Environ Earth Sci DOI 10.1007/s12665-010-0516-4
ORIGINAL ARTICLE
Temporal variations in arsenic concentration in the groundwater of Murshidabad District, West Bengal, India S. H. Farooq • D. Chandrasekharam • S. Norra • Z. Berner • E. Eiche • P. Thambidurai • D. Stu¨ben
Received: 20 July 2009 / Accepted: 5 March 2010 Ó Springer-Verlag 2010
Abstract Systematic investigations on seasonal variations in arsenic (As) concentrations in groundwater in both space and time are scarce for most parts of West Bengal (India). Hence, this study has been undertaken to investigate the extent of As pollution and its temporal variability in parts of Murshidabad district (West Bengal, India). Water samples from 35 wells were collected during premonsoon, monsoon and post-monsoon seasons and analyzed for various elements. Based on the Indian permissible limit for As (50 lg/L) in the drinking water, water samples were classified into contaminated and uncontaminated category. 18 wells were reported as uncontaminated (on average 12 lg/L As) and 12 wells were found contaminated (129 lg/L As) throughout the year, while 5 wells could be classified as either contaminated or uncontaminated depending on when they were sampled. Although the number of wells that alternate between the contaminated and uncontaminated classification is relatively small (14%), distinct seasonal variation in As concentrations occur in all wells. This suggests that investigations conducted within the study area for the purpose of assessing the health risk posed by As in groundwater should not rely on a single round of water samples. In comparison to other areas, As is mainly released to the groundwater due to Electronic supplementary material The online version of this article (doi:10.1007/s12665-010-0516-4) contains supplementary material, which is available to authorized users. S. H. Farooq (&) D. Chandrasekharam P. Thambidurai Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India e-mail:
[email protected] S. H. Farooq S. Norra Z. Berner E. Eiche D. Stu¨ben Institute of Mineralogy and Geochemistry, Karlsruhe Institute of Technology, Karlsruhe 76131, Germany
reductive dissolution of Fe-oxyhydroxides, a process, which is probably enhanced by anthropogenic input of organic carbon. The seasonal variation in As concentrations appear to be caused mainly by dilution effects during monsoon and post-monsoon. The relatively high concentrations of Mn (mean 0.9 mg/L), well above the WHO limit (0.4 mg/L), also cause great concern and necessitate further investigations. Keywords Arsenic Groundwater contamination Seasonal variations
Introduction In many regions of the world, especially India (Das et al. 1994), Bangladesh (Ahmed et al. 2004), Vietnam (Berg et al. 2001), China (Sun 2004) etc., As concentrations in groundwater are significantly higher than the limit of 10 lg/L set by World Health Organization (WHO 2006). Arsenic contamination in groundwater of West Bengal and Bangladesh is well documented by various workers (Bhattacharya et al. 1997; Chakraborti et al. 1996; Chandrasekharam et al. 2001; Norra et al. 2005; Stu¨ben et al. 2003; van Geen et al. 2008) and is described as the world’s biggest arsenic calamity (Chatterjee et al. 1995). The presence of naturally elevated levels of As in groundwater is confirmed in seven Indian states, namely West Bengal, Bihar, Uttar Pradesh, Assam, Jharkhand, Chattisgarh and Madhya Pradesh (Das et al. 1996; Chakraborti et al. 2003; Ahamed et al. 2006; Paul and Kar 2004; Bhattacharjee et al. 2005; Acharyya et al. 2005; Chakraborti et al. 1999). In West Bengal, investigations suggest that nine districts, namely Malda, Murshidabad, Burdwan, Nadia, Hoogly, Howrah, Kolkata, 24 Parganas (North) and
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24 Parganas (South), are the most severely affected; and nearly 50 million people are at risk due to consumption of As contaminated water (http://soesju.org/arsenic/wb1.htm). However, the processes causing the high As concentrations in the affected aquifers are not fully understood yet, but it is estimated that about 200 million people in Asia are exposed to As contaminated drinking water (Sun et al. 2006). The presence of As in groundwater is associated with a number of adverse effects on human health. The USEPA considers As to be a human carcinogen (US Environmental Protection Agency, 1996). Long time exposure of As may cause various diseases such as melanosis, keratosis, nonpetting oedema, gangrene, leucomelanosis, circulatory system problems and an increased cancer risk, especially of skin, bladder and lungs (Arnold et al. 1990; Chen et al. 1996; Karim 2000). A review of the health problems associated with consumption of As is given in a report by WHO (2006). Apart from clinical symptoms, a number of social and societal problems are aggravating the situation (Bhattacharya et al. 2002; Ahmed et al. 2007; Nriagu et al. 2008). Dissolution of marriages and avoidance of arsenicosis patients are reported in many areas. The severity of the As problem in West Bengal and Bangladesh (jointly called as Bengal delta plain) has prompted many governmental and non-governmental organizations to come forward and act immediately. As a result, millions of wells were sampled and, depending upon the results of analysis, areas were labeled as As contaminated or safe. These samples were, however, collected without taking into consideration that As concentrations may vary throughout the year due to seasonal effects (premonsoon, monsoon and post-monsoon). Only a few studies were conducted on seasonal variations of As concentrations in West Bengal and Bangladesh, some of them (McArthur et al. 2004; Kinniburgh and Smedley 2001; Cheng et al. 2005; Wagner et al. 2005) suggest absence of any significant change in As concentration, while others (Savarimuthu et al. 2006; Yakota et al. 2001) show very pronounced seasonal variations. Dhar et al. (2008) suggested that the temporal variability of dissolved As concentrations is a function of water age, which is closely related to specific geomorphic units in which groundwater occurs. However, additional factors might also play a role. This study has been undertaken to (1) investigate whether As concentrations in groundwater fluctuate in response to seasons in the study area, (2) assess if such fluctuations are sufficient to cause an area to be classified as uncontaminated during one season but contaminated during the rest of the year. Areas that experience large seasonal fluctuations in As concentrations might require more extensive monitoring programs before the groundwater can be labeled as safe or contaminated for the affected residents.
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Sampling and analytical methods Study area The study area is located in the eastern part of Murshidabad district (latitude: N24°1400 –N24°1700 , longitude: E88°3100 – 88°4300 ), West Bengal, India and occurs within the delta plain of the meandering river Padda (a tributary of the river Ganga). In the east, the area is confined by the international boundary with Bangladesh (Fig. 1). The general groundwater flow direction in West Bengal is north-west to southeast (Das et al. 1996). Three interconnected aquifer systems exist in the Bengal delta plain. The shallowest aquifer extends up to 12–15 m below the groundwater level and is unconfined in most parts of the delta. The shallow aquifer is mainly composed of fine to medium grained sands with occasional intercalation of clay lenses. The intermediate (35–46 m) and deep aquifers (70–150 m) are reported to be at shallower depth in the Murshidabad district (Stu¨ben et al. 2003) where the study area is situated. For all domestic purposes groundwater is mainly pumped from tube-wells placed mostly in the shallow aquifer and occasionally in the intermediate aquifer. Water sampling and analysis To investigate the magnitude of seasonal variability in As concentrations, water samples were collected from 35 wells (33 tube wells and 2 open wells) during the years 2005– 2006 on a random basis. Each well was sampled once for all the three seasons: pre-monsoon (March 2005), monsoon (August 2005) and post-monsoon (January 2006). These sampled wells (mostly shallow tube wells, B30 m; as shown in supplementary data sheet 1) are mainly used for drinking water purposes (Fig. 1). Out of the 35 sampled wells, 21 are actively used domestic wells, 11 are wells of public utility placed near schools, bus stops etc., two are open wells and one is used as irrigation well. The depth of these wells varies from 3 to 98 m (24 wells are up to 20 m deep (shallow aquifer), 4 wells are between 21 and 40 m deep (mainly intermediate aquifer), 7 wells are 41–100 m deep (deep aquifer) which means that water from all three aquifers is exploited. The two sampled open dug wells (well number 7 and 31) are 5 and 3 m deep, respectively. Information about the depth of wells was acquired through personal communication with the well owners. Each well site was geo-referenced using a Garmin Global Positioning System (GPSMAP-76S) instrument. Water samples from all wells include the collection of: (1) filtered samples (0.45 lm cellulose nitrate filter) for analyses of major anions; and (2) filtered (0.45 lm cellulose nitrate filter) and acidified (with 5 mL 14 M ultrapure HNO3/L) samples for major cation and trace element analyses. All
Environ Earth Sci
Fig. 1 Area map showing locations of water sample collection wells
samples were tightly sealed and stored at low temperature until further analyses. The tube wells, which were not in active use, were purged for a minimum of three casing volumes before sampling, while actively used ones were purged for a minimum of 2 min. pH values, temperature and electric conductivity (EC) were measured in the field instantly after collecting the samples. The pH value was measured with a portable pH-meter (Orion 261S) using a combination electrode (pH C2401–7) and EC by a conductivity meter (Orion 250A?) with an operating range between 0 and 500 mS/cm. Analysis for all major (cations) and trace elements was done by high resolution ICP-MS (Axiom, Thermo/VG Elemental, UK) at the Institute of Mineralogy and Geochemistry, Karlsruhe Institute of Technology, Germany. Sulfate (SO42-) concentrations were measured by spectrophotometer (Shimadzu UV-Visible spectrophotometer 160), alkalinity by titration and chloride (Cl-) by Expandable Ionanalyzer 940A with a combination electrode Orion ionplus 9817 BN. Drinking water guidelines The resulting dissolved As concentrations were compared with the Permissible Indian Limit (PIL) and the WHO drinking water standards. The Government of India has set a provisional water quality standard of 50 lg/L of As in drinking water, which is five times higher than the WHO limit of 10 lg/L, the limit also set by various developed countries. Effects of As consumption on human health are not uniform and are controlled by many factors, such as nutritional value of daily diet (Chen et al. 1988; National Research Council 1999), standard of living etc.; thus, no unambiguous threshold value can be defined as the
dividing line between ‘‘safe’’ and ‘‘unsafe’’. Further, WHO guidelines for water quality apply to ‘‘finished water’’. For most study wells, which were primarily domestic wells, ‘‘finished water’’ may not be equivalent to filtered water. The data presented here may be slightly biased towards low As concentrations as the samples were filtered to minimize the colloids and to remove the sediments entrained during pumping.
Results In general, groundwater in the study area is nearly neutral to mildly alkaline. The pH value ranges from 6.8 to 7.7 for all three seasons (Table 1). During pre-monsoon season pH values range from 7.2 to 7.7 (mean 7.4), while for monsoon and post-monsoon the values range from 6.8 to 7.5 (mean 7.2) and 6.9–7.6 (mean 7.2), respectively. Considerable temporal variations are observed for all major ions (Table 1). Ca2? (55.3–276 mg/L) is the dominating cation and HCO3- (231–958 mg/L) is the major anionic species present in the groundwater irrespective of seasons. Concentrations of other major cations such as Na? (2.7–108 mg/L), K? (2.5–50.6 mg/L) and Mg2? (8.7– 123 mg/L) and anions such as Cl- (7.2–157 mg/L) and SO42- (10.6–129 mg/L) also show a considerable variability. Mean SO42- concentrations for pre-monsoon, monsoon and post-monsoon are 38.1, 36.2 and 34.7 mg/L, respectively. As a whole, SO42- concentrations in the study area are quite high (mean 36.3 mg/L) compared to other As affected areas (Eiche et al. 2008; Smedley and Kinniburgh 2002), where they are frequently well below detection limit. Chloride shows considerable seasonal variations in mean concentrations (pre-monsoon 54 mg/L, monsoon 42 mg/L and post-monsoon 40.5 mg/L), as the mean Cl-
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Environ Earth Sci Table 1 Statistical summary of the chemistry of groundwater (major and trace elements; n = 35) from Nabipur block, Murshidabad, West Bengal Parameter
Pre-monsoon
Monsoon
Mean Median Min. pH
7.4
HCO3- (mg/L) 569 Cl- (mg/L) SO42?
(mg/L)
Na (mg/L) K? (mg/L) Mg2? (mg/L) Ca2? (mg/L)
54.0
7.4 559
7.2 260
Max. Standard Mean Median Min. deviation 7.7 958
39.4
7.9 157
38.1
32.6
10.6 129
27.2
21.5
9.2
7.0
45.5
40.6
162
162
Post-monsoon
2.7
89.8
2.6
46.8
0.1 151 43.4
7.2 492 42
7.2 488 27.3
25.7
36.2
27.7
19.3
27.7
19.0
6.8 312
Max. Standard Mean Median Min. deviation 7.5 847
7.8 113 19
91.0
6.9 108
0.2 112 33.8
7.2 492 40.5
7.2 470 25.5
6.9 231
Max. Standard deviation 7.6 694
7.2 137
0.2 104 37.0
20.1
34.7
25.6
15.9
83.4
19.1
22.5
25.3
17.6
5.1
97.0
19.8
7.7
8.3
6.1
3.8
50.6
8.0
5.8
5.1
2.5
19.2
3.0
15.4 123
23.4
34.2
32.8
13.6
58.3
11.1
31.6
28.7
8.7
63.7
12.3
56
276
55.2
128
123
82.6 230
30.8
121
116
55.3 207
32.9
As (lg/L)
63.2
44.7
0.4 301
70.4
59.2
24.3
1.0 341
75.7
54.9
26.4
0.9 325
71.9
Fe (mg/L)
36.0
21.4
0.1 151
33.9
23.8
10.9
1.9 150
32.4
23.8
11.9
3.8 119
27.0
Mn (mg/L)
1.1
0.9
0.0
3.0
0.7
1.0
0.8
0.0
0.6
0.8
0.7
0.0
Cu (lg/L)
22.9
18.4
1.8
86.9
17.4
14.3
6.1
2.7 142
30.0
8.5
5.6
3.3
48
8.2
U (lg/L)
2.8
1.1
0.0
27.6
5.2
3.3
1.0
0.0
5.6
3.5
1.2
0.0
28.3
5.6
concentration for pre-monsoon is 33% higher than for postmonsoon. The major ion composition, plotted on a piper diagram (Fig. 2), indicates that the groundwater is mainly of Ca–HCO3 type, which is typical for As affected water bodies in young Asian deltaic aquifers (Eiche et al. 2008). The piper diagram indicates that there are no major seasonal differences in the hydrochemical characteristics of wells. Data clustering suggests that the groundwaters evolved mainly from a rainfall derived recharge which percolated through unsaturated sandy sediments and interacted with the sediments for a relatively short period of time. Iron concentrations vary from 91 lg/L to 150 mg/L. With respect to seasonal variations, most of the samples show highest Fe concentrations during pre-monsoon. The mean concentrations during pre-monsoon, monsoon and post-monsoon are 36.0, 23.8 and 23.8 mg/L, respectively (Table 1). Considerable seasonal variations in Mn concentrations are also observed, but are less prominent when compared to Fe concentrations; and the mean seasonal concentrations range between 0.8 and 1.1 mg/L. In general, the mean Mn concentration show a relative decrease of 24% from pre-monsoon to post-monsoon, while in the case of Fe a relative decrease of 34% is noticed from premonsoon to post-monsoon. Arsenic concentrations vary from less than 1 to 341 lg/L (median 31.3 lg/L). 49% of the sampled wells shows As concentrations above the limit of 50 lg/L either in one or all the seasons throughout the year. The mean As concentrations for pre-monsoon, monsoon and post-monsoon are 63.2, 59.2 and 54.9 lg/L, respectively. Like most of the elements (except U and Na) mean As concentrations also show a clear
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2.9 29.8
2.5
0.5
Fig. 2 Plot of the major ion composition of the groundwater samples in a piper diagram
decreasing trend from pre-monsoon through monsoon to post-monsoon (Table 1).
Discussion To evaluate the seasonal variability, the water samples were divided into three groups: (1) Group-L, having As concentrations \50 lg/L throughout the year (2) Group-H, those having As concentrations C50 lg/L throughout the year and (3) Group-V, those having As concentrations \50 lg/L during some parts of the year and C50 lg/L
Environ Earth Sci
However, the absence of a strong correlation between dissolved As and Fe in the water samples collected for this study suggest that processes other than reductive dissolution are also involved in the release of As to groundwater. 86% of wells do not change their category and stay in either of the two groups (Group-L; safe or Group-H; unsafe) throughout the year. In the remaining 14% of wells (Group-III), As concentrations vary throughout the year to the extent that they change their association from one group to the other. Thus, these wells can be labeled as uncontaminated in some parts of the year whereas in the other parts of the year they can be labeled as contaminated. In both groups (Group-L and Group-H) some wells show considerably large seasonal variations with regard to As concentration. The standard deviation for the wells within these groups vary between 0.3–25 and 2.1–36.7, respectively, indicating that the seasonal variability of the As concentration is considerably higher for Group-H as compared to Group-L. The standard deviation for As concentrations in Group-V ranges from 22.6 to 99.5. These variances indicate that the extent of the change in As concentration is always relatively high when a well shifts between the L and H groups. In order to get a better understanding of the seasonal variations, samples were further grouped into (1) Group-A and (2) Group-B, based on the standard deviation of their As concentrations in different seasons. The samples that show coefficients of variation (CV) in As concentrations \20% were allocated to Group-A while the samples that show higher CV in As concentrations ([20%) were allocated to Group-B (Table 3). 17 samples (49%) belong to Group-A which include 7 samples with less than 50 lg/L of As and 10 samples with more than 50 lg/L of As. The
during other parts of the year (Table 2). The separation of these groups is based on the limit of 50 lg/L set by the Government of India to classify water as polluted or unpolluted with regard to dissolved As concentrations. In addition to low As values (mean 12.2 lg/L), Group-L is characterized by lower concentrations of redox-sensitive elements such as Fe (mean 15.3 mg/L) and Mn (0.9 mg/L), compared to Group-H (As (mean) 128 lg/L, Fe (mean) 47.5 mg/L, Mn (mean) 1.1 mg/L; Table 2). It is worth mentioning that the water of Group-L wells might be safe for drinking with regard to As concentrations but in many wells the concentration of other elements (e.g. dissolved Mn etc.) exceeded the WHO limits, which makes the water from these wells unacceptable for drinking purposes. It is noteworthy to observe the difference in uranium (U) concentrations between the two groups (Group L and H) because the redox behavior of this element is generally opposite to that of Fe and Mn. This is reflected by the roughly four times higher mean U concentrations in GroupL (5.2 lg/L) as compared to Group-H (Table 2). Group-L has also roughly 10% lower HCO3- concentrations (507 mg/L as compared to 547 mg/L in Group-H). All the other measured elements have more or less similar mean values. These hydrochemical characteristics indicate that the higher As concentrations appear preferentially in the context of a low redox hydrogeochemical environment. Consequently, it is reasonable to assume that high proportions of dissolved As and Fe results from the reductive dissolution of Fe-oxyhydroxides. It is widely accepted that Fe-oxyhydroxides in the Bengal delta plain sediments serve as the source of As, which is released under reducing conditions (Bagla and Kaiser 1996; Bhattacharya et al. 1997; Nickson et al. 1998; Ravenscroft et al. 2001).
Table 2 Statistical summary of the chemistry of groundwater (major and trace elements; n = 35) after dividing them into different groups Parameter
Group-L (Unpolluted, n = 18) Mean Median Min.
pH 7.3 7.3 HCO3- (mg/L) 507 491 Cl- (mg/L)
49.4
35.7
SO42- (mg/L)
36.9
31.1
Na (mg/L)
28.0
21.7
K (mg/L)
9.0
6.6
Mg (mg/L) Ca (mg/L)
37.8 135
35.7 122
As (lg/L)
12.2
4.9
Fe (mg/L)
15.3
Mn (mg/L)
0.9
Cu (lg/L) U (lg/L)
Group-H (Polluted, n = 12)
Max. Standard Mean deviation
7.0 7.7 0.2 231 958 128 7.2 129 12.3
84.1
2.7 108 2.6
50.6
Median Min.
Group-V (n = 5)
Max. Standard Mean Median Min. deviation
7.2 547
7.2 527
35.7
51.8
27.2
10.1 157
43.9
16.4
10.3
7.5
65.3
14.5
19.2
41.1
30.8
11.0 129
26.5
23.0
22.3
10.6
49.8
9.1
22.2
28.8
18.4
8.8
83.3
20.9
17.2
17.0
11.2
32.1
4.6
8.9
6.8
6.0
3.6
17.3
2.8
5.4
5.4
2.5
10.0
1.9
19.6
78.2
15.3
27.8
17.6
61.6
10.6
39.0
35.2
6.8 7.5 0.2 359 847 134
7.3 7.3 486 459
Max. Standard deviation
8.7 123
19.9
55.3 276
45.0
147
138
81.6 253
45.1
128
111
51.3 341
73.5
61.8
56.6
6.0 151
41.5
25.9
0.5
0.8
30.5
0.4
49.9
14.2
10.7
2.2
82.4
15.2
47.5
32.2
0.7
0.0
3.0
0.8
1.1
1.1
0.4
13.6
7.9
3.3
86.9
15.2
19.9
8.6
2.7 142
5.2
1.9
0.0
29.8
6.8
0.4
0.0
1.14
2.0 8.2
1.73
29.8 118
112
7.0 7.6 0.2 368 790 105
74.1 227
36.2
1.4 180
59.2
18.3
0.1
69.1
23.6
0.8
0.3
1.2
0.3
10.0
7.4
1.8
29.0
7.6
1.0
0.3
0.0
5.7
1.7
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Environ Earth Sci Table 3 Grouping of samples based on the variance in seasonal As concentration Parameter
Group-A (Coefficient of variance in As concentrations \20%) Group-B (Coefficient of variance in As concentrations [20%) (n = 17) (n = 18) Mean Median Min.
pH
7.2
HCO3- (mg/L) 532
7.2 503
6.8 348
Max. 7.5
Standard CV deviation range
Mean Median Min.
0.1–0.2
0.8–3.4
858
12–209
3.0–34.0 504
7.3
7.3 479
7.0 231
Cl- (mg/L)
49.6
29.4
10.1
157
0.3–29.5
2.9–62.8
41.6
29.9
7.2
SO42-
11
129
40.5
30.9
1.2–28.4
3.8–64.4
32.4
25.6
10.6
Na (mg/L)
(mg/L)
29.3
19.1
8.8
89.8
0.9–33.6
2.5–65.5
24.4
18.3
2.7
K (mg/L)
8.3
6.1
3.1
50.6
0.3–24.9
4.0–72.5
7.2
6.2
2.5
35.1
Mg (mg/L) Ca (mg/L)
39.2 141
37.7 124
19.6
123
1.9–51.6
5.7–81.3
82
270
8.7–93.6
7.6–57.8 133
30.5 120
8.7 55
Max.
Standard CV deviation range
7.7 0.1–0.3 32–265
5.0–41.0
145
0.2–27
1.7–74.7
84.1 0.6–14.8 108
0.5–28.8
22.5 0.8–8.0 2.1–48.9
7.4–79.1
3.1–93.2
2.7–55.3
180
0.3–99.5
23.5–157.5
84.4
70.8
1.2
341
0.3–28.8
1.0–19.7
35.2
20.1
0.4
31.4
16
3
151
1.4–18.5
7.7–106
24.6
13
0.1
793
83
1,990
47.1–412
143
1.4–65.5
10.5–132
12.8
7.8
1.8
86.9 2.9–47.3
0–11.7
0.9–82.1
3.2
1.2
0.0
29.8
Cu (lg/L)
17.8
7.9
3.3
U (lg/L)
3.2
1.0
0.0
27.6
4.9–87.1 961
862
23
2.9–56.3 11.7–86.9
118
Fe (mg/L)
937
2.4–51.5
276
As (lg/L) Mn (mg/L)
1.3–3.5
958
93.8 1.1–41.4 3,000
7.8–785 0–13.6
5.6–136.2 5.0–48.8 35.9–147.1 2.5–94.4
CV = SD 9 100/mean
remaining 18 samples are in Group-B. The samples in Group-B include 11 samples with less than 50 lg/L of As, 2 samples with more than 50 lg/L of As and 5 samples that have\50 lg/L of As in some parts of the year and[50 lg/L of As in the remaining part of the year. In addition to As, samples of Group-B show greater variances in redox sensitive elements (Table 3). A higher degree of variance in Fe along with Cu and U in Group-B, suggests that a wide fluctuation in the redox conditions in the aquifer is responsible for the greater seasonal variation in As concentrations. The redox potential data from many previous studies demonstrate that mild oxidizing to moderately and/ or strongly reducing conditions exists in the Bengal delta plain (BGS and DPHE 2001; Bhattacharya et al. 2002). Our study suggests a slight but continuous decrease of the As concentrations from the pre-monsoon to postmonsoon season due to dilution with percolating rain water (Table 1). During the period preceding the monsoon season, this decrease is possibly offset by processes that lead to a new episodic increase of the As concentrations. Among the possible mechanisms one may consider are (1) the mobilization of As under reducing conditions (Smedley and Kinniburgh 2002; Nickson et al. 2000) generated by the microbially mediated decay of natural organic matter (NOM) and/or dissolved organic carbon (DOC) (Harvey et al. 2002) and (2) the mobilization of As by organic acids (a fraction of DOC) that may play an important role in mineral degradation and metal mobilization (Baker 1973; Kalbitz and Wennrich 1998; Bauer and Blodau 2006). In West Bengal high quantities of organic matter are left over in the paddy fields due to traditional paddy cultivation. In
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paddy cultivation, harvested crop is cut from the middle of stem and the remaining half of the stem and roots are ploughed back for the next cultivation. During the monsoon, paddy fields are flooded with rain water and the decomposition of such plant remains increases the availability of DOC in general and of organic acids in particular. The availability of DOC and production of organic acids during the monsoon season is further enhanced by the local jute industry (biggest jute producer in the world), which follows the traditional practice of decomposing the jute crop in ponds in order to get jute fibres. In this way substantial quantities of DOC and organic acids are produced (as shown in supplementary data sheet 2), which can percolate down with rain water, and, on their way to groundwater react with mineral surfaces or act as electron donor for microbes. As a consequence, the reductive dissolution of Fe-oxyhydroxides would be enhanced. This might be one cause of the extremely high dissolved Fe-concentrations (\151 mg/L) in some wells. Furthermore, the presence of DOC modifies the solubility and mobility of many contaminants (Sensei et al. 1994). The negatively charged DOC, for example, increases the desorption of As from the binding sites through electrostatic effects (Bauer and Blodau 2006). A slight but quasi constant decrease of the pH values from pre-monsoon (mean 7.4) to monsoon/post-monsoon (mean monsoon 7.2, post-monsoon 7.2) further suggests percolation and mixing of organic acids, though some other factors may also play a considerable role in the fluctuation of pH values (Fig. 3). Higher pH values during pre-monsoon coupled with relatively minor seasonal fluctuations but with a clear
Environ Earth Sci Pre-monsoon Monsoon Post-monsoon
7.8 7.6
pH
7.4 7.2 7.0 6.8 6.6 1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35
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Fig. 3 Variations in pH values in response to seasonal changes
400 350
As (ug/L)
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2
R (H)= 0.42
250 200
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R (V)= 0.97
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100
Group-V
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R (L)= 0.34
0 0
20
40
60
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120
140
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Fig. 4 Graph showing Fe and As correlation during monsoon. The samples which are allocated to Group-L show only a weak correlation between Fe and As during monsoon (r2 = 0.34), while samples from Group-H and Group-V show a moderate (r2 = 0.42, Group-II) to very strong (r2 = 0.97; Group-V) correlation between Fe and As in the same season
decreasing trend in mean sulphate concentration (premonsoon 38.1 mg/L, monsoon 36.2 mg/L and post-monsoon 34.7) argue against the formation of sulphate by pyrite oxidation, because pyrite oxidation would imply the lowering of pH (Das et al. 1996). The samples that remain in Group-L (As \ 50 lg/L) throughout the year show only a week correlation between Fe and As (r2 = 0.34) during monsoon; however, the wells from Group-H (r2 = 0.42) and specially Group-V show a very strong correlation (r2 = 0.97) between As and Fe concentrations during monsoon period (Fig. 4). This suggests that the release of As and its high seasonal variability is controlled not only by rain water dilution, but also to some extent by changes in the redox environment. Although dilution effects alone could cause a covariance between As and Fe, the simultaneous increase of their concentrations during the pre-monsoonal season, certainly should have a different explanation. Furthermore, the wells that fall in Group-V shows a higher correlation (r2 = 0.70) between As and HCO3- during monsoon, which suggests that the release of As is associated with the degradation of organic material. The microbial degradation of organic matter leads to the formation of HCO3-. The displacing
effect of HCO3- for As sorbed onto Fe oxyhydroxides is well documented (Appelo et al. 2002). Sometimes higher concentrations of HCO3- fosters the release of As into the groundwater (Kim et al. 2000) and, consequently, be a further source for dissolved As. Apart from various other factors, distance from pools rich in organic matter and organic acids (paddy fields, ponds and waste dumping sites) may also play an important role in the seasonal variation of As concentrations in groundwater. Open (dug) wells, which are often located away from organic matter pools (agricultural fields/jute decomposing ponds), were found to have relatively low As concentrations, often well below (1.7–3.2 lg/L) than the WHO prescribed limit. These wells are just 3–5 m deep, a depth range in which direct recharge through vertical flow dominates, while mixing and recharge through horizontal flow is assumed to be negligible. The rain water/run-off which percolates from the surface into the groundwater, feeding these wells, has very low organic matter content and As concentrations. Differences among wells with respect to the seasonal fluctuation of As concentrations may additionally be controlled by various local factors, such as the screening depth of tube wells, the presence of clay intercalations/pockets in the cap-rocks of the aquifer, the proximity of organic acid production sites and of waste dumping sites, etc. There is no clear relationship between well depth and As concentration. Pockets and intermittent layers of clayey sediments, which are typical for deltaic sequences, prolongs the percolation time of surface water. Thus provides longer interaction time with the sediments that have higher specific surface area and abundant reactive sites for adsorption or desorption of As. There is no information about the hydrogeology of each well in order to further establish a possible relationship between well depth and hydrogeological units. Out of the five wells which shift their category in response to seasonal changes, well numbers 10 and 15 show a decrease in As concentration during the monsoon season probably due to dilution by rainwater. In both of these wells As concentrations increase during post-monsoon season (Fig. 5a, b), which might reflect a delay in the development of sufficiently reducing conditions. The development of reducing conditions in these wells is indicated by a significant increase of the Fe and HCO3concentrations during the post-monsoon season (as shown in supplementary data sheet 1). Wells 25 and 34 show a prominent ‘‘dilution effect’’ during post-monsoon rather than the monsoon season (Fig. 5c, d). Both of these wells are screened at a greater depth (46 and 98 m, respectively) as compared to the other sampled wells. The water table is at a very shallow depth in the study area (3 m) and the mixing of percolating rain
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Fig. 5 a–e Pattern of As concentrations in wells that exhibit a change from one group to another in response to seasonal variations
water with deeper groundwater is assumed to take longer time. Clay lenses and clay patches that are common in deltaic deposits exhibit very limited permeability. The slow infiltration through such lithologies can effectively delay the dilution of groundwater by precipitation derived water. A significant increase in the As concentration during monsoon in well 16 (Fig. 5e) could be due to some particular local conditions, which may affect As mobility. Proximity of waste dumping site, heterogeneity of aquifer characteristics, anthropogenic activity, etc. are potential causes for such special conditions. It is critical to monitor whether the inter-seasonal variability or total As concentrations increase with time; therefore, it is necessary that monitoring should be carried out over longer periods during all the three seasons. Even if only 14% of the wells are misclassified by disregarding the seasonal variability of As concentrations, the huge number of wells in the Bengal delta means that such a wrong labeling would affect the health of millions of people. Savarimuthu et al. (2006) also point out seasonal variations in As concentrations, but opposite to our results, the highest As concentrations were found during monsoon and the lowest in the pre-monsoon time. A monsoon season with less intensive rainfall is likely to trigger greater seasonal variations in As concentrations. A drop in average rainfall will not only result in lesser dilution of As, but will
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also lead to relatively high DOC and organic acids concentrations, thus inducing stronger reducing conditions. The results of this study are only partly in line with the findings of Dhar et al. (2008) conducted in Araihazar area in Bangladesh, where over a monitoring period of 2–3 years the As concentrations in some wells were increasing, while in others a decreasing trend was observed, and some wells remained practically unchanged.
Conclusions This study clearly shows that seasonal variability of dissolved As occurs in several wells but also suggests that a big majority (86%) of wells did not show such a prominent change in their As concentrations that they can be labeled as contaminated in one season and uncontaminated in the another. Nevertheless, a very high standard deviation in the seasonal As concentrations of these wells highlight a dire need for long term seasonal monitoring. Considering the high population density in West Bengal and the huge number of wells in the Bengal delta Plain, a wrong labeling of only a low percentage of the wells can pose a severe health threat to a considerably large human population. In general, this study suggests a kind of cyclical seasonal pattern, characterized by a decreasing trend of the As concentrations from the pre-
Environ Earth Sci
monsoon to post-monsoon period, followed by an increase during the shorter pre-monsoon time. The decreasing branch of the cycle is dominated by dilution through run-off and by a redox shift towards more oxic conditions, while the shorter increasing branch is characterized by a more intensive water/ sediment interaction and the development of more reducing conditions in the groundwater. There are also indications that the well depth and lithology may have an influence on the seasonal variability along with local peculiarities (dump sites etc.). Arsenic release may also be heavily influenced by the infiltration of organic carbon from agricultural processes; however, its influence on the variability of dissolved As concentrations could not be established from this study. A more detailed study on temporal variations of As is needed as it has important implications on the exposure and precision of the risk assessments. In addition to the various natural controls on As release from sediments to groundwater, it is also important to consider the role that organic matter plays in As mobility. Acknowledgments The authors gratefully acknowledge support from German Academic Exchange Service (DAAD) through research fellowship. Our thanks are also due to Mrs. Claudia Mo¨ssner of the Institute of Mineralogy and Geochemistry (IMG) for the chemical analyses.
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