DSpace at VNU: Heavy metal contamination of agricultural soils around a chromite mine in Vietnam

The present study examined the influence of chromite mining activities on the adjacent lowland paddy field by investigating heavy metal and As levels in the mine tailings, sediments, pad

ORIGINAL ARTICLE

Heavy metal contamination of agricultural soils around a chromite mine in Vietnam

1 United Graduate School of Agricultural Sciences, Ehime University, Ehime 790-8566, 2 Faculty of Chemistry, Hanoi University of Science, Hanoi, Vietnam, 3 Graduate School of Kuroshio Science and 4 Faculty of Agriculture, Kochi University, Kochi 783-8502, Japan

Abstract

In Vietnam, the Co Dinh mine is the largest chromite mine in the country Mining, ore dressing and disposal of the tailings provide obvious sources of heavy metal contamination in the mine area The present study examined the influence of chromite mining activities on the adjacent lowland paddy field by investigating heavy metal and

As levels in the mine tailings, sediments, paddy soils and water At paddy fields located near the mine tailings, the total contents of Cr, Co and Ni were 5,750, 375 and 5,590 mg kg)1, and the contents of their water-extract-able form were 12.7, 1.16 and 32.3 mg kg)1, respectively These results revealed severe contamination of low-land paddy soils with Cr, Co and Ni as a result of mining activity, suggesting serious health hazards through agricultural products, including livestock in this area The principal source of the pollution was sediment inflow owing to the collapse of the dike, which was poorly constructed by heaping up soil Moreover, water flowing out from the mining area was also polluted with Cr and Ni (15.0–41.0 and 20.0–135 lg L)1, respectively) This might raise another problem of heavy metal pollution of watercourses in the area, indicating the need for further investigation and monitoring of fluctuations of water quality with seasonal changes

Key words: contamination, heavy metal, mine, soil, Vietnam

INTRODUCTION

Mining activities produce large quantities of waste

materi-als, such as waste rock, tailings and slag, leading to metal

contamination of the environment (Adriano 2001;

Cho-pin and Alloway 2007; Jung 2001) Elevated levels of

toxic metals are often reported in agricultural soils, food

crops and stream systems as a result of the discharge and

dispersion of mine wastes into the environment (Jung

2001; Lee 2006; McGowen and Basta 2001) A number

of studies have investigated the spatial distribution and

behavior of heavy metals in and around mining areas to

assess the potential health risks and environmental

haz-ards caused by polluted agricultural products For

exam-ple, enrichment of Cr and Ni (86–358 and 21.2–

126 mg kg)1, respectively) as a result of mine tailings was

reported for surface soils taken from the Almade´n mining

district in Spain (Bueno et al 2009) Elevated levels of Cr and Ni (182–1,029 mg Cr kg)1and 15–432 mg Ni kg)1) were also found in soils collected from mining areas in southern Togo (Gnandi and Tobschall 2002) Bi et al (2006) reported Cr contamination of agricultural soils (71–240 mg kg)1), resulting from zinc smelting activities

in Hezhang County, western Guizhou, China High levels

of heavy metals in paddy fields nearby the Daduk Au–Ag– Pb–Zn mining area of Korea were caused by dispersion of the metals from the tailings by sedimentation and water-courses (Lee et al 2001) In addition, a number of studies have investigated the vertical distribution of heavy metals

in soil profiles in and round mining areas, and changes in the geochemical processes throughout the profile (Adriano 2001; Johnson et al 2000; Kim and Jung 2004; Otero

et al 2000) According to these studies, it appears that the profile distribution patterns of each heavy metal are site specific, with no consistent distribution pattern correlating with soil depth

The toxicity and bioavailability of heavy metals in soils are influenced by the metal’s mobility and reactivity with various environmental factors Therefore, information about metal speciation as well as its total content is

neces-Correspondence: C N KIEN, United Graduate School of

Agri-cultural Sciences, Ehime University, 3-5-7, Tarumi, Matsuyama,

Ehime 790-8566, Japan Email: chungockien@hotmail.com

Received 25 September 2009.

Accepted for publication 20 December 2009.

sary for the assessment of heavy metal toxicity and

bio-availability The proportion of a metal that is mobile and

bioavailable will provide practical information for

evalu-ating its potential environmental risks Kien et al (2009)

studied and reported the form and horizontal distribution

of Cu and As in paddy soils and watercourses resulting

from the exploitation of tin and tungsten ores in Daitu

dis-trict, in the northern part of Vietnam However, in

Viet-nam, few studies on the form and distribution of heavy

metals and metalloids have been carried out for

agricul-tural soils affected by mining activities

In Vietnam, a wide variety of minerals have been found

to contain various elements (e.g antimony, chromite,

cop-per, tin, tungsten), and a lot of metalliferrous mines have

been established over the country Some are currently

being mined, whereas others have been abandoned Many

of these mines are located in mountainous areas or in the

upper reaches of lowland streams, where various types of

crops are cultivated, including lowland rice, which is the

major crop Although dikes are usually constructed

around mine areas to prevent the release of tailings, waste

water and solid waste into the surrounding environment,

frequent occurrences of flooding during the rainy season

have caused some of these dikes to collapse and not

function properly, resulting in heavy metal pollution in

lower streams and farmland areas Therefore, it is

impera-tive that we accumulate more data related to heavy metal contamination around mining sites where various types of metals are extracted

In the present study, we assessed the influence of heavy metal contamination on lowland paddy fields in the downstream areas of the Co Dinh chromite mine and clar-ified possible pathways that the contaminants might take The chemical forms of the contaminants were also deter-mined to evaluate their mobility and any potential risks to the surrounding environment

MATERIALS AND METHODS

Co Dinh chromite mine Mining activity at the Co Dinh chromite mine (19o43¢N,

105o36¢E; Fig 1) was initiated in the early 20th century (1930) with ore extraction from the ground surface, fol-lowed by open-pit mining Large-scale and intensive min-ing activities commenced in the 1990s Recently, the Co Dinh Chromite Mining Company temporarily closed the mine with the aim of re-construction and installation of new technology However, local people living in the vicin-ity of the mine continue small-scale activities using small adits and open pits

The chromite concentrate produced from this mine contains 46% Cr2O3 and <27% Fe2O3, 5% SiO2 and

Figure 1 Location of the sampling sites.

0.4% H2O; the estimated ore reserve of the mine is

approximately 20.8 metric tons of Cr2O3 (Wu 2002,

2004) Annual production of the concentrate reached

80,000 tons per year

Surface and open-pit mining, followed by on-site ore

processing have left a huge number of waste rock piles,

spoil heaps and tailing ponds within approximately 2 km2

of the mining area These mine waste residues are

trans-ported by small streams, eventually reaching and being

deposited into a large lake (approximately 0.5 km2); the

lake was formed by the intensive mining activities (Fig 1)

A number of local people constructed a dike at the border

of the mining site by heaping up soil to prevent pollutant

inflow into the adjacent agricultural lands However, the

dike sometimes collapses as a result of flooding during the

rainy season, resulting in the inflow of sediments,

includ-ing mine wastes, into the adjacent drainage canals and

paddy field areas

Study sites and sampling

Field surveys and sampling were conducted in November

2006 and November 2007 at study sites selected within

the Co Dinh chromite mine and in the adjacent lowland

paddy fields (Fig 1; Table 1) The climate is tropical

mon-soon with a mean annual precipitation and temperature

of 1,600–2,000 mm and 23–24C, respectively (Thanh

Hoa Department of Culture, Sport and Tourism 2009)

At the time of the field surveys and sampling (November

2006 and November 2007), rice plants had just been har-vested and the paddy fields were submerged for the next rice cultivation

Three representative sites were examined to character-ize the soil properties of the study area and to investigate the vertical distribution of heavy metals and As with soil depth: one site was located in the mining area (Sp1) and two sites were located in the areas used for paddy fields, severely affected (Sp2) and non-affected (Sp3) by mine wastes At each site, a soil pit was prepared and its profile was described Soil samples were taken from each horizon

of the soil profiles, placed into a plastic bag and stored at 4C until analysis

To assess the influence of heavy metals and As from the mining areas and to clarify the pathways of contamina-tion, samples of mine tailings, sediment, paddy soil and water were collected from established sites at various dis-tances from the mining area Tailings was taken from a depth of 0–25 cm at the mine tailings site (designated as MT) Stream sediments were sampled from the bottom of streams running through the mining area (SD1–4 and SD6) In addition, one sediment sample was taken from the lake (SD5; Fig 1) In total, eight paddy fields were selected as study sites: six paddy fields (P1–6) were assumed to be affected by sediment inflow as a result of the dike collapse, and two paddy fields (P7–8) were

Table 1 List of the sampling sites

Distance and direction from tailings (km) Sample type Soil pits

Sp1 Located on a mountain slope of a natural laurel forest 0.00 NE Mine soil Sp2 A paddy field located immediately outside of the dike 2.70 NE Paddy soil Sp3 A paddy field located far from the dike (between sites P7 and P8) 5.30 NE Paddy soil Mine tailings and sediments

SD1 Upstream running through the mine tailings 0.30 NE Stream sediment

SD4 Downstream of site SD3 and adjacent to the reservoir lake 1.50 NE Stream sediment

SD6 Located at the drainage channel running from

the reservoir lake and flowing into paddy fields

2.40 NE Stream sediment Paddy soils

P1 A paddy field located inside of the dike,

the transition ⁄ buffering area between the lake and the dike

2.50 NE Paddy soil P2 A paddy field located immediately outside of the dike 2.60 NE Paddy soil P3 A paddy field located outside of the dike 2.80 NE Paddy soil P4 A paddy field located outside of the dike 3.00 NE Paddy soil P5 A paddy field located outside of the dike 3.50 NE Paddy soil P6 A paddy field located outside of the dike 4.50 NE Paddy soil P7 A paddy field located far from the mining sites 5.00 NE Paddy soil P8 A paddy field located far from the mining sites 6.00 NE Paddy soil

assumed to be unaffected because the transportation road

formed an effective barrier At each paddy field, surface

(0–5 cm) and subsurface (20–25 cm) soils were collected

In addition to sampling the tailings, sediments and soils,

water samples were collected from each site, including

stream water near the MT, SD1 and SD3–4 sites, lake

water at SD5 site, and standing water in the paddy fields

(P1–2 and P4–8)

Sample analyses

Soil and sediment analyses

General physicochemical properties The soil and

sedi-ment samples were air-dried at room temperature and

crushed to pass through a 2-mm mesh sieve The particle

size distribution was determined using the pipette method

(Gee and Bauder 1986) The clay mineral composition

was identified by X-ray diffraction (XRD) analysis using

CuKa radiation (XD-D1w; Shimadzu, Kyoto, Japan) The

electrical conductivity (EC) and pH (H2O) values were

determined using a platinum electrode and a glass

elec-trode, respectively, at 1:5 (w ⁄ v) ratio of soil ⁄

sedi-ment : water The pH and redox potential (Eh) of the

soils taken from the three soil profiles and from some

selected sediment samples were determined in situ

Exchangeable (Ex-) cations (K, Na, Ca and Mg) were

extracted with 1 mol L)1ammonium acetate at pH 7.0,

and the contents were determined using an atomic

absorp-tion spectrometer ([AAS] AA-6800; Shimadzu) After

removing excess NH4, the sample was extracted with

100 g L)1NaCl solution and the supernatant was used to

determine the cation exchange capacity (CEC) using the

Kjeldahl distillation and titration method (Rhoades

1982) The contents of total carbon (TC) and total

nitro-gen (TN) were analyzed on an NC analyzer (Sumigraph

NC-80; Sumitomo Chemical, Osaka, Japan)

Total heavy metals and As After the samples were

digested in a mixture of HNO3and HF (9:1) and heated

in a microwave (Multiwave, Perkin-Elmer, Yokohama,

Japan), the total concentration of the heavy metals (Cr,

Co, Cu, Ni and Pb) was determined using AAS The

accu-racy of the method was assessed using certificated

refer-ence soils (JSO-1, provided by the National Institute of

Advanced Industrial Science and Technology, Tsukuba,

Japan) and marine sediment (NIES No.12, provided by

the National Institute for Environmental Studies,

Tsu-kuba, Japan) The recoveries of Co, Cr, Cu, Ni and Pb

were in the ranges 92.6–117, 96.7–101, 96.6–101, 96.2–

104 and 92.9–96.2%, respectively

For the analysis of total As content in the soils and

sedi-ments, samples were digested in a mixture of HClO4–

HNO3–HF (2:3:5) with the addition of 20 g L)1KMnO4

in a teflon vessel at 100C The concentration of As in the

acid digest was determined using an inductively coupled

plasma-atomic emission spectrometer ([ICP-AES] ICPS-1000IV; Shimadzu) equipped with a hydride vapor gener-ator (HVG-1; Shimadzu) The standard reference materi-als JSO-1 and JSO-2 (provided by the National Institute

of Advanced Industrial Science and Technology, Tsukuba, Japan) were used to verify the accuracy of the As determi-nation The recovery rates of As were within 90–95% Water-extractable forms of the heavy metals and

As Water-extractable forms of the heavy metals and As were examined using fresh (moist) soil and sediment sam-ples The procedure for this extraction was adapted from

a previous report by Kim et al (2003) In brief, a 10-g portion of each sample and 50 mL of deionized water were placed into a 100 mL plastic vessel All samples were shaken using a platform shaker at room temperature for

1 h The suspended mixtures were filtered through 0.45 lm cellulose membrane filters The concentrations

of the heavy metals and As in the leachates were deter-mined by AAS and ICP-AES, respectively

Extraction of hexavalent chromium Extraction and determination of total hexavalent chromium (Cr(VI)) in the soils and sediments were conducted based on the EPA Method 3060A ⁄ 7196A with some modifications The protocol applied here was described in detail by James

et al (1995) In brief, 2.5 g of homogenized fresh (moist) soil or sediment sample was placed into a 250 mL beaker Fifty milliliters of a solution of 0.28 mol L)1Na2CO3in 0.5 mol L)1 NaOH (pH 12) was added and mixed (unheated) for approximately 10 min The beakers were then transferred to a preheated 150C hot plate and main-tained at 90–95C for 60 min with continuous stirring Then, the solution was cooled and replenished with dis-tilled water to the initial volume After centrifugation for phase separation (20 min, 2000 g), the supernatant was filtered through a 0.45-lm cellulose membrane filter The solution was then adjusted to a pH of 7.5 ± 0.5 with 5.0 mol L)1 HNO3 solution and topped up to 100 mL with distilled water The sample digests were then ana-lyzed using the colorimetric method One milliliter of diphenylcarbazide solution was added to 80 mL of digest

in 100 mL Erlenmeyer flasks A 1.8 mol L)1H2SO4 solu-tion was added to the digests until the solusolu-tion reached a

pH value between 1.6 and 2.2, then it was topped up to

100 mL with distilled water After 10 min, the absor-bance at 540 nm was measured using a spectrophoto-meter (V-360 BIO; Jasco, Tokyo, Japan) and the Cr(VI) concentration was determined against standard solutions ranging from 0.05 to 2 mg L)1

Water analysis Water samples were filtered through a 0.45-lm mem-brane filter and divided into two portions One portion was acidified with 0.03 mol L)1 HNO3 for analysis of heavy metal and As concentrations, and the other portion

was left unacidified for pH and EC measurements The

water samples were then stored in a refrigerator at 4C

until analysis The total concentrations of heavy metals

(Co, Cr, Cu, Ni and Pb) and As were determined by AAS

and ICP-AES, respectively

RESULTS

General physicochemical properties of the soils

The soil profile at the Sp1 site was highly disturbed as a

result of mining activity; the A horizon was mainly

com-posed of colluvial materials from the upper slope, whereas

the BC and C horizons contained abundant rock

frag-ments of various sizes The soil profiles from the Sp2 and

Sp3 sites resembled each other However, the profile at

the Sp2 site appeared to be more affected by redox

reac-tions owing to uncontrolled flooding caused by the

col-lapse of the dike The Ap horizon was thicker at the Sp2

site than at the Sp3 site It should be noted that the soil

matrix color was greenish gray or bluish gray throughout

the profile at the Sp2 site, but was brownish gray at the

Sp3 site

The physicochemical properties were determined for

soils collected from three soil profiles in the Sp1, Sp2 and

Sp3 sites (Table 2) The Sp1 soils had a clayey texture

throughout the profile, with a higher content of TC in the

A horizon The soil pH values were neutral in reaction,

with the highest Ex-Mg content among the exchangeable

bases Compared with the Sp1 soils, the Sp3 soils had

lower clay contents and a more acidic nature, with higher

Ex-Ca contents, which were the highest among the

exchangeable bases The Sp2 soils showed intermediate

properties between the Sp1 and Sp3 soils in terms of pH

and exchangeable bases However, the clay content of the

Sp2 soils was the highest throughout the profile among the three sites The XRD analysis revealed that the clay minerals were composed mainly of montmorillonite, kao-lin minerals, mica minerals and quartz for the Sp1 soils and kaolin minerals, mica minerals, quartz and gibbsite for the Sp3 soils In the case of the Sp2 soils, both mont-morillonite and gibbsite were detected in addition to the other minerals, which were detected commonly in the Sp1 and Sp3 soils Based on the USDA soil classification sys-tem, paddy soils at both the Sp2 and Sp3 sites were classi-fied as Typic Endoaquepts, whereas the soil at the Sp1 site was determined to be a Lithic Ustorthents (Soil Survey Staff 2006)

As a whole, the physicochemical properties of the mine tailings (MT) and sediments (SD1–6), the P1–6 paddy soils and the P7–8 paddy soils were found to resemble those of the Sp1, Sp2 and Sp3 soils, respectively (Table 3; Fig 1)

Vertical distribution of total heavy metals and

As in the soils The vertical distributions of total Cr, Co, Ni, Cu, Pb and

As were determined in the soil profiles at the Sp1, Sp2 and Sp3 sites There were different tendencies in the distribu-tion patterns between Cr, Co and Ni and Cu, Pb and As (Fig 2)

The levels of Cr, Co and Ni were considerably higher in the Sp1 soils than in the Sp3 soils, the respective ranges were 1,616–4,331 mg kg)1, 162–286 mg kg)1 and 4,226–4,700 mg kg)1 At the Sp1 site, the content of Cr and Co was higher in the A horizon than in the BC and C horizons, whereas the Ni content was constant with depth In contrast, at the Sp3 site, levels of Cr, Co and Ni did not vary appreciably throughout the profile The Sp2

Table 2 Physicochemical properties of the soils taken from three soil profiles

Site Horizon Depth (cm) pH † EC (mS m)1)

TC TN Ex-K Ex-Na Ex-Ca Ex-Mg CEC

Clay (%) Texture (g kg)1) (cmol c kg)1)

Mine soil

Sp1 A 0–20 ⁄ 33 6.82 4.50 42.0 3.75 0.16 0.07 0.75 8.77 32.6 39 CL

BC 20 ⁄ 33–40 ⁄ 50 7.10 2.35 18.6 2.29 0.03 0.06 0.50 6.77 36.4 40 SC

Paddy soils

B1gir 23–50 6.89 4.05 17.9 1.54 0.17 0.14 1.74 6.91 38.4 54 C B2gir 50–73 6.93 3.28 10.9 0.80 0.20 0.14 1.14 6.50 27.7 57 C B3gir 73–100+ 7.21 3.80 1.95 0.37 0.19 0.18 1.17 6.63 27.7 57 C Sp3 Ap 0–9 ⁄ 10 5.36 2.12 20.0 2.02 0.14 0.08 3.37 0.71 12.2 30 CL B1g 9 ⁄ 10–33 5.89 0.62 4.56 0.66 0.05 0.09 2.70 1.23 11.1 37 CL B2gir 33–57 5.84 0.71 1.59 0.34 0.06 0.07 2.29 1.61 9.59 28 SCL B3gir 57–95+ 5.79 1.07 1.07 0.32 0.08 0.06 2.06 1.56 8.89 23 SCL

† pH, pH(H 2 O) CEC, cation exchange capacity; C, clay ; CL, clay loam; EC, electrical conductivity; Ex-, exchangeable; SC, sandy clay; SCL, sandy clay loam ; TC, total carbon; TN, total nitrogen.

soils had intermediate levels of these heavy metals

between the Sp1 and Sp3 soils, but they were higher in the

Ap horizon than in the lower horizons

In contrast to Cr, Co and Ni, the levels of Cu, Pb and

As were lower in the Sp1 soils than in the Sp3 soils In the

Sp1 soils, Cu and As did not vary with depth, whereas Pb

content was high in the A horizon and decreased slightly

with depth At the Sp3 site, the Cu content was highest in

the Ap horizon and almost constant in the deeper

hori-zons, whereas Pb content had a minimum value in the

B2gir horizon The total As content at this site was lower

in the Ap horizon than in the B1g to B3gir horizons In

the case of the Sp2 site, although the Pb and As contents

in the Ap horizon were equivalent or similar to those at

the Sp1 site, these values increased with depth, and

reached similar levels to those of the Sp3 site in the deeper

horizons Similar to Pb and As, Cu at the Sp2 site tended

to be lowest in the Ap horizon, but with relatively high levels throughout the profile

Horizontal distribution of total heavy metals and

As in the soils Figures 3 and 4 record the levels of total Cr, Co and Ni and Cu, Pb and As, respectively, in the P1–8 soils, tailings (MT) and sediments (SD1–6) collected from the sites shown in Fig 1

High contents of Cr, Co and Ni were found in the MT tailings (10,428, 340 and 4,200 mg kg)1, respectively) and in the SD1–6 sediments (2,674–6,700, 294–1,000 and 3,477–7,800 mg kg)1, respectively) The paddy field sites could be classified into three groups based on the content of these heavy metals, which decreased

signifi-Table 3 Physicochemical properties of the mine tailings, sediments and paddy soils

Site Depth (cm) pH † EC (mS m)1)

TC TN Ex-K Ex-Na Ex-Ca Ex-Mg CEC

Clay (%) Texture (g kg)1) (cmol c kg)1)

Mine tailings

Sediment

Paddy soil

Average values

Mine tailings and sediments

(n = 7) 0–25 7.43 17.1 12.3 0.70 0.08 0.10 1.05 6.15 32.9 24

Paddy soils

Taken near mining sites (P1–P6)

(n = 6) 0–5 6.30 14.0 20.6 1.35 0.21 0.12 3.64 4.82 35.7 43

(n = 4) 20–25 6.50 8.11 14.8 1.17 0.19 0.12 3.26 2.86 27.1 42

Taken 5–6 km from the mining site (P7–P8)

(n = 2) 0–5 5.93 4.11 18.6 1.51 0.13 0.12 4.52 1.30 10.8 27

(n = 2) 20–25 6.94 3.52 5.02 0.46 0.11 0.11 5.11 1.81 10.8 25

† pH, pH(H 2 O) CEC, cation exchange capacity; C, clay ; CL, clay loam; EC, electrical conductivity; Ex-, exchangeable; SC, sandy clay; SCL, sandy clay loam ; TC, total carbon; TN, total nitrogen.

cantly with increasing distance from P1 (Fig 3) The first

group contained the P1–4 sites and the levels of these

heavy metals were high compared with the levels in the

sediment from SD6 The second group contained the P7

and P8 sites, where the soils were low in Cr, Co and Ni

content, similar to the Sp3 site The P5 and P6 sites were

in the last group, and the levels of heavy metals at these

sites were higher than those in the second group, but were

considerably lower than those in the first group

In contrast, the contents of Cu, Pb and As (ranges of

4.12–17.5, 7.14–17.2 and 1.35–4.79 mg kg)1,

respec-tively) in the MT tailings and SD1–6 sediments were

almost comparable to the levels found in the Sp1 soils In

the P1–8 sites, the paddy soil ranges were 14.4–64.9 mg

kg)1for Cu, 14.4–93.1 mg kg)1for Pb and 3.03–11.3 mg

kg)1for As in the surface layers, and 21.2–41.6 mg kg)1

for Cu, 19.2–77.0 mg kg)1for Pb and 3.41–13.8 mg kg)1

for As in the subsurface layers The contents of these

ele-ments increased significantly with increasing distance

from P1 as illustrated by the correlation coefficients in

Fig 4

Water-extractable heavy metals and As

As a whole, small amounts of heavy metals and As were

detected in water-extractable form in the sediment

samples (SD2 and SD6) and soil samples (Sp1–Sp3) com-pared with their total content (Table 4)

The levels of Cr, Co and Ni in water-extractable form were relatively high in the sediments and in the Sp1 and Sp2 soils These metals in the Sp1 and Sp2 soils decreased with depth, except for Co in the Sp1 soils The Sp3 soils showed a similar distribution pattern for Cr, Co and Ni, but the maximum levels found in the Ap horizon were lower than the minimums found in the B3gir horizon for the Sp2 site It is noteworthy that the Co content of the Sp1 and Sp2 soils was higher than that of the sediments, whereas the Ni content was highest in the Ap horizon at Sp2 site

The content of water-extractable Cu and Pb in the Sp1 and Sp2 soils was higher than that in the SD2, SD6 and Sp3 soils Within the profile, Cu content was high in the

BC and C horizons at the Sp1 site, and in the B1gir and B2gir horizons at the Sp2 site, whereas Pb content was high in the A horizon at the Sp1 site and in the B1gir and B2gir horizons at the Sp2 site At the Sp3 site, Cu and Pb tended to be high in the Ap and B1g horizons Water-extractable As was not detected in the SD2 and SD6 sediments or in the Sp1 soils, but soils in the upper two horizons at the Sp2 site showed negligible content of As

Figure 2 Vertical distribution of heavy metals (Cr, Co, Ni, Cu and Pb) and As in three soil profiles (Sp1–Sp3).

Figure 3 (A–C) Horizontal distribution of Cr, Co and Ni in the mine tailings, sediments and paddy soils , surface layer; , subsurface layer (a–c) Relationships between Cr, Co and Ni in the surface layer of the paddy soils and the distance from the P1 site.

Figure 4 (A–C) Horizontal distribution of Cu, Pb and As in the mine tailings, sediments and paddy soils , surface layer; , subsurface layer (a–c) Relationships between Cr, Co and Ni in the surface layer of the paddy soils and the distance from the P1 site.

Hexavalent chromium

Taking into consideration that the toxicity of Cr(VI) is

much higher than that of Cr(III) (Adriano 2001), the

amount of total Cr(VI) was determined for the soil

sam-ples from the Sp1, Sp2 and Sp3 sites and for the sediment

samples SD2 and SD6 (Table 5) Table 5 also gives the

results of field measurements of the pH and Eh values

No clear relationships were found between the content

of Cr(VI) and pH or Eh values The Cr(VI) content was higher at the Sp1 and Sp2 sites than at the Sp3 site The vertical distribution of Cr(VI) in the soil profile resembled that of water-extractable Cr However, the Sp1 soils had much higher levels of Cr(VI) than water-extractable Cr, although the levels of these Cr fractions were similar to each other in the Sp2 soils

Concentration of heavy metals and As in the water samples

In the water samples, as distance from the mining area increased, higher concentrations of certain elements were found; for example, moving from the MT to the P2 site for Cr and moving from the SD1 to the P4 site for Ni (Table 6) The concentration of Ni, in particular, often exceeded the regulation limit of 100 lg L)1 set by the

‘‘Vietnamese standard limitation for surface water’’ (TCVN 5942-1995 1995) In contrast, Co, Cu, Pb and As were not detected

DISCUSSION Influence of mining activity on adjacent paddy fields

Based on the results related to the vertical and horizontal distribution patterns of total heavy metals and As, as well

as the general physicochemical soil properties and the soil profile descriptions, mining activity caused Cr, Co and

Ni contamination of soils in adjacent paddy fields by sediment inflow at the time of the dike collapse, which

Table 4 Water-extractable contents of heavy metals and As in the sediments and soils taken from three soil profiles

Site Horizon Depth (cm)

(mg kg)1) Sediments

Mine soil

Paddy soils

Concentrations are calculated as the element content in the dry mass of the sample –, not detected.

Table 5 pH, redox potential and hexavalent chromium content

in the sediments and soils taken from three soil profiles

Site Horizon

Depth (cm) pH †

Eh (mV)

Cr(VI) (mg kg)1) Sediments

Mine soil

Sp1 A 0–20 ⁄ 33 6.61 242 36.0

BC 20 ⁄ 33–40 ⁄ 50 7.37 236 22.5

Paddy soils

B1gir 23–50 7.27 124 5.34

B3gir 73–100+ 7.60 141 0.50

B1g 9 ⁄ 10–33 5.80 287 0.74

B2gir 33–57 6.20 260 0.30

B3gir 57–95+ 6.13 252 0.11

† pH, pH (H 2 O) Concentrations are calculated as the element content in

the dry mass of the sample Cr(VI), hexavalent chromium; Eh, redox

potential.