- Email: [email protected]
Spatial distribution and vertical variation of arsenic in Guangdong soil profiles, China
Environmental Pollution 144 (2006) 492e499 www.elsevier.com/locate/envpol
Spatial distribution and vertical variation of arsenic in Guangdong soil profiles, China H.H. Zhang a,b, H.X. Yuan a, Y.G. Hu a,b, Z.F. Wu a,b, L.A. Zhu a,b, L. Zhu c, F.B. Li a,b, D.Q. LI a,b,* a
b
Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou, 510650, China Guangdong Public Lab of Environmental Science & Technology, Guangzhou, 510650, China c Guangzhou Institute of Geochemistry, Chinese Academic, China
Received 25 October 2005; received in revised form 13 January 2006; accepted 17 January 2006
Soil arsenic movement export is a potential threat to the water quality of the study area. Abstract Total of 260 soil profiles were reported to investigate the arsenic spatial distribution and vertical variation in Guangdong province. The arsenic concentration followed an approximately lognormal distribution. The arsenic geometric mean concentration of 10.4 mg/kg is higher than that of China. An upper baseline concentration of 23.4 mg/kg was estimated for surface soils. The influence of soil properties on arsenic concentration was not important. Arsenic spatial distributions presented similar patterns that high arsenic concentration mainly located in limestone, and sandshale areas, indicating that soil arsenic distribution was dependent on bedrock properties than anthropogenic inputs. Moreover, from A- to C-horizon arsenic geometric mean concentrations had an increasing tendency of 10.4, 10.7 to 11.3 mg/kg. This vertical variation may be related to the lower soil organic matter and soil degradation and erosion. Consequently, the soil arsenic export into surface and groundwaters would reach 1040 t year1 in the study area. Ó 2006 Published by Elsevier Ltd. Keywords: Soil; Arsenic; Baseline concentration; Spatial distribution; Vertical variation
1. Introduction Arsenic is often described as being ubiquitous in nature; it is present in soils, rocks, water, and the biological chains of animal and plant lives. It occurs naturally in air above thermally active areas such as volcanoes and thermal waters and, of course, as a contaminant in numerous areas of industrial activity. Arsenic is recognized as an element of public
* Corresponding author. Tianyuanlu 808#, Tianhe District, Guangzhou, 510650, Guangdong Province, China. Tel.: þ86 20 87024715; fax: þ86 20 87024766. E-mail addresses: [email protected] (H.H. Zhang), [email protected] (D.Q. LI). 0269-7491/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.envpol.2006.01.029
concern and is suspected to be responsible for bladder, kidney, liver, lung, and skin cancers. Many arsenic-poisoning episode induced by groundwater arsenic contamination had been reported such as in Taiwan (Thornton and Farago, 1997), West Bengal-India (Nickson et al., 1998; Acharyya et al., 1999), and China (Wang and Han, 1994). Therefore, the maximum allowed level of arsenic in drinking water is 0.050 mg/L and recommended value is 0.010 mg/L by the Environmental Protection Agency and the World Health Organization (EPA, 1998; WHO, 2001). And developing countries are struggling to reach standards of 0.050 mg/L in arsenic affected areas. Due to threaten to the human health, much work has been done on the groundwater arsenic such as arsenic pollution
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
mechanisms (MaArthur et al., 2001) and arsenic sources (Peters et al., 1999; Berg et al., 2001). Moreover, an average toxicity threshold of 40 mg/kg was established for crop plants (Sheppards, 1992). And Xu and Liu (1996) estimated the arsenic threshold of 38e47 mg/kg for Guangdong soils. As we know, the groundwater contaminations primarily come from natural sources. Soil degradation and erosion is the important pathway that soil arsenic transports into ground and surface waters. As an indicator of particulate freight of the stream for soil erosion, the global soil arsenic export estimated roughly would reach 4.4 107 kg year1 (Matschullat, 2000), compared with soil input of 2.84e9.4 107 kg year1 (Chilvers and Peterson, 1987). Many investigations for soil arsenic background concentration had been carried out in different areas (Chen et al., 1999, 2001; Reimann et al., 2001; Zhang et al., 2002). As a general summary, it can be considered that the principal factors influencing the arsenic concentration in soils are the parent rock and anthropogenic activities. Factors such as climate, the organic and inorganic components of soils and redox potential status also affect the level of soil arsenic (Mandal and Suzuki, 2002). Soils are complex mixtures of organic, inorganic materials and metal compounds from anthropogenic sources. For the analysis of these samples, chemical dissolution must often be performed, such as ICP-AES, AAS, and ICP-MS, but in most cases silicate minerals are not easily digested, which may cause analytical error in element determination. Whereas, Instrumental Neutron Activation Analysis (INAA) is a purely instrumental method, which does not require any chemical treatments, including dissolution, of the samples to be analyzed, thus reducing the possibility of contamination and losses (Naidu et al., 2003; Orvini et al., 2005). In Guangdong province the limestone and sandshale were formed dominantly, due to the large-scale invasion of the sea, during the early Carboniferous period to early Permian period, when the biological enrichment is the primary reason that high arsenic content occurred in the limestone and sandshale (GSGIO, 1993; Wang and Wei, 1995). And then, in the period of Cenozoic tectonic movements, the landform of Guangdong province was uplifted totally. Igneous rocks intruded and cut through the limestone and sandshale along regional faults extensively, at the same time, hydrothermal activities were intensive and frequent induced by the regional magmation. Geothermal researches revealed that hydrothermal activities had close relationship with high content of arsenic and mercury (Zhu et al., 1989; Zhu and Yu, 1995). As a result, many polymetal ores such as pyrite ores that formed during that period are always located in the limestone and sandshale areas cut through by regional faults. Therefore, because the arsenic belongs to the sulfur family element and has strong chemical affinity with ferrum element, arsenopyrite (FeAsS) and its secondary mineral arsenolite (As2O3) is the most abundant arsenic mineral in these areas (GSGIO, 1993; Wang and Wei, 1995). The soil and water loss due to intensive physical and chemical weathering and damage of vegetation is serious issue in Guangdong province, and the loss area reach up to about
493
7376 km2, accounted for 4.2% of total area (GSGIO, 1993). Moreover, because there exist the frequent arsenic exchange between different interfaces, and soil arsenic release via active soil microbial actions in the subtropical area (Matschullat, 2000), soil arsenic has a considerably shorter retention time in the study area, compared to retention time 1000e3000 years in moderate climates (Bowen, 1979). Therefore, the investigations of spatial distribution and vertical variation for soil arsenic are of importance to reveal the transformation rule of soil arsenic and evaluate the influence of soil arsenic on the quality of regional water body. The present study was conducted to: (i) estimate the upper baseline concentration of surface soil; (ii) evaluate influence factors controlled the spatial distribution and vertical variation of soil arsenic concentrations; and (iii) reveal that surface soil degradation and erosion was an important driving force of soil arsenic into the ground and surface water. Such information would provide useful information for establishing proper arsenic cleanup standard, and will help to evaluate the influence of natural arsenic on the quality of ground and surface water in the study area. 2. Materials and methods 2.1. Description of the study area Guangdong province is located in the southern part of the south China (Fig. 1A), between latitude 20 100 e25 310 (N) and longitude 109 410 e 117 170 (E). The area influences by a subtropical monsoon climate with an average annual precipitation of 1336 mm and averages annual evaporation 1100 mm. Its average annual temperature is 17e27 C and averages 1828 h of sunshine annually. From north to south the altitude of landform decreases and coteau, platform, and plain alternate. The coteau area is up to about 60% of the total area. Regional faults with NNE- or NE-treading are the main geological character, and always cut through the regional sandstone and sandshale areas (Fig. 1B). Granite is the most extensive parent rock of soil forming, accounted for more than 40% of Guangdong area. Moreover, the larger area of limestone and sandshale are cropped out in study area; and some basalt parent rock is in the Leizhou Peninsula (Fig. 1B). Guangdong soil profile is the typical Al-enriched weathering profile and is the product of the latest stage of weathering. It had been developed on a variety of rocks, such as, granite, sandshale, limestone, and basalt. Because of the intensive eluviation and illuviation in the hydrothermal condition of study area, the soil profiles are deficient in soluble salt, alkali metal, and alkali-earth metal, but rich in Fe and Al oxides and Hþ (Lan et al., 2003). Therefore, the average pH value in Guangdong soil profiles is acidic in nature. Moreover, acid rain is also an important impact on the pH value of soils (Larssen and Carmichael, 2000).
2.2. Sampling and chemical analysis Soil samples used in this study, as a part of the investigation of soil background value in China during the Seventh Five Years Plan, were colleted from locations shown in Fig. 1C. All samples were taken from the soil profiles with length of 1.5 m, width of 0.8 m, and depth of 1.2 m, soil samples were dried at natural temperature (22e25 C), crushed by the wooden stick, and picked out the gravel, and then sieved through a 20-mesh nylon screen (about 1 mm aperture size) and 150-mesh nylon sieve prior to analysis. In this work, the analytical technique used for elemental analysis was Instrumental Neutron Activation Analysis (INAA). Approximately 0.1 g of prepared sample precisely weighed was packed in aluminium bag of 1 cm 1 cm size, and then sealed by the high purity aluminium foil together with the National Institute for Science and Technology coal fly ash standard reference
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
494
material (NIST-1633a, Gaithersburg, Maryland, USA; standard value: 145 3 mg/kg) and the National Research Center for GeoAnalysis loess standard reference material (GSS-8, Beijing, China; Standard value: 12.7 0.4 mg/kg). The samples were irradiated in a heavy-water cooled reactor, China Institute of Atomic Energy, at the neutron flux of 5 1013 n cm2 s1. The 76As isotope of samples was detected on the HPGe Gamma-ray spectrometer (model Pop Top- EG&G ORTEC, America) with the resolution of 2.0 keV on 1332 keV 60Co line. The observed arsenic concentration of standard reference materials of NIST-1633a and GSS-8 were 145 3 mg/kg (n ¼ 32) and 12.6 0.5 mg/kg (n ¼ 32), respectively. The analytic precision is less than 1% for arsenic. The pH of soil was measured by taking 10 g of sample into 25 ml of reagent water (Chinese National Standard Agency, 1988). The soil organic matter content was calculated from the weight loss of sample by decomposing at 110 C for 2 h after a pre-treatment with 3 ml of 30% (v/v) hydrogen peroxide. The different soil grain size fractions were separated using manual dry sieving. Three nylon sieves were used and placed in series for the determination of a fine (clay) fraction (<50 mm), a sand fraction (50e2000 mm) and a coarse fraction (>2000 mm). By avoiding possible contamination by metal instrument, this simple technique allowed us to perform accurate analyses of the different fractions.
As in each horizon soil were produced based on geostatistical analysis using the software of SurferÒ 8.0.
2.3. Data analysis
3.2. Soil properties
All arsenic contents were presented on a dry matter basis. These values were transformed to logarithms (base 10) because they had positive-skewed frequency distributions (Table 1). Because the data fit a lognormal distribution (Fig. 2), the central tendency and variation of the data were expressed as the geometric mean (GM) and geometric standard deviations (GSD), respectively. Therefore, the GM and GSD were used to calculate the range and variation expected for arsenic. Baseline concentrations of arsenic were calculated using GM/GSD2 and GM GSD2 of the samples, which include 95% of sample population (Dudka et al., 1995; Chen et al., 2001). Spatial interpolation was performed using ordinary kriging. For the lowdensity and background sampling, the ordinary kriging estimate can be thought of simply as an optimally weighted average of the data. It provides a best linear unbiased prediction of spatial distribution (Cressie, 1991). The spatial interpolation and contour maps displaying the spatial distribution of
3.2.1. Organic matter content The organic matter content of soil samples from A-horizon ranged from 0.17 to 9.94% (GM, 2.37%; AM, 2.75%). In soils from B- and C-horizon the content ranges were 0.07e3.83% (GM, 0.82%; AM, 0.99%) and 0.03e3.56% (GM, 0.49%; AM, 0.65%), respectively. The high value was observed in A-horizontal soils and the lowest in soils from the C-horizon (Table 1).
3. Results 3.1. Arsenic concentration The concentrations of arsenic in soil samples from A-horizon ranged 1.2e309.2 mg/kg, with a geometric mean (GM) of 10.4 mg/kg and an arithmetic mean (AM) of 17.7 mg/kg. In soils from the B-horizon, the concentration range was from 0.6 mg/kg up to 340 mg/kg, with a GM of 10.7 mg/kg and an AM of 19.0 mg/kg. The C-horizon soil arsenic concentrations were between 1.3 and 255.2 mg/kg. Its GM and AM were 11.3 and 19.6 mg/kg, respectively. The highest concentration was observed in soils from B-horizon, up to 340 mg/kg. All of data had a high positive skewness in each horizontal soil (Table 1).
3.2.2. Grain size distribution The data of grain size fit normal distribution in each horizontal soil. From A- to C-horizon the arithmetic mean contents
Table 1 Summary statistic of the data for 260 soil profiles in Guangdong n
Min.
Median
Max.
Skewness
AM ASD*
GM GSD**
A-horizon As (mg/kg) Sand (%) Clay (%) OM (%) pH-H2O
260 256 256 260 257
1.2 12.0 0.3 0.17 4.0
10.3 51.6 17.3 2.42 4.9
309.2 90.0 46.7 9.94 8.5
5.6a; 0.68b 0.1 0.7 1.5 1.6
17.7 30.2 50.4 13.9 18.4 8.9 2.75 1.55 5.17 0.78
10.4 ± 1.5 48.2 1.2 15.8 1.3 2.37 1.29 5.12 1.06
B-horizon As (mg/kg) Sand (%) Clay (%) OM (%) pH-H2O
257 254 254 257 254
0.6 3.0 0.4 0.07 4.1
10.4 41.8 19.2 0.86 5.1
340.0 91.6 51.7 3.83 8.5
5.7a; 0.39b 0.5 0.6 1.7 1.5
19.0 32.4 43.4 15.9 20.4 9.9 0.99 0.63 5.40 0.85
10.7 ± 1.5 40.3 1.2 17.5 1.3 0.82 1.32 5.34 1.06
C-horizon As (mg/kg) Sand (%) Clay (%) OM (%) pH-H2O
258 255 254 258 256
1.3 4.60 0.30 0.03 3.5
11.3 43.9 18.0 0.50 5.2
255.2 91.6 51.7 3.56 9.0
4.6a; 0.47b 0.3 0.9 2.6 1.3
19.6 30.4 44.6 16.7 20.1 10.4 0.65 0.57 5.48 0.83
11.3 ± 1.5 41.1 1.2 17.1 1.3 0.49 1.4 5.43 1.06
*AM e arithmetic mean; ASD e arithmetic standard deviation; **GM e geometric mean; GSD egeometric standard deviation. a Untransformed data. b Log-transformed data.
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
495
Fig. 1. Location of Guangdong Province in China (A), geological sketch indicating the distribution of limestone and sandshale (B), and sampling locations (C). (Number 1 is Beijiang River, 2 is Xijiang River, 3 is Dongjiang River, 4 is Hanjiang River, and 5 is Jianjiang River.)
of sand are 50.4, 43.4, and 44.6% and that of clay are 18.4, 20.4 and 20.1%, respectively (Table 1). 3.2.3. Soil pH From A- to C-horizon the soil pH values had a slight increase from 5.14, 5.34 to 5.43. The highest and lowest values were observed in the C-horizon (Table 1).
4. Discussion 4.1. Baseline concentration of surface soils Natural background concentration is defined as the ambient concentration of chemicals in soils without human influence (Gough, 1993). But it is almost impossible to establish the true natural background level of arsenic in soils, due to long-range transport and precipitation of contaminants. As such, the baseline concentration as a reference to determine clean soils represent element concentrations specific for a given region and time period, but do not always represent true background concentration (Kabata-Pendias and Pendias, 1992; Salminen and Tarvainen, 1997). The present study showed that the arsenic background concentration of 10.4 mg/kg in Guangdong surface soils was higher than that of 9.6 mg/kg in China (Wei, 1990) and 5e 7.5 mg/kg in global soil (Goldschmidt, 1958; Koljonen, 1992; Allard, 1995). The baseline concentration was 4.7e 23.4 mg/kg calculated by using GM/GSD2 and GM GSD2. Log-normality was obvious from histograms of the data (Fig. 2). Therefore the GM and GSD are better to represent the central tendency of the distribution than AM and ASD, and upper baseline concentration (UBC) of 23.4 mg/kg can better express arsenic background concentration because of the distorting effects of a few large values had been minimized by using the log-transformation (Dudka et al., 1995; Chen et al., 2001).
To validate the data for establishing arsenic background concentration in soil, the normal probability plot of arsenic contents was adopted. A cumulative frequency plot of the log-As data followed a near straight line (Fig. 3), which means that the data come from a single statistical population and suggests that arsenic concentrations in this database are largely not affected by anthropogenic influences. This indicates that the database was a good estimate of arsenic background concentration in surface soil of Guangdong. Moreover, the upper tail concerned about 15 samples that had less high values than would be expected, suggesting there exited the arsenic loss induced by the soil degradation and erosion in high arsenic concentration areas. The arsenopyrite (FeAsS) and arsenolite (As2O3) are main minerals responsible for elevated arsenic levels in these regions. The formation of arsenopyrite is the result that As3þ and Fe3þ are prone to combine for similar ionic ˚ ; Fe3þ, 0.67 A ˚ ) during the period of the radius (As3þ, 0.69 A hydrothermal deposition induced by the regional faults cutting through the limestone and sandshale stratums (Wang and Wei, 1995). The arsenolite is the secondary mineral formed by oxidation of arsenopyrite. 4.2. Correlations with soil properties Soils containing high amounts of clay minerals and organic matter will tend to accumulate higher metal level because these compounds have pronounced metal binding properties (Bolt and Bruggenwert, 1976). Chen et al. (1999) evaluated influence factors affecting arsenic concentrations in Florida surface soils and indicated that clay content, organic matter, and soil pH had strong positive correlation with arsenic concentrations, implying the soil properties are major controlling factors for arsenic concentrations. But to the relatively young Guangdong soils, the correlations were not like so. In the present research to Guangdong soil profiles, linear trends between soil properties and arsenic contents were mostly not significant (Table 2). Only for clay contents was
496
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
Fig. 2. Distribution frequency of arsenic concentration in A-, B-, and C-horizon soils in Guangdong (left: origin data; right: log10 transformed data).
there a significant positive correlation of 0.137 with the As concentrations in A-horizontal soils. As we known, the formation of soil is a function of climate, soil organisms, landscape, vegetation, time and geology. Our results revealed that soil organic matter and clay content are not major factors in determining the distribution and content of arsenic in Guangdong soils. These weaker correlations may be the result of the very rapid weathering of parent rock. Therefore, since the short soil formational process due to intensive weathering and mineral components and texture of parent rocks, the
properties of parent rocks played the more important role than other influence factors such as soil clay and organic content in the soil arsenic content and distribution in the study area. 4.3. Spatial distributions and vertical variation of arsenic level The spatial distribution of arsenic content as estimated by kriging, is mapped in Fig. 4AeC for A-, B-, and C-horizontal
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
(Lat.) 25
497
A
24
23
22
21 110
(Lat.) 25
Fig. 3. Lognormal probability plot for total arsenic concentration in Guangdong surface soils (A-horizon soil).
111
112
113
114
115
111
112
113
114
115
111
112
113
114
115
116
117 (Long.)
B
24
soils, respectively. The A-horizon reflects the complex interplay between atmosphere, biosphere and lithosphere. The B-horizon can be used to study the influence of soil forming process, while the C-horizon represents the composition of the lithosphere at each sample site and thus the geogenic background (Reimann et al., 2001). Wang and Wei (1995) summarized soil samples (n ¼ 4095) developed on different parent rocks in China and summarized that the background concentration of soil arsenic was primarily influenced by the parent rocks and presented that average soil arsenic concentration developed on the sediment areas were 15.6 mg/kg (limestone), 12.2 mg/kg (shale), 11.0 mg/kg (sandshale), and 10.7 mg/kg (sandstone), much higher than the national background value of 9.6 mg/kg, however, far lower soil concentration than national background value of arsenic were on the igneous rock areas about 5.9e6.6 mg/kg.
23
22
21 110
116
117 (Long.)
(Lat.) 25
C
24
23
22
Table 2 Pearson correlations between log10 As content and some soil properties (significance at the 5% level is indicated with ‘‘*’’, at the 1% level with ‘‘**’’) As (mg/kg) A-horizon As (mg/kg) OM (%) Clay (%) Sand (%) pH B-horizon As (mg/kg) OM (%) Clay (%) Sand (%) pH C-horizon As (mg/kg) OM (%) Clay (%) Sand (%) pH
1.000
OM (%) 0.008 1.000
Clay (%)
Sand (%)
pH
0.137* 0.147* 1.000
0.098 0.111 0.684** 1.000
0.047 0.060 0.074 0.192** 1.000
1.000
0.013 1.000
0.119 0.290** 1.000
0.132* 0.131* 0.519** 1.000
0.031 0.223** 0.170** 0.054 1.000
1.000
0.009 1.000
0.112 0.427** 1.000
0.113 0.192** 0.475** 1.000
0.084 0.177** 0.196** 0.041 1.000
21 110
116
117 (Long.)
Log10[As]0 0.2 0.4 0.6 0.8 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 As(mg/kg) 1 1.58 2.51 3.98 6.31 10.0 12.6 15.8 20.0 25.1 31.6 40.0 50.1 63.1 79.4 100 126
Fig. 4. Spatial distributions of arsenic concentrations in A-, B-, and C-horizon soils in Guangdong, China.
In the present study, geometric mean (GM) of A-, B-, and C-horizontal soils had an increasing tendency of 10.4, 10.7 to 11.3 mg/kg from A- to C-horizontal soils (Table 1), and contours of arsenic concentration displayed similar spatial distribution patterns (Fig. 4). These similar distributional patterns were that the soil samples with high concentration of arsenic mainly located in the limestone and sandshale areas (Figs. 4 and 1B), through these parent rocks also varied in their arsenic content, from small amounts to up to 309 mg/kg. And further confirmed that the distribution of arsenic was controlled by
498
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
properties of regional parent rocks rather than clay and organic contents of soils. As such, anthropogenic arsenic inputs were not a very important factor. Although soil profiles displayed the increasing tendency of soil arsenic concentrations from A- to C-horizon, it was not obvious accumulation of the bottom layer. This phenomenon may had close relation with the followed reasons: (1) Ahorizontal soil with the lower average organic matter content of 2.75% cannot act as a natural biogeochemical barrier that suppresses the percolation of the element with the seepage water, and thus, cannot accumulate the arsenic element; (2) the subtropical hydrothermal condition, lower soil pH, and high permeability of semi-weathering granitic parent rocks cropped out extensively, all of these would induce the upper soil arsenic transferred to the much lower soil layer or groundwater; (3) it is obvious that an additional arsenic movement export will take place constantly from surface soil degradation and erosion, the products of which are being transported into ground and surface water. As a rough estimate, the soil area of Guangdong province is about 147,458 km2 with an average soil density of about 1 l03 kg/m3 (GSGIO, 1993). Annual soil erosion yield is about 1 108 t a1 and mainly distributed along Hanjiang, Beijiang, Dongjiang, and Jiangjiang watersheds (Zhang et al., 1994). According to these data at least about 1040 t arsenic would enter into the river water annually produced by the soil degradation and erosion. As a result, the average concentrations of arsenic in the sediments of the Xijiang River, Beijiang River and Dongjiang River had reached up to 45.1, 68.2, and 21.6 mg/kg, and average concentrations of Fe is up to 4.96, 4.21, and 3.96%, respectively (Zhang and Wang, 2001). The sediment-bound arsenic most probably originates from erosion and weathering processes, which result in the fluvial transport and sedimentation of arsenic-enriched iron oxyhydroxides (Peters et al., 1999), and then released to the groundwater by reductive dissolution of iron and oxidation of sulfide phases (Nickson et al., 1998; Berg et al., 2001). Moreover, the limestone and sandshale were always cut through by regional faults (Fig. 1B), which provided the pathway that soil arsenic transferred into the groundwater much more easily. Therefore, the natural soil erosion and vertical transformation process might be a potential threat to the quality of surface and groundwater in the study area.
5. Conclusions The geometric mean (GM) arsenic concentration of 10.4 mg/kg and upper baseline As concentration of 23.4 mg/kg were estimated for Guangdong surface soils. The influence of organic matter, clay contents, pH, and anthropogenic activities on arsenic concentration was not important. The results showed that regional parent bedrock properties were primary factor controlled the spatial distribution of soil arsenic concentration. About 1040 t arsenic produced by the surface soil degradation and erosion was the main arsenic contamination source of ground and surface water, and might be the potential threat to the water source in study area.
Acknowledgments We thank Professor W.J. Manning and reviewers for constructive reviews and suggestions that improved the quality of this manuscript. This work was partly funded by the Youth Talent Supported Foundation of Guangdong Academy of Sciences (2005-04), the Technologies R & D Program of Guangdong Province (2004B32501004) and the Key Program of Guangdong Natural Science Foundation (04101225).
References Acharyya, S.K., Chakraborty, P., Lahiri, S., Raymahasay, B.C., Guha, S., Bhoumik, A., 1999. Arsenic poisoning in the Ganges delta. Nature 401, 545. Allard, B., 1995. Groundwater. In: Salbu, B., Steinnes, E. (Eds.), Trace Elements in Natural Waters. CRC Press, Boca Raton, pp. 151e176. Berg, M., Tran, H.C., Nguyen, T.C., Phem, H.V., Schertenleib, R., Giger, W., 2001. Arsenic contamination of groundwater and drinking water in Vietnam: a human threat. Environmental Science and Technology 35, 2621e2626. Bolt, G.M., Bruggenwert, M.G.M., 1976. Soil Chemistry. A. Basic Elements. Elsevier, Amsterdam. Bowen, H.J.M., 1979. Environmental Chemistry of the Elements. Academic Press, London, 333 pp. Chen, M., Ma, L.Q., Willie, G.H., 1999. Baseline concentrations of 15 trace elements in Florida surfaces soils. Journal of Environmental Quality 28, 1173e1181. Chen, M., Ma, L.Q., Hoogeweg, C.G., Harris, W.G., 2001. Arsenic background concentration in Florida, U.S.A. surface soil: determination and interpretation. Environmental Forensics 2, 117e126. Chilvers, D.C., Peterson, P.J., 1987. Global cycling of arsenic. In: Hutchinson, T.C., Meema, K.M. (Eds.), Lead, Mercury, Cadmium, and Arsenic in the Environment. John Wiley & Sons, New York, pp. 279e 301. Scientific Committee on Problems of the Environment (SCOPE) 31. Chinese National Standard Agency, 1988. Determination of pH Value in Forest Soil. GB 7859-87, UDC 634.0.114:631.422, pp. 171e173 (in Chinese). Cressie, C., 1991. Statistics and Data Analysis in Geology, second ed. John Wiley & Sons, New York. Dudka, S., Ponce-Hernandez, R., Hutchinson, T.C., 1995. Current levels of total element concentrations in the surface laver of Sudbury’s soils. Science of the Total Environment 162, 161e172. Environmental Protection Agency, 1998. Integrated Risk Information System (IRIS), Arsenic, Inorganic. CASRN 7440-38-2. U.S. Cincinnati, OH. Goldschmidt, V.M., 1958. Geochemistry. In: Muir, A. (Ed.). Oxford University Press, Oxford, pp. 468e475. Gough, L.P., 1993. Understanding our Fragile Environment, Lessons from Geochemical Studies. USGS Circular 1105. US Gov. Print Office, Washington, DC. GSGIO, 1993. Guangdong Soil. In: Liu, A.S. (Ed.). Science Publish House, Guangdong Soil General Investigation Office. Kabata-Pendias, A., Pendias, H., 1992. Trace Elements in Soils and Plants. CRC Press, Boca Raton, FL. Koljonen, T. (Ed.), 1992. Geochemical Atlas of Finland, Part 2: Till. Geological Survey of Finland, Espoo, Finland, 218 pp. Lan, H.X., Hu, R.L., Yue, Z.Q., Lee, C.F., Wang, S.J., 2003. Engineering and geological characteristics of granite weathering profiles in South China. Journal of Asian Earth Sciences 21, 353e364. Larssen, T., Carmichael, G.R., 2000. Acid rain and acidification in China: the importance of base cation deposition. Environmental Pollution 110, 89e102. MaArthur, J.M., Ravenscroft, P., Safiulla, S., Thirlwall, M.F., 2001. Arsenic in groundwater: testing pollution mechanism for sedimentary aquifers in Bangladesh. Water Resources Research 37, 109e117.
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499 Mandal, B.K., Suzuki, K.T., 2002. Arsenic round the world: a review. Talanta 58, 201e235. Matschullat, J., 2000. Arsenic in the geosphere e a review. Science of the Total Environment 249, 279e312. Naidu, G.R.K., Trautmann, N., Zaunar, S., Balaji, T., Rao, K.S., 2003. Multielemental analysis of soils by instrumental neutron activation analysis. Journal of Radioanalytical and Nuclear Chemistry 258 (2), 421e425. Nickson, R.T., McArthur, J.M., Burgess, W.G., Ahmed, K.M., Ravenscoft, P., Rahman, M., 1998. Arsenic poisoning of Bangladesh groundwater. Nature 395, 338. Orvini, E., Speziali, M., Herborg, C., Salvini, A., 2005. Trace element characterization by INAA of three sediments to be certified as standard reference materials. Microchemical Journal 79, 239e242. Peters, S.C., Blum, J.D., Klaue, B., Karagas, M.R., 1999. Arsenic occurrence in New Hampshire drinking water. Environmental Science and Technology 33, 1328e1333. Reimann, C., Kashulina, G., de Caritat, P., Niskavaara, H., 2001. Multi-element, multi-medium regional geochemistry in the European Arctic: element concentration, variation and correlation. Applied Geochemistry 16, 759e780. Salminen, R., Tarvainen, T., 1997. The problem of defining geochemical baseline. A case study of selected elements and geological materials in Finland. Journal of Geochemical Exploration 60, 91e98. Sheppards, S.C., 1992. Summary of phytotoxic levels of soil arsenic. Water, Air and Soil Pollution 64, 539e550. Thornton, I., Farago, M., 1997. The geochemistry of arsenic. In: Abernathy, C.O., Calderon, R.L., Chappell, W.R. (Eds.), Arsenic: Exposure and Health Effects. Chapman and Hall, Kluwer Academic Publisher, London, pp. 1e16.
499
Wang, L.F., Han, J.H., 1994. Chronic arsenism from drinking water in some areas of Xinjiang, China. In: Nriagu, J.O. (Ed.), Arsenic in the Environment, Part II: Human Health and Ecosystem Effects. John Wiley & Sons, New York, pp. 159e172. Wang, Y., Wei, F.S., 1995. Chemistry of Elements in the Pedosphere Environment. China Environmental Science Press, Beijing, pp. 42e58 (in Chinese). Wei, F.S., 1990. Elemental Background Contents in the Soil of China. China Environmental Science Press, Beijing. (in Chinese). WHO, 2001. Arsenic Compounds, Environmental Health Criteria 224, second ed. World Health Organization, Geneva. Xu, L.F., Liu, T.H., 1996. The zonal differentiation of soil environmental background values and critical contents in Guangdong. Journal of South China Agricultural University 17 (4), 58e62. (in Chinese). Zhang, C.S., Wang, L.J., 2001. Multi-element geochemistry of sediments from the Pearl River system, China. Applied Geochemistry 16, 1251e1259. Zhang, J.W., Yao, S.X., Li, H.S., 1994. Study on Slope Land in South China. Science Press, Beijing. Zhang, X.P., Deng, W., Yang, X.M., 2002. The background concentrations of 13 soil trace elements and their relationships to parent materials and vegetation in Xizang (Tibet), China. Journal of Asian Earth Sciences 21, 167e174. Zhu, B.Q., Zhu, L.X., Shi, C.Y., Yu, H., Wang, G.Y., 1989. Application of geochemical methods in the search for geothermal fields. Journal of Geochemical Exploration 33, 171e183. Zhu, B.Q., Yu, H., 1995. The use of geochemical indicator elements in the exploration for hot water sources within geothermal fields. Journal of Geochemical Exploration 55, 125e136.
Spatial distribution and vertical variation of arsenic in Guangdong soil profiles, China H.H. Zhang a,b, H.X. Yuan a, Y.G. Hu a,b, Z.F. Wu a,b, L.A. Zhu a,b, L. Zhu c, F.B. Li a,b, D.Q. LI a,b,* a
b
Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou, 510650, China Guangdong Public Lab of Environmental Science & Technology, Guangzhou, 510650, China c Guangzhou Institute of Geochemistry, Chinese Academic, China
Received 25 October 2005; received in revised form 13 January 2006; accepted 17 January 2006
Soil arsenic movement export is a potential threat to the water quality of the study area. Abstract Total of 260 soil profiles were reported to investigate the arsenic spatial distribution and vertical variation in Guangdong province. The arsenic concentration followed an approximately lognormal distribution. The arsenic geometric mean concentration of 10.4 mg/kg is higher than that of China. An upper baseline concentration of 23.4 mg/kg was estimated for surface soils. The influence of soil properties on arsenic concentration was not important. Arsenic spatial distributions presented similar patterns that high arsenic concentration mainly located in limestone, and sandshale areas, indicating that soil arsenic distribution was dependent on bedrock properties than anthropogenic inputs. Moreover, from A- to C-horizon arsenic geometric mean concentrations had an increasing tendency of 10.4, 10.7 to 11.3 mg/kg. This vertical variation may be related to the lower soil organic matter and soil degradation and erosion. Consequently, the soil arsenic export into surface and groundwaters would reach 1040 t year1 in the study area. Ó 2006 Published by Elsevier Ltd. Keywords: Soil; Arsenic; Baseline concentration; Spatial distribution; Vertical variation
1. Introduction Arsenic is often described as being ubiquitous in nature; it is present in soils, rocks, water, and the biological chains of animal and plant lives. It occurs naturally in air above thermally active areas such as volcanoes and thermal waters and, of course, as a contaminant in numerous areas of industrial activity. Arsenic is recognized as an element of public
* Corresponding author. Tianyuanlu 808#, Tianhe District, Guangzhou, 510650, Guangdong Province, China. Tel.: þ86 20 87024715; fax: þ86 20 87024766. E-mail addresses: [email protected] (H.H. Zhang), [email protected] (D.Q. LI). 0269-7491/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.envpol.2006.01.029
concern and is suspected to be responsible for bladder, kidney, liver, lung, and skin cancers. Many arsenic-poisoning episode induced by groundwater arsenic contamination had been reported such as in Taiwan (Thornton and Farago, 1997), West Bengal-India (Nickson et al., 1998; Acharyya et al., 1999), and China (Wang and Han, 1994). Therefore, the maximum allowed level of arsenic in drinking water is 0.050 mg/L and recommended value is 0.010 mg/L by the Environmental Protection Agency and the World Health Organization (EPA, 1998; WHO, 2001). And developing countries are struggling to reach standards of 0.050 mg/L in arsenic affected areas. Due to threaten to the human health, much work has been done on the groundwater arsenic such as arsenic pollution
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
mechanisms (MaArthur et al., 2001) and arsenic sources (Peters et al., 1999; Berg et al., 2001). Moreover, an average toxicity threshold of 40 mg/kg was established for crop plants (Sheppards, 1992). And Xu and Liu (1996) estimated the arsenic threshold of 38e47 mg/kg for Guangdong soils. As we know, the groundwater contaminations primarily come from natural sources. Soil degradation and erosion is the important pathway that soil arsenic transports into ground and surface waters. As an indicator of particulate freight of the stream for soil erosion, the global soil arsenic export estimated roughly would reach 4.4 107 kg year1 (Matschullat, 2000), compared with soil input of 2.84e9.4 107 kg year1 (Chilvers and Peterson, 1987). Many investigations for soil arsenic background concentration had been carried out in different areas (Chen et al., 1999, 2001; Reimann et al., 2001; Zhang et al., 2002). As a general summary, it can be considered that the principal factors influencing the arsenic concentration in soils are the parent rock and anthropogenic activities. Factors such as climate, the organic and inorganic components of soils and redox potential status also affect the level of soil arsenic (Mandal and Suzuki, 2002). Soils are complex mixtures of organic, inorganic materials and metal compounds from anthropogenic sources. For the analysis of these samples, chemical dissolution must often be performed, such as ICP-AES, AAS, and ICP-MS, but in most cases silicate minerals are not easily digested, which may cause analytical error in element determination. Whereas, Instrumental Neutron Activation Analysis (INAA) is a purely instrumental method, which does not require any chemical treatments, including dissolution, of the samples to be analyzed, thus reducing the possibility of contamination and losses (Naidu et al., 2003; Orvini et al., 2005). In Guangdong province the limestone and sandshale were formed dominantly, due to the large-scale invasion of the sea, during the early Carboniferous period to early Permian period, when the biological enrichment is the primary reason that high arsenic content occurred in the limestone and sandshale (GSGIO, 1993; Wang and Wei, 1995). And then, in the period of Cenozoic tectonic movements, the landform of Guangdong province was uplifted totally. Igneous rocks intruded and cut through the limestone and sandshale along regional faults extensively, at the same time, hydrothermal activities were intensive and frequent induced by the regional magmation. Geothermal researches revealed that hydrothermal activities had close relationship with high content of arsenic and mercury (Zhu et al., 1989; Zhu and Yu, 1995). As a result, many polymetal ores such as pyrite ores that formed during that period are always located in the limestone and sandshale areas cut through by regional faults. Therefore, because the arsenic belongs to the sulfur family element and has strong chemical affinity with ferrum element, arsenopyrite (FeAsS) and its secondary mineral arsenolite (As2O3) is the most abundant arsenic mineral in these areas (GSGIO, 1993; Wang and Wei, 1995). The soil and water loss due to intensive physical and chemical weathering and damage of vegetation is serious issue in Guangdong province, and the loss area reach up to about
493
7376 km2, accounted for 4.2% of total area (GSGIO, 1993). Moreover, because there exist the frequent arsenic exchange between different interfaces, and soil arsenic release via active soil microbial actions in the subtropical area (Matschullat, 2000), soil arsenic has a considerably shorter retention time in the study area, compared to retention time 1000e3000 years in moderate climates (Bowen, 1979). Therefore, the investigations of spatial distribution and vertical variation for soil arsenic are of importance to reveal the transformation rule of soil arsenic and evaluate the influence of soil arsenic on the quality of regional water body. The present study was conducted to: (i) estimate the upper baseline concentration of surface soil; (ii) evaluate influence factors controlled the spatial distribution and vertical variation of soil arsenic concentrations; and (iii) reveal that surface soil degradation and erosion was an important driving force of soil arsenic into the ground and surface water. Such information would provide useful information for establishing proper arsenic cleanup standard, and will help to evaluate the influence of natural arsenic on the quality of ground and surface water in the study area. 2. Materials and methods 2.1. Description of the study area Guangdong province is located in the southern part of the south China (Fig. 1A), between latitude 20 100 e25 310 (N) and longitude 109 410 e 117 170 (E). The area influences by a subtropical monsoon climate with an average annual precipitation of 1336 mm and averages annual evaporation 1100 mm. Its average annual temperature is 17e27 C and averages 1828 h of sunshine annually. From north to south the altitude of landform decreases and coteau, platform, and plain alternate. The coteau area is up to about 60% of the total area. Regional faults with NNE- or NE-treading are the main geological character, and always cut through the regional sandstone and sandshale areas (Fig. 1B). Granite is the most extensive parent rock of soil forming, accounted for more than 40% of Guangdong area. Moreover, the larger area of limestone and sandshale are cropped out in study area; and some basalt parent rock is in the Leizhou Peninsula (Fig. 1B). Guangdong soil profile is the typical Al-enriched weathering profile and is the product of the latest stage of weathering. It had been developed on a variety of rocks, such as, granite, sandshale, limestone, and basalt. Because of the intensive eluviation and illuviation in the hydrothermal condition of study area, the soil profiles are deficient in soluble salt, alkali metal, and alkali-earth metal, but rich in Fe and Al oxides and Hþ (Lan et al., 2003). Therefore, the average pH value in Guangdong soil profiles is acidic in nature. Moreover, acid rain is also an important impact on the pH value of soils (Larssen and Carmichael, 2000).
2.2. Sampling and chemical analysis Soil samples used in this study, as a part of the investigation of soil background value in China during the Seventh Five Years Plan, were colleted from locations shown in Fig. 1C. All samples were taken from the soil profiles with length of 1.5 m, width of 0.8 m, and depth of 1.2 m, soil samples were dried at natural temperature (22e25 C), crushed by the wooden stick, and picked out the gravel, and then sieved through a 20-mesh nylon screen (about 1 mm aperture size) and 150-mesh nylon sieve prior to analysis. In this work, the analytical technique used for elemental analysis was Instrumental Neutron Activation Analysis (INAA). Approximately 0.1 g of prepared sample precisely weighed was packed in aluminium bag of 1 cm 1 cm size, and then sealed by the high purity aluminium foil together with the National Institute for Science and Technology coal fly ash standard reference
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
494
material (NIST-1633a, Gaithersburg, Maryland, USA; standard value: 145 3 mg/kg) and the National Research Center for GeoAnalysis loess standard reference material (GSS-8, Beijing, China; Standard value: 12.7 0.4 mg/kg). The samples were irradiated in a heavy-water cooled reactor, China Institute of Atomic Energy, at the neutron flux of 5 1013 n cm2 s1. The 76As isotope of samples was detected on the HPGe Gamma-ray spectrometer (model Pop Top- EG&G ORTEC, America) with the resolution of 2.0 keV on 1332 keV 60Co line. The observed arsenic concentration of standard reference materials of NIST-1633a and GSS-8 were 145 3 mg/kg (n ¼ 32) and 12.6 0.5 mg/kg (n ¼ 32), respectively. The analytic precision is less than 1% for arsenic. The pH of soil was measured by taking 10 g of sample into 25 ml of reagent water (Chinese National Standard Agency, 1988). The soil organic matter content was calculated from the weight loss of sample by decomposing at 110 C for 2 h after a pre-treatment with 3 ml of 30% (v/v) hydrogen peroxide. The different soil grain size fractions were separated using manual dry sieving. Three nylon sieves were used and placed in series for the determination of a fine (clay) fraction (<50 mm), a sand fraction (50e2000 mm) and a coarse fraction (>2000 mm). By avoiding possible contamination by metal instrument, this simple technique allowed us to perform accurate analyses of the different fractions.
As in each horizon soil were produced based on geostatistical analysis using the software of SurferÒ 8.0.
2.3. Data analysis
3.2. Soil properties
All arsenic contents were presented on a dry matter basis. These values were transformed to logarithms (base 10) because they had positive-skewed frequency distributions (Table 1). Because the data fit a lognormal distribution (Fig. 2), the central tendency and variation of the data were expressed as the geometric mean (GM) and geometric standard deviations (GSD), respectively. Therefore, the GM and GSD were used to calculate the range and variation expected for arsenic. Baseline concentrations of arsenic were calculated using GM/GSD2 and GM GSD2 of the samples, which include 95% of sample population (Dudka et al., 1995; Chen et al., 2001). Spatial interpolation was performed using ordinary kriging. For the lowdensity and background sampling, the ordinary kriging estimate can be thought of simply as an optimally weighted average of the data. It provides a best linear unbiased prediction of spatial distribution (Cressie, 1991). The spatial interpolation and contour maps displaying the spatial distribution of
3.2.1. Organic matter content The organic matter content of soil samples from A-horizon ranged from 0.17 to 9.94% (GM, 2.37%; AM, 2.75%). In soils from B- and C-horizon the content ranges were 0.07e3.83% (GM, 0.82%; AM, 0.99%) and 0.03e3.56% (GM, 0.49%; AM, 0.65%), respectively. The high value was observed in A-horizontal soils and the lowest in soils from the C-horizon (Table 1).
3. Results 3.1. Arsenic concentration The concentrations of arsenic in soil samples from A-horizon ranged 1.2e309.2 mg/kg, with a geometric mean (GM) of 10.4 mg/kg and an arithmetic mean (AM) of 17.7 mg/kg. In soils from the B-horizon, the concentration range was from 0.6 mg/kg up to 340 mg/kg, with a GM of 10.7 mg/kg and an AM of 19.0 mg/kg. The C-horizon soil arsenic concentrations were between 1.3 and 255.2 mg/kg. Its GM and AM were 11.3 and 19.6 mg/kg, respectively. The highest concentration was observed in soils from B-horizon, up to 340 mg/kg. All of data had a high positive skewness in each horizontal soil (Table 1).
3.2.2. Grain size distribution The data of grain size fit normal distribution in each horizontal soil. From A- to C-horizon the arithmetic mean contents
Table 1 Summary statistic of the data for 260 soil profiles in Guangdong n
Min.
Median
Max.
Skewness
AM ASD*
GM GSD**
A-horizon As (mg/kg) Sand (%) Clay (%) OM (%) pH-H2O
260 256 256 260 257
1.2 12.0 0.3 0.17 4.0
10.3 51.6 17.3 2.42 4.9
309.2 90.0 46.7 9.94 8.5
5.6a; 0.68b 0.1 0.7 1.5 1.6
17.7 30.2 50.4 13.9 18.4 8.9 2.75 1.55 5.17 0.78
10.4 ± 1.5 48.2 1.2 15.8 1.3 2.37 1.29 5.12 1.06
B-horizon As (mg/kg) Sand (%) Clay (%) OM (%) pH-H2O
257 254 254 257 254
0.6 3.0 0.4 0.07 4.1
10.4 41.8 19.2 0.86 5.1
340.0 91.6 51.7 3.83 8.5
5.7a; 0.39b 0.5 0.6 1.7 1.5
19.0 32.4 43.4 15.9 20.4 9.9 0.99 0.63 5.40 0.85
10.7 ± 1.5 40.3 1.2 17.5 1.3 0.82 1.32 5.34 1.06
C-horizon As (mg/kg) Sand (%) Clay (%) OM (%) pH-H2O
258 255 254 258 256
1.3 4.60 0.30 0.03 3.5
11.3 43.9 18.0 0.50 5.2
255.2 91.6 51.7 3.56 9.0
4.6a; 0.47b 0.3 0.9 2.6 1.3
19.6 30.4 44.6 16.7 20.1 10.4 0.65 0.57 5.48 0.83
11.3 ± 1.5 41.1 1.2 17.1 1.3 0.49 1.4 5.43 1.06
*AM e arithmetic mean; ASD e arithmetic standard deviation; **GM e geometric mean; GSD egeometric standard deviation. a Untransformed data. b Log-transformed data.
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
495
Fig. 1. Location of Guangdong Province in China (A), geological sketch indicating the distribution of limestone and sandshale (B), and sampling locations (C). (Number 1 is Beijiang River, 2 is Xijiang River, 3 is Dongjiang River, 4 is Hanjiang River, and 5 is Jianjiang River.)
of sand are 50.4, 43.4, and 44.6% and that of clay are 18.4, 20.4 and 20.1%, respectively (Table 1). 3.2.3. Soil pH From A- to C-horizon the soil pH values had a slight increase from 5.14, 5.34 to 5.43. The highest and lowest values were observed in the C-horizon (Table 1).
4. Discussion 4.1. Baseline concentration of surface soils Natural background concentration is defined as the ambient concentration of chemicals in soils without human influence (Gough, 1993). But it is almost impossible to establish the true natural background level of arsenic in soils, due to long-range transport and precipitation of contaminants. As such, the baseline concentration as a reference to determine clean soils represent element concentrations specific for a given region and time period, but do not always represent true background concentration (Kabata-Pendias and Pendias, 1992; Salminen and Tarvainen, 1997). The present study showed that the arsenic background concentration of 10.4 mg/kg in Guangdong surface soils was higher than that of 9.6 mg/kg in China (Wei, 1990) and 5e 7.5 mg/kg in global soil (Goldschmidt, 1958; Koljonen, 1992; Allard, 1995). The baseline concentration was 4.7e 23.4 mg/kg calculated by using GM/GSD2 and GM GSD2. Log-normality was obvious from histograms of the data (Fig. 2). Therefore the GM and GSD are better to represent the central tendency of the distribution than AM and ASD, and upper baseline concentration (UBC) of 23.4 mg/kg can better express arsenic background concentration because of the distorting effects of a few large values had been minimized by using the log-transformation (Dudka et al., 1995; Chen et al., 2001).
To validate the data for establishing arsenic background concentration in soil, the normal probability plot of arsenic contents was adopted. A cumulative frequency plot of the log-As data followed a near straight line (Fig. 3), which means that the data come from a single statistical population and suggests that arsenic concentrations in this database are largely not affected by anthropogenic influences. This indicates that the database was a good estimate of arsenic background concentration in surface soil of Guangdong. Moreover, the upper tail concerned about 15 samples that had less high values than would be expected, suggesting there exited the arsenic loss induced by the soil degradation and erosion in high arsenic concentration areas. The arsenopyrite (FeAsS) and arsenolite (As2O3) are main minerals responsible for elevated arsenic levels in these regions. The formation of arsenopyrite is the result that As3þ and Fe3þ are prone to combine for similar ionic ˚ ; Fe3þ, 0.67 A ˚ ) during the period of the radius (As3þ, 0.69 A hydrothermal deposition induced by the regional faults cutting through the limestone and sandshale stratums (Wang and Wei, 1995). The arsenolite is the secondary mineral formed by oxidation of arsenopyrite. 4.2. Correlations with soil properties Soils containing high amounts of clay minerals and organic matter will tend to accumulate higher metal level because these compounds have pronounced metal binding properties (Bolt and Bruggenwert, 1976). Chen et al. (1999) evaluated influence factors affecting arsenic concentrations in Florida surface soils and indicated that clay content, organic matter, and soil pH had strong positive correlation with arsenic concentrations, implying the soil properties are major controlling factors for arsenic concentrations. But to the relatively young Guangdong soils, the correlations were not like so. In the present research to Guangdong soil profiles, linear trends between soil properties and arsenic contents were mostly not significant (Table 2). Only for clay contents was
496
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
Fig. 2. Distribution frequency of arsenic concentration in A-, B-, and C-horizon soils in Guangdong (left: origin data; right: log10 transformed data).
there a significant positive correlation of 0.137 with the As concentrations in A-horizontal soils. As we known, the formation of soil is a function of climate, soil organisms, landscape, vegetation, time and geology. Our results revealed that soil organic matter and clay content are not major factors in determining the distribution and content of arsenic in Guangdong soils. These weaker correlations may be the result of the very rapid weathering of parent rock. Therefore, since the short soil formational process due to intensive weathering and mineral components and texture of parent rocks, the
properties of parent rocks played the more important role than other influence factors such as soil clay and organic content in the soil arsenic content and distribution in the study area. 4.3. Spatial distributions and vertical variation of arsenic level The spatial distribution of arsenic content as estimated by kriging, is mapped in Fig. 4AeC for A-, B-, and C-horizontal
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
(Lat.) 25
497
A
24
23
22
21 110
(Lat.) 25
Fig. 3. Lognormal probability plot for total arsenic concentration in Guangdong surface soils (A-horizon soil).
111
112
113
114
115
111
112
113
114
115
111
112
113
114
115
116
117 (Long.)
B
24
soils, respectively. The A-horizon reflects the complex interplay between atmosphere, biosphere and lithosphere. The B-horizon can be used to study the influence of soil forming process, while the C-horizon represents the composition of the lithosphere at each sample site and thus the geogenic background (Reimann et al., 2001). Wang and Wei (1995) summarized soil samples (n ¼ 4095) developed on different parent rocks in China and summarized that the background concentration of soil arsenic was primarily influenced by the parent rocks and presented that average soil arsenic concentration developed on the sediment areas were 15.6 mg/kg (limestone), 12.2 mg/kg (shale), 11.0 mg/kg (sandshale), and 10.7 mg/kg (sandstone), much higher than the national background value of 9.6 mg/kg, however, far lower soil concentration than national background value of arsenic were on the igneous rock areas about 5.9e6.6 mg/kg.
23
22
21 110
116
117 (Long.)
(Lat.) 25
C
24
23
22
Table 2 Pearson correlations between log10 As content and some soil properties (significance at the 5% level is indicated with ‘‘*’’, at the 1% level with ‘‘**’’) As (mg/kg) A-horizon As (mg/kg) OM (%) Clay (%) Sand (%) pH B-horizon As (mg/kg) OM (%) Clay (%) Sand (%) pH C-horizon As (mg/kg) OM (%) Clay (%) Sand (%) pH
1.000
OM (%) 0.008 1.000
Clay (%)
Sand (%)
pH
0.137* 0.147* 1.000
0.098 0.111 0.684** 1.000
0.047 0.060 0.074 0.192** 1.000
1.000
0.013 1.000
0.119 0.290** 1.000
0.132* 0.131* 0.519** 1.000
0.031 0.223** 0.170** 0.054 1.000
1.000
0.009 1.000
0.112 0.427** 1.000
0.113 0.192** 0.475** 1.000
0.084 0.177** 0.196** 0.041 1.000
21 110
116
117 (Long.)
Log10[As]0 0.2 0.4 0.6 0.8 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 As(mg/kg) 1 1.58 2.51 3.98 6.31 10.0 12.6 15.8 20.0 25.1 31.6 40.0 50.1 63.1 79.4 100 126
Fig. 4. Spatial distributions of arsenic concentrations in A-, B-, and C-horizon soils in Guangdong, China.
In the present study, geometric mean (GM) of A-, B-, and C-horizontal soils had an increasing tendency of 10.4, 10.7 to 11.3 mg/kg from A- to C-horizontal soils (Table 1), and contours of arsenic concentration displayed similar spatial distribution patterns (Fig. 4). These similar distributional patterns were that the soil samples with high concentration of arsenic mainly located in the limestone and sandshale areas (Figs. 4 and 1B), through these parent rocks also varied in their arsenic content, from small amounts to up to 309 mg/kg. And further confirmed that the distribution of arsenic was controlled by
498
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499
properties of regional parent rocks rather than clay and organic contents of soils. As such, anthropogenic arsenic inputs were not a very important factor. Although soil profiles displayed the increasing tendency of soil arsenic concentrations from A- to C-horizon, it was not obvious accumulation of the bottom layer. This phenomenon may had close relation with the followed reasons: (1) Ahorizontal soil with the lower average organic matter content of 2.75% cannot act as a natural biogeochemical barrier that suppresses the percolation of the element with the seepage water, and thus, cannot accumulate the arsenic element; (2) the subtropical hydrothermal condition, lower soil pH, and high permeability of semi-weathering granitic parent rocks cropped out extensively, all of these would induce the upper soil arsenic transferred to the much lower soil layer or groundwater; (3) it is obvious that an additional arsenic movement export will take place constantly from surface soil degradation and erosion, the products of which are being transported into ground and surface water. As a rough estimate, the soil area of Guangdong province is about 147,458 km2 with an average soil density of about 1 l03 kg/m3 (GSGIO, 1993). Annual soil erosion yield is about 1 108 t a1 and mainly distributed along Hanjiang, Beijiang, Dongjiang, and Jiangjiang watersheds (Zhang et al., 1994). According to these data at least about 1040 t arsenic would enter into the river water annually produced by the soil degradation and erosion. As a result, the average concentrations of arsenic in the sediments of the Xijiang River, Beijiang River and Dongjiang River had reached up to 45.1, 68.2, and 21.6 mg/kg, and average concentrations of Fe is up to 4.96, 4.21, and 3.96%, respectively (Zhang and Wang, 2001). The sediment-bound arsenic most probably originates from erosion and weathering processes, which result in the fluvial transport and sedimentation of arsenic-enriched iron oxyhydroxides (Peters et al., 1999), and then released to the groundwater by reductive dissolution of iron and oxidation of sulfide phases (Nickson et al., 1998; Berg et al., 2001). Moreover, the limestone and sandshale were always cut through by regional faults (Fig. 1B), which provided the pathway that soil arsenic transferred into the groundwater much more easily. Therefore, the natural soil erosion and vertical transformation process might be a potential threat to the quality of surface and groundwater in the study area.
5. Conclusions The geometric mean (GM) arsenic concentration of 10.4 mg/kg and upper baseline As concentration of 23.4 mg/kg were estimated for Guangdong surface soils. The influence of organic matter, clay contents, pH, and anthropogenic activities on arsenic concentration was not important. The results showed that regional parent bedrock properties were primary factor controlled the spatial distribution of soil arsenic concentration. About 1040 t arsenic produced by the surface soil degradation and erosion was the main arsenic contamination source of ground and surface water, and might be the potential threat to the water source in study area.
Acknowledgments We thank Professor W.J. Manning and reviewers for constructive reviews and suggestions that improved the quality of this manuscript. This work was partly funded by the Youth Talent Supported Foundation of Guangdong Academy of Sciences (2005-04), the Technologies R & D Program of Guangdong Province (2004B32501004) and the Key Program of Guangdong Natural Science Foundation (04101225).
References Acharyya, S.K., Chakraborty, P., Lahiri, S., Raymahasay, B.C., Guha, S., Bhoumik, A., 1999. Arsenic poisoning in the Ganges delta. Nature 401, 545. Allard, B., 1995. Groundwater. In: Salbu, B., Steinnes, E. (Eds.), Trace Elements in Natural Waters. CRC Press, Boca Raton, pp. 151e176. Berg, M., Tran, H.C., Nguyen, T.C., Phem, H.V., Schertenleib, R., Giger, W., 2001. Arsenic contamination of groundwater and drinking water in Vietnam: a human threat. Environmental Science and Technology 35, 2621e2626. Bolt, G.M., Bruggenwert, M.G.M., 1976. Soil Chemistry. A. Basic Elements. Elsevier, Amsterdam. Bowen, H.J.M., 1979. Environmental Chemistry of the Elements. Academic Press, London, 333 pp. Chen, M., Ma, L.Q., Willie, G.H., 1999. Baseline concentrations of 15 trace elements in Florida surfaces soils. Journal of Environmental Quality 28, 1173e1181. Chen, M., Ma, L.Q., Hoogeweg, C.G., Harris, W.G., 2001. Arsenic background concentration in Florida, U.S.A. surface soil: determination and interpretation. Environmental Forensics 2, 117e126. Chilvers, D.C., Peterson, P.J., 1987. Global cycling of arsenic. In: Hutchinson, T.C., Meema, K.M. (Eds.), Lead, Mercury, Cadmium, and Arsenic in the Environment. John Wiley & Sons, New York, pp. 279e 301. Scientific Committee on Problems of the Environment (SCOPE) 31. Chinese National Standard Agency, 1988. Determination of pH Value in Forest Soil. GB 7859-87, UDC 634.0.114:631.422, pp. 171e173 (in Chinese). Cressie, C., 1991. Statistics and Data Analysis in Geology, second ed. John Wiley & Sons, New York. Dudka, S., Ponce-Hernandez, R., Hutchinson, T.C., 1995. Current levels of total element concentrations in the surface laver of Sudbury’s soils. Science of the Total Environment 162, 161e172. Environmental Protection Agency, 1998. Integrated Risk Information System (IRIS), Arsenic, Inorganic. CASRN 7440-38-2. U.S. Cincinnati, OH. Goldschmidt, V.M., 1958. Geochemistry. In: Muir, A. (Ed.). Oxford University Press, Oxford, pp. 468e475. Gough, L.P., 1993. Understanding our Fragile Environment, Lessons from Geochemical Studies. USGS Circular 1105. US Gov. Print Office, Washington, DC. GSGIO, 1993. Guangdong Soil. In: Liu, A.S. (Ed.). Science Publish House, Guangdong Soil General Investigation Office. Kabata-Pendias, A., Pendias, H., 1992. Trace Elements in Soils and Plants. CRC Press, Boca Raton, FL. Koljonen, T. (Ed.), 1992. Geochemical Atlas of Finland, Part 2: Till. Geological Survey of Finland, Espoo, Finland, 218 pp. Lan, H.X., Hu, R.L., Yue, Z.Q., Lee, C.F., Wang, S.J., 2003. Engineering and geological characteristics of granite weathering profiles in South China. Journal of Asian Earth Sciences 21, 353e364. Larssen, T., Carmichael, G.R., 2000. Acid rain and acidification in China: the importance of base cation deposition. Environmental Pollution 110, 89e102. MaArthur, J.M., Ravenscroft, P., Safiulla, S., Thirlwall, M.F., 2001. Arsenic in groundwater: testing pollution mechanism for sedimentary aquifers in Bangladesh. Water Resources Research 37, 109e117.
H.H. Zhang et al. / Environmental Pollution 144 (2006) 492e499 Mandal, B.K., Suzuki, K.T., 2002. Arsenic round the world: a review. Talanta 58, 201e235. Matschullat, J., 2000. Arsenic in the geosphere e a review. Science of the Total Environment 249, 279e312. Naidu, G.R.K., Trautmann, N., Zaunar, S., Balaji, T., Rao, K.S., 2003. Multielemental analysis of soils by instrumental neutron activation analysis. Journal of Radioanalytical and Nuclear Chemistry 258 (2), 421e425. Nickson, R.T., McArthur, J.M., Burgess, W.G., Ahmed, K.M., Ravenscoft, P., Rahman, M., 1998. Arsenic poisoning of Bangladesh groundwater. Nature 395, 338. Orvini, E., Speziali, M., Herborg, C., Salvini, A., 2005. Trace element characterization by INAA of three sediments to be certified as standard reference materials. Microchemical Journal 79, 239e242. Peters, S.C., Blum, J.D., Klaue, B., Karagas, M.R., 1999. Arsenic occurrence in New Hampshire drinking water. Environmental Science and Technology 33, 1328e1333. Reimann, C., Kashulina, G., de Caritat, P., Niskavaara, H., 2001. Multi-element, multi-medium regional geochemistry in the European Arctic: element concentration, variation and correlation. Applied Geochemistry 16, 759e780. Salminen, R., Tarvainen, T., 1997. The problem of defining geochemical baseline. A case study of selected elements and geological materials in Finland. Journal of Geochemical Exploration 60, 91e98. Sheppards, S.C., 1992. Summary of phytotoxic levels of soil arsenic. Water, Air and Soil Pollution 64, 539e550. Thornton, I., Farago, M., 1997. The geochemistry of arsenic. In: Abernathy, C.O., Calderon, R.L., Chappell, W.R. (Eds.), Arsenic: Exposure and Health Effects. Chapman and Hall, Kluwer Academic Publisher, London, pp. 1e16.
499
Wang, L.F., Han, J.H., 1994. Chronic arsenism from drinking water in some areas of Xinjiang, China. In: Nriagu, J.O. (Ed.), Arsenic in the Environment, Part II: Human Health and Ecosystem Effects. John Wiley & Sons, New York, pp. 159e172. Wang, Y., Wei, F.S., 1995. Chemistry of Elements in the Pedosphere Environment. China Environmental Science Press, Beijing, pp. 42e58 (in Chinese). Wei, F.S., 1990. Elemental Background Contents in the Soil of China. China Environmental Science Press, Beijing. (in Chinese). WHO, 2001. Arsenic Compounds, Environmental Health Criteria 224, second ed. World Health Organization, Geneva. Xu, L.F., Liu, T.H., 1996. The zonal differentiation of soil environmental background values and critical contents in Guangdong. Journal of South China Agricultural University 17 (4), 58e62. (in Chinese). Zhang, C.S., Wang, L.J., 2001. Multi-element geochemistry of sediments from the Pearl River system, China. Applied Geochemistry 16, 1251e1259. Zhang, J.W., Yao, S.X., Li, H.S., 1994. Study on Slope Land in South China. Science Press, Beijing. Zhang, X.P., Deng, W., Yang, X.M., 2002. The background concentrations of 13 soil trace elements and their relationships to parent materials and vegetation in Xizang (Tibet), China. Journal of Asian Earth Sciences 21, 167e174. Zhu, B.Q., Zhu, L.X., Shi, C.Y., Yu, H., Wang, G.Y., 1989. Application of geochemical methods in the search for geothermal fields. Journal of Geochemical Exploration 33, 171e183. Zhu, B.Q., Yu, H., 1995. The use of geochemical indicator elements in the exploration for hot water sources within geothermal fields. Journal of Geochemical Exploration 55, 125e136.