ARSENIC CONTAMITION IN BENGAL BASIN

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ARSENIC CONTAMINATION IN BENGAL BASIN

BY

SOUMYAJIT BASU

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CONTENTS

CHAPTERS PAGE NO.

INDRODUCTION 3

STATE OF ARSENIC IN GROUND WATER 4

SOURCE OF ARSENIC IN GROUND WATER 6

ARSENIC CONTAMINATION IN BENGALBASIN

9

ARSENIC POLLUTION RELATED PLATFORMCOLOUR

14

THE PALAEOSOL MODEL FOR ARSENICPOLLUTION

16

MECHANISM FOR ARSENIC RELEASE 18

PREVENTIVE MEASUREMENTSTOCONTROLL THE ARSENIC POLLUTION INGROUNDWATER

22

CONCLUSION 24

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INDRODUCTIONIn the 1970’s it was widely held that the alluvial aquifers everywhere yielded wholesomegroundwater. Groundwater is always a safe alternative to microbiologically polluted surfacewater for domestic supply. But the groundwater is also severely polluted by naturallyoccurred dissolved Arsenic (As) and other substances like Fluoride, Methane, and Ammoniaetc. The problem was revealed successively in West Bengal, Bangladesh, Vietnam, andCambodia, and is now known to occur in 30 deltaic and coastal aquifers worldwide. It is clearthat the pollution is severe and global in extent.

Dissolved arsenic is odourless, tasteless, poisonous, and carcinogenic. Its danger lies in itslong period of carcinogenic latency, which is measured in years to decades. As a consequence,in Bangladesh alone, the affect of natural As-pollution of groundwater was termed “the worstmass poisoning of a population in history” by Smith et al. (2000).

Although the concentrations of As in drinking water are usually low, in some circumstancesthey can reach far in excess of these statutory drinking-water limits and thus cause apotentially severe threat to health. Groundwaters are generally more vulnerable to Ascontamination than surface waters because of the interaction of groundwater with aquiferminerals and the increased potential in aquifers for the generation of the physicochemicalconditions favourable for As release. Indeed, the majority of the world’s recognised As-related health problems are linked with long-term use of groundwater for drinking.

The distribution of As in groundwater is highly heterogeneous. The concentration varieswidely, both vertically and spatially within a scale of few meters. The deltaic aquifers ofBengal Basin are most widely polluted and pose biggest threat to health because of the largepopulations in Bangladesh and West Bengal (more than 200 million). By the term pollutionis meant concentrations of dissolved arsenic that exceed water-quality standards of 10 μg/LAs of the World Health Organization’s (2006) guideline or 50 μg/L As of India andBangladesh. Most industrialised countries take 10 μg/L as a statutory limit although mostdeveloping nations continue to use the pre-1993 WHO guideline value as a national standardbecause of difficulties with analytical detection and compliance.

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STATE OF ARSENIC IN GROUND WATERArsenic occurs in the environment in several oxidation states but in water is mostly found asthe inorganic forms, arsenite (+3) and arsenate (+5). Organic forms of As are rarelysignificant in groundwaters but may become more important in waters affected by industrialpollution. Under oxic conditions at thermodynamic equilibrium, aqueous As is dominated byarsenate oxyanions (H

2As(V)O

4

-or HAs(V)O

4

2-depending on pH conditions). Under

reducing conditions and over a wide range of pH values, the uncharged arsenite speciesH3AsO

3

0predominates. However, in natural systems thermodynamic equilibrium is often not

achieved because of slow As redox kinetics. Much recent evidence suggests that redoxkinetics can be significantly accelerated by microbial activity which has been implicated inboth the oxidation of arsenite and the dissimilatory reduction of arsenate. Oxidation of As(III)is also well known to be catalysed by Mn oxides. Arsenite oxidation rates are pH-dependent,the reaction being slowest in acidic conditions. Dissolved As-S species can occur in stronglyreducing environments although precipitation of As sulphide minerals limits theconcentrations of dissolved As in conditions where sulphide concentrations are high.

Sorption plays an important role in As speciation. Amorphous iron oxides in particular arewell-known to have strong sorption capacities for As and therefore exert a strong influenceon As mobility (Dzombak and Morel, 1990). Figure 2.1 shows the predominant species in thesystem As-Fe-H

2O where sorption of As species onto hydrous ferric oxide (Hfo) is taken into

consideration. The diagram is subject to the deficiencies of current As thermodynamicdatabases but illustrates the strong adsorptive capacity of Hfo for As (especially arsenate) atnear-neutral pH under oxic to mildly reducing conditions. Under oxic conditions, aqueous Asspecies have increased importance at both high and low pH, the former related to electrostaticrepulsion from negatively charged oxide surfaces and the latter to Hfo instability anddissolution. Under strongly reducing conditions, aqueous arsenite is a predominant speciesover a wide pH range, again linked to Hfo instability. The system illustrated in Figure1 doesnot consider sulphide species. The strong tendency for sorption to iron oxides at near-neutralpH in oxic conditions is a significant factor in defining the low-As status of most naturalgroundwaters. The distribution of predominant species shown in Figure1 also goes some wayto explaining the high As concentrations observed in some groundwaters under stronglyreducing conditions and at extremes of pH.

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Figure 2.1: Eh-pH diagram for an As-Fe system in which hydrous ferric oxide (Hfo: (Fe(OH)3(a)) precipitates

and adsorbs As(V) and As(III) species.

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SOURCE OF ARSENIC IN GROUND WATER

1) Arsenic in mining and mineralised areas:-

Arsenic occurs as a major constituent in more than 200 minerals, most of them ore mineralsor their weathering products. They include elemental Arsenic, Arsenides, Sulphides, Oxides,Arsenates and Arsenites. These As minerals are relatively rare in nature but are concentratedin some ore zones. The most abundant As ore minerals are Arsenopyrite (FeAsS) andArsenian Pyrite (Fe(S,As)

2). Other As-bearing sulphides in mineralised areas include Realgar

(AsS) and Orpiment (As2S3). Arsenic is also present at high concentrations in the more

common sulphide minerals, the most abundant of which is Pyrite (FeS2). High concentrations

of As, up to several weight %, can also be found in many oxide minerals and hydrous metaloxides, especially when formed as weathering products of primary sulphide minerals. Ironsulphides and iron oxides together constitute the most important mineral sources of As ingroundwaters and it follows that environmental As problems can arise in mineralised areaswhere these are particularly concentrated.

Rates of mineral dissolution in mineralised areas may be enhanced by mining activity and Ascontamination can be particularly severe in water associated with mine wastes and minedrainage. Arsenic-related health problems in mineralised areas can arise through exposure tohigh-As soils and waste piles, release of As to the atmosphere through intensive coal burningor contamination of drinking water.

EXAMPLE: Ron Phibun District, Thailand

Health problems linked to As in drinking water were first recognised in residents of RonPhibun District, Nakhon Si Thammarat Province in Thailand in 1987. Over 1000 people inthe area were diagnosed with As-related skin disorders, including keratosis and melanosis,particularly in and close to Ron Phibun town (Williams, 1997). At the time of firstrecognition of the problems, some 15,000 people were thought to be drinking water withmore than 50 μg/L As (Fordyce et al., 1995).

The affected area lies within the South-East Asian Tin Belt where primary Sn-W-Asmineralisation and alluvial placer tin deposits were mined for over 100 years. Legacies of themining operations included arsenopyrite and pyrite-rich waste piles, and waste from oredressing plants and panning. Waste piles from former bedrock mining contained up to 30%As (Williams et al., 1996).

High As concentrations have been found in both surface waters and shallow groundwatersfrom the area around the mining activity as a result of natural oxidation of arsenopyrite,mining activity and release following post-mining groundwater rebound. Williams et al.(1996) reported concentrations of As in the surface waters reaching up to 580 μg/L. Shallowgroundwater (<15 m deep) from alluvial and colluvial deposits were reported to haveextremely high As concentrations, reaching up to 5100 μg/L with 39% of samples havingmore than 50 μg/L. Conditions in the shallow aquifer were noted to be generally oxic withnitrate concentrations (as N) up to 8.9 mg/L, low Fe concentrations (<0.4 mg/L) anddissolved As being present dominantly as As(V). As well as deeper groundwaters from anunderlying carbonate aquifer (well depth >15 m) had generally lower As concentrations,

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although 15% of samples exceeded 50 μg/L (Williams et al., 1996). The presence of morereducing conditions may be responsible for the observed high As concentrations in some ofthe deeper groundwaters, though leakage of high-As groundwater from the overlying aquiferis also possible.

In Chattisgarh, India high As in ground water due to mining activity is found.

2) Arsenic in geothermal waters:-

High concentrations of As have also long been associated with some geothermal fluids.Geothermal systems are found in diverse tectonic settings including active plate margins (e.g.the Pacific Rim), continental and oceanic hot spots (e.g. Yellowstone, Hawaii respectively)and within-plate rift zones (e.g. the East African Rift). High As concentrations have beenfound in geothermal fluids from continental plate margins (e.g. Alaska, Japan, Kamchatka,New Zealand, Philippines) and some continental hotspots (e.g. Yellowstone) but aregenerally not associated with oceanic hotspots or within-plate rift zones. The reasons for thedifferences in As concentrations in geothermal fluids between continental and oceanictectonic settings are not fully understood. They may be partially related to the fact thatbasaltic rocks, which constitute the likely As sources in oceanic settings, have comparativelylow As concentrations (ca. 0.05–0.2 mg/kg compared to around 1 mg/kg for more evolvedrhyolites, Arnorsson, 2003). Potentially more importantly, increased loads of dissolved Ascan be derived by leaching from continental crustal material which is present in convergentplate margin and continental hotspot settings.

High concentrations of As in geothermal areas are more commonly reported in surface watersthan groundwaters. High-As geothermal waters are often associated with a characteristic suiteof other trace constituents, including Li, B, F, Hg, Sb, Se, Th, and H

2S. Positive correlations

with Cl and salinity have also often been reported.

EXAMPLE:- Some high-As areas which are associated with the occurrence of geothermalfluids include parts of the USA, central America, Japan and New Zealand. Contamination oflocal soils and waters can be severe in such mineralised or geothermal areas, at least locally.Some, though not all, have had an adverse effect on human health through contamination ofdrinking water.

3) Arsenic in young sedimentary aquifers:-

In recent years it has become increasingly apparent that some of the most extensive andserious groundwater As problems occur not in areas influenced by metalliferousmineralisation or geothermal activity, but in seemingly ordinary sedimentary aquifers. Indeed,this is one of the most significant reasons why As problems in regions such as the BengalBasin were not recognised earlier. An important discovery of recent years has been that thesediments composing these aquifers do not tend to contain unusually high As concentrations.Average As concentrations in soils and sediments are in the approximate range 5–10 mg/kg.This compares for example with concentrations of 1–15 mg/kg found in sediments of theBengal Basin (BGS and DPHE, 2001), 3–29 mg/kg found in the Huhhot Basin of China(Smedley et al., 2003) and 0.6–33 mg/kg in the Red River Basin of Vietnam (Berg et al.,2001). All of these areas are characterised by high groundwater As concentrations. Arsenicrelease to groundwater in such areas must therefore occur by a combination of specialgeochemical and hydrogeological conditions rather than extraordinary As-rich sources.

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Many minerals may be involved in the release of As to groundwater and it is often verydifficult in a given aquifer to distinguish the principal As mineral sources. There is also aproblem in distinguishing between primary and secondary sources. However, it is widelyrecognised that metal oxides, particularly iron oxides, can and do play an important role inthe cycling of As in sedimentary aquifers. The release of As from iron oxides under reducingconditions has been widely documented. It is also recognised that many of As problems inyoung sedimentary aquifers occur under strongly reducing conditions. Recent investigationshave shown that release of significant concentrations of As can occur under oxidisingconditions in aquifers where pH values rise sufficiently high to promote desorption of As(V)from metal-oxide surfaces, or at least inhibit sorption to such surfaces.

EXAMPLES:- Huhhot Basin, Inner Mongolia, China

The Huhhot Basin lies on the southern edge of the Gobi Desert and experiences an aridclimate with average annual precipitation of around 440 mm. The basin is composed of up to1500 m of poorly-consolidated sediments, a large thickness of which are Quaternary(Smedley et al., 2003). This Quaternary sediments consist largely of coarse-grained alluvial-fan deposits on the basin margins, but with finer-grained lacustrine deposits in the lower-lying parts of the basin further south-west.

Residents in the region depend heavily on groundwater for domestic supply and agriculture.In recent years, traditional dug wells have largely been replaced as drinking-water sources byhand-pumped boreholes which mainly abstract groundwater at shallow levels (typically <30m). Groundwater is also present within a discrete, deeper aquifer (typically >100 m depth)which is separated from the shallow aquifer by clay layers. Boreholes tapping this deeperaquifer are often artesian in the central parts of the basin.

Smedley et al. (2003) found from a study of 73 samples taken from the Huhhot Basin thatgroundwaters have a large range of As concentrations: <1–1480 μg/L in the shallow aquiferand <1–308 μg/L in the deep aquifer. They reported that 25% of shallow sources and 57% ofdeep sources had As concentrations greater than 50 μg/L. Unlike Bangladesh, the deepaquifer is actually more severely affected than the shallow aquifer. Groundwaters from thebasin margins within the coarser-grained deposits are oxic and have universally low dissolvedAs concentrations. High As concentrations are generally restricted to the low-lying part of thebasin where the sediments are finer-grained and the groundwaters strongly reducing(Smedley et al., 2003). The aquifer characteristics of the Huhhot Basin clearly have manysimilarities with those of Bangladesh although high DOC concentrations (often withdiscoloured waters reflecting an abundance of humic substances) are a particular feature ofthis region.

In West Bengal, India, a part Bengal Basin, high As in ground water is attributed to theyounger sediments deposited by Ganges river.

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ARSENIC CONTAMINATION IN BENGAL BASINBengal Basin is situated towards the northeastern part of Indian Peninsula in the state of WestBengal, lies tentatively between Latitudes 25°-20°30’ and Longitude 87°30’ – 90° 30’ andfalls in the West Bengal state of India and Bangladesh.

Bangladesh

The most serious of the world’s recognised groundwater As problems without doubt occursin Bangladesh. The region has been the subject of intensive water testing, hydrogeologicaland epidemiological investigation, patient identification and treatment and mitigation effortsince the groundwater As problem was first recognised by the national government andothers in 1993.

The high-As groundwaters of the region are mainly from aquifers of Holocene age whichcomprise unconsolidated grey micaceous sands, silts and clays deposited as alluvial anddeltaic sediments associated with the Ganges, Brahmaputra and Meghna rivers. Thesediments are derived from the upland Himalayan catchments and from basement complexesof the northern and western parts of West Bengal. Many studies have observed that thehighest concentrations of As in the shallow Holocene aquifer of Bangladesh occur at depthstypically around 15–50 m (BGS and DPHE, 2001; Harvey et al., 2002; Klump et al., 2006).Concentrations of As in excess of 1000 μg/L have been found in some shallow groundwatersfrom the region, although these are relatively rare.

A random national survey of As in groundwater (BGS and DPHE, 2001), using laboratorydata for 3208 groundwater samples from the shallow Holocene aquifer (<150 m depth),found that 27% of samples had As concentrations greater than the national standard for As indrinking water of 50 μg/L; 46% exceeded 10 μg/L. A map of smoothed groundwater Asdistributions is given in Figure 4.1. More recent data from the Bangladesh Arsenic Mitigationand Water Supply Program (BAMWSP, 2005) showed that of almost 5 million boreholestested nationally using field-test kits, some 30%, had As concentrations greater than 50 μg/L.Each dataset produced for Bangladesh groundwaters demonstrates a very variable distributionof As regionally across the country, with the greatest proportion of exceedances ingroundwaters from the south and south-east (Figure 4.1).

In some parts of southern Bangladesh, the majority of boreholes have concentrations greaterthan 50 μg/L. A recent UNICEF/DPHE survey of groundwaters from 15 upazilas in southernBangladesh (Rosenboom, 2004) found that of 316,951 boreholes tested, 66% had Asconcentrations greater than 50 μg/L. In 574 villages tested in the survey, groundwater fromevery single borehole had concentrations exceeding 50 μg/L. Van Geen et al. (2003) alsofound, from a survey of 6000 boreholes in Araihazar upazila of central Bangladesh, that some75% of the shallow boreholes deriving water from Holocene sediments (depth range 15–30 m)had As concentrations above 50 μg/L. The BGS and DPHE (2001) survey also found somevillages in southern Bangladesh where more than 90% of the boreholes had Asconcentrations greater than 50 μg/L.

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Figure 4.1. Smoothed map of As distributions in groundwater in Bangladesh showing the locations of specialstudy areas described in detail by BGS and DPHE (2001) and histograms of As concentrations in each area.Arsenic distributions based on 3208 groundwater samples from the shallow aquifer (<150 m depth).

The results clearly indicate a problem that is very large. BGS and DPHE (2001) estimated, onthe basis of the population at the time, that up to 35 million people were drinkinggroundwater with As concentrations above 50 μg/L and up to 57 millions were drinkingwater with greater than 10 μg/L. Mitigation efforts have gone some way to reducing theexposure, although this is offset somewhat by the rapid population growth rate and thecontinuing installation of new boreholes. Many millions of people in the country still remainwithout access to low-As water.

A combination of the presence of poorly-permeable sediment horizons, particularly asoverbank deposits in the upper part of the Holocene sequence and a relative abundance of co-deposited fresh organic matter leads to often poor hydraulic circulation and the generation ofstrongly reducing conditions in many parts of the aquifers. The groundwaters of the region

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typically have high concentrations of Fe, Mn and HCO3and often high NH

4-N and DOC

concentrations, as well as low concentrations of SO4and NO

3-N. In some areas, conditions

are even sufficiently reducing for methane generation and ‘flaring’ wells have beenrecognised (Ahmed et al., 1998). Such reducing conditions favour the mobilisation of As.

Arsenic speciation studies suggest that a large range in the relative proportions of dissolvedAs(V) and As(III) exists in the groundwaters of Bangladesh (Acharyya, 1997; Ohno et al.,2005; Bhattacharya et al., 2006a). BGS and DPHE (2001) found the modal proportion ofAs(III) to be between 50% and 60% of the total As. However, detailed studies ofgroundwaters with high As concentrations have generally shown a strong dominance ofAs(III) (≥70%).

Arsenic has been found in the sediments in association with mixed Fe(II)-Fe(III) oxides,phyllosilicate minerals (Breit et al., 2001) and sulphide minerals. There is as yet little overallconsensus on the detailed mechanisms involved in As mobilisation in the Bangladeshaquifers, although most workers would agree that the iron oxides exert a significant control inthe process. Under the ambient strongly reducing conditions, dissolution of sulphide mineralsis unlikely to be a major release mechanism on a regional scale.

Figure 4.2. Arsenic concentration in Bangladeshi tubewells 2005

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West Bengal

Arsenic contamination in ground water of West Bengal in the range of 0.001-3.20 mg/loccurs in isolated patches, spreading over 79 blocks in the eight districts of the state. Thedistricts of Murshidabad, Nadia, North 24 Parganas, South 24- Parganas are located to theeast of river Bhagirathi/ Hooghly and Howrah, Hooghly , Bardhaman to the west of the sameriver, and Malda to the north of river Ganga. The eastern part of Bhagirathi/ hooghli river ismuch more affected than the rest. The people staying in such areas are in the risk zone ofgetting affected by arsenic poisoning in the event of more number of ground water structuresare tested to yield high arsenic water in due course of time. At present about 162.6 lakhpopulations (35.48% of the total population of the State) occupying 17533 habitations stay inthe risk zone of potential threat in terms of arsenic related disease in the future.

Geologically the arsenic infested area of West Bengal forms a part of the GangaBhagirathi delta, comprising succession of thick Quaternary sediments. The arseniferous tractis mainly restricted in the upper delta plain within shallow depth (mainly within 100m belowground level), which is mainly built up of sediments deposited by meandering streams andlevees that are composed of sands of various grades, silt, clay and their admixtures. Theground water in the area occurs in thick zone of saturation within the unconsolidated alluvialsediments. The aquifers are made up of sands of various grades. Since the area lies in theGanga-Bhagirathi delta, lateral variation in lithology is observed, in shallow aquifers (within100m bgl). Ground water occurs, in general, under unconfined hydrogeologic condition.

Hydrogeological tests on arseniferous aquifers (within 100 mbgl) have been conducted indifferent arsenic infested areas to observe the arsenic concentration in ground waterconsequence to pumping of arsenic water from the tube well. The results indicate that there isnot much impact on arsenic concentration of ground water due to pumping when thedrawdown created remains within 6m. Impact due to higher rate of pumping is yet to beascertained. Effects of pesticides and inorganic fertilizers on the arsenic infested groundwater regime have been studied in selected areas. In this regard arsenic content in commonlyused pesticides & fertilizers concentration has been analyzed and the content of inorganicarsenic in these chemicals has not been detected to be high.

Periodic monitoring of water level & water quality in arsenic infested area reveals thatarsenic concentration varies within a season. This may be due to the rainfall and change ofwater level behavior of the aquifer. The level of arsenic in ground water is maximum duringpremonsoon period and minimum during monsoon/postmonsoon period. This may be due tothe natural recharge of arsenic free rain water in to the shallow arseniferous aquifer. Groundwater in arsenic affected area is characterized by high iron.

Three aquifer systems have been identified within 100 mbgl, 120 – 160 mbgl and 200 –250mbgl. The top aquifer within 100 m bgl is mostly arseniferous, whereas both the deeperaquifers which are separated by a thick clay (>10 m) from the overlying aquifers, capable ofyielding 5 to 20 lps. water, are arsenic free. In a recent survey of Guptipara panchayat, block-Balagarh, dist- Hooghly, total 81 of both shallow and deep tube well are tested among which16 shallow tube wells (49-180ft) yield As above 50 ppb.

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Figure 4.3: Showing distribution As in various areas of West Bengal.

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ARSENIC POLLUTION RELATED PLATFORM COLOUR

A low cost rapid screening tool for arsenic (As) and manganese (Mn) in groundwater isPlatform colour. The result shows 27 that black colored platform with 73% certainty indicatesthat well water is safe from As, while 28 with 84% certainty red colored platform indicatesthat well water is enriched with As, 29 compared to WHO drinking water guideline of 10μg/L. With this guideline the efficiency, 30 sensitivity and specificity of the tool are 79%,77% and 81% respectively. However, the 31 certainty value becomes 93% and 38%,respectively, for black and red colored platform at 50 32 μg/L, the drinking water standardsfor India and Bangladesh.

Method Adopted by Workers:The sampling of groundwater from TWs installed in shallow aquifer (<70 m), was carried outin an area of 100 km2 119 in Chakdaha block, West Bengal, India (23.02-23.14ºN; 88.49-88.62ºE). A total of 423 TWs (density 4.23 TWs/km2120 ) were sampled during March toMay, 2011. During sampling each TW was purged for few minutes prior to collection ofgroundwater sample for laboratory analysis of As, Fe and Mn. In the field, major colorationon each TW platform was examined carefully and recorded. The picture of TW platform wasalso captured by a digital camera. The platform color was re-examined in the laboratory byanother person observing these pictures to avoid any biasness. The mutual agreement oncolor classification was more than 87%. All disagreements (n = 54) were for the separationof mixed colored platforms from red (n = 46) and black (n = 8) colored platforms; whereasnone of the red colored platforms were classified as black and vice versa during re-examination. Tubewell depth and installation year were also recorded from TW owners.

Effectiveness of platform color as a tool for screening As in TWs.The TW platform color to be effective as a screening tool, it should have high probability oftrue- positive and negative values and reasonably low probability of false-positive and

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negative values with respect to a particular drinking water standard.Figure-5.1 showscomparison of the respective true- positive and negative and false- positive and negativevalues for As at WHO drinking water guideline and national standard. The results shows atWHO guideline, the positive predictive value (PPV) of red colored platform is 84%, whilethe negative predictive value (NPV) of black colored platform is 73%.The respective efficiency, sensitivity and specificity of the tool are 79%, 77% and 81%. Thissignifies that platform color can be regionally used as an initial screening tool to evaluate Asin groundwater. While if 50 μg/L standard is considered, the platform color tool ismoderately efficient (65%) to screen low and high As in TW water. However, the NPV ofblack colored platform to identify As safe TW is very high (93%) compare to PPV (38%) ofred colored platform to identify As enriched TWs. Thus the sensitivity (85%) of the tool isacceptably higher compared to the specificity (59%). This indicates that at the nationalstandard, despite having moderate efficiency of the tool, black colored platform can still beused as an excellent indicator to screen safe TWs for As rather than to identify unsafe wellsby red colored platforms.

Figure 5.1. Generic approach for the validation of platform color as a tool for screening low and high As inTWs at cut-off levels of: a, WHO drinking water guideline value (10 μg/L) and b, national drinking waterstandard (50 μg/L).

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THE PALAEOSOL MODEL FOR ARSENIC POLLUTION

The model sets As-pollution in the context of sea-level change, weathering, and the formationof ancient soils (termed palaeosols) by continental weathering. Sea-level decreased in levelby about 120 m between 125 ka and 20 ka, as the last ice-age developed to its maximum.Falling sea-level exposed the world’s coastal areas to subaerial weathering.

In deltas worldwide, this lowering caused rivers to incise into the exposed coastal plains tomaintain their base levels as close as possible to sea-level. The exposed regions between therivers, termed the interfluves (Figure 6.1) were not eroded by rivers but were weatheredsubaerially and developed a capping of impermeable clay soil, termed a palaeosol (Figures6.1 & 6.2). Sands underlying the palaeosol were also weathered by oxygenated groundwaterflowingthrough them. The sands were turned brown in colour as they became FeOOH rich. Thisweathered, oxidized, and eroded, landscape was buried by later (post LGM) sediments as sea-level rose between 20 ka and 6 ka to near its present level, pushing the sea-shore landward,backing up the rivers, and making them deposit their sediment load so as to build the moderndelta we call Bangladesh. In the Bengal Basin, the sediments included organic-rich silts andmuds to drive FeOOH-reduction and cause As-pollution in these post- LGM (Last GlacialMaximum) sands.

The key postulate of the ‘palaeosol model’ is that the weathering of interfluves between 125ka and 20 ka capped the oxidised Pleistocene brown sands with an impermeable palaeosol,termed the Last Glacial Maximum Palaeosol (LGMP). The LGMP formed regionally, ifdiscontinuously, and has a major impact on groundwater flow (Figure 6.2), thus controllingthe distribution of As-pollution. The LGMP controls flow because it is impermeable.The LGMP prevents vertical recharge reaching brown-sand aquifers beneath the palaeo-interfluves (Figure 6.2) and so protects them from both downward percolation of As-pollutedwater, and alsofrom downward migration of organic matter (OM), from overlying OM-richsediments, that would drive reduction of FeOOH and cause As-pollution. A consequence ofthis prevention of vertical flow is that FeOOH in palaeointerfluvial aquifers has suffered littlereduction, and the sands remain today both brown and FeOOH-rich, so their groundwater isAs-free. In contrast, the old river channels, now buried and so termed palaeo-channels,contain no LGMP, either because it was never deposited in active river channels, or becauseit was removed after formation by post-LGM erosion (Figures 6.1 and 6.2). Palaeo-channelscontain no barrier to downward flow of arsenic or organic matter so both have moveddownwards in them to bothpollute and reduce FeOOH in underlying sands of all ages.

The LGMP prevents vertical flow, but not horizontal flow, so why do palaeo-interfluvialaquifers of brown sand exist some 6,000 years after basin-filling largely ceased as sealevelreached its present level? Why have the palaeointerfluves not been invaded laterally byarsenic to pollute, and by organic matter to reduce FeOOH (Figure 2) and further pollute?More importantly, if it has not happened by now, will it ever: will such aquifers always besafe from lateral invasion by pollution?

One reason for the survival of palaeo-interfluvial brown sands is that their depth of >20m isbelow base-level. The elevation of much of the deltaic regions of the world is close to sea-level; in the Bengal Basin, the elevation of the land istypically only 5 to 10 metres above sea-

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level. Because of this low elevation, water at 20m depth below ground level is below sea-level. At such depth where the topography is flat, as it is in deltas, flow is slow because nohydraulic head exists to drive it. With little flow, palaeo-interfluves suffered little lateralinvasion by pollution. What little occurred would, for arsenic, have been retarded by sorptionto FeOOH in brown, palaeo-interfluvial sands. Organic matter would have been retarded byreaction with FeOOH, with the arsenic released being re-sorbed immediately downflow.

But since the introduction of pumping of groundwater for irrigation in the 1970s,groundwater flow to even 30 m depth is no longer natural or slow; both velocity andflowdepth have increased; subsurface flow is now strong. In the McArthur et al. field area,irrigation pumping has reversed the natural southerly flow direction, which is nownorthwards, from As-polluted palaeo-channels into unpolluted palaeo-interfluves. Thismodern flow carries pollution into the palaeo-interfluvial aquifers. The invadingpollution/redox front threatens the reserves of good-quality groundwater present in palaeo-interfluvial areas. Movement of the front is the reason why wells positioned near it havechanged their As-concentration with time, some increasing as the front approached and somedecreasing after it had passed.

Figure 6.1: Schematic of palaeosol development onpalaeo-interfluvial areas of the Bengal Basin between125 ka and the last glacial maximum at 20 ka.

Figure 6.2: The LGM palaeosol and its effect on groundwaterflow. Section across the field site shown in Figure 1. Openarrows show groundwater flow direction; size denotes flowmagnitude. The LGMP prevents vertical recharge to brownsands beneath it, so preserving them as unpolluted aquifers.Horizontal flow moves organic matter (to reduce FeOOH)and arsenic (to pollute) into the brown sand aquifer beneaththe LGMP, thereby threatening the palaeo-interfluvialaquifer.

This model is successfully tested by McArthur et al.(2004) on the villages of Joypur,Ardevok, and Moyna, together they are called JAM.

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MECHANISM FOR ARSENIC RELEASEOxidation of sulphide minerals

Numerous primary sulphide minerals occur in association with gold and base-metal depositsin mineralised zones. These minerals include Fe sulphides such as pyrite and arsenopyrite,complex copper sulphides such as enargite and tennantite, as well as the As sulphidesorpiment and realgar. Dissolution of these minerals, particularly the more abundant Fesulphides, has been recognised as a cause of high concentrations of aqueous As in manymineralised areas of the world. All these minerals oxidise readily in contact with theatmosphere and release As and other potentially toxic trace elements. The As released isrepartitioned between water and secondary minerals, including schwertmannite, scorodite andthe iron oxides. The oxidation reactions can lead to severe degradation of water quality,particularly in areas strongly affected by mining activity. However, aqueous As is stronglyattenuated by adsorption to iron oxides, especially under the acidic and oxidising conditionsprevalent in the vicinity of the mineral transformations. High As concentrations in surfacewaters and groundwaters are therefore typically of localised occurrence.

Release from iron oxides under reducing conditions

In many of the world’s recognised high-As aquifers, the generation of reducing conditionsappears to have played a critical role in triggering the release of As to groundwater. Thepresence of organic matter in the system drives a complex series of redox reactions, involvingprogressive and sequential loss of dissolved oxygen, production of CO

2from the oxidation of

organic carbon, reduction of nitrate, Mn(IV) and Fe(III) and subsequently reduction of SO4

and possibly production of CH4. The reductive dissolution of Fe oxides during this process

can be responsible for release of As to water. As part of the redox reaction sequence, As(V) isalso reduced to As(III). This species is normally less strongly adsorbed to Fe oxides and maytherefore trigger a further net release of As from adsorption sites as reduction proceeds. Theimportance of microbial activity in catalysing As release in aquifers has been increasinglyrecognised in recent years (Oremland et al., 2002; Islam et al., 2004). Several species ofmicrobes have been found to be capable of dissimilatory arsenate reduction and a number ofothers use arsenate reduction as a detoxification mechanism (Hoeft et al., 2002).

Another potentially important process in the sediments is diagenesis of the iron oxidesthemselves. This may involve a change in oxide structure (bulk and surface) and oxidationstate, which can affect the affinity of the minerals concerned for As binding. A change underreducing conditions from Fe(III) forms to mixed Fe(II)/Fe(III) oxides such as magnetite orgreen rust has been recognised in Bangladesh and elsewhere (Lovley et al., 1990; BGS andDPHE, 2001; Benner et al., 2002; Horneman et al., 2004).

Phosphate is often present at relatively high concentrations in reducing high-As groundwaterssuch as those in Bangladesh (BGS and DPHE, 2001), West Bengal (McArthur et al., 2004)and China (Smedley et al., 2003). Phosphate concentrations are sometimes in excess of 1mg/L and almost always in excess of the concentrations of dissolved As. Bicarbonate, alsooften present at very high concentrations in high-As groundwaters, has likewise beenimplicated as a potential competitor for As (Appelo et al., 2002; Charlet et al., 2007). Silicamay exert a control on the sorption of As(V) and As(III) (Swedlund and Webster, 1998;

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Rochette et al., 2000) although its effect was considered less significant for groundwaters inWest Bengal by Charlet et al. (2007).

The nature and origin of the organic carbon that acts as a driver for redox reactions in high-As aquifers has been much debated. Peat deposits (McArthur et al., 2001) and peaty stratamarginal to peat basins (McArthur et al., 2004) have been implicated. Alternatively,disseminated organic matter in the sediments and water have been cited as likely carbonsources (BGS and DPHE, 2001). The introduction of anthropogenic organic carbon throughdrawdown induced by groundwater pumping has also been proposed as a driver for Asrelease (Harvey et al., 2002; Harvey et al., 2006).

Release of arsenic at high pH

As noted above, under aerobic and near-neutral pH conditions characteristic of many aquifers,adsorption of arsenic to Fe oxides as arsenate is normally strong. Aqueous As concentrationsin aquifers are therefore usually low. However, at high pH the adsorption capacity for As(V)is reduced. There are a number of reasons why groundwater pH might increase, but amongthe most important are uptake of protons by mineral-weathering and ion-exchange reactions,evaporation and inputs from geothermal sources. Uptake of protons during mineralweathering and evaporation can be significant processes in arid and semi-arid regions.Observed pH increases in such environments are commonly associated with the developmentof groundwater and soil salinity. Inputs of high-pH geothermal waters may be important inmaintaining high As concentrations in some alkaline lakes.

The generation of high groundwater pH, especially above pH 8.5, is thought to be animportant criterion for the mobilisation of As(V) since sorption to Fe oxides is lessfavourable under such conditions. Such processes are likely to have been responsible formaintaining high groundwater As concentrations in oxidising Quaternary sedimentaryaquifers in the semi-arid inland basins of Argentina (Smedley et al., 2002), south-westernUSA (Robertson, 1989) and Mexico (Rosas et al., 1999) for example. As such a pH increaseinduces the desorption of a wide variety of oxyanions, other solute oxyanions such asvanadate, uranyl, phosphate and molybdate may also accumulate, as has been observed insome areas (Smedley et al., 2002; Bhattacharya et al., 2006b). As with reducing high-Asgroundwaters, specific adsorption of these anionic species to oxide binding sites can reducethe load of sorbed As(V). Calcium is likely to be the most important cation because of theirpositive charge(+2), may promote the adsorption of negatively charged arsenate in mostnatural waters. Aluminium and manganese oxides can also adsorb As to some extent andalthough less well studied in high-As groundwater contexts, these may be additional sourcesof or sinks for As in some aquifers.

Variations in groundwater flow

A high degree of spatial variability in arsenic concentrations both areally and with depth hasbeen noted in many of the recognised high-As groundwaters (BGS and DPHE, 2001;Smedley et al., 2002; van Geen et al., 2003; Charlet et al., 2007). Variations in groundwaterflow are likely to have been a factor in generating the chemical variations observed.Considerable heterogeneity in sediment texture and composition on a scale of centimetres tometres has been observed in Holocene Bangladesh sediments for instance (BGS and DPHE,2001) and can be responsible for large variations in permeability and flow. Low-flowhorizons occur in low-lying parts of deltas and the insides of river meanders. These have

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often been associated with occurrences of localised As ‘hotspots’. The accumulation of fine-grained, iron-oxide rich deposits may also be favoured in such low-flow zones.

Lack of aquifer flushing has been considered important in maintaining high groundwater Asconcentrations in both reducing aquifers and oxic, high-pH aquifers. In the latter case, aridclimatic conditions enable high pH values to be maintained as well as restricting groundwaterflow. High As concentrations are less likely to occur in well-flushed aquifers.

Impact of man’s activities

One question that has not been fully answered by the various studies on As in groundwatercarried out to date is the extent to which man’s activities have contributed to the As problemsin different aquifers. In some sulphide mining areas, man has clearly had a major impact byexcavating ore minerals, accumulating and redistributing waste piles and pumping mineeffluent. This is particularly the case where mining activity is large-scale and long-term.However, in areas where mining activity is small-scale (e.g. artisanal) and/or initiatedrelatively recently, the impact is likely to be much smaller. Such is the case in northernBurkina Faso where Smedley et al. (2007) concluded that there was no evidence for the highgroundwater As concentrations observed being affected significantly by human activity.

In sedimentary high-As aquifers the impacts of anthropogenic activity are also poorly-defined.Groundwater pumping will have an inevitable effect on groundwater flow and mixing bothwithin and between aquifers and will mean that some changes to the aquifer systems can beexpected in the medium to long term. Other potential impacts include inputs of anthropogenicorganic carbon and phosphorus to the land surface and seasonal waterlogging of soils due torice production. The significance of such processes is difficult to quantify for any givenregion.

RESULT OF MAN’S ACTIVITY ON BENGAL BASIN

Studies in the Bengal Basin during the 1990s concluded that high-As groundwaters therewere the result of recent over-pumping of groundwater for rice irrigation which wereconsidered to cause dewatering of the sediments and resultant oxidation of sulphide minerals(e.g. Das et al., 1996). Subsequent studies in the region have dismissed this as a significantmechanism as strongly reducing conditions prevail in the affected parts of the aquifers. Manyworkers have attributed the redox transformations in the Bengal aquifers to reactions withnaturally-occurring organic carbon in a natural process which may have been going on formany thousands of years (BGS and DPHE, 2001; McArthur et al., 2001; McArthur et al.,2004). Harvey et al. (2002) attributed the process to more recent introductions ofanthropogenic carbon from the land surface from pollutants, although the evidence for thishas been disputed (e.g. Klump et al., 2006).

A number of retrospective studies have suggested that groundwater As concentrations inBangladesh are higher in older boreholes. DPHE/BGS/MML (1999) suggested that anincrease in the percentage of boreholes exceeding 50 μg/L occurred as a function of well age,although possible causes of such changes were not discussed. Variations with well age couldoccur for example as a result of time-varied changes in spatial or depth distribution ofboreholes, which may have responded to increased intelligence on As spatial distributions

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since the mid 1990s. However, in Bangladesh, no obvious spatial or depth relationship isdiscernible from available groundwater-quality data (dataset of BGS and DPHE, 2001).

Van Geen et al. (2003) suggested on the basis of regression of As concentrations as afunction of year of borehole installation that small increases occurred with well age over alldepth intervals investigated in Araihazar upazila of central Bangladesh. Rosenboom (2004)also found that median As concentrations in boreholes from southern Bangladesh were higherwhere well age was >25 years old than those of <25 years. They found no significant changein spatial distribution of wells with time, although deep (low-As) tubewells were not presentamong the older wells in the dataset and shallow dug wells constituted a larger proportion ofthe old wells.

The results of these studies do not provide strong evidence that groundwater Asconcentrations in Bengal Basin boreholes have increased with time. However, they do raisesome interesting questions about temporal variability and reiterate the need for detailed long-term monitoring. The analysis of survey-type data in this way often involves large numbersof samples and invariably explains only a small proportion of the total variance. A criticalanalysis needs to be undertaken to ensure that any so-called ‘statistically significant’ trendsare indeed truly significant. Division of the Bangladesh groundwater As data from the BGSand DPHE (2001) database into districts suggests that As concentrations in most districts donot show significant trends as a function of well age, although a few exceptions occur. Acloser examination of the local circumstances is needed to establish whether a real temporalvariation exists at these sites.

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PREVENTIVE MEASUREMENTS TOCONTROLL THEARSENIC POLLUTION IN GROUNDWATER

The As contamination of ground water is attributed to geogenic origin. It is therefore difficultto conceive the spread of arsenic, both in space and time, over a period of 20-30years. Thefact of the matter is, with intense and increased monitoring of water quality by variousGovernment and non government organizations, the knowledge and awareness of the actualspread of arsenic contaminated area is coming to the notice of all in a comprehensive manner.In other words the area which was not considered to be arsenic affected earlier simplybecause the ground water samples were not analysed, have come under the arsenic belt withhigh degree of surveillance.

(i) Supply of Surface Water

Surface water based schemes are also safe options and these schemes have also reduced thestress on ground regime. But availability of such sources is a problem and thus can not beimplemented everywhere.Supply of water for drinking purposes from ponds, rivers etc. through pipe net work afterpurification by conventional method of treatment viz. coagulation, flocculation, rapid sandfiltration and disinfections, have been considered wherever easily accessible. Horizontalroughing filter with slow sand filter have also been adopted using pond water.

(ii) Tapping Alternative Aquifer for Arsenic Free Ground Water

Groundwater with arsenic contamination has been found mainly in the shallow aquifers.Deep aquifers when separated by a thick clay layer of appropriate composition & thickness,sealing off the upper arseniferous aquifer by cement, prevents percolation of arseniccontaminated ground water from the top aquifer. It was inferred from the isotopic studiescarried out in West Bengal that in alluvial formations, there is no hydraulic connectionbetween shallow and deep aquifers ( they belong to different age group) they , whenseparated by an appropriate impervious layer. Central Ground Water Board, while carryingout extensive work on this aspect, has deciphered and delineated deep arsenic free aquifers atnumber of places in the states of West Bengal. Deep arsenic free aquifers, potential enough toyield adequate water to meet the water requirement in the domestic sector, are available inmost of the arsenic affected regions for ground water exploration.

(iii)Treatment and Removal of Arsenic from ground water

While the first two options are being adopted wherever possible notwithstanding the hugefinancial requirement for the piped water scheme, the treatment of tube well water forremoval of arsenic has also been applied in a big way (especially in the state of West Bengal).The removal of arsenic depends upon its chemical state in which it occurs in the water, viz.Trivalent Arsenic (As III) and Pentavalent Arsenic (As V). As (III) is much more prevalent inground water and it is difficult to remove because it exists predominately in the non ionicform. Whereas Arsenic (V) exists in monovalent state and when water contains iron in higher

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concentration, it is easier to remove As (V) as it co precipitates with iron. Therefore, theoxidation of As(III) to As(V) improves the effectiveness of arsenic removal technology. Theoxidizing agent used for this conversion are, Oxygen, powdered active carbon, .UVirradiation, free chlorine, hypochlorite, Potassium permanganate, Ozone and also sunlight.Some of the adopted removal techniques are:1. Coagulation-flocculation-Sedimentation and Filtration using Alum, Ferric alum lime etc.2. Ion exchange Resins.3. Adsorption using activated alumina, Granulated ferric hydroxide, iron coated sand,activated carbon, laterites etc.4. Membrane Techniques using Reverse Osmosis or Electrodialysis5. Biological method using Phyto-remediation and Bacterial removal and6. Adsorption of arsenic by colloidal media suspended in water and application of membranebased separation technique using ceramic micro – filtration membrane.

(iv) Rain Water Harvesting

Rain water harvesting may be adopted if appropriate conservation structure is available tofacilitate collection. Water conserved in such a way needs to undergo filtration, anddisinfections before it is put to use for public supply. This will provide an option and the useof treated arsenic contaminated water, may be dispensed with. Artificial recharge of rainwater into the aquifer through recharging structures may also be considered whereverhydrogeological condition is found feasible to dilute the concentration of arsenic in groundwater and thereby making its arsenic content within the permissible limit.

(v) Injection of Compressed Air

Compressed Air has been injected in shallow arsenic rich aquifer and it is observed thatArsenic & Iron concentration has been reduced due to injection of compressed air.Experiments have been conducted in a shallow exploratory well at Birohi, Chakdah block,Nadia district, West Bengal.

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CONCLUSIONThe concentrations of As in natural waters, including groundwater, are usually low. Most arebelow the WHO provisional guideline value for As in drinking water of 10 μg/L and manybelow 1 μg/L. Investigations in the last few years have shown that As mobilisation can occurin many aquifers and concentrations can exceed the low drinking-water thresholds in diversehydrogeological conditions. However, very high As concentrations in groundwater,potentially orders of magnitude greater than threshold values, and extensive areas affected byhigh-As groundwater tend to be found in a rather limited number of settings.

These settings include areas of metalliferous mineralisation and mining, particularly inconnection with gold occurrence, areas of geothermal activity, and major alluvial/deltaicplains and inland basins composed of young (Quaternary) sediments. High-As groundwatersin young sedimentary aquifers occur in response to the generation of specific geochemicalconditions, among the most important of which appear to be the development of conditionswhich are either strongly reducing or oxic and high-pH. Although these two cases aregeochemically very different, they each favour As mobility in part through the reducedcapacity of metal oxides to adsorb As under such conditions. Lack of flushing of groundwaterfrom an aquifer can also be a factor in maintaining high groundwater As concentrations.Low-lying sedimentary basins and delta plains are typically areas of such slow groundwatermovement.

The extent to which anthropogenic activity has affected the distributions of As ingroundwater in sedimentary aquifers is not well-established although the evidence for asignificant deleterious effect on As concentrations is not compelling. Impacts are potentiallydiverse and include modification of groundwater flow regimes and inputs of chemicalpollutants that can affect As speciation. Pumping-induced groundwater mixing will inevitablymodify the chemistry of groundwater in a given aquifer over time but such changes couldfeasibly lead to decreased As concentrations in places as well as increased ones. Furtherinvestigations are required for any given aquifer to establish the magnitude of any impacts.

Although the most important triggers for As release in aquifers have become increasinglywell-established in recent years, newly discovered areas of contamination are still emergingas a result of increased groundwater testing. Fortunately, new examples on the scale ofBangladesh As contamination have not emerged.

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NAME: SOUMYAJIT BASU

CLASS: M.Sc. (FINAL YEAR)

SEMESTER: 3RD

SUBJECT: APPLIED GEOLOGY

CLASS ROLL No.: 70

REGISTRATION No.: 13221611014

M.Sc THESIS: ON HYDROGEOLOGY UNDER THEGUIDANCE OFPROF. PRADIP K. SIKDAR

DATE: 1st April,2015