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ISSN 0974-5904, Volume 07, No. 01
February 2014, P.P.35-40
Arsenic Contamination in the Groundwater of Thoubal and Bishnupur District of Manipur, India A K CHANDRASHEKHAR1, D CHANDRASEKHARAM1, THAMBIDURAI P1AND S H FAROOQ2 1
Department of Earth Sciences, IIT Bombay, Mumbai - 400076, INDIA School of Earth, Ocean and Climate Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar, INDIA Email:
[email protected],
[email protected],
[email protected],
[email protected] 2
Abstract: Groundwater plays an important role in shaping the economic and social health of urban and rural population throughout the globe, however the distribution of good quality groundwater is quite uneven. In many parts of the world, especially in the South and South-East Asian nations, manifold higher concentrations of arsenic (As) than the WHO permissible limit has been reported. Still it seems that the extent of the As contamination is not known completely, as in the case of North-Eastern Indian states where very limited studies has been conducted till date. A systematic study has thus been undertaken to assess the groundwater quality of Thoubal and Bishnupur district of Manipur. In total 26 water samples have been collected and analyzed for various elements, including the arsenic. More than 45 % of collected water samples have shown As concentrations well above the permissible limit (10µg/L) prescribed by WHO for drinking water. The highest concentration of arsenic (535µg/L) has been reported from Ngangkha Lawai Mamang Leikai area of Bishnupur district which is fifty fold higher than the WHO limit and tenfold higher than the Indian permissible limit (50µg/L) of As in drinking water. The study indicates that these two districts are severely affected by as contamination in the groundwater. Keywords: Groundwater, Arsenic contamination, Manipur 1.
Introduction:
The access to safe drinking water is one of the prime necessities for the survival of mankind. However, in many regions of the world, especially in underdeveloped countries, this primary requirement is not fulfilled, and significantly large population is forced to drink contaminated water, which ultimately causes many waterborne diseases [1]. Apart from anthropogenic sources, many inorganic and organic pollutants from natural sources are also contaminating the groundwater. In the case of India, it is well established that pollutants such as Arsenic [2,3] Fluoride [4] and Selenium [5] are contaminating the groundwater due to natural causes at both the intra-basin and inter-basin scale. Though, a lot of research has been carried out on the problem of arsenic contamination in several basins of India, yet many are to be explored. India being a vast country with different hydrogeological conditions, the outcome of one basin cannot be compared with other basins, thus, there seems to be a need to investigate every single basin individually. The study has, therefore, been undertaken with an aim to investigate the extent of arsenic contamination in parts of Imphal valley region of Manipur (Fig. 1). 2. Geology of the study area: Manipur (North Eastern state of India) is highly structurally deformed province, consisting of lounger sediments of quaternary age [6].
Fig 1: Geological map of Manipur (modified after [7]) The area is located between N: 23°50′-25°42′ and E: 92°58′-94°45′ longitude in the extreme eastern part of the country. The total surface area of the state is 22,327 km2, of which around 90% is covered by hills and the remaining 10% consists of central oval shaped valley surrounded by hills from all sides. Barak river basin and the Manipur river basin are two major river basins within the state of Manipur. The rivers draining both the basins originate from the surrounding northern hilly region, thus they are comparatively young while in valley area they show some degree of maturity. The geological setup of the Manipur constitutes sedimentary
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Arsenic Contamination in the Groundwater of Thoubal and Bishnupur District of Manipur, India
and meta-sedimentary rocks with ophiolite suites belonging to tertiary age. Table 1: Generalized stratigraphic succession of rocks in Manipur [8] Group Formation Quaternary Tipam Group Surma Group
Bokabil Formation Bhuban Formation Disconformable boundary Brail Renji Group Formation Jenam Formation Aisong Formation
Lithology Age Sand, silt, gravel, Recent boulder, clay etc. Sandstone, clay Miocene Shale, siltstone alternation with minor sandstone
Coarse, gritty, massive and well bedded sandstone Oligocen with current e bedding and ripple marks, with plant remains and coal streaks and conglomerates Contact gradation : boundary containing patchy conglomerate at places Disang Upper Disang Shale, siltstone, Middle Group Formation greywacke with to Upper rhytmite, Eocene olitrostome with fossils and slices of ophiolitic rocks Lower Shale, greywacke Upper Disang with rhythmites and Cretaceo Formation minor sandstone us bands to Lower Eocene Tectonic Contact Oceanic Lushat Cherts (light grey Cretaceo Pelagic Formation and pink) us to Sediments radiolarian and Lower diatom bearing Eocene cherts, limestone (fossiliferous at places), red shales, greywacke with minor volcanics Ophiolitic Volcanic rocks with Suite flow (Igneous breccias,gabbro/pla rocks) giogranite/pyroxenit e/lherzolite/harzgur gite/dunite/peridotit e with disseminated nodular and massive podiform chromite (The ultramafics in various stages of serpentinisation)
The inner Palaeogene fold belt of Manipur constitutes a large area of sedimentary rocks belongs to Disang group and Barail group. This inner Palaeogene fold belt extends to the parts of northern, central and southern of Manipur and to the west side by the ophiolite zone [9, 10, 11]. Disang rocks consist of argillaceous sediments which are grey to buff colored. A major part of the Disang group consists of splintery shale which has been locally metamorphosed to a very low degree and normally shows the anticlinal cores. In northern part of Manipur, a major syncline exists to the east of the Mao anticlinal structure and passes through Phuba-PhaibungKhullen village. The Barail rocks are classified into a lower Phaibung Formation consisting of alternative beds of shale and sandstone, and upper Phuba Formation which is mainly composed of shale [9]. In many areas of the Palaeogene fold belt, Barail group has not yet been classified, where it is recognized as an undifferentiated sedimentary group. Barails deformed into a number of regionally extended folds and faults. The major folds and severe faults follow the regional trend of N–S to NE–SW. These rocks are highly deformed and conformable sequence with the underlying Disang at many places. Shale intercalated by sandstone over upper Disang rocks is taken as a marker for beginning of the Barail sediments. The basement of the Barail group is characterized by a thin, coarse, pebbly conglomeratic horizon, which represents a disconformity at the top of Disang group [12]. Fossiliferous rich zone [13] near the contact of Disang group with Barail group in the inner Palaeogene fold belt of Manipur-Nagaland has been reported. This fossiliferous zone comprises gastropods, pelecypods, corals and sometimes foraminifers indicating an Upper Eocene age and this fossiliferous assemblage demarcating the boundary between Disang and Barail group. 3. Sample collection and method of study: To investigate the extent of As contamination, 26 water samples were collected on a random basis (Fig. 2). Out of the 26 sampled wells, 21 were actively used domestic wells (tube wells/hand pumps), 2 river water samples, 1 spring water and 1 pond water sample. Collection of all the water samples involved collection of: (1) filtered samples (0.45 µm cellulose nitrate filter) for analyses of major anions; and (2) filtered (0.45 µm cellulose nitrate filter) and acidified (with 5mL 14 M ultrapure HNO 3/L) samples for major cation and trace element analyses. All samples were tightly sealed and stored at low temperature until further analyses. Analysis for all major (cations) and trace elements was done by ICPAES (Perkin-Elmer, France). Sulphate (SO42-) concentrations were measured by spectrophotometer (Shimadzu UV-Visible spectrophotometer 160), alkalinity by titration and chloride (Cl-) by Expandable Ion Analyzer 940A with a combination electrode Orion ionplus 9817 BN. The pH and electrical conductivity
International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 07, No. 01, February, 2014, pp. 35-40
A K CHANDRASHEKHAR , D CHANDRASEKHARAM , THAMBIDURAI P AND S H FAROOQ were measured by Thermo Scientific Orion 4 Star pH conductivity portable meter. Tube wells were pumped at least 5 minutes before collection of samples, to flush out the stagnant water inside the tube and to get the water representing the aquifer.
Fig 2: Sample locations map of the study area. 4. Result and discussion: 4.1 Physicochemical parameters: Ranges of the physicochemical parameters for groundwater and surface water of Thoubal and Bishnupur districts are shown in Table 2. Both the groundwater and the surface water show slightly alkaline character and the pH values ranges between 7.2 – 8.8 and 7.9 – 8.8, respectively. The average pH value of groundwater (7.9) were slightly lower than that of the surface water (8.3),which can be attributed to the use of acid producing agricultural fertilizers such as ammonium sulphate and superphosphate of lime [14]. The WHO prescribed pH limit for drinking water ranges between 6.5 – 8.5 [15], and in the present study more than 90 % of the water samples falls in prescribed limit while in remaining 10% of the water samples slightly higher pH values were registered. Electrical conductivity (EC) and total dissolved solids (TDS) signifies the ionic strength and inorganic load of the water, respectively. The EC values in the groundwater samples ranged between 374 - 1417 (µS/cm) while for surface water it ranges between 45 262 (µS/cm). Similarly, the TDS values for the groundwater varied between 251- 949 mg/L and for the surface water it ranged between 30-176 mg/L. Higher EC and TDS values in the groundwater may be due to intense chemical weathering and long residence time of groundwater within the local lithologies of peat and older alluvium in the region [6]. The high EC and TDS in the groundwater samples may also be also due to higher ion exchange and solubilization in the aquifer [16, 17]. Bicarbonate (HCO3-), representing a major source of alkalinity and ranges between 90-120 mg/L in surface water samples and 190–560 mg/L in
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groundwater samples. Higher bicarbonate and calcium concentrations (average conc. 326 mg/L and 34.9 mg/L, respectively) in the groundwater indicates that the dominance of carbonate weathering process in the aquifer [18]. Sulphate (SO42-) does not show much variation in its concentrations and the average sulpahte concentration measured in the groundwater and surface water were 27.1 mg/L and 30.5 mg/L. This indicates that in aquifer conditions similar to the surface exists. This is quite interesting to note that in most of the As contaminated areas aquifers are quite reduced in nature and the sulphate concentrations remains frequently below the detection limit [19, 20]. More than 70% of water sample show iron concentrations higher than the WHO permissible limit (1 mg/L) [15]. The concentrations of iron in the groundwater (av. conc. 16.8 mg/L) and the surface water (av. conc. 1.1 mg/L) ranges between 0.1 - 166 mg/L and 1.1mg/L to below detection level (bdl). Generally, high concentrations of iron in As contaminated areas are attributed to reductive dissolution As bearing iron minerals [19]. However, in the study area, due to existence of non-reducing conditions in the aquifer (as hinted by higher sulphate concentrations), it is quite unlikely that the dissolution of iron can cause higher iron concentrations in the groundwater. The presence of alfisols, red in color and rich in nutrients especially iron is characteristic of the region [6]. During the monsoon period, considerable amount of iron may leach through the soil and may cause elevated level of iron in the aquifer. 4.2 Water classification: The major ion composition, plotted on piper diagram (Fig. 3), clearly differentiates the groundwater water from the surface water. Groundwater is mainly of NaHCO3 type while the surface water (including river, lake, pond and spring) is Ca-Mg-HCO3 type. Scattered groundwater data points suggests that the groundwater evolved through different processes and encountered different rocks in the process of evolution [22].
Fig3: Piper diagram showing the geochemical variation of different water type from the study area. 4.3. Arsenic contamination in the study area:
International Journal of Earth Sciences and Engineering ISSN 0974-5904, Vol. 07, No. 01, February, 2014, pp. 35-40
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Arsenic Contamination in the Groundwater of Thoubal and Bishnupur District of Manipur, India
The present study clearly shows that the Thoubal and Bishnupur districts of Manipur are highly affected by As contamination in the groundwater. Around 50% of the groundwater samples collected from Thoubal district shows As concentrations higher than the WHO limit. In case of Bishnupur district more than 65% of groundwater samples show As contaminated with respect to the WHO limit. All surface water samples collected from river, pond, lake and spring shows As concentrations with safe limit (
