Risk Assessment of Arsenic Mitigation Options in Bangladesh
Risk Assessment of Arsenic Mitigation Options in Bangladesh
Risk Assessment of Arsenic Mitigation Options in Bangladesh
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J HEALTH POPUL NUTR 2006 Sep;24(3):346-355ISSN 1606-0997 $ 5.00+0.20© 2006 International Centre forDiarrhoeal Disease Research, <strong>Bangladesh</strong><strong>Risk</strong> <strong>Assessment</strong> <strong>of</strong> <strong>Arsenic</strong> <strong>Mitigation</strong><strong>Options</strong> <strong>in</strong> <strong>Bangladesh</strong>Guy Howard 1 , M. Feroze Ahmed 2 , Abu Jafar Shamsudd<strong>in</strong> 3 ,Shamsul Gafur Mahmud 3 , and Daniel Deere 41<strong>Arsenic</strong> Policy Support Unit, 5th Floor, DPHE Bhaban, 14 Shaheed Capt. Monsur AliSharani, Kakrail, Dhaka 1000, <strong>Bangladesh</strong>, 2 <strong>Bangladesh</strong> University <strong>of</strong> Eng<strong>in</strong>eer<strong>in</strong>gand Technology, Dhaka 1000, 3 International Tra<strong>in</strong><strong>in</strong>g Network, <strong>Bangladesh</strong> University<strong>of</strong> Eng<strong>in</strong>eer<strong>in</strong>g and Technology, Dhaka 1000, and 4 Water Futures,32 Sirius Street, Dundas Valley, NSW 2117, AustraliaABSTRACTThe provision <strong>of</strong> alternative water sources is the pr<strong>in</strong>cipal arsenic mitigation strategy <strong>in</strong> <strong>Bangladesh</strong>,but can lead to risk substitution. A study <strong>of</strong> arsenic mitigation options was undertaken to assess waterquality and sanitary condition and to estimate the burden <strong>of</strong> disease associated with each technology<strong>in</strong> disability-adjusted life years (DALYs). Dugwells and pond-sand filters showed heavy microbialcontam<strong>in</strong>ation <strong>in</strong> both dry and monsoon seasons, and the estimated burden <strong>of</strong> disease was high. Ra<strong>in</strong>waterwas <strong>of</strong> good quality <strong>in</strong> the monsoon but deteriorated <strong>in</strong> the dry season. Deep tubewells showedmicrobial contam<strong>in</strong>ation <strong>in</strong> the monsoon but not <strong>in</strong> the dry season and was the only technology to approachthe World Health Organization's reference level <strong>of</strong> risk <strong>of</strong> 10 -6 DALYs. A few dugwells and one pondsandfilter showed arsenic <strong>in</strong> excess <strong>of</strong> 50 µg/L. The f<strong>in</strong>d<strong>in</strong>gs suggest that deep tubewells and ra<strong>in</strong>waterharvest<strong>in</strong>g provide safer water than dugwells and pond-sand filters and should be the preferred options.Key words: <strong>Arsenic</strong>; <strong>Arsenic</strong> mitigation: Water pollution; Water supply; Water quality; <strong>Risk</strong> assessment;DALYs; Pathogens; Technology choice: <strong>Bangladesh</strong>INTRODUCTIONThe contam<strong>in</strong>ation <strong>of</strong> shallow groundwater used forwater supply with arsenic <strong>in</strong> <strong>Bangladesh</strong> represents asignificant public-health concern for the country. Fromthe <strong>in</strong>itial detection <strong>in</strong> 1993, the extent <strong>of</strong> arsenic contam<strong>in</strong>ationhas become better characterized and has beendescribed by some as one <strong>of</strong> the most significant poison<strong>in</strong>gthreats from water worldwide (1). A nationwide surveyundertaken <strong>in</strong> the late 1990s suggested that 27% <strong>of</strong>all shallow tubewells were likely to have arsenic contam<strong>in</strong>ationabove the <strong>Bangladesh</strong> standard <strong>of</strong> 50 µg/L withCorrespondence and repr<strong>in</strong>t requests should be addressed to:Dr. Guy HowardDFIDAbercrombie HouseEaglesham RoadEast KilbrideGlasgow G75 8EAUKEmail: g-howard@dfid.gov.ukFax: 44 1355 84 409946% <strong>in</strong> excess <strong>of</strong> the provisional World Health Organization(WHO) guidel<strong>in</strong>e value <strong>of</strong> 10 µg/L (2). Subsequently,blanket screen<strong>in</strong>g <strong>of</strong> shallow tubewells has beenundertaken <strong>in</strong> 271 affected upazilas, with about fivemillion tubewells hav<strong>in</strong>g been tested (3). This screen<strong>in</strong>gshowed that 29% <strong>of</strong> the tubewells <strong>in</strong> the affected upazilashad arsenic <strong>in</strong> excess <strong>of</strong> the <strong>Bangladesh</strong> standard.These results suggest that the overall scale <strong>of</strong> theproblem may, <strong>in</strong> fact, be less than orig<strong>in</strong>ally suspected(4). Nonetheless, where arsenic contam<strong>in</strong>ation does occur,it represents a potentially serious public-health threatfor many ord<strong>in</strong>ary <strong>Bangladesh</strong>is. The provision <strong>of</strong> watersupplies with acceptable levels <strong>of</strong> arsenic is the only provenmeans <strong>of</strong> reduc<strong>in</strong>g the threat to exposed populations.Progress on mitigation <strong>of</strong> arsenic risks has laggedbeh<strong>in</strong>d the def<strong>in</strong>ition <strong>of</strong> the scale and nature <strong>of</strong> the problem,and provision <strong>of</strong> mitigation options rema<strong>in</strong>s a majorchallenge. The major emphasis for the mitigation <strong>of</strong>arsenic <strong>in</strong> <strong>Bangladesh</strong> is the provision <strong>of</strong> alternativewater sources. A number <strong>of</strong> alternative water-supply
<strong>Risk</strong> assessment <strong>of</strong> arsenic mitigation options 347technologies have been identified and tested on a pilotbasis <strong>in</strong> several areas <strong>of</strong> <strong>Bangladesh</strong>. <strong>Options</strong> <strong>in</strong>cludeimproved dugwells, deep tubewells, pond-sand filters,and ra<strong>in</strong>water harvest<strong>in</strong>g. In addition, piped water suppliesfrom various sources have been piloted, and workis ongo<strong>in</strong>g to develop improved designs for multi-stagefiltration units that can take water from larger waterbodies, such as rivers, canals, and baors.The key policy lesson for public-health protectionthat emerges from the arsenic crisis <strong>in</strong> <strong>Bangladesh</strong> isthat, <strong>in</strong> improv<strong>in</strong>g water-supply services, considerationmust be given to the degree <strong>of</strong> public-health risk substitutionthat may result from the <strong>in</strong>tervention (5). In thecase <strong>of</strong> <strong>Bangladesh</strong>, the provision <strong>of</strong> tubewells tapp<strong>in</strong>gthe shallow aquifer substituted one public-health risk(diarrhoeal disease from microbial pathogens) by another(from arsenic). This risk substitution was not predictedat that time, and the evidence <strong>of</strong> the potential for sucha substitution was certa<strong>in</strong>ly not adequate for an evaluation<strong>of</strong> the probability and nature <strong>of</strong> potential risk substitutes.In the mitigation <strong>of</strong> arsenic, however, the potentialrisk substitution is well-known and can be quantified,albeit with<strong>in</strong> estimable levels <strong>of</strong> uncerta<strong>in</strong>ty.Understand<strong>in</strong>g risk substitution is critical to technologyselection and susta<strong>in</strong>able arsenic mitigation. Indevelop<strong>in</strong>g a mitigation option, public-health protectionwould demand that a key criterion for technology selectionshould be based on the potential public-health consequence<strong>of</strong> us<strong>in</strong>g a particular technology. Although suchdecisions are normally based on a number <strong>of</strong> considerations(f<strong>in</strong>ancial, environmental, social, etc.), as a generalpr<strong>in</strong>ciple, the technology represent<strong>in</strong>g the least publichealthrisk would usually be selected. This is consistentwith approaches to establish<strong>in</strong>g health-based targetsfor water supply outl<strong>in</strong>ed <strong>in</strong> the third edition (2004) <strong>of</strong>the WHO Guidel<strong>in</strong>es for Dr<strong>in</strong>k<strong>in</strong>g-water Quality (6).<strong>Risk</strong> substitution is a relatively common phenomenon<strong>in</strong> water supply, and attempts are <strong>in</strong>creas<strong>in</strong>gly made toquantify 'compet<strong>in</strong>g' risks to <strong>in</strong>form decision-mak<strong>in</strong>g.Good examples <strong>of</strong> risk substitution are the risks associatedwith dis<strong>in</strong>fection by-products result<strong>in</strong>g from <strong>in</strong>troduc<strong>in</strong>gchlor<strong>in</strong>ation and ozonation <strong>of</strong> water supplies, or the <strong>in</strong>creas<strong>in</strong>gexposure to Legionella as a result <strong>of</strong> <strong>in</strong>troduc<strong>in</strong>gpiped water supplies. In the case <strong>of</strong> dis<strong>in</strong>fection byproducts,the use <strong>of</strong> chemicals to remove pathogenicbacteria leads to a small <strong>in</strong>crease <strong>in</strong> risk <strong>of</strong> cancers fromchemicals formed dur<strong>in</strong>g the oxidation reactions but thisis justified because <strong>of</strong> the much greater decrease <strong>in</strong> risksfrom microbial pathogens. In the case <strong>of</strong> Legionella, thisenvironmental organism is unlikely to present more thanvery limited human health risks unless warm water environmentsare created, such as through piped hot watersystems operat<strong>in</strong>g at relatively low temperature or throughsome types <strong>of</strong> evaporative cool<strong>in</strong>g systems. Therefore,<strong>in</strong> both the cases, the risk substitutes are generally consideredto present much lower risks than those risks thatwould otherwise be present.In the context <strong>of</strong> arsenic mitigation, three pr<strong>in</strong>cipaltypes <strong>of</strong> hazard may potentially substitute for arsenicfrom water supply (5). These are: (a) Microbial hazards:pathogens derived from human and animal faeces thatcause diarrhoea and a range <strong>of</strong> other diseases, some withsignificant chronic sequelae; (b) Tox<strong>in</strong>s derived fromcyanobacteria that may lead to adverse health effects,<strong>in</strong>clud<strong>in</strong>g liver cancer; and (c) Other chemical contam<strong>in</strong>ants<strong>in</strong> source water from natural sources or <strong>in</strong>troducedfrom pollution.In consider<strong>in</strong>g the causes <strong>of</strong> reported waterbornediseases, microbial hazards are more commonly associatedwith greater levels <strong>of</strong> risk than chemical hazards.In develop<strong>in</strong>g countries, microbial hazards account fora very significant proportion <strong>of</strong> the burden <strong>of</strong> disease.Diseases due to microbial hazards from poor water, sanitation,and hygiene are responsible for an estimated 3.7%<strong>of</strong> the total global burden <strong>of</strong> disease (7). In addition, formicrobial hazards, as for carc<strong>in</strong>ogenic risks from chemicals,such as arsenic, it is generally assumed that thereis no safe threshold s<strong>in</strong>ce any exposure has the potentialto <strong>in</strong>itiate <strong>in</strong>fection.When compar<strong>in</strong>g the risks associated with arsenicand microbial hazards, several important po<strong>in</strong>ts emerge.Both are strongly <strong>in</strong>fluenced by poverty and nutrition(5,8). <strong>Risk</strong>s <strong>of</strong> <strong>in</strong>fection by microbial hazards <strong>in</strong>creasemarkedly with <strong>in</strong>creas<strong>in</strong>g poverty, and the overall healthburden from microbial hazards is significantly greater<strong>in</strong> poorer communities. <strong>Arsenic</strong>osis also appears to berelated to poverty and has a greater <strong>in</strong>cidence amongpoorer households exposed to elevated concentrations<strong>of</strong> arsenic. There is a l<strong>in</strong>k between nutritional status andillness from pathogen <strong>in</strong>fection and arsenicosis. Forpathogens, this is a two-way process with under-nutrition<strong>in</strong>creas<strong>in</strong>g susceptibility to disease and repeated <strong>in</strong>fections,result<strong>in</strong>g <strong>in</strong> cont<strong>in</strong>ued under-nutrition (5). The situationwith arsenic is less clear, but is likely to operate<strong>in</strong> a similar way (8).Cyanobacterial blooms occur <strong>in</strong> some ponds <strong>in</strong><strong>Bangladesh</strong>. The risks associated with tox<strong>in</strong>s derivedfrom cyanobacteria <strong>in</strong>clude gastro<strong>in</strong>test<strong>in</strong>al diseases fromacute exposure and liver cancer from chronic exposure.In addition to direct adverse health effects, algal blooms<strong>of</strong>ten lead to significant taste and odour problems. The
348 J Health Popul Nutr Sep 2006 Howard G et al.overall health impact <strong>of</strong> cyanobacterial tox<strong>in</strong>s rema<strong>in</strong>sunclear, but the potential risk substitution is sufficientlylarge to suggest that only protected surface water sourcesare used or that the water is clarified prior to filtration.Other potential hazards <strong>in</strong>clude chemicals derived frompollution and levels <strong>of</strong> manganese <strong>in</strong> groundwater. However,the overall risks associated with such pollution willgenerally be significantly lower than for microbial hazardsand arsenic.To provide some quantification <strong>of</strong> the public-healthrisks associated with the use <strong>of</strong> alternative water sources,a study was undertaken with support from the <strong>Arsenic</strong>Policy Support Unit over 12 months on the quality <strong>of</strong>water, sanitary condition, and social acceptability <strong>of</strong> watersupplies provided as arsenic mitigation. In addition, aquantitative health risk assessment (QHRA) model wasdeveloped and used for provid<strong>in</strong>g an estimation <strong>of</strong> diseaseburden associated with each technology.MATERIALS AND METHODSThe four pr<strong>in</strong>cipal alternative water-supply technologiesused <strong>in</strong> arsenic mitigation to date are: dugwells, deeptubewells, pond-sand filters, and ra<strong>in</strong>water harvest<strong>in</strong>gsystems. Dugwells are large diameter wells manuallyconstructed, l<strong>in</strong>ed with concrete r<strong>in</strong>gs, and covered bya concrete slab or a metal sheet with ventilation. A handpumpis used for withdraw<strong>in</strong>g the water. A small apronto protect aga<strong>in</strong>st short-circuit<strong>in</strong>g by contam<strong>in</strong>ants surroundsdugwells. The deep tubewells <strong>in</strong> the study allhad hand-pumps for water withdrawal and were sunk<strong>in</strong>to deeper aquifers (typically 300 feet or more). Allthe deep tubewells had small aprons around the wellheadto provide protection aga<strong>in</strong>st short-circuit<strong>in</strong>g by contam<strong>in</strong>ants.The pond-sand filters are designed as a slowsand filtration system with water drawn from an adjacentpond. In this study, the pond-sand filters had an <strong>in</strong>itialrough<strong>in</strong>g filter to reduce turbidity and a f<strong>in</strong>al sand-bedfor slow sand filtration. The ra<strong>in</strong>water harvest<strong>in</strong>g systemswere all <strong>in</strong>dividual household systems with foulflushmechanisms and taps for removal <strong>of</strong> water. Thedesigns for each technology were consistent with<strong>in</strong> eachgroup. Although the water supplies were constructedunder different programmes, <strong>in</strong> many cases, the constructionagency did not vary. The study did not attemptto analyze performance by mitigation programme, as thiswould have required additional survey design criteriaand was not the primary <strong>in</strong>terest <strong>of</strong> the study.In all cases, ongo<strong>in</strong>g operation and management <strong>of</strong>the mitigation options was the responsibility <strong>of</strong> the community,and caretakers had been identified and tra<strong>in</strong>edfor all water sources visited, although their activity levelvaried. <strong>Assessment</strong> <strong>of</strong> the performance <strong>of</strong> operation andma<strong>in</strong>tenance was undertaken through sanitary <strong>in</strong>spection,and a social assessment was performed at the same time.Water sources <strong>in</strong>cluded <strong>in</strong> the study were selectedus<strong>in</strong>g a modified cluster survey approach, with the confidencelevel set at 0.95, power set at 0.8, precision at0.1, and a design effect <strong>of</strong> 2 (9). Two sets <strong>of</strong> water sourceswere selected. The first set were dugwells and deep tubewells<strong>in</strong>stalled as arsenic mitigation options. For thisset, 72 water supplies were calculated as be<strong>in</strong>g requiredfor statistical representivity. In addition, sampl<strong>in</strong>g <strong>of</strong> 24shallow tubewells was undertaken <strong>in</strong> the dry season toprovide a comparison <strong>in</strong> quality <strong>of</strong> water and health risk.The second set were pond-sand filters and ra<strong>in</strong>waterharvest<strong>in</strong>g systems, for which it was calculated that 84water sources were required. For both the sets, the totalwater sources were divided <strong>in</strong>to equal numbers <strong>of</strong> eachwater source type. This resulted <strong>in</strong> 36 dugwells and 36deep tubewells, and 42 pond-sand filters and 42 ra<strong>in</strong>waterharvest<strong>in</strong>g be<strong>in</strong>g sampled.Based on a review <strong>of</strong> data on water supplies <strong>in</strong>stalledacross the country, the dugwells and deep tubewellswere divided <strong>in</strong>to six clusters each and the pond-sandfilters and ra<strong>in</strong>water harvest<strong>in</strong>g <strong>in</strong>to seven clusters each.Clusters were randomly selected across the country us<strong>in</strong>ga proportional weight<strong>in</strong>g table. Samples for water-qualityanalysis were taken <strong>in</strong> both dry and monsoon seasonsfrom all sources dur<strong>in</strong>g 2004-2005.All water sources were tested for thermotolerant coliformsas the pr<strong>in</strong>cipal <strong>in</strong>dicator <strong>of</strong> faecal contam<strong>in</strong>ation<strong>in</strong> l<strong>in</strong>e with recommendations <strong>of</strong> the WHO (6). Thethermotolerant coliforms were analyzed us<strong>in</strong>g a portablefield test-kit employ<strong>in</strong>g membrane filtration and membranelauryl sulphate broth. The plates were <strong>in</strong>cubatedfor 14-18 hours, and yellow colonies were counted asthermotolerant coliforms. Confirmatory test<strong>in</strong>g <strong>of</strong>Escherichia coli was undertaken <strong>in</strong> the laboratory <strong>of</strong><strong>Bangladesh</strong> University <strong>of</strong> Eng<strong>in</strong>eer<strong>in</strong>g and Technology(BUET) us<strong>in</strong>g the multiple tube method. Analysis <strong>of</strong>arsenic was undertaken by atomic adsorption spectrophotometer<strong>in</strong> the BUET laboratory. All other chemicalanalyses were performed us<strong>in</strong>g a HACH spectrophotometer<strong>in</strong> the BUET laboratory. The exist<strong>in</strong>g qualitycontrolprocedures <strong>of</strong> BUET laboratory were rigorouslyfollowed <strong>in</strong> all analyses.Sanitary <strong>in</strong>spections were carried out on all watersupplies us<strong>in</strong>g a standardized format (10). Questionnaireswith a mixture <strong>of</strong> observations and community <strong>in</strong>terviewquestions were prepared based on exist<strong>in</strong>g examples andfilled <strong>in</strong> for each water source at the time <strong>of</strong> sampl<strong>in</strong>g.
<strong>Risk</strong> assessment <strong>of</strong> arsenic mitigation options 349A range <strong>of</strong> physical and chemical parameters was testedfor each technology, although only arsenic data are reportedhere, as this was the only chemical parameterfor which a likely disease-burden estimate was derived.The data for thermotolerant coliforms and arsenicwere used for estimat<strong>in</strong>g the likely burden <strong>of</strong> diseaseus<strong>in</strong>g disability-adjusted life years (DALYs), which isthe globally applied metric used for compar<strong>in</strong>g differentdisorders and diseases with different health outcomes.The model structure and assumptions were describedelsewhere (9) and are only summarized here.The WHO recommends the use <strong>of</strong> reference pathogenswhen undertak<strong>in</strong>g a quantitative assessment <strong>of</strong> microbialrisk (6). A reference pathogen is one whose characteristics(<strong>in</strong>fectivity, burden <strong>of</strong> disease, and ubiquity) wouldmean that its control would have controlled the risks fromall similar pathogens (11). This means that detailed riskassessments for each <strong>in</strong>dividual pathogen that could bepresent, which would be difficult (because <strong>of</strong> limiteddata) or expensive, are avoided. In this study, three referencepathogens were selected. A generalized pathogenicE. coli was developed draw<strong>in</strong>g on characteristics<strong>of</strong> Salmonella ubiquity, Shigella dysenteriae <strong>in</strong>fectivity,and a disease burden from E. coli O157:H7 to act as areference for bacterial pathogens. Rotavirus was usedto act as a reference for viral pathogens and Cryptosporidiumparvum to act as a reference for protozoanpathogens. Data for unboiled water consumption wastaken from recent literature (12) and dose-responseswere selected from published literature (13-16), and wellvalidatedmodels were applied. Only the effect <strong>of</strong> diarrhoea(morbidity and mortality) was taken <strong>in</strong>to account,given the limited data on sequelae <strong>of</strong> <strong>in</strong>fection <strong>in</strong> <strong>Bangladesh</strong>.The degree <strong>of</strong> acquired immunity to these pathogenswas built <strong>in</strong>to the model tak<strong>in</strong>g <strong>in</strong>to account localdata on <strong>in</strong>fection patterns and other routes <strong>of</strong> transmission.No direct assessment <strong>of</strong> presence <strong>of</strong> pathogens wasmade <strong>in</strong> the water samples and, therefore, assumptionswere made regard<strong>in</strong>g the relationship between pathogensand <strong>in</strong>dicator bacteria. Ratios were derived from longtermmonitor<strong>in</strong>g <strong>of</strong> pathogens and E. coli <strong>in</strong> sewage fromtwo large studies <strong>in</strong> Australia. In the absence <strong>of</strong> localdata, these were considered to be sufficiently reliable foruse <strong>in</strong> the assessment.<strong>Arsenic</strong> end-po<strong>in</strong>ts considered were early onset <strong>of</strong>symptoms (keratosis and melanosis), sk<strong>in</strong> cancer, lungcancer, and bladder cancer as these were the end-po<strong>in</strong>ts forwhich reliable data were available. Other effects <strong>of</strong> kidney,liver, and prostate cancer and cardiovascular, endocr<strong>in</strong>e,reproductive and cognitive effects were not <strong>in</strong>cludedas less conclusive evidence is currently available (17-19).The arsenic dose-response models <strong>of</strong> Yu et al. (16) wereused for the cancer end-po<strong>in</strong>ts <strong>in</strong>cluded. An <strong>in</strong>itial attemptwas made to use the West Bengal data <strong>of</strong> Mazumderet al. (20) to predict early onset <strong>of</strong> symptom development,but these were subsequently omitted because there is noagreement on severity weights to be applied for thesesymptoms. Life expectancy was based on the estimatedaverage life expectancy <strong>in</strong> <strong>Bangladesh</strong> <strong>in</strong> 2004 <strong>of</strong> 62(which was based on an average life expectancy <strong>of</strong> 60years <strong>in</strong> 1999 and likely improvement s<strong>in</strong>ce then).RESULTSThe results <strong>of</strong> the test<strong>in</strong>g <strong>of</strong> thermotolerant coliforms andarsenic are shown for each technology <strong>in</strong> the dry and wetseasons <strong>in</strong> Table 1. Figure 1 shows the percentage <strong>of</strong>samples <strong>in</strong> the wet and dry seasons that exceeded the<strong>Bangladesh</strong> standards for each parameter.The data revealed that all the alternative water sourcesshowed some degree <strong>of</strong> microbial contam<strong>in</strong>ation on, atleast, a seasonal basis. The dugwells and pond-sand filtersshowed the most regular and <strong>in</strong>tense microbial contam<strong>in</strong>ation.Even <strong>in</strong> the dry season, when contam<strong>in</strong>ationwould be expected to be lowest, over 90% <strong>of</strong> samplesfrom these technologies exceeded the <strong>Bangladesh</strong> standardfor thermotolerant coliforms. Average and medianconcentrations for both the technologies were very high<strong>in</strong> the monsoon season and rema<strong>in</strong>ed relatively high <strong>in</strong>the dry season. However, both the technologies had am<strong>in</strong>imum count <strong>of</strong>
350 J Health Popul Nutr Sep 2006 Howard G et al.Table 1. Water-quality results for technology <strong>in</strong> dry and monsoon seasonsDugwell Deep tubewell Pond-sand filter Ra<strong>in</strong>water harvest<strong>in</strong>gParameter Dry season Monsoon Dry season Monsoon Dry season Monsoon Dry season Monsoonseason season season season seasonTTC cfu/100 mL M<strong>in</strong>imum
<strong>Risk</strong> assessment <strong>of</strong> arsenic mitigation options 351that the burden <strong>of</strong> disease derived from pathogens wasgreater than that from arsenic and that viral risks weremost dom<strong>in</strong>ant at lower concentrations <strong>of</strong> <strong>in</strong>dicator organisms.The disease-burden estimates <strong>in</strong>creased significantlyfor dugwells and pond-sand filters <strong>in</strong> the monsoonand to a lesser extent for deep tubewells. TheFig. 1. Quality <strong>of</strong> water <strong>in</strong> relation to the <strong>Bangladesh</strong> standard for TTCs and arsenicPercentage100908070605040302010TTC<strong>Arsenic</strong>0 WHO BDS WHO BDS WHO BDS WHO BDS WHO BDS WHO BDS WHO BDS WHO BDSGV GV GV GV GV GV GV GVMonsoon Dry Monsoon Dry Monsoon Dry Monsoon Dryseason seasonPSF RWH DW DTWBDS=<strong>Bangladesh</strong> standard; DTW=Deep tubewell; DW=Dugwell; GV=Guidel<strong>in</strong>e value; PSF=Pond-sandfilter; RWH=Ra<strong>in</strong>water harvest<strong>in</strong>g; TTCs=Thermotolerant coliforms; WHO=World Health OrganizationTable 2. Sanitary <strong>in</strong>spection data for the options <strong>in</strong>cluded <strong>in</strong> the assessmentwater harvest<strong>in</strong>g were around one order <strong>of</strong> magnitudeless than pond-sand filters and dugwells but were stillaround three orders <strong>of</strong> magnitude greater than the referencelevel <strong>of</strong> risk. Dur<strong>in</strong>g the dry season, the upperboundrisk estimates for ra<strong>in</strong>water harvest<strong>in</strong>g were similarto (slightly higher than) those estimated for the monsoonTechnologyM<strong>in</strong>imum sanitary Median sanitary Maximum sanitary<strong>in</strong>spection score (%) <strong>in</strong>spection score (%) <strong>in</strong>spection score (%)Dugwell 2 4 8Deep tubewell 2 4 7Pond-sand filter 1 4 6Ra<strong>in</strong>water harvest<strong>in</strong>g 0 1 5disease-burden estimates <strong>in</strong>creased for ra<strong>in</strong>water harvest<strong>in</strong>g<strong>in</strong> the dry season. To determ<strong>in</strong>e the level <strong>of</strong> risk <strong>in</strong>comparison to the WHO reference level <strong>of</strong> risk <strong>of</strong> 10 -6DALYs (6), comparisons were made with the upper 95%confidence <strong>in</strong>terval on the burden <strong>of</strong> disease from bacterialpathogens because bacterial pathogens dom<strong>in</strong>atedthe burden <strong>of</strong> disease. The f<strong>in</strong>d<strong>in</strong>gs are summarized <strong>in</strong>Table 4.The upper bounds <strong>of</strong> the disease-burden estimates fordugwells and pond-sand filters were at around fourorders <strong>of</strong> magnitude greater than the WHO referencelevel <strong>of</strong> risk (10 -6 ) <strong>in</strong> the monsoon season and the dryseason. In the monsoon season, the upper bounds <strong>of</strong> thedisease-burden estimates for deep tubewells and ra<strong>in</strong>season,whereas the upper-bound estimates for deeptubewells <strong>in</strong> the dry season were almost down at thereference level <strong>of</strong> risk.For the arsenic DALYs, the dugwells, deep tubewells,and pond-sand filters had associated upper risk estimatesonly slightly above the WHO reference levels <strong>of</strong> risk <strong>in</strong>the dry and wet seasons, whereas ra<strong>in</strong>water harvest<strong>in</strong>ghad associated upper risk estimates below the WHOreference level <strong>of</strong> risk.DISCUSSIONThe results showed that the microbial quality <strong>of</strong> watersupplied from all types <strong>of</strong> water sources was prone to
352 J Health Popul Nutr Sep 2006 Howard G et al.deterioration and that the public-health risks were predictedto <strong>in</strong>crease as a result. Therefore, the delivery <strong>of</strong>safe dr<strong>in</strong>k<strong>in</strong>g-water cont<strong>in</strong>ues to represent a significantpriority <strong>in</strong> rural areas <strong>of</strong> <strong>Bangladesh</strong>. Data collected <strong>in</strong>the dry season under this study also showed that shallowtubewells were commonly contam<strong>in</strong>ated and had a higherlevel <strong>of</strong> contam<strong>in</strong>ation than deep tubewells, althoughlower than dugwells.The deterioration <strong>in</strong> microbial quality dur<strong>in</strong>g the monsoonseason was most pronounced for dugwells andpond-sand filters. The results suggest that, <strong>in</strong> many cases,the designs used were not adequate to provide publichealthprotection, and ma<strong>in</strong>tenance was poor. In thelatter cases, this clearly suggests that communities hadfollowed, more <strong>in</strong>formation is required on overall costs<strong>of</strong> both hardware and s<strong>of</strong>tware.The pond-sand filters cont<strong>in</strong>ue to perform poorly and,although no estimate was made <strong>of</strong> the burden <strong>of</strong> diseaseassociated with cyanobacterial tox<strong>in</strong>s, ponds frequentlyshowed algal blooms, and it is unlikely that the technologycurrently used would provide any significant removal.Chlor<strong>in</strong>ation <strong>of</strong> pond-sand filters will improve thequality, but the susta<strong>in</strong>ability <strong>of</strong> this approach is questionableand the benefits <strong>in</strong> terms <strong>of</strong> cyanotox<strong>in</strong>s are likelyto be m<strong>in</strong>imal.The deep tubewells and ra<strong>in</strong>water harvest<strong>in</strong>g appearedto <strong>of</strong>fer the best-quality water and the lowest overall bur-PercentageFig. 2. Comb<strong>in</strong>ed risk assessment for microbial contam<strong>in</strong>ation1009080706050403020100No action actionLow risk riskIntermediate to high riskVery high high risk riskIntermediate to high riskMonsoon Dry Monsoon Dry Monsoon Dry Monsoon Dryseason seasonDW DTW PSF RWHDTW=Deep tubewell; DW=Dugwell; PSF=Pond-sand filter; RWH=Ra<strong>in</strong>water harvest<strong>in</strong>gnot received sufficient ongo<strong>in</strong>g support <strong>in</strong> ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>gtheir water supplies. These f<strong>in</strong>d<strong>in</strong>gs for dug wells aresimilar to those reported by other organizations (22-25).The authors are aware <strong>of</strong> other data for dugwellsshow<strong>in</strong>g a relatively better performance that have beencollected by the Dhaka Community Hospital, althoughthese only covered the dry season. Furthermore, subsequentstudies (that are yet unpublished) <strong>in</strong> Jessoredistrict by United Nations Children's Fund and JapanInternational Cooperation Agency have found a betterperformance <strong>of</strong> dugwells with an improved design andchlor<strong>in</strong>ation, although aga<strong>in</strong> the data are restricted todry season sampl<strong>in</strong>g. This suggests that it is possible toimprove performance <strong>of</strong> dugwells and that these rema<strong>in</strong> aviable option <strong>in</strong> areas where communities prefer these toother options and where providers are able to <strong>of</strong>ferpost-construction support. However, if this approach isden <strong>of</strong> disease. However, both the technologies did showmicrobial contam<strong>in</strong>ation, and aga<strong>in</strong>, this reflects poorma<strong>in</strong>tenance, and by implication, <strong>in</strong>adequate support andtra<strong>in</strong><strong>in</strong>g <strong>of</strong> communities. Contam<strong>in</strong>ation <strong>of</strong> the deeptubewells <strong>in</strong>creased <strong>in</strong> the monsoon season and was almostcerta<strong>in</strong>ly due to use <strong>of</strong> contam<strong>in</strong>ated prim<strong>in</strong>g water.The quality <strong>of</strong> ra<strong>in</strong>water harvest<strong>in</strong>g deteriorated <strong>in</strong>the dry season, and the disease-burden estimate also<strong>in</strong>creased a little. The mechanism <strong>of</strong> contam<strong>in</strong>ation <strong>of</strong>ra<strong>in</strong>water tanks is not certa<strong>in</strong>, but may be either due tore-growth <strong>of</strong> thermotolerant coliform bacteria with<strong>in</strong>the tanks or because the water sampled was derived fromra<strong>in</strong> from one <strong>of</strong> the occasional storms that occur <strong>in</strong> thepre-monsoon period. If the contam<strong>in</strong>ation results fromthe former process, the health risk may be lower thanpredicted from the disease-burden tool. A review byHunter <strong>of</strong> the evidence <strong>of</strong> gastro<strong>in</strong>test<strong>in</strong>al diseases derived
<strong>Risk</strong> assessment <strong>of</strong> arsenic mitigation options 353from heterotrophic bacteria that re-grow <strong>in</strong> water suppliesshowed that there is no credible epidemiological evidence<strong>of</strong> a l<strong>in</strong>k between re-growth and gastro<strong>in</strong>test<strong>in</strong>al diseases(26). It is quite possible that the thermotolerant coliformsfound <strong>in</strong> the dry season are environmental organisms and<strong>of</strong> limited public-health significance. If contam<strong>in</strong>ation isdue to recent ra<strong>in</strong>fall, it demonstrates that householdshave not taken up the recommended practice <strong>of</strong> firstflushdiversion and have allowed contam<strong>in</strong>ated waterto enter the tanks. The later improvement dur<strong>in</strong>g themonsoon season would then reflect that faecal materialdoes not build up on the catchment.had arsenic <strong>in</strong> excess <strong>of</strong> the <strong>Bangladesh</strong> standard (24).In the BGS-DPHE national study <strong>in</strong> 1999, 1% <strong>of</strong> deeptubewells across the country were found to have raisedarsenic (2). Although, <strong>in</strong> our study, the deep tubewellshad low arsenic, there rema<strong>in</strong> concerns about their unrestricteduse <strong>in</strong> the absence <strong>of</strong> reliable data on the location<strong>of</strong> the Pleistocene aquifer and an overly<strong>in</strong>g aquiclude.The disease-estimation tool used provided a usefulmeans <strong>of</strong> compar<strong>in</strong>g different technologies, but, as <strong>in</strong>many risk-assessment methodologies, relies heavily onthe assumptions made. Sensitivity analysis showed thatFig. 3. Burden <strong>of</strong> disease by technology and seasonTechnologyDALY/1,000 person-years0 2 4 6 8 10 12 14 16 18DW D MedLCLUCLDW W MedLCLUCLSTW D MedLCLUCLDTW D MedLCLUCLDTW W MedLCLUCLViralBacterialProtozoalSk<strong>in</strong>LungBladderPSF W MedLCLUCLRW W MedLCLUCLDALY=Disability-adjusted life year; DTW=Deep tubewell; DW=Dugwell; LCL=Lower confidence levelDALYs; Med=Median DALYs; PSF=Pond-sand filter; RW=Ra<strong>in</strong>water; UCL=Upper confidence levelDALYs; W=Wet seasonFor most samples, none <strong>of</strong> the mitigation options showedsignificant arsenic contam<strong>in</strong>ation, and all options can beconsidered to be effective <strong>in</strong> reduc<strong>in</strong>g arsenic exposure.A study by the Asia <strong>Arsenic</strong> Network has shownsignificant arsenic contam<strong>in</strong>ation <strong>of</strong> dugwells <strong>in</strong> Jessore,when 46% <strong>of</strong> 51 dugwells tested, had arsenic <strong>in</strong> excess<strong>of</strong> the <strong>Bangladesh</strong> standard (23). Likewise, the Asia<strong>Arsenic</strong> Network found that 10% <strong>of</strong> deep tubewells alsothe model was sensitive to the assumed relationshipsbetween thermotolerant coliforms and E. coli, <strong>in</strong>fectivity<strong>of</strong> pathogenic E. coli, the volume <strong>of</strong> unboiled waterconsumed, and immunity to rotavirus. These are allissues that would benefit from further research.Whether the choice <strong>of</strong> rotavirus as a reference pathogen<strong>in</strong> <strong>Bangladesh</strong> is correct is an area <strong>of</strong> significantdiscussion, given its strong relationship to hygiene as
354 J Health Popul Nutr Sep 2006 Howard G et al.opposed to water. Hepatitis E virus may <strong>of</strong>fer a bettermeans <strong>of</strong> assess<strong>in</strong>g viral risks, particularly as it is acommon pathogen <strong>in</strong> <strong>Bangladesh</strong> and l<strong>in</strong>ked to regularoutbreaks. The lack <strong>of</strong> data on sequelae is a further issue<strong>of</strong> concern, as this may under-estimate overall burdens<strong>of</strong> disease, particularly for bacterial pathogens. However,the exclusion <strong>of</strong> sequelae for bacteria can be justified<strong>in</strong> the light <strong>of</strong> limited data on the impact <strong>of</strong> immunestatus due to HIV status and poor nutrition that may<strong>in</strong>crease the burden <strong>of</strong> disease from viral and protozoanpathogens (27).In terms <strong>of</strong> the outcomes from the upper diseaseburdenestimates, only the deep tubewells <strong>in</strong> the dryand heavy microbial contam<strong>in</strong>ation, particularly <strong>in</strong> themonsoon season. The cont<strong>in</strong>ued use <strong>of</strong> these technologiesrequires that designs be improved and that agencies providemore tra<strong>in</strong><strong>in</strong>g and better support to communities <strong>in</strong>operation and ma<strong>in</strong>tenance.The use <strong>of</strong> tools to estimate burdens <strong>of</strong> disease associatedwith different technologies has proved to be valuable<strong>in</strong> support<strong>in</strong>g technology choice. This tool wouldbenefit from further development, but already has provento be robust enough to assist decision-mak<strong>in</strong>g. Theadvantage <strong>of</strong> us<strong>in</strong>g such tools is that they re<strong>in</strong>force thepublic-health focus <strong>of</strong> water-supply provision and canbe used for assess<strong>in</strong>g the degree to which <strong>in</strong>terventionsTable 4. Summary <strong>of</strong> upper 95% confidence limits <strong>of</strong> the risk estimate expressed as DALYs per person peryear. The WHO reference level <strong>of</strong> risk is 1x10 -6Technology Dry season Monsoon seasonDugwell 1.11x10 -2 1.34x10 -2Deep tubewell 6.98x10 -5 1.26x10 -3Pond-sand filter 1.07x10 -2 1.30x10 -2Ra<strong>in</strong>water harvest<strong>in</strong>g 6.48x10 -3 3.83x10 -3DALY= Disability-adjusted life year; WHO=World Health Organizationseason approach the WHO reference level <strong>of</strong> risk. Theother supplies showed the upper disease-burden estimatesto be significantly higher than this level <strong>of</strong> risk.Furthermore, the use <strong>of</strong> all these supplies entails transport<strong>of</strong> water to home and, thus, re-contam<strong>in</strong>ation is acommon problem. This may result <strong>in</strong> a significantly raisedrisk at the po<strong>in</strong>t <strong>of</strong> consumption, which has been found<strong>in</strong> similar sett<strong>in</strong>gs (27).All the water supplies <strong>of</strong>fered as arsenic mitigationsolutions showed significant problems with microbialquality. By undertak<strong>in</strong>g the disease-burden estimates,it can be shown that none <strong>of</strong> the supplies can be said todeliver 'safe' dr<strong>in</strong>k<strong>in</strong>g-water under all circumstances asthey are currently employed. On the other hand, all thetechnologies had low arsenic show<strong>in</strong>g that all are effectiveat least <strong>in</strong> reduc<strong>in</strong>g arsenic exposure. Furthermore,under good situational and operational conditions, allcan prove effective at reduc<strong>in</strong>g burdens <strong>of</strong> microbialdiseases.Look<strong>in</strong>g at the body <strong>of</strong> evidence, overall, deep tubewellsand ra<strong>in</strong>water harvest<strong>in</strong>g appear to <strong>of</strong>fer the lowestpublic-health risk, although neither is free <strong>of</strong> problems.The development <strong>of</strong> deep tubewells is also constra<strong>in</strong>edby the limited <strong>in</strong>formation on the Pleistocene aquiferand its susta<strong>in</strong>able development. 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