−1) AsE (mg kg−1) AsS (mg kg−1) APE (%) SPS (%)

7.4 0.4 0.2 4.3 1.6

± ± ± ± ±

SG (n = 22)

6.1a 0.5a 0.2a 4.0a 1.9a

13.8 0.3 0.2 3.6 2.4

± ± ± ± ±

9.0b 0.3a 0.2a 3.2a 2.1b

Ftest

P-value

5.218 1.380 1.040 1.521 4.283

0.022 0.240 0.308 0.217 0.038

(B) As-forms

NS (n = 15)

AsT (mg kg−1) AsE (mg kg−1) AsS (mg kg−1) APE (%) SPS (%)

10.6 0.4 0.1 3.8 1.1

± ± ± ± ±

8.8a 0.3a 0.1a 4.0a 1.6a

AGS (n = 14) 10.1 0.3 0.3 3.4 2.3

± ± ± ± ±

6.2a 0.2a 0.2b 1.9a 1.6ab

IUS (n = 11) 10.1 0.5 0.3 5.1 3.2

± ± ± ± ±

8.1a 0.3a 0.3b 3.9a 2.8b

Ftest

P-value

0.387 4.171 10.198 2.347 10.573

0.824 0.124 0.006 0.309 0.005

A different letter indicates statistically significant differences (P b 0.05) after Krustall– Wallis and Bonferoni's post-hoc tests. AsT—total content, AsE—extractable EDTA content, AsS—soluble content. APE, and SPS are the extractable and soluble fraction percentage, respectively. AM ± STD.

Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003

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available fraction (AsE) in a contaminated soil from SE Spain was very low (b2%). In addition, Abreu et al. (2012) and Koo et al. (2012) also found that in soils contaminated by As the available fraction concentrations of As was in the same order of magnitude (b5%). With respect to SPS values, García-Sánchez et al. (2010) observed similar levels to those obtained in this study (Table 3). These data suggest that the AsT levels in these soils are not of anthropogenic origin because they are in the same order as the levels reported for natural soils in the vicinity. However, AsT is not a good indicator of soil contamination levels as far as risks to human health are concerned (Sarkar and Datta, 2006). Although, the AsS levels are higher or in the same order than founded by other authors, the APE and SPS values were b5%, which indicate non-contaminated soils. The nature of the soil parent material appears to be the main factor that determines As concentration in soils. However, due to the low As mobility, soils were slightly enriched in As if compared with their soil parent rocks (García-Sánchez and Álvarez-Ayuso, 2003). 3.1. Assessing the origin of As in soils Distinguishing between different sources of potentially toxic elements in soils is a difficult task, especially concerning As (Banning et al., 2009). The data discussed in the previous section indicate that the As levels in the study soils were normal, as confirmed by the values obtained for APE and SPS. To determine the possible impact of soil use or parent material on the variability of As concentrations (total, extractable with EDTA and water soluble) in the analysed soils, a Kruskall–Wallis test, followed by a Bonferroni's post hoc test, were carried out (Tables 4 and 5). Table 4 provides the soil property data of the soils grouped according to their parent material (granite or alluvium–colluvium) and of the soils grouped according to their use. Except for clay, the results indicated significant differences between granite and alluvium–colluvium soils for all the parameters. When this analysis was done with the soils grouped according to their use (natural soils, agricultural soils and industrial soils; see Table 4B), the results indicated significant differences for the clay, SOM, CEC, available P and pH values. These differences founded in soil parameters could be related to the different soil uses (natural soils, agricultural soils and industrial soils). Fig. 2A shows a boxplot of the data, where the difference between SG and SA is apparent. The Kruskall–Wallis test results for As forms and parent material (Table 5) showed only significant differences for AsT between soils SA and SG, and for the SPS levels (Table 5A). García-Sánchez et al. (2010) obtained a similar result in an area close to our study area, in the province of Salamanca, as did Díez et al. (2007) in soils from the province of Granada (S Spain). The differences noted for Al, Fe and Mn contents in soils grouped by parent material could explain this fact (Table 4A), because as Sarkar and Datta (2006) stated, As is strongly adsorbed by non-crystalline iron and aluminium oxides. Regarding AsT, AsE and AsS contents and land use (Table 5B), the statistical analysis showed that AsS content in natural soils was significantly lower than in the other soil groups considered, while, no significant differences in AsT and AsE were found by land use. Fig. 2B clearly illustrates the difference among NS, AS and IUS. The order of abundance for AsS was: IUS ≈ AGS N NS. A similar behaviour for the SPs index was also observed, while no significant differences for APE were found among the soil groups. However, APE was higher in IUS than for the other soil groups (Table 5B), which seems to indicate increased As mobility associated with soil management and soil use. According to Dias et al. (2009) and García-Gómez et al. (2013) this fact might be related with higher soil pH (Table 4B). All these findings indicate that anthropogenic activity does not affect the AsT levels in these soils, but suggest that parent material characteristics are the most influential factor on the total concentration of this element in soil. In fact, Chen et al. (2002) indicated that the main source of As in non-polluted soil was parent material. These data also indicate

Fig. 2. Box-and-whisker plots for water soluble (AsS) concentration and total As content (AsT) in Amblés Valley soils (Ávila, Spain). A): Grouped as soil use (natural soils (NS), agricultural soils (AS) and industrial urban soils (IUS)), B): Grouped as parent materials (granite bedrocks (SG) and alluvium–colluvium materials (SA)). The average value and outliers are also indicated.

that anthropogenic activity may influence As mobility, and suggest that this element's mobility and bioavailability are affected to a certain extent by not only edaphic properties, but also soil use and management. Soil fertilisation, especially organic amendment application, results in higher As concentrations in soil pore water as was stated by Moreno-Jiménez et al. (2013). 3.2. Baseline concentrations Geochemical baseline concentrations (GBC) and BL have been used by many authors to assess soil contamination (Dudka et al., 1995; Galán et al., 2008; Gil et al., 2004, 2010; Ramos-Miras et al., 2011; Tack et al, 2005; Tume et al., 2008). The element content in soils can vastly vary, so the use of universal GBC and BL might prove inappropriate in a specific soil or area (Horckmans et al., 2005). Thus, it is important to locally determine GBC and BL values to assess soil contamination. Following the method described by Fleischhauer and Korte (1990), the approach ranges from the data that were used to estimate the GBC and BL of As for the studied soils. The log-normal population was assumed and the Q–Q plots of the AsT, AsE and AsS concentrations were drawn (Fig. 3). For AsT, two distinct linear segments revealed the presence of two different populations from the change in plot slope (threshold points), which correspond to the soils grouped according to parent material. The threshold point delineating GBC from soils to different populations was located at 7.0 mg kg−1. So the GBCs for SG soils were established between 7.3 and 35 mg As kg−1, and they ranged from 2.2 to 6.8 mg As kg−1 for SA soils. These baseline levels are similar to those previously reported for As by García-Sánchez and ÁlvarezAyuso (2003) in soils from the same area (2.6–13.2 mg As kg− 1 for soils on alluvium–colluvium materials; 2.0–51 mg As kg−1 for soils on

Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003

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granite bedrocks), and by Roca-Pérez et al. (2010) in some Mediterranean soils (b10 mg As kg−1). All these data confirm that each subpopulation has a characteristic background range and that it represents a specific combination of soil formation factors and processes. A different behaviour was observed for the EDTA extractable As contents, and only one population was obtained (Fig. 3). The AsE levels ranged from 0.06 mg kg−1 to 1.5 mg kg−1 for all soils, with a BL of 0.41 mg As kg− 1. For AsS, two different linear segments revealed the presence of two distinct populations from the slope changes. The threshold point was 0.14 mg As kg− 1. So the BL for NS soils was established at 0.06 mg As kg−1, with values ranging between 0.01 and 0.12 mg As kg−1, and their range was 0.15–0.63 mg As kg−1 for AGS and IUS soils, with a BL of 0.37 mg As kg−1. The BL for AsS and AsE was similar in soils used for human activity, which indicates that As is slightly retained in these soils. 3.3. Correlation analysis A correlation analysis is a useful tool for analysing similarities between paired data and is widely used in trace element database analyses

(Gil et al., 2010; Ramos-Miras et al., 2011; Roca-Pérez et al., 2010). In this study, a statistical correlation analysis was performed between the As form concentrations and the determined edaphic properties (pH, clay, SOM, EC, CEC, Al, Fe and Mn) of the 40 Amblés Valley soils (Table 6). Most correlations were significant (α = 0.01 and α = 0.05). Positive and significant correlations were found among AsT and clay, and Fe–Al. In accordance with what several authors have previously reported (Fritzsche et al., 2011; Gan et al., 2014; García-Sánchez et al., 2010; Makris et al., 2009; Ravenscroft et al., 2005; Sahu et al., 2011; Smedley and Kinniburgh, 2002; Wey et al., 2006), this may be explained by As being frequently associated with Fe and Al compounds. Thus, higher Fe–Al content in the soil may be responsible for increase of As sorption in soil. Furthermore, the correlation between AsT and clay may be related with the presence of this element in soil clay minerals. The positive correlations of SOM–AsE, CEC–AsE, EC–AsE, EC–AsS and pH–AsS not only indicate the influence of edaphic factors on the As concentrations in the Amblés topsoils, but also imply that soil properties are the main controlling factors of As concentrations, which corroborate Section 3.1. Similar results were obtained by Chen et al. (1999), who assessed the influence factors affecting As concentrations in Florida

Fig. 3. Normal probability plot of As-forms (AsT, AsE and AsS) in soils as the log of concentration to estimate the GBC values.

Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003

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Table 6 Spearman's correlation coefficients (rho) among soil properties and arsenic forms (n = 40); significant levels: *α b 0.05, **α b 0.01.

Clay pH CEC EC SOM P2O5 Al Fe Mn AsT AsE AsS

Clay

pH

CEC

EC

SOM

P

Al

Fe

Mn

AsT

AsE

1 n.s. 0.237(*) n.s. n.s. −0.229(*) 0.561(**) n.s. n.s. 0.716(**) n.s. n.s.

1 n.s. 0.428(**) n.s. 0.246(*) −0.435(**) n.s. n.s. n.s. n.s. 0.377(**)

1 0.282(*) 0.372(**) −0.426(**) n.s. 0.430(**) 0.499(**) n.s. 0.450(**) n.s.

1 n.s. n.s. n.s. n.s. n.s. n.s. 0.390(**) 0.426(**)

1 −0.274(*) n.s. 0.396(**) 0.397(**) n.s. 0.342(*) n.s.

1 n.s. −0.535(**) 0.318(*) n.s. n.s. n.s.

1 0.614(**) 0.348(*) 0.660(**) n.s. n.s.

1 0.571(**) 0.459(**) n.s. n.s.

1 n.s. n.s. n.s.

1 0.390(**) 0.538(**)

1 0.561(**)

SOM—soil organic matter, CEC—cationic exchange capacity, EC—electric conductivity, P—available phosphorous, Al—total soil aluminium, Fe—total soil iron, Mn—total soil manganese, AsT—arsenic total content, AsE—arsenic extractable EDTA content, AsS—arsenic soluble content; n.s.—not significant.

topsoils. Those authors also indicated that clay content, organic matter and soil pH are strongly and positively correlated with As concentrations. It is also interesting to note that the relationship obtained between pH and AsS concentration, and between SOM and AsE, might indicate a positive transfer mechanism of significant As concentrations to groundwater, and may reduce As sorption by soils. Although this mechanism has not yet been elucidated, it has been suggested by several authors (e.g.: Girouard and Zagury (2009), Lin et al. (2008), Madeira et al. (2012); and Mukherjeea et al. (2008)). Furthermore, it is well-established that pH is one of the most important soil properties to determine mobility of elements in soils due to the pH-dependence of the variable charge components of soil, pH-dependent cation selectivity of sorption sites, and hydrolysis and precipitation reactions (Bohn et al., 1985). The pH appears to influence only AsS, and higher pH values general increase available As in soil solution (Fayiga et al., 2007; García-Sánchez et al., 2010; Vázquez et al., 2008; Wey et al., 2006). Finally, significant positive correlations (α b 0.01) have also been found among AsT, AsE and AsS concentrations. This fact suggests that the combined determination of the total, water soluble and extractable concentrations with EDTA is a good indicator to estimate the distribution of these elements in soils, and that AsS is a good indicator of mobility given its relationship with pH and soil use; this is in good agreement with García-Sánchez et al. (2010), Naidu et al. (2009) and Pérez-Sirvent et al. (2013).

4. Conclusions In this work the AsT and AsS levels in the Amblés Valley soils are related with parent material and soil use. Thus, the GBCs of AsT in those soils on granitic materials (7.3–35.1 mg kg−1) have been distinguished from those on alluvium–colluvium materials (2.2–6.8 mg kg− 1). Similarly, two BLs have been distinguished for AsS according to soil use: 0.06 mg kg−1 for NS and 0.37 mg kg−1 for AGS and IUS, respectively. For AsE, no differences between the soil groups considered have been observed. As far as the meaning and usefulness of As soil forms are concerned, the fact that AsS in NS obtained significantly lower concentrations than AGS and IUS soils suggests that human activity may increase the mobility of this element in soil. The BLs for AsE are similar in all the soil groups, whereas those for AsS in natural soils are lower than AsS in AGS and IUS. Once again, this suggests that human activity favours As mobility in these soils. The APE and SPs values can indicate that the concentration of this element falls within the normal range for non-contaminated soils. These findings, along with soil characteristics, confirm the geogenic origin of As. The correlations found between the As soil-form concentrations and soil properties obtained herein indicate the influence of edaphic factors on As distribution in Amblés soils. Soil properties, together with granite bedrock, are the main controlling

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Please cite this article as: Ramos-Miras, J.J., et al., Influence of parent material and soil use on arsenic forms in soils: A case study in the Amblés Valley (Castilla-León, Spain), J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.09.003