Arsenic contamination in groundwater and its possible sources in Hanam, Vietnam

Arsenic contamination in groundwater and its possiblesources 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 thegroundwater, As concentration showed significantcorrelations with the total concentrations of Fe andHCO3

−. Significant correlations between HCl-extractable As and non-crystalline Fe oxide, total C,N, and S were also observed in the profiles. Theresults support the hypothesis that under favorablereductive conditions established by the degradation oforganic matter, the dissolution of iron (hydr)oxidesreleases adsorbed As into the groundwater. Thedeposition of As in the sediments from the Red Riverwere significantly higher than that in the Chau GiangRiver, suggesting that the Red River is the mainsource of As-containing substances deposited in thestudy area.

Keywords Arsenic . Bore core . Groundwater . River .

Sediment . Vietnam

Introduction

Arsenic is unique among the heavy metalloids andoxyanion-forming elements (e.g., As, Se, Mo) in itssusceptibility to mobilization under the pH conditionstypically found in groundwater (pH=6.5–8.5) and overa wide range of redox conditions (Hossain 2006). Tensof millions of people in South and Southeast Asiaroutinely consume groundwater that has unsafe Aslevels (Smith et al. 2000; Chowdhury et al. 2000; Berget al. 2001, 2007; Hossain 2006). As a main water

Environ Monit Assess (2012) 184:4501–4515DOI 10.1007/s10661-011-2281-6

N. M. Phuong : C. N. KienUnited Graduate School of Agricultural Sciences,Ehime University,Matsuyama 790-8566, Japan

Y. Kang :K. Sakurai :M. SugiharaFaculty of Agriculture, Kochi University,Monobe, Nankoku,Kochi 783-8502, Japan

N. M. Phuong (*) :N. D. Bang :H. M. NgocFaculty of Chemistry, Hanoi University of Science,Hanoi, Vietname-mail: [email protected]

Abstract This study investigated the arsenic (As)level in groundwater, and the characteristics of aquifersediment as related to the occurrence of As ingroundwater in Hanam, Vietnam. The deposition andtransport of As-containing substances through riverswere also examined. Arsenic concentrations in 88%of the groundwater samples exceeded the As limit fordrinking water based on the WHO standards. Thedominating form of arsenic was As(III). The maxi-mum total As content in bore core sediment wasfound in a peat horizon of the profiles and generally,elevated levels of As were also found in other organicmatter-rich horizons. Total As contents of the borecore sediments were significantly correlated withcrystalline iron oxide, silt and clay contents, suggest-ing that As in aquifer sediment was mainly associated

source for local communities, groundwater has beenexploited in Vietnam since the 1900s. The firstpublication on As contaminations in groundwater ofHanoi, Vietnam, in 2001 reported contamination levelsfrom 1 to 3,050 μg l−1 (average 159 μg l−1) (Berg et al.2001). Such elevated As concentrations were found innumerous regions throughout Vietnam (Berg et al.2001; Chander et al. 2004; Agusa et al. 2006; Nguyenet al. 2009). A random survey of As levels in tube wellwater from 12 Vietnamese provinces indicated thatHanam is one of the most seriously As-contaminatedarea in the Red River Delta. In this area, Asconcentrations exceeded the WHO guideline for Asin drinking water (10 μg l−1) (Chander et al. 2004) in52% of the tube wells surveyed.

Arsenic-bearing groundwater in Vietnam has beennoted because of the geological similarity with theGanges–Brahmaputra, Mekong, and Red River basinswhich are built up with alluvium from the rapidlyweathering Himalayas and are characterized bycomplex lithological structures of the aquifers whichdo not show a full separation between upper andlower aquifers (Laurent and David 2006). Someresearchers have argued that oxidation of As-richsulfide minerals is one possible mechanism for therelease of As into groundwater. Others have suggestedthat reductive dissolution of iron oxyhydroxides orarsenate sorbed by detrital organic carbon is anotherpossible mechanism of As mobilization (Nickson etal. 1998; Smedley and Kinniburgh 2002). However,the dissolution of iron oxide is regarded the primaryprocess responsible for high As concentrations in thegroundwater in some areas. Arsenic is naturallyderived from eroded Himalayan sediments, and isbelieved to become mobile following reductiverelease from solid phases under anaerobic conditions(Polizzotto et al. 2008). A study of the hydrologicaland sedimentary conditions of river bank deposits inthe Hanoi area indicated that elevated groundwaterlevels of As are caused by reductive dissolution underiron-reducing conditions (Berg et al. 2008).

Hanam Province with a total area of 849.5 km2 anda population of 820,100 is a productive agriculturalregion located in the lower part of the Red RiverDelta. The topography is dominated by limestonemountains, hills, and forests with some sloping areasin the west (10–15% of the total area), whereas theeast is a plain that mainly consists of alluvium fromthe Red River (85–90% of the total area). About 38.6

km of the Red River form the eastern border of theprovince. The Red River plays an important role inthe fertility and irrigation of the roughly 10,000 ha ofagricultural land. However, there is little informationon the characteristics and degree of As contamination,and the causes of As release to the groundwater in thisarea. In this study, we examined As concentrations ingroundwater and the geochemical parameters ofaquifer sediment related to the occurrence of As inthe groundwater. Our study area in the Lynhan districtof Hanam Province represents alluvium from tworivers, the Red River and the Chau Giang River.Therefore, we also investigated the deposition andtransport 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 theLynhan district, Hanam Province, in November 2006(dry season) (Fig. 1). Hop Ly and Xuan Khe arelocated near the Chau Giang River, while Chan Ly islocated 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 tosampling, water from tube wells was flushed awayuntil crystal clear water was obtained (Berg et al.2001). Immediately after collection, pH, electricalconductivity (EC), oxidation–reduction potential (Eh),and dissolved oxygen (DO) were measured. Thesamples were passed through small disposable ionexchange cartridges packed with 2.5 g selectivealuminosilicate adsorbent (Metalsoft Center, HighlandPark, NJ; Meng and Wang 1998). This adsorbentretained As(V) but not As(III). The filtrates then wereacidified with 1% (volume) concentrated HCl for As(III) analysis. The cartridges have been widely used inthe field to separate As(V) from As(III) in watersamples because of their convenience and reliability.The average recovery of As(III) in the filtrates was98% (Meng and Wang 1998). Water samples for theanalysis of total As, Fe, and Mn were acidified with 1ml concentrated HCl acid and preserved in 100-mlpolypropylene bottles. For major ions analysis,polypropylene bottles were filled completely with

4502 Environ Monit Assess (2012) 184:4501–4515

sampled water, all bubbles were removed, and thebottles were tightly capped. A set of 50-ml sampleswas used to determine HCO3

− in the laboratory (seebelow for details). Another set of 50-ml samples werefiltered through 0.45-μm membrane filters to removesuspended organic matter and acidified to pH<2 withconcentrated HCl for DOC analysis conductedaccording to Standard Methods 5310 (see below fordetails). All water samples were kept at 4°C untilanalysis.

To clarify the origin of As contaminations in tubewell water, bore cores were obtained in the XK andHL communes to depths of about 20 m, the commondepth of household tube wells in the study area. Thepre-survey was conducted to select the location of thebore cores. The locations of the bores were selectedbased on first, the As levels we had examined in 15tube wells using the Hach As test kit (the data is notshown), and second, on observations of dark peathorizons made by local people when they drilled theirwells. Samples from the same, clearly differentiatedhorizon were combined for analysis. Water sampleswere collected from the bore holes after 1 h pumping.

In addition, sediment and water samples from 5points along the Red River and 6 points along theChau Giang River were sampled. The sediments wereair-dried, ground with a ceramic pestle, passedthrough a 2.0-mm sieve, and stored in plastic bottlesuntil analysis. The water samples were filteredthrough 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/pHmeter (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 aportable DO meter (YSI 55, YSI, USA). In thelaboratory, water samples were analyzed for totalconcentration of As using an inductively coupledplasma atomic emission spectrometer (ICP-AES;ICPS-1000 IV, Shimadzu, Kyoto, Japan) equippedwith 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 theprecision of the measurement, reference standardsolution with a known concentration of each mea-sured element, which was prepared from the differentsource of the stock standard solution used forcalibration standard, were used as a control sample.After every ten samples during analysis, the controlsample was analyzed to check the accuracy ofanalysis. All samples were measured at least twotimes in order to assess the repeatability of themeasurement. Samples were reanalyzed if the errorof the control sample exceeded 10% or the relativestandard deviation of the measurement exceeded 5%.Dilution was made with 2% nitric acid, when theconcentration of the sample was over the upper

Chan LyChan Ly

XuanKheXuanKhe

Hop LyHop Ly

Chau Giang River

106o00' 106o05' 106o10' 106o15' 106o20'

20o50'

20o55'

20o60'

0 2 km

Hanoi

Vietnam

Hanam

Lynhan

river, canal

commune border

Legend

Fig. 1 The study area, LyNhan district, Ha Namprovince, Vietnam. Furtherdetails of the location ofsampling sites (bore core,groundwater, river water,and river sediment sampledsite) are as in Figs. 2, 3, 4,8, and 9

Environ Monit Assess (2012) 184:4501–4515 4503


limitation of the standard range. HCO3− was mea-

sured by titration method using methyl orange andbromcresol green indicators, and DOC was analyzedwith a TOC analyzer (TOC-VCPH/TNM; Shimadzu).The concentrations of Cl−, NO3

−, SO42−, PO4

3−,NH4

+, Na+, K+, Mg2+, and Ca2+ ions were determinedby ion chromatography (IA −300, DKK-TOA, Japan).

Sediment

For the analysis of total As, P, and S contents, a 0.15-gsoil sample was digested at 100°C in a Teflon vesselcontaining a mixture of 2 ml 60% HClO4, 3 ml conc.HNO3, 5 ml concentrated HF, and 2 ml of a 20 gl–1

KMnO4 solution. If the purple color of the KMnO4 haddisappeared after 20 min of heating, 1 ml of theKMnO4 solution was added, and this procedure wasrepeated until the mixture remained colored (Terashima1984). The concentrations of As in the digests weredetermined by using an ICP-AES (ICPS-1000 IV;Shimadzu) equipped with HVG-1 (Shimadzu). Forthe determination of P and S, the ICP-AES system wasused. The standard reference materials (JSO-1 andJSO-2 from the Geological Survey of Japan) were usedto verify the accuracy of As determination. Therecovery rates of As were within 95–105%. Bore coresediments were extracted with 1 M HCl over 30 min todetermine HCl-extractable As. Physicochemical prop-erties of the bore core sediments including particle sizedistribution, total carbon (TC), total nitrogen (TN),dithionite–citrate–bicarbonate (DCB)-extractable andammonium oxalate-extractable Fe oxides and hydrox-ides (Fed and Feo, respectively) were examined by themethods described by Phuong et al. (2008).

Results

Chemistry of groundwater

Arsenic concentrations in the groundwater samplesranged from <5 to 703 μg l−1 (178±170 μg l−1); thegeographic distribution of As in the three communesis shown in Figs. 2, 3, and 4. The average Asconcentrations in the groundwater of HL, XK and CLwere 196, 256, and 43 μg l−1, respectively; the valuefor CL was significantly lower than those for HL andXK. On average, about 76% of the total As in thegroundwater existed in the As(III) form.

The groundwater was characterized by a neutralpH and high EC (Table 1). The low Eh values (−157to 11.0 mV) demonstrated the reducing nature of theaquifer (Table 1). Concentrations of total Fe in thewater samples ranged from 1.17 to 41.6 mg l−1

(average, 15.0 mg l−1). The total Mn concentrationvaried from <0.1 to 2.82 mg l−1 (average,0.66 mg l−1). A wide range of NH4

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

−

and SO42− in most samples were lower than the

detection limit. Except for one sample, the concen-trations of PO4

3− were lower than 2.4 mg l−1. The DOvalues were lower than 1.76 mg l−1. Major ioncomposition 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 averageconcentrations of HCO3

− and Ca2+ in the groundwaterof CL were significantly lower, and the averagevalues of Eh and total Mn were significantly higherthan at the other two sites. Compared to HL and CL,significantly higher levels of EC, DOC, NH4

+, K+ andMg2+ were observed in XK. Concentrations of Cl−

and Na+ were significantly lower in HL than in XKand CL. Furthermore, the concentration of Ascorrelated 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 correlatedwith 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, muddyclay and silty clay layers were observed from thesurface horizon to 4.7 m. A sequence of grey or darkgrey silty sand and fine grained sand were collectedfrom 4.7 to 20 m, interrupted by some plant remainsand 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 siltysand and fine grained sand layers were observed to adepth of 23 m. A thin and dark grey peat layerenriched with plant residuals and organic matter wascollected at 6.6–7.0 m depth. A lot of shells and snailswere 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 andHL bore cores ranged from 5.51 to 20.1 and from7.37 to 25.1 mg kg−1, respectively. In the XK profile,elevated levels of total As were detected in clay layersfrom the surface to 3.7 m, at 4.0–4.7 m, and in ahorizon containing plant residuals (14.0–14.8 m). Onthe other hand, the highest total As content (25.1 mgkg−1) in the HL profile was found in a peat horizon(6.6–7.0 m) (Figs. 6b and 7b).

In the XK profile, high proportions of HCl-extractable As were observed in the layers containingplant residuals or organic matters (Fig. 6b). A highproportion of HCl-extractable As was detected in apeat horizon of the HL profile (Fig. 7b).

In XK profile, the distribution of Fed showedsimilar trends as the total As content throughout theXK profile (Fig. 6b). Except for the surface horizon

(0–1 m), the distribution of P and Feo roughlyparalleled the total As in the profile (Fig. 6b). Inparts of the profile, total As also correlated withHCl-extractable As (6.8–20 m), total C (6.8–20 m),and clay (0–12 m) (Fig. 6). Furthermore, thedistribution of HCl-extractable As and total Scontent were quite similar throughout the profile(Fig. 6b).

In HL profile, the P distribution in the HL profileresembled that of total As contents (Fig. 7b). Exceptfor 0–3 m depth, the distributions of HCl-extractableAs, total S, C, and N contents were similar to that oftotal As (Fig. 7b). At 8–22 m, a correlation betweenclay 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 totalP, S, C and N contents were detected in a peathorizon, where the highest total and HCl-extractableAs contents were detected.

Legend

ChauGiangRiver

river, lake, canal

paddy field

commune border

bore core sampling location

< 1010 -100100 -300> 300

Arsenic (µµg L-1)

500 m

Legend

Fig. 2 As concentrationsin the groundwater in XuanKhe. Filled star, bore core;empty circle, As levellower than 10 μg l−1;shaded circle, As level 10–100 μg l−1; diagonallystriped circle, As level 100–300 μg l−1; filled circle, Aslevel greater than 300 μg l−1


Results of the correlation analysis between As andother parameters of the XK and HL bore coresediments (except for the peat horizon) are shown inTables 2 and 3. In the XK bore core, the total Ascontents of the sediments were positively correlatedwith Fed, N, P, clay, silt contents, and were correlatednegatively with sand contents (p≤0.01). A significantcorrelation at a level of 5% was also obtained betweenthe total As and Feo contents. On the other hand, theHCl-extractable As contents were significantly corre-lated with total C and N at a level of 1%, and with Feoand total S at a level of 5% (Table 2). In the HL borecore, 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%, andwith total N at a level of 5% (Table 3).

Levels of As in river water and sediments

River water

The level of As in the river water ranged from <5 to13 μg l−1 (Fig. 8). The highest As concentration wasobserved in the sample W6 from the Hop Ly area, butthe levels decreased downstream along the ChauGiang River. In the Red River, no unambiguoustrends were observed along the stream. The differ-ences in As concentration between the two riverbranches were not significant (paired t-test, p≤0.05).

Sediments

The content of As in the river sediments ranged from15.2 to 92.1 mg kg−1 (average, 47.3 mg kg−1)

Legend

500 m

< 1010 -100100 -300> 300

Arsenic (µg L-1)

river, lake, canal

commune borderbore core sampling location

paddy field

Legend

Fig. 3 As concentrationsin the groundwater in HopLy. Further details as inFig. 2


(Fig. 9). The highest accumulation of As (92.1 mgkg−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

Water compositions Xuan Khe (n=11) Hop Ly (n=12) Chan Ly (n=8)

Range Mean SD Range Mean SD Range Mean SD

pH 6.37 to 7.44 6.83 0.30 6.64 to 7.06 6.84 0.15 6.32 to 7.35 6.72 0.32

Eh (mV) −154 to −102 −134 15.4 −153 to −101 −135 16.1 −157 to 11.0 −68.0 68.7

EC (mS m−1) 94.6 to 596 342 196 72.8 to 108 90.7 11.9 87.5 to 435 198 108

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

HCO3− (mg l−1) 211 to 980 541 257 459 to 683 578 73.5 56.1 to 564 225 211

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

NO3− (mg l−1) nd to 8.11 1.21 2.33 nd to 8.13 1.96 3.06 nd nd –

SO42− (mg l−1) nd to 0.08 0.01 0.03 nd to 3.5 0.73 1.20 nd nd –

PO43− (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

Mg2+ (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

2− , <0.2 mg l−1 for NH4+ )

< 1010 -100100 -300> 300

Arsenic (µg L-1)

500 m

river, lake, canal

commune border

paddy field

LegendLegend

Fig. 4 As concentrations inthe 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 RedRiver, the S3 sample from the Chan Ly area containedthe lowest As level (39.8 mg kg−1). Statistically, theaverage content of As in the Red River sediments wassignificantly higher than that in the Chau Giang Riversediments (paired t-test, p≤0.05).

Discussion

Arsenic concentration in groundwater

The present results lead to similar conclusions as aprevious study on groundwater in Hanam Province(Nguyen et al. 2009): the groundwater in the studied

(a) (b)

Medium to coarse grained sand

Fine grained sand

Muddy clay

Clay

Silty clay

Silty sand

Shell or snail

Organic matter

0 5 10 15 20 25

Tamm extractable Fe(g kg-1)

0 3 6 9 12 15

DCB extractable Fe (g kg-1)

Fed Feo

0.0 0.2 0.4 0.6 0.8

0 1 2 3 4

P S

P (g kg-1)

S (g kg-1)

0 1 2 3 4 5

0.00 0.06 0.12

C (%)

N (%)

CN

0 10 20 30 40 50 60

0 20 40 60 80 100

Clay/Silt (%)

Sand (%)

ClaySiltSand

0 10 20 300

-4

-8

-12

-16

-20

Total As (mg kg-1)

0 3 6 9 12 15HCl extractable As

(mg kg-1)

To

tal A

sH

Cle

x. A

sDep

th (

m)

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

Tota

l As

(mg

L-1

)

0

200

400

600

800

0 10 20 30 40 50

r = 0.68**

0

200

400

600

800

0 200 400 600 800 1,000

Total Fe (mg L-1) HCO3- (mg L-1)

r = 0.43*

0

200

400

600

800

6 6.4 6.8 7.2 7.6 8

r = 0.46**r = - 0.55**

0

200

400

600

800

-160 -120 -80 -40 0 40

pH Eh (mV)

Tota

l As

(mg

L-1

)

Tota

l As

(mg

L-1

)

Tota

l As

(mg

L-1

)

(a) (b)

(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, Mnand NH4

+. The concentration of Fe in all samplesexceeded the Vietnamese standard limit of 0.5 mg l−1

for drinking water (Ministry of Science, Technologyand Environment 2002). As concentrations (average,178 mg kg−1) in the majority (88%) of thegroundwater samples exceeded the WHO guidelineas well as the Vietnamese standard limit for drinkingwater (10 μg l−1). Similar levels of As in ground-water (159 μg l−1 in average) were reported from theHanoi area, where 72% of the tube wells containedAs levels higher than 10 μg l−1 (Berg et al. 2001).Comparable levels of As contamination were observedin Bangladesh, India, and Taiwan (Chowdhury et al.

2000; Nath et al. 2008; Wang et al. 2007). We detectedmuch lower As levels in the groundwater at sites closeto the Red River than at sites located on the banks ofthe Chau Giang River; this has also been observed byNguyen et al. (2009).

Sixty eight and 32% of the samples, respectively,contained NH4

+ and Mn concentrations above theVietnamese standard limit for drinking water (4.0 and0.5 mg l−1, respectively). The WHO guidelines forMn 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 SO42−, and the

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

Total As HCl-extractable As

Fed Feo C N P S Clay Silt Sand

Total As 1

HCl-extractable As

0.320 1

Fed 0.873** 0.114 1

Feo 0.514* 0.536* 0.514* 1

C 0.238 0.611** 0.010 0.383 1

N 0.642** 0.698** 0.637** 0.680** 0.439 1

P 0.874** 0.264 0.929** 0.580* 0.151 0.595* 1

S 0.230 0.562* −0.021 0.435 0.834** 0.516* −0.014 1

Clay 0.816** 0.005 0.941** 0.473 −0.172 0.508* 0.903** −0.208 1

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

(a) (b)

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

0 2 4 6 8 10

DCB extractable Fe (g kg-1)

Fed Feo

Tamm extractable Fe(g kg-1)

0 2 4 6 8 10

0.0 0.1 0.2

CN

N (%)

C (%)0.0 0.2 0.4 0.6 0.8 1.0

0 1 2 3 4 5S (g kg-1)

P (g kg-1)

P S

0 10 20 30 40

0 20 40 60 80 100Sand (%)

Clay/Silt (%)

ClaySiltSand

0 10 20 300

-4

-8

-12

-16

-20

0 3 6 9 12 15

Total As (mg kg-1)

To

tal A

sH

Cle

x. A

s

HCl extractable As(mg kg-1)

Dep

th (

m)

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 observedin Hanoi and some areas of the Red River Delta (Berget al. 2001, 2007; Postma et al. 2007). On the otherhand, compared to the data obtained in this study,higher levels of sulfate and slightly lower Fe concen-

trations in the groundwater were reported from theMekong Delta, southern Vietnam, where acid, sulfate-rich soils are abundant (Nguyen and Itoi 2009).Moreover, the chemical features of the groundwaterobserved in the present study are quite similar tothose in Bangladesh and West Bengal, India (Nicksonet al. 2000; Anawar et al. 2003; Nath et al. 2008).

0 1 2 km

W6

W10

W7

W8

W9

W11

W5

W4

W3W2

W1

Chan Ly

Xuan Khe

Hop Ly

15

05

10

(As µg L-1)

river, canal

commune border

Legend

Fig. 8 Distribution of Asin river water. Bars in themap indicate As contents

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

Total As HCl-extractable As

Fed Feo C N P S Clay Silt Sand

Total As 1

HCl-extractable As

−0.213 1

Fed 0.879** −0.094 1

Feo −0.525 0.832** −0.504 1

C 0.108 0.824** 0.255 0.632* 1

N 0.413 0.682* 0.588 0.305 0.892** 1

P 0.888** −0.210 0.926** −0.508 0.158 0.520 1

S −0.098 0.806** 0.017 0.736** 0.955** 0.762** −0.019 1

Clay 0.825** −0.083 0.974** −0.504 0.233 0.599 0.956** 0.018 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


0 1 2 km

Chan Ly

Hop Ly

(As mg kg-1)

0

20

4060

80

100

S6

S10

S7S8

S9

S11

S5

S4

S3

S2

S1

river, canal

commune border

Legend

Fig. 9 Distribution of As inriver sediments. Bars in themap indicate As contents


Occurrence of As in aquifer sediments

The contents of As in the solid phase of the XK andHL sediment profiles were higher than the average Ascontent of sediments (7.7 mg kg−1) reported byBowen (1979), and were comparable with the Ascontent (2–20 mg kg−1) in alluvial sediments fromAs-contaminated regions in Bangladesh (Hossain2006). Similar degrees of As enrichment (0.85–37.7mg As kg−1) were observed in vertical profiles ofdown to 36 depth of the Ganges and Meghna floodplains (Tareq et al. 2003). However, the As content inthe sediments of the XK and HL bore cores (5.51–25.1mg kg−1) was higher than in the groundwater ofcontaminated areas of Bangladesh (0.08–12.8 mg kg−1)(Anawar et al. 2003). Swartz et al. (2004) andHorneman et al. (2004) also reported lower total Aslevels (≤10 mg kg−1) in bore core sediments from areasinfluenced by As-contaminated groundwater in Ban-gladesh. A slightly higher total As content (4–45 mgkg−1) was found in core samples taken in the MekongDelta, Vietnam (Nguyen and Itoi 2009).

The highest total As content (25.1 mg kg−1) in theHL profile was found in a peat horizon (6.6–7.0 m),which also contained the maximum amounts of C(9.4%) and N (0.2%). Similar to these results, Root etal. (2005) found the highest As content (21 mg kg−1) in

an organic matter-rich horizon at a depth of 262 m atan As-contaminated site in Wisconsin, USA. Meharg etal. (2006) also discussed the co-deposition of organiccarbon and As in Bengal Delta aquifers. Peat wasfound extensively in As-affected areas in south andsouthwestern Bangladesh at depths of about 10 m(Ishiga et al. 2000). Sediments containing 6.0 and7.8% total organic carbon have been reported fromdepths of 2.1 m at Gopalganj (southwestern Dhaka)and 23 m at Tepakhola (Faridpur municipality, Bangla-desh) (Nickson et al. 1998; McArthur et al. 2004).

Possible sources of As contamination in groundwater

Oxidation of As-rich sulfide minerals; reductivedissolution of As-rich iron oxyhydroxides; and ex-change of adsorbed As with other competitive anions(phosphate, bicarbonate, and silicate) are supposed tobe main processes releasing As into groundwater(Nickson et al. 2000; Root et al. 2005).

In our study, no relation between the contents oftotal As and S could be found throughout the borecores. In addition, the low concentrations of SO4

2− aswell as the negative correlation between concentra-tions of As and Eh values in the groundwaterindicated that oxidation of As-rich sulfide mineralsmight not occur in the study area.

On the other hand, the similarities in the distribu-tions of total As and Fe oxides in the XK and HLsediment profiles suggested that As in the solid phasewas strongly adsorbed by iron (hydr)oxide. Given theanaerobic condition of the studied groundwater, thegood correlations between As concentrations and thelevels of Fe, HCO3

−, and pH values are someevidences supporting the hypothesis that the dissolu-tion of arseniferous iron oxyhydroxide may occurredand releases the As sorbed on Fe oxyhydroxide(Ahmed et al. 2004), according to:

4FeOOHþ CH2Oþ 7H2CO3 ¼ 4Fe2þ þ 8HCO3� þ 6H2O

The hypothesis that As is released to groundwaterthrough the reduction of arseniferous iron oxyhydr-oxides under anoxic conditions has been widelyaccepted for Bangladesh and West Bengal (Nicksonet al. 1998, 2000; Acharyya et al. 1999; Harvey et al.2002; Anawar et al. 2003). Some evidence for theassociation of As with iron oxyhydroxides in aquifersediments was also found in certain areas of the RedRiver Delta (Berg et al. 2001, 2008; Postma et al.2007).

Reduction of (hydr)oxides is often coupled tomicrobial oxidation of organic matter (Nickson et al.2000). In our study, the good correlations between thecontent HCl-extractable As and total C, N, S and Feowere observed, indicating the importance of organicmatter in mobilization of As. The total S significantlycorrelated with total C and N in HL and XK bore coresediments, and noncrystalline Fe oxides (Feo) in HLbore core sediments, suggesting that the occurrence ofS in the solid phases may not in association withmineral lattice. In addition, similar to the abundanceof total S in the organic matter-rich solid aquifers ofsedimentary basins of West Bengal (McArthur et al.2004), we observed the highest accumulation of totalS in the peat horizon of the HL bore core. Reductionof SO4

2− is supposed to be driven by microbialmetabolism of organic matter. Therefore, S is almostabsent from aquifer sands but is relatively abundant inhorizons that contain organic matter, where SO4

2− isreduced and early diagenetic Fe sulfides are formed(McArthur et al. 2004). The high proportion of HCl-extractable As, which accounted for 38% of the totalAs content was detected in the peat horizon; andrelatively high As contents were also found in someorganic matter-rich horizons of the XK and HL bore

cores. The results suggested that upon burial of thesediment, the degradation of organic matter providedthe reducing conditions that enhance the mobility ofAs. Therefore, these organic matter-rich horizons arepossible sources of As.

Beside the incorporation between As and Feoxides, the good correlations between total As andclay or silt in the XK and HL sediment profilesindicated a strong occlusion of As in fine silt and clayparticles. Clay mineral particles tend to adsorb Asbecause of the oxide-like character of their edges(Smedley and Kinniburgh 2002). Desorption of Asmay occur via reductive mechanisms or competitionfrom other species (phosphate, bicarbonate, silicate)for adsorption sites on mineral surfaces. The similar-ity in chemical behavior of As and P became evidentby their similar distributions in the sediment profilesand the correlation data. Hence, mobilization of Asfrom the aquifer sediments to the groundwater mightbe controlled by the competition for the adsorptionsites between P and As.

Taken together, the current study supports theprevious hypothesis that As is adsorbed by iron(hydr)oxides, and that the dissolution of iron (hydr)oxides and the release of As to the groundwater isaccelerated under favorable reductive conditionsestablished by the degradation of organic matterunder the reductive conditions established by thedegradation of organic matter (Meharg et al. 2006).

Transport of As-containing materials through surfacestreams

Arsenic concentrations in river water vary accordingto the composition of the surface recharge, thecontribution from base flow, and the bedrock lithol-ogy. However, As concentration in river water iscommonly low (<8 μg l−1) (Smedley and Kinniburgh2002), and our results were in agreement with thisfigure. The concentration of toxic elements in riverwater depends on the adsorption affinity between thetoxic elements and the sediments. If the adsorptionaffinity is strong, the self-purification capability of theriver water is enhanced (Chunguo and Zihui 1988).

Studying the distribution of As in surface sedi-ments is important for understanding the depositionand transportation of this pollutant in aquatic environ-ments. Sediment-bound As most probably originatesfrom erosion and weathering processes, which result


in the enrichment of As on ferric oxyhydroxidesfollowed by fluvial transport and sedimentation(Welch et al. 1988; Bowell 1994). The Red Riverdelta, located on the west coast of the Gulf of Tokin,is one of the largest deltas in Southeast Asia. Initiallythe delta was located in the vicinity of Hanoi, but itsubsequently expanded to reach its present area ofapproximately 10,300 km2, mainly as a result ofsediment supply from the Red River (Tanabe et al.2003, 2006). The total sediment discharge and waterdischarge of the Red River system is 100–130 millionton year−1 and 120 km3 year−1, respectively, and theaverage sediment concentration of the river is 0.83–1.08 kg m−3 (Tanabe et al. 2003). According toLaurent and David (2006), the Ganges–Meghna–Brahmaputra delta, the Mekong basin, and the RedRiver basin all are part of the drainage system of therapidly weathering Himalayas whose sulfide rockscontains up to 0.8% As (Acharyya et al. 2000).Hanam Province is located in the lower part of theRed River system, and thus receives huge amounts ofsuspended solids carried by the Red River and othersmaller rivers. The suspended solids mostly originatefrom erosion and weathering processes of parentrocks from upstream regions including the Himalayas.The load of As and other metal oxide contained in thesuspended solids results in fluvial transport anddownstream sedimentation of As-enriched metalhydroxides.

Intriguingly, the distribution of As in the two riverstreams running through Hanam is quite irregular(Fig. 9), which resembled findings made in bedsediments of the Ganges–Meghna–Brahmaputra riversystem (Tareq et al. 2003). The combined effects ofwater flow patterns, sediment load and geologicalcharacteristics may lead to such irregular distributionpatterns (Tareq et al. 2003). However, the As levelswe found in the river bed sediments of the Red River(63.4 mg kg−1 on average) and the Chau Giang River(33.8 mg kg−1 on average) were higher than those ofthe Ganges–Meghna–Brahmaputra system (12.2–27.5mg kg−1). The values obtained were also significantlyhigher than those reported for various unconsolidatedsediments in the world (0.6–50 mg kg−1; average,3 mg kg−1) or for river bed sediments in Bangladesh(1.2–5.9 mg kg−1), and for stream and lake silt inCanada (<1–72 mg kg−1; average, 6 mg kg−1)(Smedley and Kinniburgh 2002). The increasingaccumulation of As towards the lower intersection

of the Red River and Chau Giang River implied thatunder the aerobic conditions that prevail duringtransport in the river system, sediment particles enablethe growth of metal oxides on their surfaces, and thusact as As adsorbents. Furthermore, the river systemalso transports weathered organic matter-rich sedi-ments (Tareq et al. 2003). Long-term chemicalweathering and turbulent physical processes occurringin the Red River and Chau Giang River system couldbe significant factors that promote the biogeochemicaland sedimentary cycling of As during early diagene-sis. The abundance of As in the sediments of the RedRiver suggested that the Red River plays a moreimportant role than the Chau Giang River in thetransport of As-containing particles to the study area.Further long-term and large-scale study on hydrolog-ical and sedimentary processes should be conductedto elucidate the influence of river bank deposits.

Conclusion

Groundwater is the main water source for drinking,cooking, watering and other household purposes inthe study area. The present study revealed that thegroundwater was seriously contaminated with As,exceeding the limit given by the WHO (10 μg l−1) 20-fold on average. Elevated concentrations of Fe, NH4

+

and Mn were also found in the groundwater. Ourresults suggested that the Fe oxyhydroxide reductionprocess might be considered as a source for As ingroundwater. The relatively high accumulation of Asin peat and organic matter-rich horizons suggestedthat these horizons are possible sources of As, wherethe decomposition of organic matter can providereductive conditions that favor the mobilization ofAs. The significant correlations between the contentsof total As and crystalline Fe oxides, and the silt andclay fraction in sediments of two bore cores indicatethe strong affinity of iron (hydr)oxide or fine silt andclay particles for As. HCl-extractable As was relatedto total C, N, S, and Feo, suggesting that the reductionof iron (hydr)oxides which releases adsorbed As isoften coupled to microbial oxidation of organicmatter. Although the levels of As in the river waterswas low, sediment particles enriched with As arebeing carried by the Red River and the Chau GiangRiver and probably have been deposited in the studyarea which, consequently, might become a source of



As itself. Based on our present understanding of theproblem, the only viable measure to control andreduce As levels in the groundwater is the develop-ment of suitable treatment techniques to remove Asfrom the water in this area.

Acknowledgements The authors thank the officers of thesampling sites and colleagues in the Faculty of Geology, HanoiUniversity of Science, Vietnam, for their valuable help andsupport with sample collection.

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Arsenic contamination in groundwater and its possible sources in Hanam, Vietnam

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