P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 5. 6.2. Ghana 5.6.3. United States 5.6. 4. Other areas 6. Common features of groundwater arsenic problem areas 6. 1. A hydrogeochemical perspective 6.2. The source of arsenic 6.3. Arsenic mobilisation--the necessary geochemical trigger .3.1. Desorption at high ph under oxidising conditions 6.3.2. Arsenic desorption and dissolution due to a change to reducing conditions 6.3.3. Reduction in surface area of oxide minerals 6.3.4. Reduction in binding strength between arsenic and mineral surfaces 6.3.5. Mineral dissolution 6.4. Transport--historical groundwater flows 6.5. Future developments in arsenic research 6.6. Identification of at riskaquifers 7. Concluding remarks 559 Acknowledgements… References 1. ntroduction a variety of sources depending on local availability: sur- face water(rivers, lakes, reservoirs and ponds), ground- The recent finding that groundwaters from large areas water(aquifers) and rain water. These sources are very of West Bengal, Bangladesh and elsewhere are heavily variable in terms of As risk. Alongside obvious point enriched with As has prompted a reassessment of the sources of As contamination, high concentrations are factors controlling the distribution of As in the natural mainly found in groundwaters. These are where the environment and the ways in which As may be mobilised. greatest number of, as yet unidentified, sources are likely Arsenic is a ubiquitous element found in the atmosphere to be found this review therefore focuses on the factors ils and rocks, natural waters and organisms. It is mobi- controlling As concentrations in groundwaters. Hoy ed through a combination of natural processes such ever. the authors also review the occurrence of s weathering reactions, biological activity and volcanic broad range of natural waters since these may indirectly f anthropogenic be involved in the formation of As-rich groundwaters and activities. Most environmental As problems are the result can also provide a useful background against which to of mobilisation under natural conditions. However, view groundwater As concentrations. Furthermore, many man has had an important additional impact through of the processes involved in the uptake and release of A mining activity, combustion of fossil fuels, the use of are common to a wide range of natural environments rsenical pesticides, herbicides and crop desiccants and Following the accumulation of evidence for the the use of As as an additive to livestock feed particularly chronic toxicological effects of As in drinking water for poultry. Although the use of arsenical products such recommended and regulatory limits of many authorities as pesticides and herbicides has decreased significantly in are being reduced. The Who guideline value for As in the last few decades, their use for wood preservation drinking water was provisionally reduced in 1993 from still common. The impact on the environment of the use 50 to 10 ug l-l. The new recommended value was based of arsenical compounds, at least locally, will remain for on the increasing awareness of the toxicity of As, parti some cularly its carcinogenicity, and on the ability to measure Of the various sources of as in the envir nt, it quantitatively(WHO, 1993). If the standard basis for drinking water probably poses the greatest threat to risk assessment applied to industrial chemicals were human health. Airborne As, particularly through occu- applied to As, the maximum permissible concentration pational exposure, has also given rise to known health would be lower still. The ec maximum admissible con problems in some areas. Drinking water is derived from centration (MAC)for As in drinking water has been1. Introduction The recent finding that groundwaters from large areas of West Bengal, Bangladesh and elsewhere are heavily enriched with As has prompted a reassessment of the factors controlling the distribution of As in the natural environment and the ways in which As may be mobilised. Arsenic is a ubiquitous element found in the atmosphere, soils and rocks, natural waters and organisms. It is mobi￾lised through a combination of natural processes such as weathering reactions, biological activity and volcanic emissions as well as through a range of anthropogenic activities. Most environmental As problems are the result of mobilisation under natural conditions. However, man has had an important additional impact through mining activity, combustion of fossil fuels, the use of arsenical pesticides, herbicides and crop desiccants and the use of As as an additive to livestock feed, particularly for poultry. Although the use of arsenical products such as pesticides and herbicides has decreased significantly in the last few decades, their use for wood preservation is still common. The impact on the environment of the use of arsenical compounds, at least locally, will remain for some years. Of the various sources of As in the environment, drinking water probably poses the greatest threat to human health. Airborne As, particularly through occu￾pational exposure, has also given rise to known health problems in some areas. Drinking water is derived from a variety of sources depending on local availability: sur￾face water (rivers, lakes, reservoirs and ponds), ground￾water (aquifers) and rain water. These sources are very variable in terms of As risk. Alongside obvious point sources of As contamination, high concentrations are mainly found in groundwaters. These are where the greatest number of, as yet unidentified, sources are likely to be found. This review therefore focuses on the factors controlling As concentrations in groundwaters. How￾ever, the authors also review the occurrence of As in a broad range of natural waters since these may indirectly be involved in the formation of As-rich groundwaters and can also provide a useful background against which to view groundwater As concentrations. Furthermore, many of the processes involved in the uptake and release of As are common to a wide range of natural environments. Following the accumulation of evidence for the chronic toxicological effects of As in drinking water, recommended and regulatory limits of many authorities are being reduced. The WHO guideline value for As in drinking water was provisionally reduced in 1993 from 50 to 10 mg l
1 . The new recommended value was based on the increasing awareness of the toxicity of As, parti￾cularly its carcinogenicity, and on the ability to measure it quantitatively (WHO, 1993). If the standard basis for risk assessment applied to industrial chemicals were applied to As, the maximum permissible concentration would be lower still. The EC maximum admissible con￾centration (MAC) for As in drinking water has been 5.6.2. Ghana................................................................................................................................................ 551 5.6.3. United States ..................................................................................................................................... 551 5.6.4. Other areas ........................................................................................................................................ 551 6. Common features of groundwater arsenic problem areas...................................................................................... 552 6.1. A hydrogeochemical perspective ................................................................................................................... 552 6.2. The source of arsenic..................................................................................................................................... 552 6.3. Arsenic mobilisation—the necessary geochemical trigger ............................................................................. 552 6.3.1. Desorption at high pH under oxidising conditions ........................................................................... 553 6.3.2. Arsenic desorption and dissolution due to a change to reducing conditions .................................... 554 6.3.3. Reduction in surface area of oxide minerals ..................................................................................... 555 6.3.4. Reduction in binding strength between arsenic and mineral surfaces ............................................... 555 6.3.5. Mineral dissolution............................................................................................................................ 556 6.4. Transport—historical groundwater flows...................................................................................................... 556 6.5. Future developments in arsenic research....................................................................................................... 558 6.6. Identification of ‘at risk’ aquifers .................................................................................................................. 558 7. Concluding remarks ............................................................................................................................................... 559 Acknowledgements...................................................................................................................................................... 560 References ................................................................................................................................................................... 560 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 519