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Preliminary Report on Arsenic and Some RelatedComponents in Tube-well Waters from the High andLow Arsenic Groundwater Areas Near Dhaka,Bangladesh

著者Author(s)

Terakado, Yasutaka / Asagoe, Yuma / Koga, Toshiki /Nasher, N.M.Refat

掲載誌・巻号・ページCitat ion

神戸大学大学院人間発達環境学研究科研究紀要,9(1):73-80

刊行日Issue date 2015-09

資源タイプResource Type Departmental Bullet in Paper / 紀要論文

版区分Resource Version publisher

権利Rights

DOI

URL http://www.lib.kobe-u.ac.jp/handle_kernel/81009126

Create Date: 2017-12-18

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神戸大学大学院人間発達環境学研究科 研究紀要 第９巻 第１号 2015

Bulletin of Graduate School of Human Development and Environment, Kobe University, Vol.9 No.1 2015REPORT

* Graduate School of Human Development and Environment, Kobe University, Japan

** Faculty of Human Development, Kobe University, Japan

*** Department of Geography and Environment, Jagannath University, Bangladesh.（Received March 31, 2015

Accepted April 15, 2015）

1. Introduction

It is well known that arsenic is a toxic element and drink-

ing arsenic-rich water over a long period results in various

health problems, which leads to establishing standards for

drinking waters. For example, the World Health Organiza-

tion has given a provisional guideline value of 10 µg/L for ar-

senic in drinking-water (WHO, 2011). Arsenic poisoning is es-

pecially serious when people have no choice but to get

drinking waters from wells, because concentrations of arsenic

in some groundwaters are significantly elevated (sometimes

up to 12 mg/L) (WHO, 2011).

Such examples are seen in the Bangladesh and West Ben-

gal area, and a tragic history is briefly as follows (e.g., Smith

et al., 2000; Ravenscroft et al., 2009): People in Bangladesh

used river or pond water for drinking and cooking and conse-

quently infants and children suffered from acute gastrointes-

tinal disease resulting from bacterial contamination of water.

To provide bacteriologically-safe source of drinking water, the

use of tube-well accelerated particularly after 1970, partly as a

result of a deliberate policy promoted by the United Nations

Children's Fund (UNICEF). However, in these days, arsenic

was not recognized as a problem in water supplies. In 1983,

the first cases of arsenic-induced skin lesions were identified in

India (Saha, 1995). The increased number of tube-wells in

1990s caused serious arsenic-induced diseases, and the problem

of arsenic contaminated water received extensive attention. It

was in middle 1990s that such arsenic poisoning was recog-

nized in Bangladesh, and it has been considered that the

transformation took place in India and Bangladesh between

1994 and 1998. Afterwards, many geochemical researches on

groundwaters have been performed focusing on mechanism of

arsenic release to groundwater (e.g., Saha and Chakrabarti,

1995; Dhar et al., 1997; Nickson et al., 2000; BGS/DPHE, 2001;

Smedley and Kinniburgh, 2001; Itai, 2011).

Sources, mechanisms and other related factors on arsenic

pollution of groundwater have been examined (e.g., Smedley

and Kinniburgh, 2001: Ravenscroft et al., 2009), which include

(1) Mobilization by reductive dissolution; (2) Mobilization by

alkali desorption; (3) Mobilization by sulfide oxidation; (4)

Source minerals, etc. However, we feel that there seem to re-

main unsolved problems of how such groundwaters are

formed. Moreover, in relevance to the arsenic problem, behav-

iors of other elements are interesting, and Bangladesh is a

nice research field for various investigations of groundwaters,

because there are many wells from which we can collect sam-

ples.

Abstract：The major and some trace elements of 18 groundwater samples from near Dhaka, Bangladesh were analyzed

to examine high arsenic groundwater genesis and to obtain fundamental data for further investigations on trace

element geochemistry. The results are consistent with those reported previously for the groundwaters from the

Bangladesh and West Bengal areas. Thus the model concerning reductive dissolution of Fe-hydroxides is considered to

be plausible as a basic formation process of arsenic-rich groundwaters. However, the data obtained suggest

participation of some other processes which overlap with the basic process.

Keywords: groundwater, arsenic, Bangladesh, well-water, aquifer

Preliminary report on arsenic and some related components in tube-well waters

from the high and low arsenic groundwater areas near Dhaka, Bangladesh

Yasutaka TERAKADO*　Yuma ASAGOE**　Toshiki KOGA**　N. M. Refat NASHER***

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The study area, the middle part of Bangladesh, is located

in a delta plain at the confluence of the Ganges (Padma),

Brahmaputra (Jamuna), and Meghna Rivers and their tribu-

taries (Fig. 1). The areal distribution of the arsenic concentra-

tions of well-waters in Bangladesh is previously established,

and the contrast between high and low As concentrations are

conspicuous as shown in Fig. 1-B (DPHE/BGS/DFID, 2001).

Moreover, the low As concentration area almost corresponds

to the residual deposit area as shown in Fig. 1-C (USGS, 2001).

The aims of our Bangladesh groundwater project is to reex-

amine the various genetic models proposed previously and to

obtain better understanding of the genesis of these groundwa-

ters using trace element data, such as rare earth elements

which have scarcely been examined. As part of such project,

we would like to report some concentration data of commonly

analyzed components and briefly discuss about the genesis of

these groundwaters.

2. Samples and Analytical Methods

Samples collections were made during January 6 to Janu-

ary 11, 2015, and the sampling sites are indicated in Fig. 1-A.

The pH measurements were made on-site for unfiltered water

with a handy pH meter (PH-6011A CUSTOM) calibrated with

pH 7 and pH 4 standard solutions. The waters were filtered

using 0.45μm membrane filter. Alkalinity was titrated on a

filtered sample using 0.02 N sulfuric acid to pH 4.8. The sam-

ples were stored in polyethylene bottles which were washed in

advance using boiled hydrochloric acid and hot pure water.

The depth data were heard from people around the wells.

The sample bottles were transported to the laboratory in

Kobe University under ambient temperatures. After arrival

of the samples, the samples were stored in refrigerator imme-

diately for anion measurements, and for the sample bottles

for cation analyses, purified hydrochloric acid were added and

stored at room temperature.

The concentrations of F-, Cl-, Br-, NO2-, NO3

- and SO42-

were measured by ion chromatograph with Hitachi HPLC

system during January 16 to 18. The Ca, Mg, Na, K, Fe and

Mn were determined by atomic absorption method using Sr as

an interference suppressor on Hitach Z-2010. Fe, Mn, Si and P

were measured by ICP optical emission spectrometry with

Shimadzu ICPS-1000IV. Total inorganic arsenic concentra-

tions were analyzed by hydride generation atomic absorption

spectrometry using a hydride generator HYD-10 (Nippon Jar-

rell Ash Co., Japan) and with Hitachi Z-2010. In order to re-

duce arsenate, 3.75 mL of 50% KI solution and 8.75 mL of 6 M

HCl were added to 50 mL sample before starting the HYD-10

processing, and pre-heating furnace of HYD-10 was applied.

The solutions of 50% KI and 2.5% sodium tetrahydroborate

were newly prepared at time of use. The detection limits were

estimated from the variations of the blank measurements.

Overall, reproducibility and precision for each analysis were

less than 3%. The ionic balance errors were usually less than

±2% aside from a few samples.

Fig. 1 Maps showing the sampling localities (A), the spatial arsenic distribution data reported in the literatures (B), and the geologic map for the study area (C).

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3. Results

The analytical data are listed in Table 1. The pH values

for all of the groundwater samples are almost neutral rang-

ing from 6.7 to 7.2. The alkalinities (Alk) for the northern

samples (N1~N7) range from 1.95 to 3.42 meq/L, and are

smaller than those for the southern samples (S1~S5 and

B1~B4) ranging from 4.02 to 9.04 meq/L. These ranges for the

northern and southern samples are clearly discriminated,

which is consistent with the previously obtained tendency for

the low and high As groundwaters (Fig. 1-B) and with the ge-

ologic difference as seen in Fig. 1-C (DPHD/BGS/DFID, 2000;

USGS, 2001). The lowest Alk of the groundwater samples of

this study is 1.95 meq/L from the northern area, and this val-

ue is close to 1.81 meq/L of Padma river sample (R1).

In Fig. 2, the concentrations of various components are

plotted against Alk. Most elements show vague positive

trends. The plots on Mg and F show relatively good positive

correlations (see Fig. 2). The points of the Na, P, K, Ca, Fe

and Mn plots are scattered largely, but lack of relatively high

concentration points of these elements in the lower Alk range

Table 1. Analytical results.

Date Depth T pH Alk Na K Mg Ca Mn

sample M/D m (℃ ) meq/L ppm ppm ppm ppm ppm

N1 1/7 76 23 7.0 3.42 23.5 0.89 11.0 31.7 0.13

N2 1/7 61 23 6.9 3.37 24.1 1.10 11.1 29.3 0.12

N3 1/8 91 23 6.9 3.25 24.3 1.05 8.91 29.2 b.d.

N4 1/8 0 23 6.7 1.95 13.6 1.12 6.26 17.1 b.d.

N5 1/8 61 26 6.8 3.38 23.2 1.10 11.7 29.4 0.16

N6 1/8 0 26 6.8 3.39 26.4 0.91 9.71 30.1 0.21

N7 1/8 37 25 6.7 3.35 21.6 1.02 10.9 33.3 0.19

S1 1/9 76 7.4 7.11 80.1 1.09 21.1 48.6 1.4

S2 1/9 67 25 7.1 7.56 19.8 4.80 35.5 51.0 0.12

S3 1/9 229 25 6.9 5.57 212 2.12 12.2 24.5 0.18

S4 1/10 21 23 7.3 9.04 149 2.59 22.4 40.2 0.91

S5 1/10 0 22 7.2 8.75** 29.3 2.93 36.2 84.8 0.59

S6 1/10 61 25 7.1 7.27 20.5 4.56 21.6 94.0 0.18

S7 1/10 73 26 7.3 7.20 185 3.32 13.6 31.6 0.059*

R1 1/10 0 8.1 1.81 4.86 2.19 6.42 29.0 b.d.

B1 1/11 37 25 7.2 7.30 166 1.89 20.3 31.1 0.90

B2 1/11 46 24 7.3 4.02 15.4 2.57 13.9 65.4 0.71

B3 1/11 37 23 7.2 6.73 14.1 4.03 20.4 95.0 0.36

B4 1/11 46 21 7.0 8.96 59.8 5.33 28.4 138 3.6

Fe Si P As F Cl NO2 Br NO3 SO4

sample ppm ppm ppm ppb ppm ppm ppm ppm ppm ppm

N1 b.d. 32.2 0.11 b.d. 0.24 2.83 b.d. b.d. 0.27 0.21

N2 b.d. 33.8 0.13 b.d. 0.21 1.74 b.d. b.d. 0.058 0.29

N3 b.d. 34.3 0.13 b.d. 0.18 1.35 b.d. 0.047 0.050 0.30

N4 b.d. 38.0 0.123 b.d. 0.16 0.78 b.d. 0.17 0.35 0.51

N5 0.33 32.6 0.13 b.d. 0.28 1.67 b.d. b.d. 0.70 0.27

N6 0.17* 32.0 0.17 b.d. 0.29 1.48 b.d. b.d. 0.42 0.31

N7 0.16* 33.3 0.17 b.d. 0.22 2.14 b.d. b.d. 0.034 0.12

S1 b.d. 21.8 0.13 b.d. n.d. 15.3 b.d. 0.31 0.033 0.52

S2 8.8 20.6 1.36 134 n.d. 4.62 0.017 0.045 0.16 0.060

S3 2.4 25.2 0.18 b.d. 0.36 206 b.d. 1.1 0.14 6.3

S4 1.9 13.6 0.59 73 0.50 50.2 b.d. 0.77 0.044 0.38

S5 3.4 17.9 0.44 112 0.21 26.1 b.d. 0.20 0.16 0.037

S6 5.7 19.8 1.96 193 n.d. 17.0 b.d. 0.26 0.057 0.11

S7 1.4 12.5 0.53 190 0.28 127 b.d. 1.0 0.084 0.076

R1 b.d. 5.5 0.056 1.2* 0.069 1.91 b.d. b.d. 0.14 17.5

B1 0.74 19.8 0.40 b.d. n.d. b.d. 1.1 0.041 7.6

B2 1.1 18.3 0.59 60 n.d. 11.8 b.d. 0.13 0.18 30.8

B3 7.5 15.1 1.20 222 n.d. 14.9 b.d. 0.083 0.10 0.015

B4 15.1 25.0 1.45 203 n.d. 106 b.d. 0.60 0.088 34.0

b.d. : below detection limit. Mn=0.028 ppm, Fe=0.067 ppm, As=1.1 ppb, *: below determination limit**: value based on ion balancebalnak : not determined

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indicates positive tendency rather than negative one. On the

other hand, Si and NO3- seem to show negative tendencies.

The data of chloride and sulfate ions are considerably ambigu-

ous.

The total arsenic concentrations of the northern samples

(N-1 to -7) are below the detection limit (1.1 ppb), which is con-

sistent with the geographical distribution pattern of As (Fig.

1-B). On the other hand, the south area samples (S1-S7 & B1-

B4) include the high As groundwaters (60.5－222 ppb), which is

also consistent with the regional As distributions. However,

some samples from the south areas have concentrations less

than the detection limit, and there may be clear distinction

between high and low As wells. In Fig. 3, the relationships

between As concentrations and other parameters are shown.

The As-Alk, As-K, As-P and As-Fe relationships suggest posi-

tive correlations, while negative tendency may exist in the As-

Mn plot. The As-SO42- data are very ambiguous.

The NO2- signal on the chromatographic charts was not

obtained for most samples, but an obvious peak was observed

on the S2 chart, and this sample have second highest Fe con-

centration, suggesting reducing state. In Fig. 4, Na or Br

data are plotted against Cl. The rough correlations are seen

Fig. 2 Relatioships between alkalinity and various concentration data.

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previously obtained tendency for the low and high As groundwaters (Fig. 1-B) and with the geologic difference as seen in Fig. 1-C (DPHD/BGS/DFID, 2000; USGS, 2001). The lowest Alk of the groundwater samples of this study is 1.95 meq/L from the northern area, and this value is close to 1.81 meq/L of Padma river sample (R1). In Fig. 2, the concentrations of various components are plotted against Alk. Most elements show vague positive trends.

The plots on Mg and F show relatively good positive correlations (see Fig. 2). The points of the Na, P, K, Ca, Fe and Mn plots are scattered largely, but lack of relatively high concentration points of these elements in the lower Alk range indicates positive tendency rather than negative one. On the other hand, Si and NO3− seem to show negative tendencies. The data of chloride and sulfate ions are considerably ambiguous.

The total arsenic concentrations of the northern samples (N-1

0

100

200

300

0

2

4

6

Na (ppm)

K (ppm)

0

10

20

30

40 Mg (ppm)

0

50

100

150 Ca (ppm)

0

20

40Si (ppm)

0.0

0.5

1.0

1.5

2.0 P (ppm)

1

2

3

4Mn (ppm)

0

10

Fe (ppm)

0 2 4 6 8 100

100

200

R1B1 - B4

Cl- (ppm)

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

N1 - N7S1 - S7

NO3- (ppm)

0 2 4 6 8 100

20

40

SO42- (ppm)

Alkalinity (meq/L)

0.0

0.2

0.4

0.6

F- (ppm)

Fig. 2 Relatioships between alkalinity and various concentration data.

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in both plots. It is interesting that some samples such as S3,

S7 and B4 have very high Na and Cl concentrations, suggest-

ing involvement of seawater. However, these data points devi-

ate from the line corresponding to the Cl/Na weight ratio of

1.8 for seawater (e.g., Turekian, 1968) (Fig. 4A). Moreover,

the data points on the Cl-Br plot also deviate from the seawa-

ter weight Cl/Br ratio of 288 (e.g., Turekian, 1968) (Fig. 4B).

These observations suggest that Cl- ions in these groundwa-

ters are not simply originated from seawater. As for Fe-Mn

data (Fig. 5A), a negative tendency is seen aside from one high

Fe and Mn sample (B4), although such tendency is crucial and

more data are necessary. A broad positive correlation is seen

in the Fe-P plot (Fig. 5B).

4. Discussion

The general mechanisms causing arsenic-rich groundwa-

ters have been pointed out in order of decreasing importance

as follows (e.g., Ravenscroft et al., 2009):

(1) Dissolution of As adsorbed iron oxides under reducing

condition mainly due to break down of organic matter.

The groundwater which belongs to this type is character-

ized by high iron and bicarbonate concentrations, while

nitrate and sulfate are absent.

(2) Desorption of As from iron oxides or other minerals under

elevated pH condition, which can explain alkali-oxic wa-

ters.

(3) Decomposition of iron sulfide minerals which contain As

via oxidation. Pyrite and arsenopyrite are the well known

As containing minerals.

(4) As release from geothermal fluids such as hot spring wa-

ters.

The forth mechanism is not adequate for the Bangladesh

groundwaters, because geothermal activity is not common in

Bangladesh. Two hot springs are known, Boga lake and

Sitakunda, but both areas are incompatible with the high As

areas of the spatial distribution map such as shown in Fig.

Fig. 3 Relationships between arsenic and various elements. The symbols are the same as those in Fig. 2.

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to -7) are below the detection limit (1.1 ppb), which is consistent with the geographical distribution pattern of As (Fig. 1-B). On the other hand, the south area samples (S1-S7 & B1-B4) include the high As groundwaters (60.5−222 ppb), which is also consistent with the regional As distributions. However, some samples from the south areas have concentrations less than the detection limit, and there may be clear distinction between high and low As wells. In Fig. 3, the relationships between As concentrations and other parameters are shown. The As-Alk, As-K, As-P and As-Fe relationships suggest positive correlations, while negative tendency may exist in the As-Mn plot. The As-SO42− data are very ambiguous. The NO2− signal on the chromatographic charts was not obtained for most samples, but an obvious peak was observed on the S2 chart, and this sample have second highest Fe concentration, suggesting reducing state. In Fig. 4, Na or Br data are plotted against Cl. The rough correlations are seen in both plots. It is interesting that some samples such as S3, S7

and B4 have very high Na and Cl concentrations, suggesting involvement of seawater. However, these data points deviate from the line corresponding to the Cl/Na weight ratio of 1.8 for seawater (e.g., Turekian, 1968) (Fig. 4A). Moreover, the data points on the Cl-Br plot also deviate from the seawater weight Cl/Br ratio of 288 (e.g., Turekian, 1968) (Fig. 4B). These observations suggest that Cl− ions in these groundwaters are not simply originated from seawater. As for Fe-Mn data (Fig. 5A), a negative tendency is seen aside from one high Fe and Mn sample (B4), although such tendency is crucial and more data are necessary. A broad positive correlation is seen in the Fe-P plot (Fig. 5B).

4. Discussion

The general mechanisms causing arsenic-rich groundwaters have been pointed out in order of decreasing importance as follows (e.g., Ravenscroft et al., 2009): (1) Dissolution of As adsorbed iron oxides under reducing

Mn (ppm)

0 1 4

As (ppb)

100

200

Mg (ppm)0 20 40

As (ppb)

0

100

200

P (ppm)0 1 2

As (ppb)

0

100

200

K (ppm)0 2 4 6

As (ppb)

0

100

200

SO42- (ppm)

0 20 30

As (ppb)

0

100

200

0.5

Alkalinity (meq/L)0 2 4 6 8 10

As (ppb)

100

200

pH6.5 7.0 8.0

As (ppb)

100

200

Fe (ppm)

0 5 10 15

As (ppb)

0

100

200

Depth (m)0 50 100 250

As (ppm)

0

100

200

Fig. 3 Relationships between arsenic and various elements. The symbols are the same as those in Fig. 2.

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1B. Moreover, the features of Sitakunda hot spring, for ex-

ample, are much lower grade than typical hot spring ones, and

therefore its effect must be limited: the temperature is rather

low (26.1 to 29.8 ℃ during winter) and the total alkalinity and

chlorine content range from 12 to 20 mg/L and 13.9 to 18.3

mg/L, respectively (Hossain and Anwar, 2009).

As a whole, it can be said that the mutual relationships

among various concentration data obtained in the present

study are similar to those reported by the previous investiga-

tors (e.g., McArthur et al., 2004). Therefore, on the basis of

the summary of Ravenscroft et al., (2009), the first model (re-

ductive dissolution) seems to be plausible. The fundamental

reaction of the first model can be represented by the following

redox reaction which has been examined by many investiga-

tors (e.g., Hem, 1977; Langmuir, 1997):

8 FeOOH + CH3COOH + 14 H2CO3 ⇒

8 Fe2+ + 16 HCO3- + 12 H2O

Combining this reaction to another fundamental fact that As

and P are tend to be involved in Fe hydroxides (e.g., Williams

et al., 1976; Antelo et al., 2005; Violante et al., 2009), the cor-

relations among Fe, As, P and HCO3- concentrations can be

interpreted by dissolution of Fe hydroxides which contain As

and P. In this relation, the third model (sulfide oxidation

model) in which the opposite redox condition is assumed does

not meet the observed data.

The second desorption model seems to be unclear in litera-

tures. For example, adsorption of arsenate onto most oxides

and clays decreases significantly at pH>8.0, but there tends to

be a peak in the region of pH 6 to 8 for Al oxide and Al-rich

clays (Ravenscroft et al., 2009). Moreover, the adsorption of

As(V) on ferrihydrite is sensitive to the concentrations of

phosphate, silica and organic acids, which could explain de-

sorption of As(V) in oxic waters at slightly lower pH than

otherwise expected (Ravenscroft et al., 2009). Thus, it is diffi-

cult to draw a definite conclusion about this mode, but a

broad negative tendency between As concentration and pH

(Fig. 3) is not consistent with this desorption mode.

Thus, at present, the first model seems to be plausible.

However, there are several uncertain points which we must

consider:

(1) The negative correlation between As and Mn concentra-

tions in contrast to the positive correlation between As and Fe

concentrations (Fig. 3) despite similar dissolution nature of

both Mn and Fe oxides in reducing conditions, seems to be cu-

rious. This is related to the negative correlation between Fe

and Mn concentrations (Fig. 5). These observations might be

attributed to the difference in the boundary positions of Fe2+

and Mn2+ on the Eh-pH regime (e.g., Brookins, 1988), that is,

manganese can be dissolved under mildly reducing conditions.

However, details of geological and hydrological views should

Fig. 4 Plots for Na vs Cl and Br vs Cl. The dashed line in (A) indicates Cl/Na ratio of 1.8, corresponding to the seawater value (Turekian, 1968). The dashed line in (B) indicates Cl/Br ratio of 288, corresponding the seawater value (Turekian, 1968). The symbols are the same as those in Fig. 2.

- 6 -

condition mainly due to break down of organic matter. The groundwater which belongs to this type is characterized by high iron and bicarbonate concentrations, while nitrate and sulfate are absent. (2) Desorption of As from iron oxides or other minerals under elevated pH condition, which can explain alkali-oxic waters. (3) Decomposition of iron sulfide minerals which contain As via oxidation. Pyrite and arsenopyrite are well known As containing minerals. (4) As release from geothermal fluids such as hot spring waters.

The forth mechanism is not adequate for the Bangladesh groundwaters, because geothermal activity is not common in Bangladesh. Two hot springs are known, Boga lake and Sitakunda, but both areas are incompatible with the high As areas of the spatial distribution map such as shown in Fig. 1B. Moreover, the features of Sitakunda hot spring, for example, are much lower grade than typical hot spring ones, and therefore its effect must be limited: the temperature is rather low (26.1 to 29.8 ºC during winter) and the total alkalinity and chlorine content

range from 12 to 20 mg/L and 13.9 to 18.3 mg/L, respectively (Hossain and Anwar, 2009). As a whole, it can be said that the mutual relationships among various concentration data obtained in the present study are similar to those reported by the previous investigators (e.g., McArthur et al., 2004). Therefore, on the basis of the summary of Ravenscroft at al., (2009), the first model (reductive dissolution) seems to be plausible. The fundamental reaction of the first model can be represented by the following redox reaction which has been examined by many investigators (e.g., Hem, 1977; Langmuir, 1997): 8 FeOOH + CH3COOH + 14 H2CO3 ⇒ 8 Fe2+ + 16 HCO3− + 12 H2O Combining this reaction to another fundamental fact that As and P are tend to be involved in Fe hydroxides (e.g., Williams et al., 1976; Antelo et al., 2005; Violante et al., 2009), the correlations among Fe, As, P and HCO3− concentrations can be interpreted by dissolution of Fe hydroxides which contain As and P. In this relation, the third model (sulfide oxidation model) in which the opposite redox condition is assumed does not meet the observed data. The second desorption model seems to be unclear in literatures. For example, adsorption of arsenate onto most oxides and clays decreases significantly at pH>8.0, but there tends to be a peak in the region of pH 6 to 8 for Al oxide and Al-rich clays (Ravenscroft et al., 2009). Moreover, the

Br (ppm)0.0 0.5 1.0

Cl (ppm)

0

100

200 Seawater Cl/Br

Na (ppm)0 100 200

Cl (ppm)

0

100

200

Seawater Cl/Na

A

B

Fig. 4 Plots for Na vs Cl and Br vs Cl. The dashed line in (A) indicates Cl/Na ratio of 1.8, corresponding to the seawater value (Turekian, 1968). The dashed line in (B) indicates Cl/Br ratio of 288, corresponding the seawater value (Turekian, 1968). The symbols are the same as those in Fig. 2.

Mn (ppm)0.0 0.5 1.0 3.5

Fe (ppm)

0

5

10

15

P (ppm)0 1 2

Fe (ppm)

0

5

10

15

A

B

Fig. 5 Relationship between Fe and Mn or P.

Fig. 5 Relationship between Fe and Mn or P.

- 6 -

condition mainly due to break down of organic matter. The groundwater which belongs to this type is characterized by high iron and bicarbonate concentrations, while nitrate and sulfate are absent. (2) Desorption of As from iron oxides or other minerals under elevated pH condition, which can explain alkali-oxic waters. (3) Decomposition of iron sulfide minerals which contain As via oxidation. Pyrite and arsenopyrite are well known As containing minerals. (4) As release from geothermal fluids such as hot spring waters.

The forth mechanism is not adequate for the Bangladesh groundwaters, because geothermal activity is not common in Bangladesh. Two hot springs are known, Boga lake and Sitakunda, but both areas are incompatible with the high As areas of the spatial distribution map such as shown in Fig. 1B. Moreover, the features of Sitakunda hot spring, for example, are much lower grade than typical hot spring ones, and therefore its effect must be limited: the temperature is rather low (26.1 to 29.8 ºC during winter) and the total alkalinity and chlorine content

range from 12 to 20 mg/L and 13.9 to 18.3 mg/L, respectively (Hossain and Anwar, 2009). As a whole, it can be said that the mutual relationships among various concentration data obtained in the present study are similar to those reported by the previous investigators (e.g., McArthur et al., 2004). Therefore, on the basis of the summary of Ravenscroft at al., (2009), the first model (reductive dissolution) seems to be plausible. The fundamental reaction of the first model can be represented by the following redox reaction which has been examined by many investigators (e.g., Hem, 1977; Langmuir, 1997): 8 FeOOH + CH3COOH + 14 H2CO3 ⇒ 8 Fe2+ + 16 HCO3− + 12 H2O Combining this reaction to another fundamental fact that As and P are tend to be involved in Fe hydroxides (e.g., Williams et al., 1976; Antelo et al., 2005; Violante et al., 2009), the correlations among Fe, As, P and HCO3− concentrations can be interpreted by dissolution of Fe hydroxides which contain As and P. In this relation, the third model (sulfide oxidation model) in which the opposite redox condition is assumed does not meet the observed data. The second desorption model seems to be unclear in literatures. For example, adsorption of arsenate onto most oxides and clays decreases significantly at pH>8.0, but there tends to be a peak in the region of pH 6 to 8 for Al oxide and Al-rich clays (Ravenscroft et al., 2009). Moreover, the

Br (ppm)0.0 0.5 1.0

Cl (ppm)

0

100

200 Seawater Cl/Br

Na (ppm)0 100 200

Cl (ppm)

0

100

200

Seawater Cl/Na

A

B

Fig. 4 Plots for Na vs Cl and Br vs Cl. The dashed line in (A) indicates Cl/Na ratio of 1.8, corresponding to the seawater value (Turekian, 1968). The dashed line in (B) indicates Cl/Br ratio of 288, corresponding the seawater value (Turekian, 1968). The symbols are the same as those in Fig. 2.

Mn (ppm)0.0 0.5 1.0 3.5

Fe (ppm)

0

5

10

15

P (ppm)0 1 2

Fe (ppm)

0

5

10

15

A

B

Fig. 5 Relationship between Fe and Mn or P.

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－　　－79

be examined further combining with NO3- and SO4

2- behaviors.

(2) There are some samples having high Na and Cl concentra-

tions, which suggests involvement of seawater for the genesis

of these graoundwaters. However, the data points deviate

from the seawater lines in the Na-Cl and Br-Cl diagrams (Fig.

5). Such tendencies have not been well examined for the Bang-

ladesh groundwaters, but close scrutiny may provide useful

information about the genetic models (c.f., Alcalá and Custo-

dio, 2008).

(3) In addition to the some common components studied here

as well as in other investigations, some trace elements such as

rare earth elements (REE) are interesting. It has been pointed

out that Eu and Ce anomalies in water samples may reflect

redox states of their genetic processes (e.g., Otsuka and Ter-

akado, 2003; Johannesson et al., 2005). However, data of REE

abundances for the groundwaters from Bangladesh are sparse

(c.f., Bibi et al., 2006). The data of this preliminary report

should be important for screening the samples for further de-

velopment of the trace element studies.

5. Acknowledgements

We thank the Center for Environmental Management of

Kobe University for allowing us to use the atomic absorption

spectrophotometer. We also thank Dr. K. Okushi for checking

the manuscript.

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