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Procedia Earth and Planetary Science 17 (2017) 622 - 625
15 th Water-Rock Interaction International Symposium, WRI-15
Groundwater Arsenic in the Bengal Delta Plain: Geochemical and
Geomorphological Perspectives
D. Chatterjee^1, A. Kundua, D. Sahaa, S. Barmana, U. Mandala
aDepartment of Chemistry, University of Kalyani, Nadia, Pin-741235 , India
Abstract
The presence of arsenic (As) in groundwater is a wide-spread problem in south-east Asia, especially West Bengal (India) and
Bangladesh. Water quality and chemistry of the groundwater have been investigated by a systematic screening operation of tube
wells in a locality in West Bengal (India). Groundwater is predominantly of Ca-HCO3- type and is occasionally enriched with Na
and Cl- in shallow wells (15-20 m) which may be an indicator of local pollution source. The groundwater As distribution and
their relationship with land-use characteristics suggest that As release is also influenced by local land use conditions (e.g.,
sanitation, surface water bodies and agricultural practices). The interconnectivity of these land use conditions with the aquifer
underneath often facilitates As release creating heterogeneity in their distribution.
© 2017 The Authors.Publishedby Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of WRI-15
Keywords: Groundwater; Arsenic; Heterogeneity; Land use pattern; (Hydro)chemistry.
* Corresponding author. Tel.: +91 33 2582 3883; fax: +91 33 2582 8282. E-mail address: debashis.chatterjee.ku@gmail.com
1878-5220 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of WRI-15
doi:10.1016/j.proeps.2016.12.166
1. Introduction
Drinking water has always been considered as the major ingestion route of arsenic (As) to human system. Simultaneously, soil and crops have now drawn more attention due to transfer of As in the food chain via groundwater-soil-plant system1. Several attempts (mostly hypothesis) have been made to find out the primary (origin) and secondary (mobilization) causes of this natural calamity (high As groundwater) and the spatial distribution pattern. In addition, the regional extension of the As distribution anomaly (spatio-temporal heterogeneity of As patches) has also been significantly noticed in various part of the Bengal Delta Plain (BDP). The magnitude of the problem is momentous and unprecedented, causing a serious threat to the health of several millions of peoples by arsenic induced diseases. The groundwater As concentration very often exceeds the normed national drinking water standards (10 ^g/L and 50 ^g/L where no other option is available) as well as the WHO guideline value (10 ^g/L)2. Sporadic cases of arsenicosis (and other As induced diseases) were first discovered in the mid eighties in West Bengal, India, and it is also widespread in the neighboring areas of Bangladesh3. The spatial distribution of As in the in the vast alluvial deposits of BDP does not follow any specific trend and is often very heterogeneous, with irregularly distributed As-rich pockets over the whole area. In BDP, the spatial distribution of groundwater As is often heterogeneous and patchy (contaminated areas interspersed with non-contaminated areas).
Groundwater being safe (low risk) in terms of microbial contamination, easy to tap under the direct control of the user, has heightened social dependence in meeting the ever growing demands of irrigation and drinking. The pressing need for irrigation water to meet targets under 'Green Revolution' where shallow tubewells mushroomed in the alluvial lower deltaic plain. As a result, tubewells have been installed rather indiscriminately throughout the soft alluvial tract.
Despite of intensive scientific research there is no adequate answer to the question on the ultimate cause for the occurrence of the high arsenic concentrations in the groundwater of these areas and none of the proposed models can explain in a satisfactory way all the observed features in the distribution of arsenic concentrations in groundwater, such as their spatio-temporal heterogeneity, their incidental variation with local geomorphologic elements, with aquifer lithology, geochemistry, and microbial activity. Only a detailed understanding of the complex interrelationships among the different parameters can shed light on the ultimate cause responsible for the occurrence of arsenic contaminated groundwater reservoirs with regional extent.
2. Methods and materials
Hydrogeochemical screening was carried out during 2007 - 2008 over an area of about 400 km2 in Chakdaha Block (~ 55 km north of Calcutta), Nadia District, West Bengal, India, collecting ground water samples from 181 tube wells screened at depths between 7 and 271m. Later on multivariate statistical methods have been applied to interpret the hydrogeochemical data. In this study, we demonstrate the usefulness of some multivariate methods (factor and cluster analysis) in the interpretation of the hydrogeochemical data from an arsenic affected groundwater system4. Water samples were filtered (0.45^m membrane filter) and collected in three separate bottles: one for the analysis of total As (AsT), cations and trace elements (acidified with HNO3), one for the major anions and another for determination of As(III)/As(V). As(III) and As(V) were separated on-site by passing the sample through a cartridge charged with an ion exchanger (Metal Soft Centre, Highland Park, USA) which adsorbs As(V) selectively. Major and trace elements were determined by HR-ICP-MS, and anions by UV spectrophotometry and Ion Chromatography.
3. Results and discussion
The study reveals that the groundwater in the area is by-an-large moderately reducing in character (Eh: min. 161 mV, max. 459 mV, mean 278 mV) with high FeT (min. 0.009 mg/L, max. 45.9 mg/L, mean 37.5 mg/L), AsT (min. 1 ^g/L, max. 333 ^g/L, mean 52 ^g/L), HCO3- (min. 83 mg/L, max. 388 mg/L, mean 222 mg/L) and relatively low in nitrate, sulphate and low to moderate in Cl- content (min. 1 mg/L, max 147 mg/L, mean 21 mg/L). The Piper
diagram (Fig. 1) shows that groundwater is predominantly Ca-HCO3" type, though some samples are slightly shifted towards waters of Na-Cl- type (not common in the alluvial aquifer of the BDP), but with an evident Na deficit. The bulk of the sampled wells (84.5%) are of Ca-HCO3- type, as common for the BDP3. It is interesting to note the detection of Na-Cl- type groundwaters in the alluvial aquifer of the BDP. The much more common Ca-HCO3- type is promoted by the breakdown of organic matter and dissolution of carbonate minerals in the sediments, a process which largely accounts for the high HCO3- load of the groundwater. The Na-Cl- type wells are dominantly shallow (depth range 17-24 m) and are mostly privately owned (household wells). These wells are also relatively young (<15 years) and are located very near (~1-3 m) to pit latrines. The Cl- in the groundwater is most probably derives from the sanitary system, as found in other areas of the BDP4.
Fig. 1. Piper diagram consisting of major ions [o indicating Ca-HCO3- type water, A indicating Na-Cl- type water, □ indicating high Na (and in
some cases low Cl-) type water].
Land use characteristics survey was also conducted during sampling campaign. Arsenic distribution in four major land use characteristics is shown in Table 1. The highest number of wells (41%) is located adjacent to sanitations (or sanitary installations) followed by sanitation and surface water bodies (28%), agricultural fields (19%) and surface water bodies (12%). Mean As concentration varies among the selected land use characteristics (Table 1). Tube wells located adjacent to agricultural fields have the highest mean As concentrations (67 ^g/L). It is interesting to note that number of contaminated wells (>50 ^g/L) are associated with sanitations (n = 26) followed by sanitation and surface water bodies (n = 17), agricultural fields (n = 12) and surface water bodies (n = 9). However, the wells located near the surface water bodies are mostly affected when percentage of affected wells (>50 ^g/L) is considered (Table 1). Relatively older wells are located adjacent to sanitation installations and sanitation and surface water bodies. Deeper wells are located adjacent to surface water bodies and agricultural fields, whereas, relatively shallower wells are located adjacent to sanitary installations and sanitation and surface water bodies.
Table 1. Arsenic distribution in relation to local land use characteristics.
Land use characteristics Number of wells As (^g/L) Mean As (Hg/L) As >50 ^g/L (number of wells) % of affected wells
Sanitation 74 0-217 44 26 35
Sanitation and surface water 50 0-290 49 17 34
bodies
Surface water bodies 22 0-285 60 9 41
Agricultural fields 35 0-333 67 12 34
4. Conclusion
Generally the groundwaters in the study area are of Ca-HCO3" type. However, a Na-Cl- type is occasionally also found. In BDP, the cause for the spatio-temporal heterogeneity of the As concentrations is an important issue to be understood. Present study demonstrates the groundwater chemistry of As affected aquifers in Chakdaha block, Nadia district, West Bengal, is predominantly under anoxic condition. Critical evaluation of groundwater chemical composition reveals that Fe-oxyhydroxide reduction and As mobilization may not be the only process to explain high As concentrations in the studied groundwater. The major breakthrough is the morphology of land-use pattern (surface water bodies, sanitations and agriculture-aquifer-sediment interactions). This can suitably explain the heterogeneity of As distribution in the groundwater. Apart from land-use pattern, the local (hydro)geological regime (sediment-water interaction under local scale flow pattern) is important to explain patchy distribution of As under flat basinal topography.
References
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2. RGNDWM (Ministry of Rural Water and Development, Government of India), 2001. Phase-II Report, University of Kalyani, Nadia, West Bengal, India.
3. Bhattacharyya R, Jana J, Nath B, Sahu SJ, Chatterjee D, Jacks G. Groundwater As mobilization in the Bengal Delta Plain, the use of ferralite as a possible remedial measure - a case study. Appl Geochem 2003;18:1435-51.
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