Journal of Contaminant Hydrology 99 (2008) 1–7

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Journal of Contaminant Hydrology

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Editorial

Distribution of geogenic arsenic in hydrologic systems: Controlsand challenges

Abhijit Mukherjee a,⁎, Prosun Bhattacharya b, Kaye Savage c, Andrea Foster d, Jochen Bundschuh e

a Bureau of Economic Geology, University of Texas at Austin, 10100 Burnet Road, Austin, TX 78758, USAb KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH),Teknikringen 76, Stockholm, SE-10044, Swedenc Earth and Environmental Sciences, Vanderbilt University, VU Station B 35-1705, 2301, Vanderbilt Place, Nashville, TN 37235-1705, USAd U.S. Geological Survey, 345 Middlefield Road MS 901, Menlo Park, CA 94025, USAe Instituto Costarricense de Electricidad (ICE), UEN, PySA, Apartado , Postal 10032, San José, 1000, Costa Rica

a r t i c l e i n f o

⁎ Corresponding author.E-mail addresses: abhijit.Mukherjee@beg.utexas.ed

prosun@kth.se (P. Bhattacharya), k.savage@vanderbilt.afoster@usgs.gov (A. Foster), jochenbundschuh@yahoo

0169-7722/$ – see front matter © 2008 Elsevier B.V.doi:10.1016/j.jconhyd.2008.04.002

a b s t r a c t

Article history:Accepted 14 April 2008Available online 22 April 2008

The presence of elevated concentration of arsenic (As) in natural hydrologic systems is regardedas the most formidable environmental crisis in the contemporary world. With its substantialpresence in the drinking water of more than thirty countries worldwide, and with an affectedpopulation of more than 100 million, it has been termed as the largest mass poisoning inhuman history. In this special issue, we have tried to provide the most recent research advanceson controls and challenges of this severe groundwater contaminant. The articles in this issue,originally presented in the 2006 Geological Society of America Annual Meeting, address thedistribution of As in various geologic and geographic settings, the controls of redox and othergeochemical parameters on its spatial and temporal variability, the influence of sedimentologyand stratigraphy on its occurrence, and mechanisms controlling its mobility. The knowledgeavailable from these studies should provide a roadmap for future research in arseniccontamination hydrology.

© 2008 Elsevier B.V. All rights reserved.

Keywords:ArsenicGroundwaterDrinking waterRedoxSediment

1. Introduction

Water is an essential commodity for the nourishment ofhuman life as well as sustainable functioning of our globalecosystem. Only a small fraction of the available water in theearth can be considered potable, which can either havesurface water (e.g. rivers, lakes etc.) or groundwater sources.However, in recent years, the presence of elevated concentra-tions of various organic and inorganic pollutants from naturaland anthropogenic sources has limited the amount of safe,potable water available. Ingestion of these pollutants for overlong-time can severely impair the healthy functioning of bothhuman and other animal lives.

u (A. Mukherjee),edu (K. Savage),.com (J. Bundschuh).

All rights reserved.

Among drinking water pollutants, arsenic (As) is regardedas the most formidable contaminant on a global scale (ATSDR,2005). Until the last couple of decades, As was often notregularly monitored in drinking water for water qualityassessment. Hence, the existing scientific knowledge fordistribution of As is limited and has evolved mostly fromvery recent studies. Arsenic can range from b0.1 µg/L to~5000 µg/L in natural water. Although the first cases ofwidespread As poisoning inwater were discovered as early aslate 19th century [(e.g. northern Appalachian mountains(Hitchcock, 1878) black-foot disease in Taiwan (Tseng et al.,1968)], it was not since the discovery of As in the Bengal basin,beginning in late 1980s, that the study of As in surface andgroundwater gained its impetus. Following the increasingevidence for the chronic toxicological effects of As in drinkingwater, water quality regulatory bodies started setting permis-sible limits of As for safe drinking water consumption. In viewof this increasing awareness of the toxicity of As (e.g.

2 A. Mukherjee et al. / Journal of Contaminant Hydrology 99 (2008) 1–7

carcinogenicity), and the ability to measure it quantitatively(Smedley and Kinniburgh, 2002), the World Health Organiza-tion (WHO) provisionally reduced the guideline value for Asin drinking water from 50 to 10 µg/L in 1993 (WHO, 1993),which wasmadeMaximum Contaminant Level (MCL) in 2001(WHO, 2001). Present estimates show that elevated concen-trations of As in natural hydrologic systems with geogenicsources occur in at least twenty geographic provinces aroundthe globe (Fig. 1). An insurgence of water quality studies inlast two decades has now showed that parts of all of the sixcontinents (Asia, Africa, Australia, Europe, North and SouthAmerica) hosting most of the population of the world, arecontaminated (Nriagu et al., 2007; Mukherjee et al., in press).Probably more than 100 million people living in these regionsare at risk of adverse health effects from ingesting detrimentallevels of As for prolonged periods. Some of the recent studieshave shown that there may not be any safe concentration forAs in drinking water.

Most of the hydrologic As contamination problems aroundthe globe are the result of its mobilization under naturalconditions. The processes related to the mobilization andretention of As operate in a wide variety of natural environ-mental systems. Arsenic is an ubiquitous element and one ofthe very few among other heavy metalloids and oxyanion-forming elements (e.g. As, Se, Sb, Mo, V, Cr, U, Re) in itssusceptibility to mobilization at common groundwater pHranges (6.5 to 8.5), under both oxidizing and reducingenvironments (e.g. Smedley and Kinniburgh, 2002; Bhatta-charya et al., 2007). Some researchers suggests that thepresence of elevated As in each geographic location is aunique result of its geology, morphology, hydrology and landuse pattern and needs site-specific attention to assess its

Fig.1.WorldMap showing the presently-known,major naturally arsenic contaminat3. Terai plains, Nepal, 4. Donargarh rift zone, central India, 5. Middle Gangetic plaindelta, Vietnam, 8. Yellow river plain, Inner Mongolia, 9. Irrawady delta, Myanmar, 10.Wales, Australia, 13. Taiwan, 14. Shanxi, China, 15. Kurdistan, Iran, 16. Nigeria, 17.Lagunera region, northernMexico, 21. Southwestern USA, 22. Gulf Coastal Plain, southDakota and Minnesota, USA, 25. New England, USA, 26. Nova Scotia, Canada, 27. Ont30. Atacama dessert area, northern Chile and Argentina, 31. El Zapote, Peru 32. And

controls and challenges (Hasan et al., 2007; Mukherjee et al.,2007).

Most of the areas with arsenic contaminated groundwateracross the globe are of Quaternary age (e.g. Smedley, 2005),however contamination in Pre-Cenozoic provinces has alsobeen documented (e.g. Peters et al., 1999). The contaminatedareas can also be subdivided on the basis of some geomorphicand geologic similarities (Mukherjee et al., in press). Forexample, in Asia, most of the affected areas are located in theflood and delta plains of the major Himalayan rivers e.g. theGanges–Brahmaputra–Meghna plain and delta in India andBangladesh (e.g. Bhattacharya et al., 1997, 2007), Indus plainin Pakistan (e.g. Naseem et al., 2001), Irrawady delta ofMyanmar (e.g. WRUD, 2001), Red river delta in Vietnam (e.g.Berg et al., 2001), Mekong river delta of Cambodia and Laos(e.g. Feldman and Rosenboom, 2001). Similarly there aresome acute As contaminated areas that are located in arid andsemi arid climatic regions. The best example for these types ofsettings would be the Atacama Desert area of the northernChile (e.g. Borgono and Greiber, 1971), Argentina (e.g.Bundschuh et al., 2000, 2004; Bhattacharya et al. 2006) andNevada of USA (Fontaine, 1994). Contaminated areas of InnerMongolia (e.g. Luo et al., 1997) would be a good example forAs pollution of lacustrine deposits. Based on source fromgeothermal and igneous sediments, the areas of northernChile, Chaco-Pampean aquifers of Argentina, contaminatedareas of California, Nevada, Yellowstone National Park of USAand Donargarh rift belt of Central India (Acharyya, 2002) canbe grouped together. Polluted aquifers in the southeasternparts of Australia (e.g. Smith et al., 2003) and Taiwan (e.g.Tseng et al., 1968) are probably related to As sourced frommarine sediments. Contaminated sites in the Appalachian

ed areas of theworld: 1. Xinjiang, China, 2. Bengal Basin, India and Bangladesh,, India, 6. Mekong plain and delta, Cambodia, Vietnam, and Laos, 7. Red riverIndus alluvial plains, Pakistan, 11. Perth, Australia, 12. Stuart Point, New SouthGhana, 18. Great Hungarian Plain, Hungary and Romania, 19. Nicaragua, 20.ern Texas, USA 23. Southern High Plains, northern Texas, USA 24. Inner Plains,ario, Canada, 28. Fairbanks, Alaska, USA, 29. Chaco-Pampean plain, Argentina,ean highlands, Bolivia, 33. Serbia, 34. Albania.

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belts of northeast USA (Peters, 2008-this issue), Central BalkanPeninsula in Siberia (e.g. Dangic' and Dangic', 2007); Albania(e.g. Lazo et al., 2007), Ghana (e.g. Smedley, 1996), Nigeria (e.g.Gbadebo, 2005), and northern Chile (e.g. Oyarzun et al., 2004)are related to possible contamination from As-enriched sulfidemineralization. The As contaminated aquifers also show awidevariation in the distribution of polluted water with depth.

At present, the Ganges–Brahmaputra–Meghna plain anddelta in India and Bangladesh represents the most acutely Ascontaminated site in theworld,with concentrations some timesN4000 µg/L (Ghosh andMukherjee, 2002; Rahman et al., 2006).More than 50 million people are estimated to be ingestingcontaminated water in this area alone (Smedley and Kinni-burgh, 2002; Mukherjee, 2006). Contaminated water occursmostly as groundwater at shallow depth (BGS/DPHE/MML,2001); however deeper aquifers have also been recently foundto be considerably contaminated (Mukherjee and Fryar, 2008).Because of this severity, most of the groundwater As studies areconcentrated in this part of the world. However, other veryimportant contaminated areas exist in the Chaco-Pampa plainsof Argentina, various parts of the USA, Chia-Nan plains ofTaiwan, and elsewhere.

2. Topical session and themes

2.1. Background

In the present and preceding decade, research on theoccurrence and fate of As in environmental systems and itsremediation has been very active and includes assessment ofthe studies on various interactions at scales ranging frommolecular bonding to sub-continental distribution, studies ofphysical and biogeochemical processes controlling spatial andtemporal heterogeneities in concentration and speciation,understanding of the role of microbes and other biota inarsenic cycling, and development of technologies to remediatecontamination at various scales. As a result, each year severalinternational conferences are organized in various parts of theworld, where researchers present their recent findings.

The 2006 Geological Society of America (GSA) AnnualMeeting (22–25 October, 2006) held in Philadelphia, PA, USAprovided an opportunity for an international platform forexchange of knowledge and recent development in geologicstudies around the globe. The topical session T19: Distributionof Arsenic and Related Metalloids in Surface and GroundWaters:Controls and Challenges, (http://gsa.confex.com/gsa/2006AM/finalprogram/session_18041.htm) chaired by Dr. Kaye Savage,Dr. Dr. Andrea Foster, Dr. Prosun Bhattacharya, and Dr. AbhijitMukherjee was organized for exchange of ideas amongresearchers working on issues related to the presence ofelevated arsenic and other metalloids in hydrologic systems.The session was sponsored by GSA Hydrogeology Division,and co-sponsored by Geobiology and Geomicrobiology Divi-sion, Geology and Health Division, and the GeochemicalSociety. The oral and poster sessions were focused on severalmajor themes: (1) Sources and characterization of arsenicand related metalloids in the environment at various scales;(2) Processes that control mobility and speciation of arsenic inwater of contaminated areas around the globe; (3) Predictionof the fate of arsenic in natural environments in response togeochemical and hydrologic triggers, (4) Remediation and

management of arsenic in agricultural and drinkingwater, andnatural ecosystems. The four sub-sessions was attended by alarge number of audiences, where 56 papers were presentedby scientists and workers from more than 15 countries.

2.2. Layout and prelude of the papers

Key research contributions presented in these sessionswere subsequently compiled in this special issue of theJournal of Contaminant Hydrology. The nine papers selectedfor this special volume discuss recent research findings fromthe Ganges–Brahmaputra–Meghna plain and delta (fivepapers), eastern, central and southern USA (three papers)and Chia-Nan plains of Taiwan (one paper), and have beenauthored by scientists from Bangladesh, Czech Republic,India, Italy, Taiwan, Sweden, and USA. The articles havebeen distributed in the following five sections:

2.2.1. Distribution of arsenic and controlling processesThefirst article of this special issue reviews the distribution

of As in the groundwater of the Northern AppalachianMountains in eastern USA (Peters, 2008-this issue). Thepaper is one of the first compilations of previous As relatedstudies in this geographical province. Up to 22% of the privatedrinking water wells contains As above 10 µg/L. Generally, thefamily of As bearing pyrites and occasionally minerals likewesterveldite forms the primary source of most of the As inthe water of this area. The author proposes that the geologicprovenance of the As host rocks may be in the accretionaryterrains of the northeastern US,whichmayalso be responsiblefor higher As concentrations in the Mesozoic rift basins bycrustal recycling during continued tectonism. In most cases,pH dependent adsorption to ironmineral surfaces controls theconcentration of As. The reductive dissolution of ironminerals,which usually increase As concentrations in groundwater hasbeen identified to be related to anthropogenic activities.

The second article in this section documents the processescontrolling As distribution in two highly contaminated areasof the western part of the Bengal basin, India (Mukherjee-Goswami et al., 2008-this issue). The first of the two sites isinner delta while the other site is closer to the Bengal deltafront of the river Bhagirathi. The groundwater has beenobserved to be mostly of fresh water (Ca–Mg–HCO3) type.However the southern site has more Na and Cl enrichmentsuggesting possible saline water intrusion. The anoxicgroundwater in both locations was found to be enriched inAs, Fe, Mn, HCO3

− and PO43-. The study re-confirmed by

hydrochemical and statistical analyses that the evidence ofabsence of dissolved O2, nitrates, and enrichment of reducedsolutes suggests microbially mediated, thermodynamicallyfavored redox processes, dominated by Fe/Mn-reduction. Thestudy also observes that the near-surface clay aquitards in thesouthern site enhances such anoxia, promoting enhancedreductive dissolution. The authors postulated that the highconcentrations of HCO3

− indicate the weathering of carbonateand the degradation of organic matter under local reducingcondition (redox traps).

2.2.2. Spatial variability and redox processesThe two articles in this section discuss the spatial variability

of As in response to redox processes. The first article provides

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results of one of the first comparative studies on the hydro-geochemistry from the two extreme fringes of the acutely Ascontaminated Bengal basin: the Bhagirathi sub-basin inwestern margin, in India and the Meghna sub-basin in theeastern margin, in Bangladesh (Mukherjee et al., 2008-thisissue). The study areas were selected according to theirdifference in sediment provenance, geologic settings andhydrogeomorphology. The groundwater in the two studyareas showed that the two areas have considerable differencesin major and minor solute trends indicating varying pathwaysof hydrogeochemical evolution and influence of hydrostrati-graphy. However, in spite of this considerable difference indistribution of solutes, both sites have similar reducingenvironments indicating dominantlymetal-reducing processesand similarity in As mobilization mechanisms. The study alsohypothesizes that the redox zones (as indicated by the trends ofredox-sensitive solutes e.g. As, CH4, Fe, Mn, NO3

−, NH4+, SO4

2−)indicate substantial overlapping along flow paths to partialredox equilibrium conditions. As a result, As, once liberatedfromsourceminerals, tends to remain in solutionbecauseof thecomplex interplay among the electron acceptors.

The following article describes variability of As and Fedistribution in solid phase and solution in the shallow depthsof the Mississippi River Valley alluvial aquifer in southeasternArkansas, south-central U.S.A (Sarif et al., 2008-this issue).They found that ~18% of their sampled water from irrigationwells had As levels above 10 µg/L. The solid phase As wasmostly found to be associated with amorphous Fe and Mnphases and mostly in the shallow depths (b1 m), withinsignificant association with carbonates and organic matter.The reducible As associated with Fe/Mn amorphous phasesdecreases significantly with depth as compared to Asassociated with metal oxide mineral phases. Although Fe(II)/Fe increases with depth, the relative abundance ofreducible As and Fe decreases noticeably with depth. Thedecrease in reducible Fe and As with depth might haveresulted from flushing from hydrous ferric oxides (HFO) bymicrobially-mediated reductive dissolution and aging of HFOto crystalline phases. The study also noted that the SO4

2−

reduction to form sulfide with As co-precipitation is animportant limiting process controlling the concentration of Asin groundwater in the area.

2.2.3. Geochemical evolution and transport of ArsenicThe first article of this section discusses the evolution of As

in the Aquia aquifer along a flow path in betweenWashingtonD.C. and Chesapeake Bay of U.S.A (Haque et al., 2008-thisissue). The study explores the reason for distinct differencesbetween the total dissolved As load in the northern andsouthern portions of the study area. Aquia groundwaterswerefound to be suboxic, and non-sulfidogenic. About 50% of thetested wells in this study resulted in As concentrations higherthan the WHO MCL. However, the Aquia aquifer sedimentswere analyzed to have average solid phase As concentration is94% lower than the average crustal abundance of As. Theauthors hypothesize that the elevated As concentrationsinitially resulted from dissimilatory Fe(III) reduction that ledto release of a substantial amount of As from the non-labilepool of As associated with the Fe(III) oxide/oxyhydroxides,which form coatings on aquifer mineral surfaces (e.g., quartz,glauconite). Along the flow path recrystallization of amor-

phous Fe(III) oxide/oxyhydroxides into more thermodynami-cally stable crystalline forms resulted substantial furthermobilization of As into the groundwaters due to decrease inspecific surface area. Near the final stretch of the flow path, Feconcentration decreases in relation to As, as the former startsre-adsorbing onto sediment grain surfaces. There re-adsorbedFe subsequently forms a sink of the As, lowering its overallconcentrations. The study also suggests that this wholeevolution process may be substantially microbially-catalyzed.However, direct proof of such microbial activity would needfurther studies.

In the second article of this section, Nath et al. (2008-thisissue) explores the sources and transport mechanisms ofelevated As in the Chia-Nan plain of Taiwan. The black-footdisease in the rural population in this region has long beenassociated with ingestion of As contaminated drinking waterand external exposure through skin. The study suggests thatthe elevated concentrations of trace metals in groundwaterare largely influenced by seawater intrusion. The annualdecrease in redox potential with simultaneous increase in Fe,Mn, Sr, and Ba concentration have been attributed to activebacterial Fe(III) and Mn(V) reduction by along with avail-ability of marine SO4. However, the study hypothesizes thatthe primary transport of the As in groundwater has probablyresulted from interaction of circulating groundwater with thedeep source rocks. In light of geochemical calculations, theauthors suggest that biogenic carbonate and sulfide mineralsmay form a local As sink. They further argued that thechemical evolution of the groundwater may be a result ofconcurrent occurrence of several biogeochemical reactions(i.e., bacterial iron reduction, sulfate reduction, etc.) alongwith mineral precipitation and dissolution.

2.2.4. Temporal variability and comparison between shallowand deep groundwater

The sole article in this section, Dhar et al. (2008-this issue),is the latest contribution from the prolonged studies of theColumbia University arsenic group in the As contaminatedarea of Araihazar, Bangladesh. The paper discusses thetemporal variability in trends of As and other solutes inshallow (b30 m) and relatively deep aquifers (N30 m) of theirstudy area over a period of continuous monitoring for threeyears. The study reports that over time, the shallow ground-water of Araihazar has had low variations in concentration ofAs and P (b30%) in contrast to ~90% variation in concentra-tions of the major ions and other redox-sensitive elements(e.g. Fe, Mn, and S). In the deeper groundwater As varied byonly b10%. The concentrations of major cations also varied byb10% in these deep aquifers in comparison to up to 97%variation of Fe, S and Mn. The authors suggest that thisindicates strong decoupling between As and Fe concentra-tions. Their groundwater age data (by 3H/3He and 14C dating)also suggest that large seasonal variations inmajor and redox-sensitive solute concentrations are restricted to shallowaquifers with groundwater recharged b5 years ago. However,As does not display much corresponding variation in theseyoung groundwaters, probably due to chemical buffering byrapid equilibration between solute and solid As. Systematiclong-term decline in As in selected sites probably suggeststhat flushing may slowly deplete an aquifer of As. The studyalso observed that the shallowgroundwater older than 5 years

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shows a constant As:Pmolar ratio over time, consistent with acommon mechanism of mobilization.

2.2.5. Sedimentologic and stratigraphic control on elevatedarsenic concentrations

The two articles selected for this section explore thesedimentation, geomorphic and mineralogical controls ondistribution of As in two highly contaminated local-scalestudy areas in eastern parts of Bengal basin (Bangladesh). Thefirst article focuses on characterizing the aquifer sedimentchemistry and related hydrochemistry of Manikganj area(Shamsudduha et al., 2008-this issue). While the uppermostpart of the aquifer of this area (10 m to 80 m) has a mean Asconcentration of 35 μg/L, the deeper aquifer (N100mdepth) hasa mean arsenic concentration of 18 μg/L in groundwater.Analyses of sediment core, collected from the upper aquifer,show that the sediments aremostly gray to dark-gray, overbankand floodplain deposits enrichedwith organic matter and peat.The sands in As contaminated shallow aquifers are mainlyarkosic to sub-arkosic. High arsenic concentrations in theaquifer are mostly associated with fine-grained sedimentsthat were probably derived mostly from the recycled orogensand relatively rapidly deposited during the geomorphic evolu-tion of Holocene sea-level change. Authigenic goethite concre-tions in these sediments, speculated to be of bacterial origin,were found to have very high As concentration. Arsenic in thesesediments was found to have strong statistical correlationwith,Fe, Mn, Ca, P, Cd, Cu, Ni, Zn, and Bi.

The last article in this section and also in this special issueprovides the results of the latest studies carried out in theMatlab area by the researchers of the Royal Institute ofTechnology (KTH) arsenic research group, where they arelooking for safe, alternate groundwater options such asdeeper, low As aquifers (von Brömssen et al., 2008-thisissue). In order to study the sustainability of this option, theresults presented here describe the lithofacies and genesis ofthe sediments in the shallow aquifer and establish therelationship between aqueous and solid phase geochemistry.The groundwater in the contaminated aquifers is character-ized by high concentrations of PO4

3−tot, NH4

+, DOC, Fe and lowSO4

2− ,whereas the targeted low As aquifers have groundwaterfrom the characterized by high Mn, low NH4

+, DOC, Fe, andPO4

3−tot. Sediment chemistry analyses of the high-and-low As

aquifers show that Fetotal and Mntotal correlates well for boththe high and the lowAs unit. However, the FeOx/MnOx (oxalateextracted) is distinctively higher for high As unit indicatingthat amorphous Fe oxides and hydroxides are more inclinedto weathering and oxidation than amorphous Mn oxides andhydroxides. Geochemical modeling indicates that the con-centrations of Fe, Mn and PO4

3−tot in groundwater are also

influenced by secondary minerals in addition to redoxprocesses. As adsorption in the system was found to belargely influenced by dissolved PO4

3−tot as the most important

competing ion. Simulation using the Diffuse Layer Modelshowed a good fit with the observed groundwater and sorbedconcentrations of As (III) in the contaminated aquifer.

3. Conclusions

The selected nine articles for this special issue areexpected to provide further understanding regarding the

distribution, transport mechanisms and sediment–waterinteraction of As contaminated groundwater in some of themost polluted areas of the world. Because of the severity andextent, the Bengal basin received the most attention;however, interesting results and discussion also evolvedfrom relatively less studied sites e.g. northeastern andsouth-central U.S., and Taiwan. It can be concluded from theresults of these studies that a single mechanistic process (e.g.reductive dissolution) can't address all the intricacies asso-ciated with the fate and transport of As in contaminatedaquifer. Similarly, the absence of As from an aquifer in thepresent doesn't necessarily indicate that it wouldn't have thepotential to get contaminated in the future due to change ofnatural hydrological or geochemical regime or anthropogenicinfluences. The provenance of As bearing sediments are notyet well understood, but the source minerals can be primarilyor secondarily derived from a diverse range of setting andconditions, varying from crustal recycling to geomorphicevolution. Thus, these studies together explore the controlsand challenges of As pollution and provides a more informedpodium for undertaking future research to remediate thissevere environmental contaminant and reduce its tragicimpact on human society.

Acknowledgements

We would like to convey our sincere thanks to theGeological Society of America (GSA) for providing us theopportunity to convene the arsenic topical session at the 2006GSA Annual Meeting. We would also like to acknowledge oursession sponsors, namely GSA Hydrogeology Division, GSAGeobiology and Geomicrobiology Division, GSA Geology andHealth Division, and the Geochemical Society. We are gratefulto the GSA officers, especially Dr. Nancy Carlson, Dr. LauraToran, our speakers, and knowledgeable audience for helpingus in everyway to convene these sessions. PB would like tothank the Swedish Development Cooperation Agency (Sida),Swedish Research Council (VR), Knut Alice and Wallenberg'sFoundation of the Royal Institute of Technology (KTH) and theMISTRA for the support for participation and editorialpreparation of this special issue. AM would like to thank theJackson School of Geosciences, University of Texas at Austinand Dr. Bridget Scanlon for the support. We would also like toexpress our gratitude to the Journal of Contaminant Hydrol-ogy editorial board, especially Prof. Al Valocchi, Elsevierjournal manager Ms. Yvonne Philippo, and our authors,without whose hard work, interest and willingness to help,this special issue would never have been possible.

Finally, we would like to extend our special thanks andname our reviewers who have served in this special issue:

Dr. Douglas Kent (U.S. Geological Survey, USA), Dr. StephenPeters (Leigh University, USA), Dr. Alan Fryar (University ofKentucky, USA), Dr. W. Berry Lyons (Ohio State University,USA), Dr. Roger Beckie (University of British Columbia,Canada), Dr. Karen Johannesson (Tulane University, USA), Dr.Anindya Sarkar (Indian Institute of Technology-Kharagpur,India), Dr. Kenneth Stollenwork (U.S. Geological Survey, USA),Dr. Saugata Datta (University of Calcutta, India), Dr. Jean-Phillipe Nicot ( University of Texas at Austin, USA), Dr.Christopher Matocha (University of Kentucky, USA), Dr.Chuxia Lin (South China Agricultural University, China), Dr.

6 A. Mukherjee et al. / Journal of Contaminant Hydrology 99 (2008) 1–7

Dilip Datta ( Chittagong University, Bangladesh), Dr. Md. SalahSarif (University of Arkansas, USA), Dr. Alexander Desbarats(Geological Survey of Canada, Canada).

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