【正文】 nd 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。 Chen and Ahsan, 2004). Most ofthe ingested As is rapidly excreted via the kidney within a few days (Tam et al., 1979。 Vahter, 1994). However, high levels of As are retained for longer periods of time in the bone, skin, hair, and nails of exposed humans (Karagas et al., 2000。 Vahter et al., 1995。 Del Razo et al., 2001。 Das et al., 2004。 Correll et al., 2006。 Jacks and Bhattacharya, 1998。 Matschullat, 2000。 Smith et al., 1998。 Sanok et al., 1995。 Neal and Davies, 2003). The study revealed that the concentration of dissolved As in the rural areas averaged between and mg/L, while for the rivers influenced by industrial discharges the average between and mg/L, while suspended particulate As is much lower (average to mg/L for the rural and to mg/L for the industrial rivers). However, for the industrialized rivers dissolved As concentrations can be as high as mg/L.The possible mobilization of As in the soils, and subsequent leaching into ground or surface water or entry into the human food chain, should always be considered as a serious hazard. Detailed investigations are therefore necessary to estimate the total concentrations of As in soils in such areas, its chemical fractionation, and potential solubility to evaluate the potential risks from As mobilization.. Microbial transformations of arsenicMobilization of As in natural ecosystems is predominantly driven by microbially mediated biogeochemical interactions. Microbial reduction of As(V) to the more toxic and mobile As(III) species occurs via detoxification (Cervantes et al., 1994) or respiration processes (Ahmann et al., 1994). The genes that encode the proteins involved in As resistance are either plasmid or chromosomally borne, and have been best studied in Escherichia coli. Plasmid R773 prises of five genes arsRDABC organized in an operon (Chen et al., 1986). The arsC gene encodes the As(V)reductase。 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。 Jones et al., 2000。 Naidu and Bhattacharya, 2006。 Fostner and Haase, 1998). Thus phytoremediation has the potential to bee an environmentally friendly and lowcost alternative remediation technique. It is well documented that some tropical and subtropical plant species can tolerate and uptake various inorganic and organic forms of As (Meharg and HartleyWhitaker, 2002). Mesquite is am plant that grows well in humid and desert environments that has been shown to absorb Cr(VI) and other metals such as Pb (Aldrich et al., 2004). Xray absorption spectroscopic (XAS) studies revealed that mesquite can bioreduce Cr(VI) to the less toxic Cr(III) (Aldrich et al., 2003). However, a significant gap of information exists on the ability of desert plant species to uptake As or other toxic elements.. Current researchResearch on As is currently very active and includes assessment of interactions at scales ranging from molecular bonding to subcontinental, As speciation in inorganic and organic materials through a wide variety of chemical and spectroscopic approaches, and an emerging understanding of the role of microbes and other biota in As cycling. A recent review on health impacts of As resulted in drinking water standards of 10 μg/L or even lower in some countries (Kapaj et al., 2006). These lowered standards are projected to greatly increase water supply costs in many regions. The increased pressure on society to protect human health and the ecosystem has stimulated research using a wide multitude of approaches and techniques (Naidu et al., 2006。 2) processes that control mobility and speciation of As in soil, water and biota。 4) analytical techniques and speciation studies。 and 6) impact of As on agriculture and water supply management. The articles included in this special issue address many of these issues and pave the way through recent findings on the environmental behaviour of As in terms of its occurrence, sources, health impacts, and remediation. Besides understanding the fundamental processes of As mobilization, the articles discuss a wide variety of chemical and spectroscopic approaches, and increased understanding of the importance of microbes and other biota in As cycling. Although much has been learned about As in the environment the ability to predict the impact of intentional and unintentional changes to hydrologic and geochemical regimes often remains elusive. Key research contributions from several international teams of scientists working on As in the environment, groundwater in the Bengal Delta Plain and elsewhere in the world were presented and discussed during the symposium and are amalgamated in this Special Issue of The Science of the Total Environment.3. Layout and summary of the articlesThis special issue prises 14 articles and 1 short munication, grouped into four sections. 1) Arsenic in the groundwater environment。 3) biogeochemistry of As and toxicity, and 4) remediation of Ascontaminated soils and sediments.. Arsenic in the groundwater environmentThis section has five articles. The first two contributions deal with the specific issues related to the occurrence of geogenic