DSpace at VNU: Arsenic contamination in groundwater and its possible sources in Hanam, Vietnam

The deposition of As in the sediments from the Red River were significantly higher than that in the Chau Giang River, suggesting that the Red River is the main source of As-containing su

Arsenic contamination in groundwater and its possible

sources in Hanam, Vietnam

Nguyen Minh Phuong&Yumei Kang&Katsutoshi Sakurai&Miyuki Sugihara&

Chu Ngoc Kien&Nguyen Dinh Bang&Ha Minh Ngoc

Received: 3 November 2010 / Accepted: 27 July 2011 / Published online: 10 August 2011

# Springer Science+Business Media B.V 2011

with iron (hydr)oxides and clay mineral In the groundwater, As concentration showed significant correlations with the total concentrations of Fe and HCO3 − Significant correlations between HCl-extractable As and non-crystalline Fe oxide, total C,

N, and S were also observed in the profiles The results support the hypothesis that under favorable reductive conditions established by the degradation of organic matter, the dissolution of iron (hydr)oxides releases adsorbed As into the groundwater The deposition of As in the sediments from the Red River were significantly higher than that in the Chau Giang River, suggesting that the Red River is the main source of As-containing substances deposited in the study area

Keywords Arsenic Bore core Groundwater River Sediment Vietnam

Introduction Arsenic is unique among the heavy metalloids and oxyanion-forming elements (e.g., As, Se, Mo) in its susceptibility to mobilization under the pH conditions typically found in groundwater (pH=6.5–8.5) and over

a wide range of redox conditions (Hossain2006) Tens

of millions of people in South and Southeast Asia routinely consume groundwater that has unsafe As levels (Smith et al.2000; Chowdhury et al.2000; Berg

et al 2001, 2007; Hossain 2006) As a main water

Environ Monit Assess (2012) 184:4501 –4515

DOI 10.1007/s10661-011-2281-6

N M Phuong:C N Kien

United Graduate School of Agricultural Sciences,

Ehime University,

Matsuyama 790-8566, Japan

Y Kang:K Sakurai:M Sugihara

Faculty of Agriculture, Kochi University,

Monobe, Nankoku,

Kochi 783-8502, Japan

N M Phuong ( *):N D Bang:H M Ngoc

Faculty of Chemistry, Hanoi University of Science,

Hanoi, Vietnam

e-mail: nmphuong81@yahoo.com

Abstract This study investigated the arsenic (As)

level in groundwater, and the characteristics of aquifer

sediment as related to the occurrence of As in

groundwater in Hanam, Vietnam The deposition and

transport of As-containing substances through rivers

were also examined Arsenic concentrations in 88%

of the groundwater samples exceeded the As limit for

drinking water based on the WHO standards The

dominating form of arsenic was As(III) The

maxi-mum total As content in bore core sediment was

found in a peat horizon of the profiles and generally,

elevated levels of As were also found in other organic

matter-rich horizons Total As contents of the bore

core sediments were significantly correlated with

crystalline iron oxide, silt and clay contents,

suggest-ing that As in aquifer sediment was mainly associated

source for local communities, groundwater has been

exploited in Vietnam since the 1900s The first

publication on As contaminations in groundwater of

Hanoi, Vietnam, in 2001 reported contamination levels

from 1 to 3,050μg l−1(average 159μg l−1) (Berg et al

2001) Such elevated As concentrations were found in

numerous regions throughout Vietnam (Berg et al

2001; Chander et al.2004; Agusa et al.2006; Nguyen

et al.2009) A random survey of As levels in tube well

water from 12 Vietnamese provinces indicated that

Hanam is one of the most seriously As-contaminated

area in the Red River Delta In this area, As

concentrations exceeded the WHO guideline for As

in drinking water (10μg l−1) (Chander et al 2004) in

52% of the tube wells surveyed

Arsenic-bearing groundwater in Vietnam has been

noted because of the geological similarity with the

Ganges–Brahmaputra, Mekong, and Red River basins

which are built up with alluvium from the rapidly

weathering Himalayas and are characterized by

complex lithological structures of the aquifers which

do not show a full separation between upper and

lower aquifers (Laurent and David 2006) Some

researchers have argued that oxidation of As-rich

sulfide minerals is one possible mechanism for the

release of As into groundwater Others have suggested

that reductive dissolution of iron oxyhydroxides or

arsenate sorbed by detrital organic carbon is another

possible mechanism of As mobilization (Nickson et

al 1998; Smedley and Kinniburgh 2002) However,

the dissolution of iron oxide is regarded the primary

process responsible for high As concentrations in the

groundwater in some areas Arsenic is naturally

derived from eroded Himalayan sediments, and is

believed to become mobile following reductive

release from solid phases under anaerobic conditions

(Polizzotto et al 2008) A study of the hydrological

and sedimentary conditions of river bank deposits in

the Hanoi area indicated that elevated groundwater

levels of As are caused by reductive dissolution under

iron-reducing conditions (Berg et al.2008)

Hanam Province with a total area of 849.5 km2and

a population of 820,100 is a productive agricultural

region located in the lower part of the Red River

Delta The topography is dominated by limestone

mountains, hills, and forests with some sloping areas

in the west (10–15% of the total area), whereas the

east is a plain that mainly consists of alluvium from

the Red River (85–90% of the total area) About 38.6

km of the Red River form the eastern border of the province The Red River plays an important role in the fertility and irrigation of the roughly 10,000 ha of agricultural land However, there is little information

on the characteristics and degree of As contamination, and the causes of As release to the groundwater in this area In this study, we examined As concentrations in groundwater and the geochemical parameters of aquifer sediment related to the occurrence of As in the groundwater Our study area in the Lynhan district

of Hanam Province represents alluvium from two rivers, the Red River and the Chau Giang River Therefore, we also investigated the deposition and transport of pollutants through these streams

Materials and methods Sample collection and preparation This survey was conducted in the Xuan Khe (XK), Hop Ly (HL) and Chan Ly (CL) communes of the Lynhan district, Hanam Province, in November 2006 (dry season) (Fig 1) Hop Ly and Xuan Khe are located near the Chau Giang River, while Chan Ly is located near the Red River

Groundwater samples were taken from 31

random-ly chosen tube wells in the three communes (Hop Ly, n=12; Xuan Khe, n=11; Chan Ly, n=8) Prior to sampling, water from tube wells was flushed away until crystal clear water was obtained (Berg et al

2001) Immediately after collection, pH, electrical conductivity (EC), oxidation–reduction potential (Eh), and dissolved oxygen (DO) were measured The samples were passed through small disposable ion exchange cartridges packed with 2.5 g selective aluminosilicate adsorbent (Metalsoft Center, Highland Park, NJ; Meng and Wang 1998) This adsorbent retained As(V) but not As(III) The filtrates then were acidified with 1% (volume) concentrated HCl for As (III) analysis The cartridges have been widely used in the field to separate As(V) from As(III) in water samples because of their convenience and reliability The average recovery of As(III) in the filtrates was 98% (Meng and Wang 1998) Water samples for the analysis of total As, Fe, and Mn were acidified with 1

ml concentrated HCl acid and preserved in 100-ml polypropylene bottles For major ions analysis, polypropylene bottles were filled completely with

sampled water, all bubbles were removed, and the

bottles were tightly capped A set of 50-ml samples

was used to determine HCO3 −in the laboratory (see

below for details) Another set of 50-ml samples were

filtered through 0.45-μm membrane filters to remove

suspended organic matter and acidified to pH<2 with

concentrated HCl for DOC analysis conducted

according to Standard Methods 5310 (see below for

details) All water samples were kept at 4°C until

analysis

To clarify the origin of As contaminations in tube

well water, bore cores were obtained in the XK and

HL communes to depths of about 20 m, the common

depth of household tube wells in the study area The

pre-survey was conducted to select the location of the

bore cores The locations of the bores were selected

based on first, the As levels we had examined in 15

tube wells using the Hach As test kit (the data is not

shown), and second, on observations of dark peat

horizons made by local people when they drilled their

wells Samples from the same, clearly differentiated

horizon were combined for analysis Water samples

were collected from the bore holes after 1 h pumping

In addition, sediment and water samples from 5

points along the Red River and 6 points along the

Chau Giang River were sampled The sediments were

air-dried, ground with a ceramic pestle, passed

through a 2.0-mm sieve, and stored in plastic bottles

until analysis The water samples were filtered

through filter paper, acidified with 1% (volume)

concentrated HCl, and kept at 4°C until analysis

Analysis Water

EC and pH were measured on-site by potable EC/pH meter (WM-22EP, DKK-TOA, Japan) Redox poten-tial (Eh) was also recorded on-site with an ORP meter (RM-20P, DKK-TOA), and DO was measured with a portable DO meter (YSI 55, YSI, USA) In the laboratory, water samples were analyzed for total concentration of As using an inductively coupled plasma atomic emission spectrometer (ICP-AES; ICPS-1000 IV, Shimadzu, Kyoto, Japan) equipped with a hydride vapor generator (HVG-1; Shimadzu) The total concentrations of Fe and Mn were deter-mined using an atomic absorption spectrometer (AAS; AA-6800, Shimadzu) In order to assure the precision of the measurement, reference standard solution with a known concentration of each mea-sured element, which was prepared from the different source of the stock standard solution used for calibration standard, were used as a control sample After every ten samples during analysis, the control sample was analyzed to check the accuracy of analysis All samples were measured at least two times in order to assess the repeatability of the measurement Samples were reanalyzed if the error

of the control sample exceeded 10% or the relative standard deviation of the measurement exceeded 5% Dilution was made with 2% nitric acid, when the concentration of the sample was over the upper

Chan Ly

Xuan Khe Xuan Khe

Hop Ly

Chau Giang River

106 o 00' 106 o 05' 106 o 10' 106 o 15' 106 o 20'

20 o 50'

20 o 55'

20 o 60'

0 2 km Hanoi

Vietnam

Hanam Lynhan

river, canal commune border Legend

Fig 1 The study area, Ly

Nhan district, Ha Nam

province, Vietnam Further

details of the location of

sampling sites (bore core,

groundwater, river water,

and river sediment sampled

site) are as in Figs 2 , 3 , 4 ,

8 , and 9

4504 Environ Monit Assess (2012) 184:4501 –4515 limitation of the standard range HCO3 − was

mea-sured by titration method using methyl orange and

bromcresol green indicators, and DOC was analyzed

with a TOC analyzer (TOC-VCPH/TNM; Shimadzu)

The concentrations of Cl−, NO3 −, SO4

2 −, PO4

3 −,

NH4 , Na+, K+, Mg2+, and Ca2+ions were determined

by ion chromatography (IA−300, DKK-TOA, Japan)

Sediment

For the analysis of total As, P, and S contents, a 0.15-g

soil sample was digested at 100°C in a Teflon vessel

containing a mixture of 2 ml 60% HClO4, 3 ml conc

HNO3, 5 ml concentrated HF, and 2 ml of a 20 gl–1

KMnO4solution If the purple color of the KMnO4had

disappeared after 20 min of heating, 1 ml of the

KMnO4 solution was added, and this procedure was

repeated until the mixture remained colored (Terashima

1984) The concentrations of As in the digests were

determined by using an ICP-AES (ICPS-1000 IV;

Shimadzu) equipped with HVG-1 (Shimadzu) For

the determination of P and S, the ICP-AES system was

used 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 95–105% Bore core

sediments were extracted with 1 M HCl over 30 min to

determine HCl-extractable As Physicochemical

prop-erties of the bore core sediments including particle size

distribution, total carbon (TC), total nitrogen (TN),

dithionite–citrate–bicarbonate (DCB)-extractable and

ammonium oxalate-extractable Fe oxides and

hydrox-ides (Fedand Feo, respectively) were examined by the

methods described by Phuong et al (2008)

Results

Chemistry of groundwater

Arsenic concentrations in the groundwater samples

ranged from <5 to 703μg l−1(178±170μg l−1); the

geographic distribution of As in the three communes

is shown in Figs 2, 3, and 4 The average As

concentrations in the groundwater of HL, XK and CL

were 196, 256, and 43μg l−1, respectively; the value

for CL was significantly lower than those for HL and

XK On average, about 76% of the total As in the

groundwater existed in the As(III) form

The groundwater was characterized by a neutral

pH and high EC (Table1) The low Eh values (−157

to 11.0 mV) demonstrated the reducing nature of the aquifer (Table 1) Concentrations of total Fe in the water samples ranged from 1.17 to 41.6 mg l−1 (average, 15.0 mg l−1) The total Mn concentration varied from <0.1 to 2.82 mg l−1 (average, 0.66 mg l−1) A wide range of NH4

+

concentration was found in the groundwater (<0.2–76.0 mg l−1; average, 20.7 mg l−1) The concentrations of NO3 −

and SO4 − in most samples were lower than the detection limit Except for one sample, the concen-trations of PO4 −were lower than 2.4 mg l−1 The DO values were lower than 1.76 mg l−1 Major ion composition was dominated by HCO3 − (56.1–

683 mg l−1; average, 474 mg l−1), followed by Na+ (14.7–816 mg l−1; average, 202 mg l−1) and Ca2+ (37.9–175 mg l−1; average, 97.4 mg l−1) The average concentrations of HCO3 −and Ca2+in the groundwater

of CL were significantly lower, and the average values of Eh and total Mn were significantly higher than at the other two sites Compared to HL and CL, significantly higher levels of EC, DOC, NH4, K+and

Mg2+ were observed in XK Concentrations of Cl− and Na+ were significantly lower in HL than in XK and CL Furthermore, the concentration of As correlated significantly with the concentrations of Fe (r=0.678; p≤0.01); HCO3 − (r=0.426; p≤0.05); pH (r=0.460; p<0.01); while it was negatively correlated with Eh values (r=−0.550; p≤0.01) (Fig.5)

Geochemical characteristics of aquifer sediments Description of the bore cores

In XK bore core, brown to brownish grey clay, muddy clay and silty clay layers were observed from the surface horizon to 4.7 m A sequence of grey or dark grey silty sand and fine grained sand were collected from 4.7 to 20 m, interrupted by some plant remains and shells or snails (Fig 6a)

The drilling site of HL bore core is overlain by a 2-m-thick brown clay layer Below this layer, grey silty sand and fine grained sand layers were observed to a depth of 23 m A thin and dark grey peat layer enriched with plant residuals and organic matter was collected at 6.6–7.0 m depth A lot of shells and snails were found in a fine grained sand horizon at 19–23 m (Fig 7a)

Chemistry of aquifer sediments

The total As contents in the sediments of the XK and

HL bore cores ranged from 5.51 to 20.1 and from

7.37 to 25.1 mg kg−1, respectively In the XK profile,

elevated levels of total As were detected in clay layers

from the surface to 3.7 m, at 4.0–4.7 m, and in a

horizon containing plant residuals (14.0–14.8 m) On

the other hand, the highest total As content (25.1 mg

kg−1) in the HL profile was found in a peat horizon

(6.6–7.0 m) (Figs.6b and7b)

In the XK profile, high proportions of

HCl-extractable As were observed in the layers containing

plant residuals or organic matters (Fig 6b) A high

proportion of HCl-extractable As was detected in a

peat horizon of the HL profile (Fig 7b)

In XK profile, the distribution of Fed showed

similar trends as the total As content throughout the

XK profile (Fig.6b) Except for the surface horizon

(0–1 m), the distribution of P and Feo roughly paralleled the total As in the profile (Fig 6b) In parts of the profile, total As also correlated with HCl-extractable As (6.8–20 m), total C (6.8–20 m), and clay (0–12 m) (Fig 6) Furthermore, the distribution of HCl-extractable As and total S content were quite similar throughout the profile (Fig.6b)

In HL profile, the P distribution in the HL profile resembled that of total As contents (Fig.7b) Except for 0–3 m depth, the distributions of HCl-extractable

As, total S, C, and N contents were similar to that of total As (Fig 7b) At 8–22 m, a correlation between clay or silt and total As distribution was observed (Fig 7b) In addition, HCl-extractable As, total S, C, and N showed parallel trends (Fig.7b) Highest total

P, S, C and N contents were detected in a peat horizon, where the highest total and HCl-extractable

As contents were detected

Legend

ChauGiangRiver

river, lake, canal paddy field

commune border bore core sampling location

< 10

10 -100

100 -300

> 300

Arsenic (µµg L -1 )

500 m

Legend

Fig 2 As concentrations

in the groundwater in Xuan

Khe Filled star, bore core;

empty circle, As level

lower than 10 μg l −1 ;

shaded circle, As level 10 –

100 μg l −1 ; diagonally

striped circle, As level 100 –

300 μg l −1 ; filled circle, As

level greater than 300 μg l −1

Results of the correlation analysis between As and

other parameters of the XK and HL bore core

sediments (except for the peat horizon) are shown in

Tables 2 and 3 In the XK bore core, the total As

contents of the sediments were positively correlated

with Fed, N, P, clay, silt contents, and were correlated

negatively with sand contents (p≤0.01) A significant

correlation at a level of 5% was also obtained between

the total As and Feocontents On the other hand, the

HCl-extractable As contents were significantly

corre-lated with total C and N at a level of 1%, and with Feo

and total S at a level of 5% (Table2) In the HL bore

core, the total As contents were significantly

corre-lated with Fed, P, clay, silt and sand contents (p≤0.01)

HCl-extractable As contents showed significant

cor-relations with Feo, total C, S at a level of 1%, and

with total N at a level of 5% (Table3)

Levels of As in river water and sediments River water

The level of As in the river water ranged from <5 to

13μg l−1(Fig.8) The highest As concentration was observed in the sample W6 from the Hop Ly area, but the levels decreased downstream along the Chau Giang River In the Red River, no unambiguous trends were observed along the stream The differ-ences in As concentration between the two river branches were not significant (paired t-test, p≤0.05) Sediments

The content of As in the river sediments ranged from 15.2 to 92.1 mg kg−1 (average, 47.3 mg kg−1)

Legend

500 m

< 10

10 -100

100 -300

> 300 Arsenic (µg L -1 )

river, lake, canal

commune border bore core sampling location

paddy field Legend

Fig 3 As concentrations

in the groundwater in Hop

Ly Further details as in

Fig 2

(Fig 9) The highest accumulation of As (92.1 mg

kg−1) was found in the S5 sample from the

intersec-tion of the two rivers Contrary to the river water data, the lowest sediment content of As (15.2 mg kg−1) was

Table 1 General chemical properties of the groundwater

DO (mg l−1) 0.41 to 1.30 0.83 0.25 0.35 to 1.46 0.86 0.30 0.38 to 1.76 0.93 0.49 DOC (mg l−1) 2.97 to 18.9 9.32 5.14 0.09 to 4.62 2.15 1.26 0.43 to 12.9 2.81 4.25

Fe (mg l−1) 8.55 to 41.6 20.6 10.3 4.37 to 34.5 13.8 10.4 1.17 to 26.9 9.26 10.1

Mn (mg l−1) nd to 1.93 0.32 0.55 nd to 1.79 0.57 0.48 0.00 to 2.82 1.26 1.08

Cl−(mg l−1) 24.2 to 2,980 924 911 9.16 to 69.8 35.7 18.3 6.70 to 1,310 552 410

PO4−(mg l−1) 0.44 to 9.09 1.81 2.48 0.20 to 2.52 0.90 0.66 0.10 to 0.65 0.31 0.22

NH4 (mg l−1) 9.40 to 76.0 46.2 22.0 nd to 18.3 4.75 7.00 nd to 48.4 14.8 14.8

Na+(mg l−1) 17.1 to 816 406 287 9.03 to 38.7 19.8 8.14 14.7 to 567 194 171

K + (mg l−1)` 8.45 to 36.4 16.2 10.0 2.23 to 17.0 5.34 3.94 4.28 to 20.6 8.31 5.17

Mg 2+ (mg l−1) 25.7 to 93.5 62.1 23.8 20.8 to 38.4 28.8 5.04 28.6 to 58.8 41.8 12.8

Ca2+(mg l−1) 76.5 to 175 121 33.3 89.0 to 132 102 11.8 37.9 to 109 58.6 26.3

nd none detected (<0.1 mg l−1 for Mn, <0.01 mg l−1 for NO3−, <0.03 mg l−1 for SO4−, <0.2 mg l−1 for NH4 )

< 10

10 -100

100 -300

> 300

Arsenic (µg L -1 )

500 m

river, lake, canal commune border

paddy field Legend

Fig 4 As concentrations in

the groundwater in Chan Ly.

Further details as in Fig 2

observed in the S6 sample from the Hop Ly area,

locating at the upper Chau Giang River In the Red

River, the S3 sample from the Chan Ly area contained

the lowest As level (39.8 mg kg−1) Statistically, the

average content of As in the Red River sediments was

significantly higher than that in the Chau Giang River

sediments (paired t-test, p≤0.05)

Discussion Arsenic concentration in groundwater The present results lead to similar conclusions as a previous study on groundwater in Hanam Province (Nguyen et al 2009): the groundwater in the studied

Medium to coarse

grained sand

Fine grained sand

Muddy clay

Clay

Silty clay

Silty sand

Shell or snail

Organic matter

Tamm extractable Fe (g kg -1 )

P S

P (g kg -1 )

S (g kg -1 )

C (%)

N (%)

C

0 10 20 30 40 50 60

Clay/Silt (%)

Sand (%)

Clay Silt Sand

0

-4

-8

-12

-16

-20

HCl extractable As (mg kg -1 )

Fig 6 Description of a the Xuan Khe bore core and b the geochemistry of bore core sediments

-1 )

0

200

400

600

800

r = 0.68**

0 200 400 600 800

r = 0.43*

0

200

400

600

800

r = 0.46**

r = - 0.55**

0 200 400 600 800

-1 )

-1 )

-1 )

(d) (c)

Fig 5 Relation between As concentration and a Fe, b HCO3−, c pH, c Eh in groundwater

area is seriously contaminated with As, Fe, Mn

and NH4 The concentration of Fe in all samples

exceeded the Vietnamese standard limit of 0.5 mg l−1

for drinking water (Ministry of Science, Technology

and Environment2002) As concentrations (average,

178 mg kg−1) in the majority (88%) of the

groundwater samples exceeded the WHO guideline

as well as the Vietnamese standard limit for drinking

water (10 μg l−1) Similar levels of As in

ground-water (159μg l−1in average) were reported from the

Hanoi area, where 72% of the tube wells contained

As levels higher than 10 μg l−1 (Berg et al 2001)

Comparable levels of As contamination were observed

in Bangladesh, India, and Taiwan (Chowdhury et al

2000; Nath et al.2008; Wang et al.2007) We detected much lower As levels in the groundwater at sites close

to the Red River than at sites located on the banks of the Chau Giang River; this has also been observed by Nguyen et al (2009)

Sixty eight and 32% of the samples, respectively, contained NH4 and Mn concentrations above the Vietnamese standard limit for drinking water (4.0 and 0.5 mg l−1, respectively) The WHO guidelines for

Mn concentrations in drinking water is 0.4 mg l−1, and the threshold of NH4 in water is 1.5 mg l−1 (WHO 2008)

The high level of NH4 , low Eh and DO values, negligible levels of NO3 − and SO4 −, and the

Table 2 Correlation between As and other chemical parameters in Xuan Khe bore core sediments

Total As

HCl-extractable As

HCl-extractable As

Silt 0.842** −0.016 0.957** 0.514* −0.079 0.526* 0.907** −0.100 0.978** 1

Sand −0.861** −0.038 −0.963** −0.536* 0.035 −0.561* −0.926** 0.072 −0.982** −0.997** 1

*p ≤0.05; **p≤0.01

Medium to coarse

grained sand

Fine grained sand

Muddy clay

Clay

Silty clay

Silty sand

Shell or snail

Organic matter

0 2 4 6 8 10 12 14

Fe Fe

Tamm extractable Fe (g kg -1 )

C N

N (%)

C (%) 0.0 0.2 0.4 0.6 0.8 1.0

S (g kg -1 )

P (g kg -1 )

Sand (%)

Clay/Silt (%)

Clay Silt Sand

0

-4

-8

-12

-16

-20

Total As (mg kg -1 )

HCl extractable As (mg kg -1 )

Fig 7 Description of a the Hop Ly bore core and b the geochemistry of bore core sediments

dominance of As(III) represented typical

character-istics of groundwater under reductive conditions

Anoxic conditions of groundwater were also observed

in Hanoi and some areas of the Red River Delta (Berg

et al.2001, 2007; Postma et al.2007) On the other

hand, compared to the data obtained in this study,

higher levels of sulfate and slightly lower Fe

concen-trations in the groundwater were reported from the Mekong Delta, southern Vietnam, where acid, sulfate-rich soils are abundant (Nguyen and Itoi 2009) Moreover, the chemical features of the groundwater observed in the present study are quite similar to those in Bangladesh and West Bengal, India (Nickson

et al.2000; Anawar et al.2003; Nath et al.2008)

0 1 2 km

W6

W10

W7 W8

W9

W11

W5

W4

W3 W2

W1

Chan Ly

Xuan Khe

Hop Ly

15

0 5 10 (As µg L -1 )

river, canal commune border Legend

Fig 8 Distribution of As

in river water Bars in the

map indicate As contents

Table 3 Correlation between As and other chemical parameters in Hop Ly bore core sediments

Total As

HCl-extractable As

HCl-extractable As

−0.213 1

Silt 0.857** −0.113 0.973** −0.538 0.176 0.548 0.954** −0.042 0.993** 1

Sand −0.847** 0.077 −0.977** 0.504 −0.225 −0.589 −0.955** −0.008 −0.997** −0.998** 1

*p ≤0.05; **p≤0.01