TECHNICAL REPORTS 1598 High levels of naturally occurring arsenic are found in the shallow reducing aquifers of West Bengal, Bangladesh, and other areas of Southeast Asia. Tese aquifers are used extensively for drinking water and irrigation by the local population. Mechanisms for its release are unclear, although increasing evidence points to a microbial control. Te type of organic matter present is of vital importance because it has a direct impact on the rate of microbial activity and on the amount of arsenic released into the ground water. Te discovery of naturally occurring hydrocarbons in an arsenic-rich aquifer from West Bengal provides a source of potential electron donors for this process. Using microcosm-based techniques, seven sediments from a site containing naturally occurring hydrocarbons in West Bengal were incubated with synthetic ground water for 28 d under anaerobic conditions without the addition of an external electron donor. Arsenic release and Fe(III) reduction appeared to be microbially mediated, with variable rates of arsenic mobilization in comparison to Fe(III) reduction, suggesting that multiple processes are involved. All sediments showed a preferential loss of petroleum-sourced n-alkanes over terrestrially sourced sedimentary hydrocarbons n-alkanes during the incubation, implying that the use of petroleum-sourced n-alkanes could support, directly or indirectly, microbial Fe(III) reduction. Samples undergoing maximal release of As(III) contained a significant population of Sulfurospirillum sp., a known As(V)-reducing bacterium, providing the first evidence that such organisms may mediate arsenic release from West Bengali aquifers. The Role of Indigenous Microorganisms in the Biodegradation of Naturally Occurring Petroleum, the Reduction of Iron, and the Mobilization of Arsenite from West Bengal Aquifer Sediments H. A. L. Rowland and C. Boothman The University of Manchester R. Pancost University of Bristol A. G. Gault, D. A. Polya, and J. R. Lloyd* The University of Manchester T he presence of elevated concentrations of arsenic in shallow, reducing ground waters extensively exploited for irrigation and drinking water is causing massive human health impacts in Bangladesh and West Bengal (DPHE, 1999; Smedley and Kinniburgh, 2002); in similar aquifers in SE Asia, notably Cambodia and Vietnam (Berg et al., 2001; Polya et al., 2005; Charlet and Polya, 2006; Berg et al., 2007; Feldman et al., 2007); and other locations worldwide (Smedley and Kinniburgh, 2002). Te specific process of arsenic release in these aquifers is a source of intense academic debate, with various mobilization strategies being postulated (Nickson et al., 2000; Das et al., 1996; Harvey et al., 2002; Horneman et al., 2004; Islam et al., 2004; Polizzotto et al., 2006). It appears that microbes pay a vital role in arsenic release within these aquifers, indirectly via the oxidation of organic matter coupled to the reduction of arsenic-bearing Fe(III) oxides causing dissolution and subsequent arsenic release (Nickson et al., 2000; Ravenscroft et al., 2001; McArthur et al., 2001; Zheng et al., 2004) or directly via microbial utilization of As(V) as an electron acceptor sequentially after microbial Fe(III) reduction or in the absence of Fe(III) reduction (Zobrist et al., 2000; Akai et al., 2004; Islam et al., 2004; van Geen et al., 2004). An important factor in both of these processes is the organic matter within the aquifer because this is used as an electron donor for metal reduction by the indigenous microbial community and because its availability and type have a direct impact on the rate of microbial processes. Te source of organic matter available to indigenous microbes and leading directly or indirectly to arsenic release is not clear. Within the arsenic-affected aquifers of Bangladesh and West Bengal, organic matter from within the aquifers can potentially be sourced inter- nally (e.g., from peat commonly found within the delta sediments [Umitsu, 1993; McArthur et al., 2001; Ravenscroft et al., 2001] and original organic matter deposited at the same time as the sediment [Smedley and Kinniburgh, 2002; Ravenscroft et al., 2001; Meharg Abbreviations: CPI, carbon preference index; RFLP, restriction fragment length polymorphism. H.A.L. Rowland, C. Boothman, A.G. Gault, D.A. Polya, and J.R. Lloyd, School of Earth, Atmospheric and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science, The Univ. of Manchester, M13 9PL, UK; H.A.L. Rowland, present address: Swiss Federal Institute of Aquatic Science and Technology, EAWAG, CH 8600 Dübendorf, Switzerland; A.G. Gault, present address: Dep. of Earth Sciences, Univ. of Ottawa, 140 Louis Pasteur, Ottawa, ON, K1N 6N5, Canada; R. Pancost, School of Chemistry, Univ. of Bristol, BS8 1TS, UK. Copyright © 2009 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Published in J. Environ. Qual. 38:1598–1607 (2009). doi:10.2134/jeq2008.0223 Received 12 May 2008. *Corresponding author (jon.lloyd@manchester.ac.uk). © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA TECHNICAL REPORTS: GROUND WATER QUALITY Published July, 2009