【正文】 Jakariya et al., 2007). The paper discusses the relation between the colour of the sediments and groundwater redox conditions. This study revealed that the sediment colour is a reliable indicator of high and low As concentrations that can be used by local drillers to target lowAs groundwater. The presence of As contamination of shallow fluviodeltaic aquifers in the Bengal Basin has also resulted in increasing exploitation of groundwater from deeper aquifers that generally contain low concentrations of dissolved As (Stollenwerk, 2003). However, infiltration of highAs groundwater induced by increased pumping of these aquifers clearly indicate the possible risks for an increase in As concentrations. The following paper (Stollenwerk et al., 2007this volume) presents a study on the investigation of the adsorption capacity for As of sediment from a lowAs aquifer near Dhaka, Bangladesh. At this site a shallow, chemically reduced aquifer with 900 μg/LAs overlies a more oxidized aquifer with b5 μg/L As. Since no thick layer of clay was present at the site to inhibit vertical transport of groundwater, there was an apparent risk for an increase in the concentration of dissolved As in the deeper aquifers. Laboratory experiments and geochemical modeling were used to show that oxidized sediments have a substantial but limited capacity for removal of As from groundwater.The problem of geogenic As is not only restricted to the Bengal Basin and its surrounding region. DissolvedAs in groundwaters from coastal aquifers used extensively for human consumption has led to widespread concern in eastern Australia. In the next paper O39。 2) processes that control mobility and speciation of As in soil, water and biota。 arsR and arsD regulate the ars operon. Only a handful of microorganisms capable of respiring As(V) have been isolated (Oremland and Stolz, 2003). The As(V) reductase genes (arrA and arrB) involved in As(V) reduction have been identified in a number of bacteria, and they share high sequence identities (Santini and Stolz, 2004). The As(V)respiring microorganisms can use different electron donors (. acetate, hydrogen), and range from mesophiles to extremophiles (Oremland and Stolz, 2003). These laboratory studies indicate that microbial processes involved in As(V) reduction and mobilization is many times faster than inorganic chemical transformations (Ahmann et al., 1997。 Matschullat, 2000。 Del Razo et al., 2001。 Steinmaus et al., 2006). Field kits provide semiquantitative results and the reliability of several field kits are questioned because of poor accuracy (Rahman et al., 2002). Thus, there is a need for further evaluation of the screening results by the field kit, prior to its remendation for wide scale use in Bangladesh and elsewhere in the world.. EpidemiologyIngestion of groundwater with elevated As concentrations and the associated human health effects are prevalent in several regions across the world. Arsenic toxicity and chronic arsenicosis is of an alarming magnitude particularly in South Asia and is a major environmental health disaster (Chakraborti et al., 2004。 2005。 Mukherjee et al., 2006). During the past few years, As has also been detected in groundwaters of the sedimentary aquifers of the Terai Belt in Southern Nepal (Bhattacharya et al., 2003。 Mukherjee et al., 2006). Arsenic in groundwater is often associated with geologic sources, but in some locations anthropogenic inputs can be extremely important. Ingestion of geogenic As from groundwater sources is manifested as chronic health disorders in most of the affected regions of the world (BGS amp。 Bundschuh et al., 2005。 Vuki et al., 2007this volume), a small island in Western Pacific Ocean. Arsenic is also found in widely scattered geographical areas in the United States and Canada as well as in many other countries of Latin America such as Mexico, Argentina, Bolivia, Brazil andNicaragua,where the sources of As are geogenic as well as anthropogenic sources (Matschullat, 2000。 Khandaker, 2004。 Mandal et al., 2003). Studies of As speciation in the urine of exposed humans indicate that the metabolites prise 10–15% inorganic As (iAs) and monomethylarsonic acid (MMAV) and a major proportion (60–80%) of dimethylarsenic acid (DMAV) (Tam et al., 1979。 Naidu et al., 2006). Crop quality and the effect of As on crop quality and yield is being a major worldwide concern, particularly for rice which forms the staple for many SouthAsian countries where groundwater is widely used for irrigation (Meharg and Rahman, 2003). In a recent study it was reported that irrigation has increased in Bangladesh since 1970, while since 1980, the area under groundwater irrigation for the cultivation of Boro rice has increased by almost an order of magnitude (Harvey et al., 2005). Based on available information on the distribution of As concentration in groundwater (BGS and DPHE, 2001) and the area under shallow tubewell irrigation (BADC, 2005), Saha (2006)n estimated that approximately 1000 metric tons of As is cycled with irrigation water during the dry season of each year. Rice yield has been reported to decrease by 10% at a concentration of 25 mg/kg As in soil (Xiong et al., 1987). A greenhouse study by Abedin et al. (2002) revealed reduced yield of a local variety of rice (BR11) irrigated with water having As concentrations in the range of to 8 mg/L. The accumulation of As in rice field soils and its introduction into the food chain through uptake by the rice plant is of major concern (Duxbury et al., 2003).. Anthropogenic arsenicLarge quantities of As are released into the environment through industrial activities, which can be dispersed widely and as such play an important role in the contamination of soils, waters, and air (Nriagu,1989。 Smith et al., 1998). In addition, during manufacturing of Ascontaining pesticides and herbicides, release of waste and Asladen liquids near the manufacturing areas may contaminate soil and water bod