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

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.. The r

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

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

1 Introduction

Mining can be a significant source of metal contam-ination 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; Jung2001; Lee 2006) With growing public concern throughout the world over health hazards caused by polluted agricultural products, many studies have been con-ducted 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

Water Air Soil Pollut (2009) 197:75 –89

DOI 10.1007/s11270-008-9792-y

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

soluble, exchangeable and chelated metal species in

the soils are the available fractions for plant uptake

(Kabata-Pendias and Pendias1992; Chen et al.2007)

The proportion of a metal which is mobile and

bio-available will provide more practical information for

evaluating its potential environmental risks In

Viet-nam, 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

Investment2005) 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

acceler-ating 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

be-cause 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

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

2 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 crop-pings 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

potassi-um chloride fertilizers are supplied additionally at the

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

2.2 Sampling

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

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) 2.3 Soil and Water Analysis

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 Bauder1986) The electrical conductivity (EC) and pH (H2O) values were deter-mined using a platinum and glass electrode at 1:5 (w/ v) ratio of soil to water, respectively Exchangeable (Ex-) cations were extracted with 1 mol l−1

ammoni-um acetate at pH 7.0 and the contents were determined using an atomic absorption spectrometer

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

P1 P6

P2 P3

P4 P5

T1 T6

T7 T5 T4 T3

T2

Legend

Forest soil tream water

River, stream Mining cavity

180 Altitude line

Standing water Paddy soil

Tea field soil

Co n ri ve

Thai Nguyen

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

P1 P6

P2 P3

P4 P5

T1 T6

T7 T5 T4 T3

T2

Forest soil tream water

River, stream Mining cavity

18 Altitude line

Standing water Paddy soil

Tea field soil

Co n ri ve

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

P1 P6

P2 P3

P4 P5

T1 T6

T7 T5 T4 T3

T2

Forest soil tream water

River, stream Mining cavity

18 Altitude line

Standing water Paddy soil

Tea field soil

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

P1 P6

P2 P3

P4 P5

T1 T6

T7 T5 T4 T3

T2

Forest soil tream water

River, stream Mining cavity

18 Altitude line

Standing water Paddy soil

Tea field soil

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

P1 P6

P2 P3

P4 P5

T1 T6

T7 T5 T4 T3

T2

Forest soil

L Forest soil Stream water tream water

River, stream Mining cavity

18 Altitude line

18 Altitude line

Standing water Paddy soil

Tea field soil

Co n ri ve

Thai Nguyen

I

G

I

G

Fig 1 The location of

sam-pling sites

(AAS; AA-6800, Shimadzu, Kyoto, Japan) After

removing the excess NH4

+ , the soil was extracted with

100 g l−1NaCl solution and the supernatant was used

to determine the cation exchangeable capacity (CEC)

with the Kjeldaghl distillation and titration method

(Rhoades1982) 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

modifica-tions (Van et al.2006; Table1) 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

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; ICPS-1000IV, 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 HNO3and HF (9:1) by microwave heating (Multiwave, Perkin-Elmer, Yoko-hama, Japan) The accuracy of the method was assessed using the certificated reference soils (JSO-1) 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 mod-ifications The reagents employed and shaking period for the extraction of seven different fractions of soil

Cu are summarized in Table 2 The respective fractions were designated as water soluble (Ws-), exchangeable (Ex-), acid soluble (Aci-), Mn oxide-occluded (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,

Table 1 Methods for the sequential extraction of As from soil

Solution Ratio

Condition

Water

soluble

(Ws-)

2 h MgCl 2

extractable

(Mg-)

1.0 mol l−1MgCl 2

(pH 7.0)

2 h NaH 2 PO 4

extractable

(P-)

1.0 mol l−1NaH 2 PO 4

(pH 5.0)

24 h HCl

extractable

(HCl-)

1.0 mol l−1HCl 1:100 Shaken

1 h Residual

(Res-)

HClO 4 –HNO 3 –HF–

KMnO 4 digestion

Cr, Cu, Mn, Ni, Pb and Zn) were determined by

ICP-AES 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

con-ducted 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

3 Results and Discussion

3.1 General Characteristics of Soils

General physicochemical properties of soils are given

in Table3 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 Bauder1986) TC and OM contents tended to be higher in the tea field soils than in the paddy soils Soil pH ranged from about 4 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 < 0.05) Both for the surface and subsurface soils, the amounts of Ex–Ca in the paddy soils were significantly higher than in the tea field soils (p<0.05) The same tendency was observed for the amounts of Ex–Mg and Ex–Na These results can

be ascribed to the application of liming materials containing CaCO3 and MgCO3 for neutralization of soil acidity No distinct differences in general proper-ties were observed between the paddy soils around the tin and tungsten mining areas

3.2 As and Heavy Metals in Soils

Total contents of As and heavy metals (Cd, Cr, Cu,

Mn, Ni, Pb and Zn) in the collected soils and their

Table 2 Methods for the sequential extraction of Cu from soil

solution ratio

Condition

Water

soluble (Ws-)

Exchangeable

(Ex-)

Acid soluble

(Aci-)

Mn-oxide

occluded

(MnO-)

Organically bound

(OM-)

Fe-oxide occluded

(FeO-)

0.175 mol l−1(NH 4 ) 2 C 2 O 4 +0.1 mol l−1H 2 C 2 O 4 +0.1 mol l−1ascorbic acid (pH 3.1)

1:50 Shaken 4 h, then stirred occasionally in

boiling water for 0.5 h Residual (Res-) HClO 4 –HNO 3 –HF digestion

EC (dS

EC (dS

Table 4 Total As and heavy metal contents in soils close to the tin and tungsten mines (Dai Tu district, N Vietnam)

(mg kg−1)

Soils from the tin mining area

Forest soil

Paddy fields

Soils from the tungsten mining area

Tea fields

Paddy fields

Average values

Soils from the tin mining area

Paddy fields (n=3)

Soils from the tungsten mining area

Tea fields (n=7)

Paddy fields (n=3)

Average values followed by the same capitalized letter are not significantly different at a 5% level for surface soil, and those followed

by the same small letter are not significantly different at a 5% level for subsurface soil, according to a Tukey’s method.

There is no significant difference from the average values for the surface and subsurface soils in each soil group at 5% or 1% level a

u: surface soil (0 –5 cm)

b

l: subsurface soil (20 –25 cm)

concentrations in the water samples are shown in

Tables 4 and 5, respectively Among the examined

elements, contamination by As and Cu in the

agricultural soils was more marked than other

elements Therefore, the spatial distributions of their

total contents in the studied area were investigated

They are provided in Fig 2 Sequential extraction

procedures (vide supra) were applied in order to

identify the chemical forms of As and Cu and their

mobility The amounts of these elements in various

soil chemical fractions are given in Figs 3 and 4,

respectively

3.2.1 Soils Around the Tin Mining Area

Extremely high levels of As, Cd, Cu, and Pb were

recorded in both surface and subsurface layers of the

natural forest soil These values were roughly

com-parable to the previously reported values for the soils

in mining areas; Lee et al (2001) reported elevated

levels of Cd, Cu and Pb, in the range of 0.80–2.20,

13.6–6.00 and 33.0–708 mg kg−1

, respectively, for forest soil around a Au–Ag–Pb–Zn mine area in

Korea O’Neill (1990) showed that the surface soils

(0–5 cm) of a mineralized area in Southwest England

contained 424 mg kg−1of As However, the contents

of Cr, Mn, Ni, and Zn in the forest soil of the present

study were roughly within the ranges for

uncontam-inated soils reported by Bowen (1979) In addition,

there was no substantial difference in the contents of

As and heavy metals between the surface and subsurface forest soil

In the surface forest soil, the results of sequential extraction showed that more than 80% of total As was extracted in the Res-fraction, followed by the P- and HCl-fractions The amounts of Cu in the Res-, FeO-, and OM-fractions of the surface soil accounted for 64,

17, and 16% of the total Cu content, respectively The residual fraction was mainly composed of primary and secondary minerals containing metals in the crystalline lattice (Gleyzes et al 2002) This fraction

is considered relatively inactive, therefore, could reflect the native metal concentration in soil (Burt et

al 2003; Kaasalainen and Yli-Halla 2003) On the other hand, the distribution patterns of both As and

Cu in the subsurface soil were quite similar to those

of the surface soil (Figs 3 and 4) These results indicated that large parts of As and Cu in the forest soil were strongly bound by Fe oxides and clay minerals, which suggested that the high contents of

As and Cu originated from the weathered metal ores

In the paddy fields (P1−P3), the contents of As and Cu exceeded the maximum allowable limit values considerably (12 and 50 mg kg−1 for As and Cu, respectively) provided by the “Vietnamese Soil Quality—Maximum allowable limits of heavy metals

in the soil” for agricultural soils (TCVN 7209–2002; Table 4) Various studies have been reported As and

Table 5 pH, EC and concentrations of As and heavy metals in water sampled around the tin and tungsten mines (Dai Tu district, N Vietnam)

(dS m−1) ( μg l −1 ) (mg l−1) Water from the tin mining area

Water from the tungsten mining area

Vietnamese standard limitation

for surface water (TCVN 5942-1995)

a

Not detected

b Cr (VI)

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

F

P1 P6

P2 P3

P4 P5 T1 T6 T7 T5 T4 T3

T2

C n riv er

(mg kg -1 )

Surface Subsurface soil 200

0

400 600 800 1000

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

F

P1 P6

P2 P3

P4 P5 T1 T6 T7 T5 T4 T3

T2

C n riv er

(mg kg -1 )

Surface Subsurface soil 200

0

400 600 800 1000

200

0

400 600 800 1000

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

F

P1 P6

P2 P3

P4

T1 T6

T7 T5 T4 T3

T2

C n

ri v er

100

0

200 300

(mg kg -1 ) 400

P5

Surface Subsurface soil

120

100

160

180

160 140

200

80

(Tin) (Tungsten)

200 m

F

P1 P6

P2 P3

P4

T1 T6

T7 T5 T4 T3

T2

C n

ri v er

100

0

200 300

(mg kg -1 ) 400

P5

Surface S

(b)

(a)

Fig 2 Distribution of As

(a) and Cu (b) in soils

close to tin and tungsten

mines (Dai Tu district, N.

Vietnam)