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Geochemical study of major and trace elements in the arseniccontaminated groundwaters in Bangladesh

著者Author(s)

Asagoe, Yuma / Terakado, Yasutaka / Kishibe, Katsuya / 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,10(2):141-149

刊行日Issue date 2017-03-30

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

JaLCDOI

URL http://www.lib.kobe-u.ac.jp/handle_kernel/E0041234

PDF issue: 2020-10-23

（141）

－　　－141

神戸大学大学院人間発達環境学研究科 研究紀要 第10巻 第２号 2017

Bulletin of Graduate School of Human Development and Environment, Kobe University, Vol.10 No.2 2017REPORT

* Graduate School of Human Development and Environment, Kobe University （Received September 30, 2016Accepted October 31, 2016）

1. Introduction

Arsenic (As) is known as a strong toxic element affecting

nearly all organ systems especially causing dermal effects.

Before 1960’s, people in Bangladesh mainly had utilized the

river water, but it was significantly contaminated with bacte-

ria. As a result, infant mortality was high (Smith et al.,

2000), and the Bangladesh government encouraged the use of

groundwater to improve mortality. Before 1970’s, arsenic

contamination had not been found in Bangladesh, because ar-

senic was not included in water quality inspection items.

Since 1970’s, arsenic contamination problem in the Bangla-

desh and west Bengal areas has received attention and many

investigations have been performed (e.g., BGS/DPHE, 2001;

Ravenscfort et al., 2009). At present, the guideline value for

arsenic in drinking water in Bangladesh is 50 ppb, although

the value of WHO is 10 ppb (UNICEF, 2010; WHO, 2011). It

has been pointed out that about 50% of Bangladesh wells con-

tain more than 10 ppb arsenic and 28% contain more than 50

ppb arsenic (Yu et al., 2003).

The concentrations of arsenic and major elements in

groundwaters have been analyzed extensively (e.g., BGS/

DPHE, 2001). From the geological and geochemical view-

points, mechanisms of arsenic contaminated groundwaters are

interesting, and some genetic models have been proposed: for

example, arsenic bearing sulfide oxidation, arsenic adsorbed

ferric oxyhydroxides dissolution under reducing condition,

alkaline desorption, and contribution of geothermal waters

(e.g., Ranenscroft et al., 2009). In the previous studies, the

positive tendency between arsenic and iron and the negative

tendency between arsenic and sulfate were reported for several

high arsenic areas in Bangladesh (BGS/DPHE, 2001). The

positive tendency between arsenic and iron was considered as

reflecting congruent dissolution of iron oxides and the associ-

ated release of arsenic under the reducing conditions (BGS/

DPHE, 2001). The negative tendency between arsenic and sul-

fate was interpreted by strong reduction of sulfate in the for-

mation processes of high arsenic concentration groundwaters

(BGS/DPHE, 2001). These observations may support the re-

ductive dissolution of ferric oxyhydroxides, but both observed

correlations are not enough clear, and correlations between

arsenic and other elements such as manganese, phosphorus,

and bicarbonate are ambiguous (BGS/DPHE, 2001). There-

fore, more investigations are necessary.

Mineralogical studies on the sediments from the arsenic

contaminated areas have shown that Fe-oxides and Fe-hy-

droxides and pyrite are the most important arsenic adsorbent

minerals (Ravenscroft et al., 2009). Contrary to the water

sample cases, the clear positive correlations between iron and

arsenic concentrations in the sediments of the contaminated

areas were pointed out (BGS/DPHE, 2001), suggesting the re-

Abstract：Major and some trace elements including rare earth element (REE) in 15 groundwater samples collected from

Bangladesh were analyzed to examine the formation mechanisms of arsenic contaminated groundwaters using the

relationships among chemical elements. Several features were observed: (1) the positive tendency between arsenic and

iron, (2) the negative tendency between arsenic and sulfate, (3) relatively high barium and low REE concentrations.

These observations are consistent with the reductive dissolution of ferric oxyhydroxides. Moreover, it was considered

that the contrasting positive and negative relationships between Fe and other elements such as As, Mn, Ba, and REE

are reconciled by different adsorption ability of ferric hydroxides for these elements.

Geochemical study of major and trace elements in the arsenic

contaminated groundwaters in Bangladesh

Yuma ASAGOE*, Yasutaka TERAKADO*, Katsuya KISHIBE* and N. M. Refat NASHER*

（142）

－　　－142

ductive dissolution of ferric oxyhydroxides. However, the re-

lease mechanisms of arsenic from arsenic contained sediment

to groundwater are not completely understood due to the

complex relationships among arsenic and major component

concentrations.

The previous geochemical studies have been performed

mainly on the basis of major element concentrations, but some

trace elements such as rare earth elements (REE) have not

been investigated. It is known that REEs have unique chemi-

cal characteristics and their abundances are considered to re-

flect redox conditions of groundwaters. In oxidizing condi-

tions, Ce(III) is expected to be oxidized to Ce(IV), being

precipitated as CeO2. Therefore, the abundance of Ce ion in

water sample may decrease relative to the other trivalent

REEs, and negative Ce anomaly should be seen in the REE

pattern. Thus REE data might be a useful tool for redox con-

ditions, but REE data on the Bangladesh groundwaters have

not been reported.

In this study, we analyzed major and some trace elements

including REE of 15 Bangladesh groundwater samples collect-

ed near Dhaka City. The purpose of the study is to reveal the

relationships among the chemical elements and to discuss the

formation mechanisms of the arsenic contaminated ground-

waters in Bangladesh.

2. Samples and analytical methods

The samples used were collected near Dhaka city, Bangla-

desh, during March 6 to March 7, 2015 in addition to those

collected in early January, 2015 for the previous study (Ter-

akado et al., 2015). The sampling points are shown in Fig. 1-A

and 1-B. The pH measurements were made on-site using un-

filtered water with a pH meter (PH-6011A CUSTOM) calibrat-

ed with pH 7 and pH 4 standard solutions. The waters were

filtered using 0.45 µm membrane filter, and were stored in

polyethylene bottles which were washed by using boiled hy-

drochloric acid and boiled distilled water. The depths were

heard from people around the wells.

The sample bottles were transported to the laboratory in

Kobe University within about 10 days. The samples for meas-

urement of anion concentrations and alkalinity were frozen as

soon as possible after the arrival. The samples for major cati-

on and REE measurements, pure hydrochloric acid were added

Table 1. Analytical data.

Date Depth T pH Alkakinity As Ca Mg K NaSample M/D m ℃ meq/L ppb ppm ppm ppm ppm

D1 3/6 18 25 7.2 4.40 b.d. 47.7 15.3 1.70 19.9D2 3/6 15 25 6.8 6.06 30.1 85.2 23.8 1.99 10.2D3 3/6 16 25 7.0 6.88 22.3 114 23.6 2.11 11.1D4 3/6 24 26 7.0 6.09 25.5 83.7 23.3 2.86 12.5D5 3/6 24 27 7.1 4.92 7.07 72.9 17.2 2.09 9.26D6 3/6 23 25 6.8 5.08 13.4 70.5 24.7 3.14 16.3D7 3/6 17 25 7.1 4.99 68.5 68.0 19.0 1.61 8.17D8 3/6 21 26 7.0 4.66 3.5* 48.2 26.7 2.14 9.63S8 3/7 61 24 6.9 10.4 43.0 67.1 57.6 8.92 77.5S9 3/7 55 24 6.9 7.71 57.5 54.3 38.8 7.15 37.6S10 3/7 67 26 7.2 9.26 41.4 38.8 39.9 7.50 108S11 3/7 67 24 7.1 8.50 73.1 82.5 41.4 5.25 21.5S12 3/7 55 25 6.9 8.34 72.4 95.5 28.6 6.11 36.4S13 3/7 70 25 7.1 8.29 77.0 53.8 47.5 6.09 83.1S14 3/7 55 28 7.0 9.30 80.2 96.7 38.9 6.31 34.5

Mn Fe Si P F－ Cl－ NO2－ Br－ NO3

－ SO4－

Sample ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm

D1 0.42 b.d. 17.2 0.41 0.20 4.11 b.d. b.d. b.d. 0.45D2 1.42 2.26 13.6 0.78 n.d. 12.0 b.d. 0.034 0.0080 15.3D3 1.49 0.94 13.2 0.52 n.d. 25.6 b.d. b.d. b.d. 18.2D4 0.41 5.74 20.1 2.09 n.d. 6.15 b.d. b.d. 0.12 0.95D5 0.27 1.40 15.8 1.23 0.25 2.14 b.d. b.d. 0.075 1.66D6 0.43 9.82 21.3 1.21 n.d. 34.1 b.d. 0.40 0.020 8.19D7 0.43 2.94 16.4 1.66 0.51 4.92 b.d. 0.065 0.029 0.10D8 0.21 2.12 17.4 0.57 n.d. 12.5 b.d. b.d. 0.14 0.49S8 0.17 3.42 29.8 4.53 n.d. 49.7 b.d. 0.28 0.061 0.019S9 0.12 10.8 34.2 2.18 n.d. 13.1 b.d. 0.14 0.037 0.014S10 0.10 4.52 16.9 2.36 n.d. 38.8 b.d. 0.23 0.057 1.24S11 0.11 2.79 16.6 1.08 n.d. 5.83 b.d. 0.064 0.12 0.046S12 0.09* 7.31 15.2 1.78 n.d. 26.7 b.d. 0.46 0.15 0.11S13 0.04* 4.45 11.2 0.89 n.d. 78.5 b.d. 0.50 b.d. 0.11S14 0.20 8.72 11.7 1.23 n.d. 26.8 b.d. 0.16 0.036 b.d.

b.d. : below detection limit; As = 1.12 ppb, Fe = 0.067 ppm* : below determination limit; As = 3.75 ppb, Mn = 0.094 ppm, Fe = 0.223 ppmn.d. : not determinated

（143）

－　　－143

and stored at room temperature.

Alkalinity was measured by titration using 0.02 N sulfuric

acid to pH 4.8. The anion concentrations (F －, Cl －, NO2－, Br －,

NO3－, and SO4

2－) were analyzed by ion chromatograph with

Hitachi HPLC system. The cation concentrations (Ca, Mg, K,

Na, Fe, and Mn) were measured by flame atomic absorption

method using Sr to suppress chemical interferences on Hitachi

Z-2010. Si and P were analyzed by ICP optical emission spec-

trometry with Shimadzu ICPS-1000IV. Total inorganic arse-

nic concentrations were measured by hydride generation

atomic absorption spectrometry using a hydride generator

HYD-10 (Nippon Jarrell Ash Co., Japan) with the atomic ab-

sorption spectrometer. Potassium iodide solution (50wt.%)

was used to reduce arsenate to arsenite, and sodium borohy-

dride (2.5wt.%) was used to generate arsenic hydride.

REEs and Ba were measured by mass-spectrometric iso-

tope dilution using a thermal ionization mass-spectrometer

(JEOL JMS-05RB). Using one litter sample, the REE were

firstly coprecipitated with ferric hydroxide, and light, middle,

and heavy REEs were separated by cation exchange column

(e.g., Masuda and Ikeuchi, 1979). To avoid Ba interference in

La measurements, nitric acid was used for elution (e.g., Jahn

et al., 1980).

3. Results

The obtained data in the present study are listed in Table

1 and 2. The temperature and pH values of all the samples fall

within narrow ranges of 24-28℃ and 6.8-7.2, respectively. The

alkalinity values of the western samples (D1～D8) range from

4.40 to 6.88 meq/L, whereas those of the southern ones (S8～

S14) range from 7.71 to 10.4 meq/L. The arsenic concentra-

tions of the western samples (D1-D8) range from 3.5 ppb to

68.5 ppb, while those of the southern ones (S8～S14) range

from 41.4 ppb to 80.2 ppb.

The relationships between alkalinity and some measured

elements are plotted in Fig. 2 together with the previously ob-

tained data. Broad positive correlations are seen for Mg, K,

and F－. As for Ca, Mn, Fe and P plots, lack of relatively high

values for relatively low alkalinity range shows positive ten-

dency. Similarly, lack of relatively high values for relatively

low alkalinity range suggests negative tendency for the NO3－

plot.

In Fig. 3 and Fig. 4, the relationships between As and

other elements are rather ambiguous, but weak negative cor-

relations can be seen for As-Mn, As-NO3－, As-SO4

2－ and As-La

(REE). In Fig. 5, Cl－-Na and Cl－-Br－ relationships show posi-

tive correlations, but most data points deviate from the sea-

water lines. In Fig. 6, P-Fe and Mn-Fe data are plotted. The

P-Fe data show positive correlation, whereas Mn-Fe show

negative tendency.

The REE patterns normalized to silicic rock average are

shown in Fig. 7 together with the Ba data. The REE patterns

Table 2. Analytical data of Ba and REEs.

Ba La Ce Nd Sm Eu Gd Dy Er Yb LuSample ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm

N4 0.013 4.51E-06 1.24E-05 3.41E-06 6.87E-07 3.97E-07 1.15E-06 1.89E-06 2.11E-06 1.89E-06 3.77E-07N7 0.021 3.97E-06 6.94E-06 3.42E-06 5.53E-07 1.47E-07 8.47E-07 1.30E-06 1.59E-06 4.84E-06 3.05E-07S1 0.048 3.49E-06 1.11E-05 3.87E-06 1.23E-06 4.00E-07 6.84E-06 1.01E-05 1.34E-05 1.58E-05 2.35E-06S2 0.135 9.03E-07 2.73E-06 n.d. 2.38E-07 4.80E-08 1.62E-07 2.25E-07 2.93E-07 4.43E-07 8.70E-08S4 0.032 8.64E-07 3.76E-06 9.84E-07 2.38E-07 2.21E-07 6.30E-07 1.36E-06 2.02E-06 2.44E-06 5.33E-07S6 0.163 8.23E-07 1.57E-06 n.d. 9.20E-08 5.37E-08 1.91E-07 2.57E-07 3.40E-07 2.73E-07 1.07E-07S7 0.115 5.72E-07 2.24E-06 n.d. 1.13E-07 7.15E-08 2.86E-07 3.05E-07 n.d. 2.16E-07 n.d.S14 0.147 7.63E-06 5.00E-06 7.76E-07 4.41E-07 3.89E-07 3.08E-07 4.62E-07 2.47E-07 1.92E-07 9.37E-08R1 0.022 7.33E-06 1.93E-05 1.00E-05 2.07E-06 6.33E-07 2.57E-06 2.66E-06 1.86E-06 1.19E-06 2.68E-07B1 0.102 2.92E-06 9.55E-06 2.65E-06 6.20E-07 2.55E-07 1.03E-06 2.00E-06 3.60E-06 3.89E-06 8.48E-07B4 0.265 9.41E-07 1.78E-06 6.62E-07 1.58E-07 2.60E-07 2.27E-07 3.08E-07 4.08E-07 8.85E-07 1.21E-07D6 0.158 6.16E-06 1.58E-05 n.d. 2.67E-07 6.70E-08 3.65E-07 4.97E-07 4.11E-07 4.48E-07 1.02E-07

Fig. 1-A　Map showing the position of Bangladesh in the　　　south Asia region.

Asagoe et al.

Fig. 1-A Map showing the position of Bangladesh in the south Asia region. Fig. 1-B Map showing sampling points.

A

B

Fig. 1-B Map showing sampling points.

Asagoe et al.

Fig. 1-A Map showing the position of Bangladesh in the south Asia region. Fig. 1-B Map showing sampling points.

A

B

（144）

－　　－144

Fig. 2 Plots of major element concentrations vs. alkalinity.Asagoe et al.

Alkalinity (meq/L)

0

5

10

0 2 4 6 8 10 120

20

40

60

0 2 4 6 8 10 12

N1-N7

S1-S14

R1

B1-B4

D1-D8

0

40

80

120

160

0 2 4 6 8 10 12

Ca (ppm) Mg (ppm) K (ppm)

0

50

100

150

200

250

0 2 4 6 8 10 12

Na (ppm)

0

1

2

3

4

0 2 4 6 8 10 12

Mn (ppm)

0

4

8

12

16

0 2 4 6 8 10 12

Fe (ppm)

0

10

20

30

40

50

0 2 4 6 8 10 12

Si (ppm)

0

1

2

3

4

5

0 2 4 6 8 10 12

P (ppm)

0

50

100

150

200

250

0 2 4 6 8 10 120.0

0.3

0.6

0.9

0 2 4 6 8 10 12

Cl－ (ppm) NO3－ (ppm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0 2.0 4.0 6.0 8.0 10.012.0

0

10

20

30

40

0 2 4 6 8 10 12

F－ (ppm)

SO42－ (ppm)

Fig. 2 Plots of major element concentrations vs. alkalinity.

（145）

－　　－145

Fig. 3 Relationships between arsenic and some major elements.

Asagoe et al.

0

100

200

300

0 50 100Depth (m)

0

100

200

300

6.5 7 7.5pH

0

100

200

300

0 50 100 150Ca (ppm)

0

100

200

300

0 2 4 6 8 10 12Alkalinity (meq/L)

0

100

200

300

0 20 40 60Mg (ppm)

0

100

200

300

0 2 4 6 8 10K (ppm)

0

100

200

300

0 4 8 12 16Fe (ppm)

0

100

200

300

0.0 1.0 2.0Mn (ppm)

0

100

200

300

0.0 1.0 2.0 3.0P (ppm)

0

100

200

300

0 10 20SO4

2－ (ppm)

0

100

200

300

0.0 0.1 0.2 0.3 0.4 0.5SO4

2－ (ppm)

As (ppb)

As (ppb)

As (ppb)

As (ppb) As (ppb) As (ppb)

As (ppb)

As (ppb) As (ppb)

As (ppb)

0

100

200

300

0.0 0.1 0.2 0.3 0.4 0.5NO3

－ (ppm)

As (ppb)

As (ppb)

Fig. 3 Relationships between arsenic and some major elements.

（146）

－　　－146

of light REEs are relatively flat with small or no Ce anoma-

lies. The heavy REE values are relatively higher than the

light REE ones. Some samples show positive Eu anomalies.

However, the Eu data with higher Ba concentrations include

large positive errors due to interference of BaO peak on 153Eu.

Thus, these Eu data indicate maximum values. As seen in

Fig. 7, note that positive Eu anomalies are not large. It is in-

teresting that in the high As samples, the differences between

Ba and REE values are relatively large, in contrast to the dif-

ferences in the low As samples.

4. Discussion

Many investigators have proposed the mechanisms of ar-

senic contamination of groundwaters. As a whole, there are

four major mechanisms for high As groundwaters (e.g., Ra-

venscroft et al., 2009);

(1) [Alkali desorption] Under alkaline conditions, arsenic may

be directly desorbed from As adsorbed metal oxides or clays,

because the experimental data showed that adsorption of arse-

nate onto oxides and clays decreases significantly at pH≥8.0.

(2) [Reductive dissolution] Under reducing conditions, the As

adsorbed Fe oxides or Fe hydroxides break down and release

arsenic into solution, because reducing states promote dissolu-

tion of Fe bearing compounds.

(3) [Sulfide oxidation] Because arsenic tends to be combined

with reduced sulfide minerals such as pyrite and arsenopyrite,

and therefore their oxidation and break-down can release As

to the solution.

(4) [Contribution of geothermal waters] It is known that

many geothermal waters contain high concentration As, and

mixing or involvement of such water results in high As wa-

ters.

According to the present data, pH values are restricted to

near neutral (6.8～7.2), and therefore alkali desorption mecha-

nism is not adequate. The sulfide oxidation mechanism is not

suitable, because of negative correlation between As and SO42－

(Fig. 3). The measured water temperatures are around 24～

28℃，therefore geothermal activities seem to be minor in the

studied areas. Moreover, many researchers studying Bangla-

desh groundwaters seem to consider the reductive dissolution

of As bearing minerals most plausible (e.g., Mcarthur et al.,

2001), and organic matter may play an important role as re-

ducing agent as follows (e.g., Hem, 1977; Langmuir, 1977):

8FeO(OH)(+As) + CH3COOH + 14H2CO3→8Fe2+ + 16HCO3－

+ 12H2O+(As)

In this case, arsenic contaminated groundwater in strongly

reducing conditions should be characterized by high concen-

trations of bicarbonate, Fe, As, dissolved organic carbon

(DOC), and possibly ammonium. Conversely, oxidized species

such as dissolved oxygen and nitrate are almost absent in the

Fig. 4 Plots of As vs. Ba and As vs. La.

Asagoe et al.

Fig. 4 Plots of As vs. Ba and As vs. La.

0

100

200

300

0 0.1 0.2 0.3Ba (ppm)

As (ppb)

0

100

200

300

0.E+00 5.E-06 1.E-05La (ppm)

As (ppb)

Fig. 5 Plots of Cl－ vs. Na and Cl－ vs. Br－ .The straight lines are intended to show the mixing line with seawater (Turekian, 1968).(Cl－/Na = 1.8, Cl－/ Br－ = 288)

Asagoe et al.

0

50

100

150

200

250

0 50 100 150 200 250Na (ppm)

Cl－ (ppm)

0

50

100

150

200

250

0 0.4 0.8 1.2Br－ (ppm)

Cl－ (ppm)

Fig. 5 Plots of Cl－ vs. Na and Cl－ vs. Br－. The straight lines are intended to show the mixing line with seawater (Turekian, 1968). (Cl－/Na = 1.8, Cl－/ Br－ = 288)

Fig. 6 Plots of P vs. Fe and Mn vs. Fe.

Asagoe et al.

0.0

0.5

1.0

1.5

2.0

0 4 8 12Fe (ppm)

Mn (ppm)

0.0

1.0

2.0

3.0

0 4 8 12Fe (ppm)

P (ppm)

Fig. 6 Plots of P vs. Fe and Mn vs. Fe.

Fig. 7 Silicic rock-normalized Ba-REE patterns.Normalizing values are taken from Terakado and Masuda (1988).

Asagoe et al.

Fig. 7 Silicic rock-normalized Ba-REE patterns. Normalizing values are taken from Terakado and Masuda(1988).

（147）

－　　－147

aquifers, and sulfate concentrations are usually very low (Ra-

venscroft et al., 2009).

As for the present data, weak positive tendencies of As-Fe

and As-Alkalinity and broad negative correlations of As-NO3－

and As-SO42－ are observed, which are consistent with the

characteristics of the reductive dissolution mechanism. Con-

sequently, the reductive dissolution mechanism seems to more

or less to be involved in the formation processes of the high As

groundwaters.

The large differences between Ba and REE values are seen

in silicic rock normalized Ba-REE patterns (Fig. 7). In order

to examine the Ba and REE concentration difference, the Ba

and La data in the literatures are compiled (Fig. 8), and Fig. 8

shows that the values of the Bangladesh Ba data seem to be

relatively high and those of La are relatively low. As a result,

the Ba/La weight ratios which range from 2.8×103 to 2.8×105

seem to be larger than those from the other areas (Fig. 8), al-

though the number of data are limited. Relatively high Ba

concentrations suggest the effect of barite (BaSO4) dissolution

in the aquifer, because barite is a common component in ma-

rine sediments (e.g., Libes, 1992), reflecting its low solubility

product (logKsp = －9.773, T = 25℃ , Truesdell and Jones, 1974)

and high sulfate concentration in the seawater, and it is

known that marine transgression had occurred in the Bengal

Basin area during the last post-glacial maximum (BGS/

DPHE, 2001). The Ba and SO42－ data of the samples are plot-

ted in Fig. 9, together with the solubility line of barite (solid

line). In barite dissolution case, it is expected that the ob-

served Ba concentrations should positively correlate with

SO42－ concentrations, but such relationship is not clear. The

data points of B1, B4 and D6 are close to the solubility line,

suggesting that barite is present in these aquifers, and Ba and

SO42－ concentrations are restricted by barite saturation. The

other data points are located in the lower side of the solubility

line. If barite saturation is operated, such deviations can be

interpreted by an effect of dilution of the barite saturated

groundwater by rain water. However, other possibilities

should be considered such as scarceness of barite in the aqui-

fers.

As seen in Fig. 8, relatively low REE values suggest copre-

cipitation of REEs with Fe hydroxides, because it is known

that Fe hydroxide has a strong ability of coprecipitation for

REEs and has been used for preconcentration of REEs for wa-

ter sample analyses as actually used in this study (e.g., Masu-

　　Nishinomiya, Japan (Otsuka and Terakado, 2003）　　Carnmenellis, England（Smedley, 1991）　　Brittany, France（Dia et al., 2000）　　present data

Fig. 8 Comparisons for Ba, La abundances and Ba/La ratios 　　　in some groundwaters.

Asagoe et al.

Fig. 8 Comparisons for Ba, La abundances and Ba/La ratios in some groundwaters.

3.1×10－3

3.5×10－2

5.0×10－3

2.4×10－2

8.5×10－2

1.8×10－3

1.3×10－2

2.7×10－1

0.001

0.01

0.1

1Ba (ppm)

1.6×10－6

1.6×10－4

4.6×10－4

9.5×10－2

8.0×10－6

5.1×10－3

5.7×10－7

7.6×10－6

1.E-07

1.E-05

1.E-03

1.E-01La (ppm)

3.5×103

2.1×10244

1.2×10－1

1.8

6.1×1022.8×103

2.8×105

1.E-02

1.E+00

1.E+02

1.E+04

1.E+06Ba/La

Nishino

miya,

Japan

(Otsuka

and

Carnmen

ellis,

England

(Smedley,

1991)

Brittany,

France

(Dia et

al., 2000)

present

data Fig. 9 Relationship between log [SO4

2－] and log[Ba]. The straight line indicates the boundary between saturated and unsaturated areas for barite.

Asagoe et al.

Fig. 9 Relationship between log[SO42－] and log[Ba]. The straight line indicates to show the boundary between saturated and unsaturated areas for barite.

Log[

SO42－

] (pp

m)

（148）

－　　－148

da and Ikeuchi, 1979). In such coprecipitation case, when Fe

hydroxides dissolve under reducing condition, Fe and REE

should be released to the aquifer, which results in the positive

correlation between Fe and REE. However, Fe and La (REE)

data do not show positive correlation (Fig. 10). Therefore

more careful considerations are necessary.

Given that relatively high Fe concentrations (ppm level)

indicate dissolution of Fe hydroxides under reducing condi-

tions, to achieve REE depletion via coprecipitation, some Fe

hydroxide precipitation should occur somewhere under the

wells or re-adsorption of REE on some residual Fe hydroxides

due to relatively strong adsorption power of Fe hydroxides

for REE might have occurred.

Similarly, negative tendency between Mn and Fe (Fig. 6)

suggests Mn coprecipitation on Fe hydroxides. However, be-

cause Mn is a redox sensitive element, the negative tendency

may be attributed to the difference in redox potentials of Mn

and Fe components. Considering the boundary positions on

Eh-pH diagram, Mn can be released under more oxidizing con-

ditions than Fe, i.e., high Mn waters were formed under rela-

tively oxidized conditions. On the other hand, low Mn and

high Fe waters can be created by dissolution of the Mn scav-

enged Fe hydroxides.

In contrast to the negative tendencies seen in the Fe-REE

and Fe-Mn relationships, the positive correlation between Fe

and Ba is obvious (Fig. 10). Such positive correlation can be

simply explained by dissolution of Ba coprecipitated Fe hy-

droxides under reducing conditions. This is consistent with

“Reductive dissolution” as has been considered by many re-

searchers for high As and P groundwaters (e.g., Ravenscroft

et al., 2009).

However, the problem is that there occur two contrasting

relationships such as Fe-REE (negative) and Fe-Ba (positive) in

the same areas. As for the reason of this problem, the ad-

sorption abilities of Fe hydroxides for REE and Mn might be

stronger than those for Ba, As, and P. In such cases, if re-ad-

sorption might occur, REE and Mn should be adsorbed on Fe

hydroxides more strongly than Ba, As, and P, and thus REE

and Mn were scavenged.

As seen in Fig. 7, the REE patterns lack Ce anomalies or

show small positive Ce anomalies. This suggests reducing

condition in the aquifer, because Ce(III) is known to be oxi-

dized to Ce(IV) under oxidizing conditions. If Ce(IV) may oc-

cur, larger Ce anomalies should be observed. The REE pat-

terns show small positive Eu anomalies, which is consistent

with the reduction of Eu(III) to Eu(II), but it is known that

Eu(III) will not be reduced under earth’s near surface condi-

tions (Bau, 1991). As for positive Eu anomalies, because feld-

spars have large positive Eu anomalies (Smith, 1974), such

high Eu concentration in feldspars may contribute to the posi-

tive Eu anomaly via weathering or water-rock interactions in

the aquifers.

On the Cl－ vs. Na and Cl－ vs. Br－ plots, the deviations of

the points from the seawater lines suggest addition of Na and

Br － or depletion of Cl － in groundwaters. However, depletion

of Cl－ is unlikely, because Cl－ is known as a most conservative

component in aquatic processes due to weakness of Cl － bear-

ing compound formation (Feth, 1981). Addition of Na can be

explained by the weathering of minerals which contain Na

(i.e., albite), but the sources of Br－ cannot be identified at pre-

sent. The possibility of artificial addition of Br － (e.g., pesti-

cides, medicines, industrial solvents, gasoline additives, and

water purification compounds) was suggested by Price et al.,

(1988).

Acknowledgements

We thank the Center for Environmental Management of

Kobe University for the use of the atomic absorption spec-

trometer. We also thank Dr. K. Ohkushi for checking the

manuscript.

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