EVIDENCE OF IRON IN ARSENIC MOBILIZATION

GROUNDWATER OF BENGAL DELTA PLAIN

S. J. Sahu, S. Roy, J. Jana, R. Bhattacharya, D. Chatterjee* S.S.Dey Dalal^**

* Department of Chemistry, University of Kalyani, Kalyani, Nadia – 741235, West Bengal, India e.mail : dbchat@hotmail.com

** River Research Institute, Haringhata, West Bengal, India

ABSTRACT

Groundwater abstraction in Bengal Delta Plain (West Bengal, India and Bangladesh) has been envisaged as a problem of global concern due to contamination of arsenic in drinking water. The current situation is a threat to human health. A proper understanding of the geochemical processes may provide suitable remediation option to mitigate the problem (short as well as long term basis).

Groundwater chemistry studied in a number of water wells reveals that the water is anoxic with high arsenic, dissolved iron (ii), manganese, phosphate, bicarbonate as well as low nitrate and sulphate. Arsenic is derived as a consequences of desorption and reductive dissolution of Fe-oxyhydroxide occurring as coatings on sedimentary grains. Possible electron donors in the reduced zone of the aquifer are organic matter, present in the sediments of Quaternary age. Fe-oxide, Sulphate and nitrate reduction have been studied in shallow aquifers with the main focus on understanding of the mechanism of mobilization of arsenic in groundwater.

INTRODUCTION

Recently the occurrence of high arsenic groundwater in Bengal Delta Plain (BDP, Latitude 21° 30¢ and 27° 10¢ N ; Longitude 86° and 90° E) has been reported by various workers (Bhattacharyya et.al 1997, BGS 1998). Eight (Malda, Murshidabad, Nadia, North and South 24 Parganas, Howrah, Hooghly, Bardhaman) out of eighteen districts of West Bengal are identified with elevated level of arsenic in groundwater. Sixty-eight blocks, nine municipalities (up to December, 1998) and more than thousand villages have been affected with high arsenic groundwater.

Adverse health effects such as hyperkeratosis, hyperpigmentation and skin lesions are manifested among the population of the BDP within two decades due to the consumption of groundwater with high concentration of arsenic and a large section of population (~6 million) are also now prone to risk (Guha Majumder et. al 1988).

The predominant oxidation states such as pentavalent arsenate (H₂AsO₄^- or HAsO₄^2-) are available under oxidizing conditions (pe +pH < 8), on the other hand trivalent species (H₃AsO₃, H₂AsO₃^-) occur under reducing conditions (Pe + pH > 8). In natural As-H₂O – O₂ system the thermodynamically stable inorganic arsenic species H₃AsO₃, H₂AsO₄^-, HAsO₄^2- or As(s) are governed by the environmental conditions.

Arsenopyrite / Pyrite may be the primary sources of arsenic in natural system. A good amount of arsenic may also be found to remain adsorbed on the active surface of secondary oxides and hydroxides of iron, aluminium, manganese (HFO, HAlO, HMO). These secondary phase of mineral grains are mostly available in the more permeable zone of the aquifer where sand / silt is present. The two hypotheses aerobic and anaerobic (oxidation and reduction model) have been put forward to understand the mobilisation of arsenic in groundwater. The first model is linked with rise of water demand for irrigation due to increase of rice production and population. The second model has been put forward because of high concentration of ferrous iron in the groundwater and the anaerobic condition of most of the affected aquifers. The present contribution is aimed to represent the salient aspect of the geochemical investigation of arsenic affected alluvial aquifers of the BDP. The groundwater chemistry and prevailing geochemical processes have also been discussed to understand the mechanism of arsenic mobilization in sedimentary aquifers during groundwater development. The on-going mechanism has been put forward to delineate the high occurrence of arsenic in groundwater.

METHOD

UV-Vis spectrophotometer (Perkin Elmer model no. Lambda 20) with 1 cm. match cuvette is used for the measurement of various water quality parameters between the wavelength range 200 – 900 nm with normal operating bandwidth 1nm. An atomic absorption spectrophotometer (Perkin Elmer 306) is used to measure the arsenic total as well as speciation. . A subset of the samples are also analyzed in the field such as pH, Eh, DO, conductivity. Arsenic speciation with the help of ion exchange column (10cm.x7cm.) has been carried out in the field (Ficklin 1983) The chemical parameters such as nitrate, chloride, hardness, bicarbonate phosphate, manganese, arsenic, iron are analysed in the laboratory. Routine control samples are also analyzed side by side for standardization and calibration. The analytical results of groundwater samples have been shown in table A.

RESULT AND DISCUSSION

The groundwater is characterized by typically reduced water with high Iron (II) and Manganese and very low sulphate concentration. Phosphate content is relatively high to moderate. High bicarbonate, low chloride and nitrate are the finger prints of typically reduced condition. Low Eh (-ve), low to very low DO and neutral to slightly alkaline pH also establish the reducing nature of groundwater. Thus it can be confirmed that groundwater are anoxic with high concentration of dissolved iron and very low concentration of nitrate (even below detection limit) in general, except where surface pollution is suspected.

Fig.-1 Relationship As Vs Fe

Fig-2 Relationship As Vs HCO₃^-

The profile of water quality show local as well as partly regional hydrochemical patterns reflecting the influence of geology, sedimentology and related geochemical factors. Significantly high arsenic shows overall positive correlation with high dissolved iron (Fe II) (Fig. 1). The phosphate also shows positive correlation with arsenic. Nitrate as well as to some extent sulphate show negative correlation with arsenic and iron. The alkalinity also shows positive correlation with arsenic and iron (Fig. – 2). The dissolved oxygen has negative correlation with arsenic and iron.

Speciation of the arsenic shows that there is a wide range of As(III) to As(V) ratios and little relationship with other measured parameters. However, As(III) is increasing in many folds with increasing concentrations of total arsenic. This confirms earlier experience in other parts of West Bengal and Bangladesh (BGS 1998).

The scrutiny of the lithologs revealed that a good amount of arsenic is found in the finely grained sediment specially yellow / yellowish silty clay / sand generally found interlayered with blue / black clayey layers and / or with black and bluish white sand with fining upward sequences. High arsenic ground water is also associated with yellow / orange sand rather then grey sand.

The drilling experience emphasizes that the subsurface lithology of the worst affected area comprises of medium to coarser channel materials (yellow / orange sand) and the migration of the river seems to influence the land formation as well as type of alluvium.

Historically, the source of arsenic was the primary arsenic bearing minerals which had oxidised and was brought to the delta plain by the suspended load of rivers. Arsenic remained in soluble phase while sulphate might have been carried away to the sea. Arsenic (mostly oxidised form AS(V)) would subsequently have been sorbed and co-precipitated with secondary Fe and Al phases mainly present as colloidal suspensions along with the flow of the river. These suspensions were ultimately deposited in the lower part of the delta depending on the physiography of the basin as well as topography of the area. Thus arsenic is buried while alluvium is being deposited.

The types of sediment deposited in the delta region during Holocene age have responded to changes in sea level. More recent flooding has left the delta region with a widespread surface cover of fine grained clay and silts. This is significant because it forms a partial barrier to the entry of air and helps to maintain reducing conditions in the aquifer.

Once deposited, arsenic may undergo reduction more at depth as compared to the near surface. A redox sequence can be visualized when organic matter are degraded by bacteria. This redox sequence is due to the availability of the oxidant (electron acceptor) to use another expression. The distribution of redox stages may be very irregular both in local and regional scale. It depends on the presence of organic matter (quantity and quality) and on the availability of oxidants. In a meandering river bed sediment, the texture varies largely in space horizontally as well as vertically. So does the presence of organic matter. Thus the patchiness of arsenic distribution is quite logical.

Oxidation of pyrite / arsenopyrite FeS₂ – As(s) + ⁷/₂ O₂ + H₂O = Fe²⁺(aq) + 2SO₄^2- + As(aq) is a process that would increase dissolved sulfate in groundwater with consequent decrease in the bicarbonate content. On the contrary in reality, the concentration of bicarbonate is high and sulfate is low. This invokes the possibility of sulfate reduction as a possible mechanism where sulfides are formed authogenically particularly in the organically rich fine grained sediments.

Arsenic is mobilized under reducing conditions due to desorption from the ferric hydroxides as well as reductive dissolution

(CH₂O)_n + Fe(III)(OH)₃ à CO₂ + H₂O + Fe²⁺

(CH₂O)_n + SO₄^2- à CO₂ + H₂O + H₂S

This is substantiated by the fact that arsenic concentration remained in ground water with high dissolved Iron and Bicarbonate and low Nitrate. Shallow wells are relatively low in arsenic and the arsenic content is high at intermediate depth.

The arsenic is probably brought to the delta plain by the suspended loads of rivers, originated from uphill of Bihar plateau and / from the Himalayas. During transportation the dissolved predominant arsenic species are in (+5) oxidation state and adsorbed onto ferric hydroxide or goethite. These river bound sediments are slowly deposited in the deltaic alluvium under sluggish movement of flow. Once deposited the arsenic may undergo reduction more at depth than near the surface and likely in impermeable strata like clay than in sand. In the sediments there will be a redox sequence when organic matter (plant remnants, algae etc.) are degraded by bacteria and derive energy from the organics. This redox sequence is due to the availability of oxidants or electron acceptors to use another expression. The distribution of the redox stages may be very irregular both in the small scale as well as on a large scale. The factors governing the redox are presence of organic matter and oxidants.

REFERENCES

Prosun Bhattacharyya, Debasis Chatterjee , Gunnar Jacks. (1997)Water Resources development, 13: 79 – 92.

British Geological Survey (1998), Groundwater Studies for Arsenic Contamination in Bangladesh, U.K, BGS.

D.N Guha Majumder, A.K Chakraborty (1988), Bulletin of the World Health Organisation. 66: 499–506.

Walter H Ficklin (1983), Talanta, 30: 371 –373.

Table A : Chemical composition of groundwater of Eastern India (West Bengal)

Sample	pH	EH mV	E.C (mcm^-1)	HCO^- (mg l^-1)	Cl^- (mg l^-1)	No3^- (mg l^-1)	SO4^2- (mg l^-1)	PO4^3- (mg l^-1)	As(t) (mg l^-1)	Fe(t) (mg l^-1)	Mn(t) (mg l^-1)
S1	6.90	-152	1000	447	109.6	<0.03	0.57	1235	173.0	8.3	0.20
S2	7.30	-138	790	459	25.0	<0.02	BDL	1187	176.7	7.2	0.45
S3	7.60	-165	870	442	89.2	BDL	0.89	70	224.3	5.1	0.52
S4	7.30	-133	720	397	80.7	BDL	BDL	37	206.7	2.5	0.68
S5	7.90	-202	780	398	67.7	<0.01	0.31	460	209.5	3.98	0.82