DSpace at VNU: Arsenic and Heavy Metal Concentrations in Agricultural Soils Around Tin and Tungsten Mines in the Dai Tu district, N. Vietnam

Water Air Soil Pollut (2009) 197:75–89 DOI 10.1007/s11270-008-9792-y Arsenic and Heavy Metal Concentrations in Agricultural Soils Around Tin and Tungsten Mines in the Dai Tu district, N Vietnam Kien Chu Ngoc & Noi Van Nguyen & Bang Nguyen Dinh & Son Le Thanh & Sota Tanaka & Yumei Kang & Katsutoshi Sakurai & Kōzō Iwasaki Received: 17 March 2008 / Accepted: 29 June 2008 / Published online: 20 July 2008 # Springer Science + Business Media B.V 2008 Abstract This study assessed the arsenic and heavy metal contaminations of agricultural soils around the tin and tungsten mining areas in Dai Tu district in northern Vietnam Among the examined elements, high total contents of As and Cu were found in the agricultural fields at both tin and tungsten mining sites Although the major part of the accumulated As and Cu were bound by various soil constituents such as Fe and Mn oxides, organic matter, and clay minerals, increases in water soluble As and Cu were observed, especially for the paddy fields The results suggest that, in the studied area, As and Cu dispersion from their pollution sources into farmlands is mainly via fluvial transportation of mine waste through streams that cross the paddy fields around the tin K Chu Ngoc United Graduate School of Agricultural Sciences, Ehime University, Ehime 790-8566, Japan N Van Nguyen : B Nguyen Dinh : S Le Thanh Faculty of Chemistry, Hanoi University of Science, Hanoi, Vietnam S Tanaka Graduate School of Kuroshio Science, Kochi University, Kochi 783-8502, Japan Y Kang : K Sakurai : K Iwasaki (*) Faculty of Agriculture, Kochi University, Kochi 783-8502, Japan e-mail: kozo@kochi-u.ac.jp mining area, and soil erosion at the tea fields located at lower positions of the slope in the tungsten mining area Keywords Arsenic Heavy metal Soil contamination Tin mine Tungsten mine Vietnam Introduction Mining can be a significant source of metal contamination of the environment owing to activities such as mineral excavation, ore transportation, smelting and refining, disposal of the tailings and waste water around mines (Adriano 2001; Jung 2001; Razo et al 2004; Chopin and Alloway 2007) Due to discharge and dispersion of mine wastes from the metalliferous mines, agricultural soils, food crops and stream systems are often contaminated by elevated levels of toxic metals (McGowen and Basta 2001; Jung 2001; Lee 2006) With growing public concern throughout the world over health hazards caused by polluted agricultural products, many studies have been conducted on metal and metalloid contamination in soils, water and sediments from metalliferous mines (Merrington and Alloway 1994; Iwasaki et al 1997; Jung et al 2002; Lee 2006; Chopin and Alloway 2007; Anawar et al 2008) According to these studies, metal contaminations of agricultural soils should be evaluated based on the results of metal speciation as well as their total contents, because only 76 soluble, exchangeable and chelated metal species in the soils are the available fractions for plant uptake (Kabata-Pendias and Pendias 1992; Chen et al 2007) The proportion of a metal which is mobile and bioavailable will provide more practical information for evaluating its potential environmental risks In Vietnam, however, only few studies on the forms and distributions of heavy metals and metalloids have been carried out for the agricultural soils affected by mining activities Vietnam is well endowed with a wide range of mineral resources located mainly in the northern regions The Dai Tu district, situated in northern Vietnam, is one of the largest areas rich in ferrous and non-ferrous ore deposits in the country The mines have produced ores containing Fe, Ti, Zn, Sn, W, Cu, and Pb (Thai Nguyen Department of Planning and Investment 2005) and the common minerals are pyrite (FeS2), chalcopyrite (CuFeS2), wolframite [(Fe, Mn) WO4], accessory galena (PbS) with minor amounts of arsenopyrite (FeAsS), and bismuthinite (Bi2S3; Jung et al 2002; Chopin and Alloway 2007) Recently, involvement of foreign companies has been accelerating the development of export-orientated minerals with high values, and several important ore deposits were newly discovered in this district Besides such large-scale exploitation, traditional mining operations are still continued by local farmers living in the vicinity of the mine although many of the mines have been abandoned due to the lack of modern mining technologies In the Dai Tu district, over the last few decades, traditional manual mining has been operated through small adits and open pits Unfortunately, this mining operation has potential for releasing toxic elements such as As, Cd, Cr, Cu, Ni, and Pb to the surrounding environment during digging and washing ores because small quantities of these elements are present as minor constituents and impurities in the ores In addition, the abandoned mines, without appropriate measures, can become important point sources of the toxic element contaminations Further, allocations of mines and farmlands may pose a potential health risk from intake of heavy metals derived from soils and irrigated water from the mines, because settlements and farmlands of rural communities are located as close as hundreds of meters from the mine sites It is therefore of prime importance to assess potential environmental risks originating from the Water Air Soil Pollut (2009) 197:75–89 mining activities in order to establish a proper pollution management plan Therefore, in this study, we focused on the tin and tungsten mining areas in Dai Tu district, about 1.5 km from each other across the valley, where the ores have been mined using traditional methods The aims of this study were (1) to evaluate the degree of contamination in agricultural soils and waters by toxic elements (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn) and to clarify the contaminant pathways from the tin and tungsten mining areas, and (2) to determine the distribution of As and Cu among various soil chemical fractions in order to assess the potential risks Materials and Methods 2.1 Study Area The survey was conducted around tin and tungsten mining areas at Hung Son commune (21°38′33″ N, 105°38′58″ E) in Dai Tu district, Thai Nguyen province, situated in northern Vietnam on 18–20 February, 2006 (Fig 1) This area is located in a monsoon tropical climate zone with two distinct seasons The rainy season is from May to September with an annual average temperature of 27–29°C, and the dry season is from November to March with an annual average temperature of 16–20°C The average precipitation is approximately 1,700–1,800 mm per year, and the annual evaporation is about half of the annual precipitation (The Hydrometeorological Data Center, Vietnam 2005) The main agricultural practices in the study area include lowland rice cultivation and tea plantation The rice cultivation system involves two rice croppings per year; from February to June and from July to October Before crop establishment, the fields are shallowly submerged, plowed, and puddled After puddling, the fields are left flooded for several days with the water depth of 10–15 cm After transplanting rice seedlings, the soils are kept submerged until 1–2 weeks before harvest The depth of the standing water is normally 5–10 cm As a basal dressing, N–P–K fertilizers (6–11–2) are supplied at the rate of approximately 0.5 Mg ha−1 Sometimes, farmers also apply limes or composts (e.g green manures) before transplanting During crop growth, urea and potassium chloride fertilizers are supplied additionally at the Water Air Soil Pollut (2009) 197:75–89 77 Fig The location of sampling sites Legend 180 Altitude line 18 Mining cavity Thai Nguyen Vietnam ▲ Forest soil ■ Paddy soil River, stream ● Tea field soil Stream tream water ■ Standing water 140 T2 T5 T4 T6 ● T7 T1 (Tungsten) P4 T3 160 120 P5 P6 100 P1 80 P2 P3 160 ng Co e riv rate of 0.05–0.1 Mg ha−1 In contrast, the tea plantations are usually located on the hilly area The common fertilizers supplied to the tea field are N–P– K fertilizers (16–8–4) The dosages vary largely upon each field and year by year In addition, water for tea plantation is mostly supplied by rainfall (Tin) 180 200 m 200 chosen Three paddy fields (P4−P6) located in the valley below the slope of the mountain were also investigated At each site, surface (0–5 cm) and subsurface (20–25 cm) soils were sampled In addition, water samples were taken from the stream running through the tin mining area (Sw) and the standing water of the paddy fields (P1w−P6w) 2.2 Sampling 2.3 Soil and Water Analysis Agricultural lands were selected around the mining areas based on toposequential location (Fig 1) At the tin mining area, three paddy fields (P1−P3) were selected They were located along a stream running through the tin mine area at different distances from the main adit, and were irrigated with water from the stream A natural forest (F) on the mountain slope near the main adit of the mine was also selected At the tungsten mining area, seven tea fields located at different elevations of the slope (T1−T3, located at higher positions; T4−T7, at lower positions) were Soil samples were air-dried at room temperature, and crushed to pass through a 2-mm mesh sieve Soil particle size distributions were determined with a pipette method (Gee and Bauder 1986) The electrical conductivity (EC) and pH (H2O) values were determined using a platinum and glass electrode at 1:5 (w/ v) ratio of soil to water, respectively Exchangeable (Ex-) cations were extracted with mol l−1 ammonium acetate at pH 7.0 and the contents were determined using an atomic absorption spectrometer 78 Water Air Soil Pollut (2009) 197:75–89 (AAS; AA-6800, Shimadzu, Kyoto, Japan) After removing the excess NH4+, the soil was extracted with 100 g l−1 NaCl solution and the supernatant was used to determine the cation exchangeable capacity (CEC) with the Kjeldaghl distillation and titration method (Rhoades 1982) The contents of total carbon (TC) and total nitrogen (TN) were analyzed on an NC analyzer (Sumigraph NC-80, Sumitomo Chemical, Osaka, Japan) The organic matter contents (OM) were calculated by multiplying the TC values by 1.724 (Nelson and Sommers 1982), as it was assumed that the amounts of carbonate salts would be negligible under the relatively acidic nature of the soils For the analysis of total As content, the soil sample was digested in a mixture of HClO4–HNO3–HF (2:3:5) with the addition of 20 g l−1 KMnO4 in a teflon vessel at 100°C The standard reference materials (JSO-1 and JSO-2 from the Geological Survey of Japan) were used to verify the accuracy of As determination The recovery rates of As were within 90–95% The chemical forms of As were evaluated using a sequential extraction method according to Keon et al (2001) with some modifications (Van et al 2006; Table 1) Briefly, five kinds of extracting solution were sequentially employed to divide the total As into water soluble (Ws-), MgCl2 extractable (Mg-), NaH2PO4 extractable (P-), HCl extractable (HCl-), and residual (Res-) fractions The As in these operationally defined fractions was Table Methods for the sequential extraction of As from soil Fractions Reagents Soil/ Solution Ratio Condition Water soluble (Ws-) MgCl2 extractable (Mg-) NaH2PO4 extractable (P-) HCl extractable (HCl-) Residual (Res-) Water 1:100 Shaken 2h 1.0 mol l−1 MgCl2 (pH 7.0) 1:100 Shaken 2h 1.0 mol l−1 NaH2PO4 (pH 5.0) 1:100 Shaken 24 h 1.0 mol l−1 HCl 1:100 Shaken 1h HClO4–HNO3–HF– KMnO4 digestion assumed to correspond to water soluble As (Ws-), ionically bound As (Mg-), As strongly bound by monodentate or bidentate ligand exchange (P-), As specifically adsorbed or occluded by Mn oxides and amorphous Fe oxides (HCl-), and As occluded by crystalline Fe oxides, organic matter and secondary minerals (Res-), respectively The concentration of As in the acid digests and in the fractions were determined by using an inductively coupled plasma atomic emission spectrometer (ICP-AES; ICPS1000IV, Shimadzu, Kyoto, Japan) equipped with a hydride vapor generator (HVG-1, Shimadzu, Kyoto, Japan) The averaged ratio of sum amounts of As in each fraction to the total As content for all the selected soil samples was 101% For the analysis of total contents of heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn), the soil samples were digested in a mixture of HNO3 and HF (9:1) by microwave heating (Multiwave, Perkin-Elmer, Yokohama, Japan) The accuracy of the method was assessed using the certificated reference soils (JSO1) and marine sediment (NIES No.12, provided by the National Institute for Environmental Studies, Japan) The recoveries of Cd, Cr, Cu, Mn, Ni, Pb, and Zn were in the ranges 92.6–117%, 96.7–101%, 96.6– 101%, 96.2–104%, 84.2–95.6%, 92.5–100% and 92.9–96.2%, respectively Chemical forms of Cu were estimated by the sequential extraction method reported by Iwasaki et al (1997) with some modifications The reagents employed and shaking period for the extraction of seven different fractions of soil Cu are summarized in Table The respective fractions were designated as water soluble (Ws-), exchangeable (Ex-), acid soluble (Aci-), Mn oxideoccluded (MnO-), organically bound (OM-), Fe oxide-occluded (FeO-), and residual (Res-) fractions The total concentration of heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) in the acid digests and in the Cu fractions were measured by AAS After fractionation, the average recovery of Cu for all selected soil samples was 92% Water samples were filtered through a 0.45 μm membrane filter and divided into two portions One portion was acidified with HNO3 (0.2% v/v) for the analysis of As and heavy metal concentrations, while the other was left un-acidified for pH and EC measurements Water samples were stored in a refrigerator at 4°C until physical-chemical analyses The total concentration of As and heavy metals (Cd, Water Air Soil Pollut (2009) 197:75–89 79 Table Methods for the sequential extraction of Cu from soil Fractions Reagents Soil/ solution ratio Condition Water soluble (Ws-) Exchangeable (Ex-) Acid soluble (Aci-) Mn-oxide occluded (MnO-) Organically bound (OM-) Fe-oxide occluded (FeO-) Residual (Res-) Water 1:5 Shaken h 1.0 mol l−1 CH3COONH4 (pH 7.0) 1:10 Shaken h 25 g l−1 CH3COOH (pH 2.6) 1:10 Shaken h 0.1 mol l−1 NH2OH.HCl (pH 2.0) 1:50 Shaken 0.5 h 0.1 mol l−1 Na4P2O7 (pH 10.0) 1:50 Shaken 24 h 0.175 mol l−1 (NH4)2C2O4 +0.1 mol l−1 H2C2O4 +0.1 mol l−1 ascorbic acid (pH 3.1) HClO4–HNO3–HF digestion 1:50 Shaken h, then stirred occasionally in boiling water for 0.5 h Cr, Cu, Mn, Ni, Pb and Zn) were determined by ICPAES and AAS, respectively 2.4 Statistical Analysis Using data on the physicochemical properties, total contents of As and heavy metals, and amounts of As and Cu in different chemical forms in each soil layer, Tukey’s multiple comparisons were performed on three kinds of the fields (paddy fields around the tin mining area, tea fields, and paddy fields around the tungsten mining area) Student’s t-tests were conducted between surface and subsurface soils in each soil group The SPSS (Statistics Program for Social Science) statistical program package (Release 13.0 for Windows; SPSS Inc.) was used for these statistical analyses Results and Discussion 3.1 General Characteristics of Soils General physicochemical properties of soils are given in Table Based on the USDA classification system, the soils in the forest and tea fields were classified into Typic Haplustults (Soil Survey Staff 2006) Due to the use of irrigation water during the growing season, soil profile description could not be carried out at the paddy fields of the studied areas However, based on the general characteristics of the paddy soils, it was assumed to be classified as Typic Endoaquents or its relatives At the tin and tungsten mining areas, the soils collected at the forest and tea fields showed relatively clayey texture while those at the paddy fields had a sandy texture (Gee and Bauder 1986) TC and OM contents tended to be higher in the tea field soils than in the paddy soils Soil pH ranged from about to 5, with the forest and tea field soils being slightly more acidic than those of the paddy soils The values of TC, OM, pH and EC showed significant differences between the paddy soils around the tin mining area and the tea field soils around the tungsten mining area (p