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Chapter 1. Source and Behaviour of Arsenic in Natural Waters 1.1 Importance of Arsenic in Drinking Water

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Chapter 1. Source and Behaviour of Arsenic in Natural Waters 1.1 Importance of Arsenic in Drinking Water Chapter 1. Source and behaviour of arsenic in natural waters Pauline L Smedley and David G Kinniburgh British Geological Survey, Wallingford, Oxon OX10 8BB, U.K. 1.1 Importance of arsenic in drinking water..........................................................................2 1.2 Arsenic in natural waters ..................................................................................................2 1.2.1 Abundance and distribution .......................................................................................... 2 1.2.2 Aqueous speciation of arsenic....................................................................................... 2 1.2.3 Redox and kinetics...................................................................................................... 13 1.3 Sources of arsenic ...........................................................................................................14 1.3.1 Minerals ..................................................................................................................... 14 1.3.2 Rocks, sediments and soils.......................................................................................... 17 1.3.3 The atmosphere .......................................................................................................... 22 1.4 Mineral-water interactions .............................................................................................22 1.4.1 Relevance to arsenic mobilisation ............................................................................... 22 1.4.2 Arsenic interactions in sediments................................................................................ 26 1.4.3 Iron in reduced sediments and aquifers ....................................................................... 27 1.4.4 Arsenic release from soils and sediments following reduction ..................................... 28 1.4.5 Transport of arsenic .................................................................................................... 29 1.5 Groundwater environments showing enhanced arsenic concentrations......................30 1.5.1 World distribution of groundwater arsenic problems ................................................... 30 1.5.2 Reducing environments .............................................................................................. 35 1.5.3 Arid oxidising environments....................................................................................... 40 1.5.4 Mixed oxidising and reducing environments ............................................................... 43 1.5.5 Geothermal sources .................................................................................................... 44 1.5.6 Arsenic mineralisation and mining-related arsenic problems ....................................... 45 1.6 Common features of groundwater arsenic problem areas............................................46 1.6.1 A hydrogeochemical perspective................................................................................. 46 1.6.2 The source term.......................................................................................................... 46 1.6.3 The necessary geochemical trigger .............................................................................. 47 1.6.4 The hydrogeological dimension – aquifer flushing...................................................... 52 1.7 Outstanding questions and further needs ......................................................................54 1.7.1 Field analytical methods for arsenic determination...................................................... 54 1.7.2 Surveys of groundwater quality................................................................................... 55 1.7.3 Will the problem groundwaters get better or worse with time? .................................... 55 1.7.4 Transport under natural flow conditions...................................................................... 56 1.7.5 Interaction of arsenic with sediments .......................................................................... 56 1.8 Strategy for locating arsenic problems in groundwater................................................57 1.9 Non-technical Summary.................................................................................................59 Acknowledgments.................................................................................................................... 61 1 1.1 Importance of arsenic in drinking water Arsenic (As) is a ubiquitous element found in the atmosphere, soils and rocks, natural waters and organisms. It is mobilised in the environment 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 arsenic problems are the result of mobilisation under natural conditions, but man has had an important impact through mining activity, combustion of fossil fuels, the use of arsenical pesticides, herbicides and crop desiccants and the use of arsenic 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 arsenic in the environment, drinking water probably poses the greatest threat to human health. Airborne arsenic, particularly through occupational 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: surface water (rivers, lakes, reservoirs and ponds), groundwater (aquifers) and rain water. These sources are very variable in terms of arsenic risk. Alongside obvious point sources of arsenic 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 arsenic concentrations in groundwaters. 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 µg l–1 to 10 µg l–1. The new recommended value is based largely on analytical capability. If the standard basis for risk assessment applied to industrial chemicals were applied to arsenic, the maximum permissible concentration would be lower still. The US-EPA limit was also reduced from 50 µg l–1 to 10 µg l–1 in January 2001 following prolonged debate over the most appropriate limit. The EC maximum admissible concentration (MAC) for As in drinking water is also to be reduced to 10 µg l–1. The Japanese limit for drinking water is 10 µg l–1 and the interim maximum acceptable concentration for Canadian drinking water is 25 µg l–1. Whilst many national authorities are seeking to reduce their limits in line with the WHO guideline value, many countries and indeed all affected developing countries, still operate at present to the 50 µg l–1 standard, in part because of lack of adequate testing facilities for lower concentrations. Until recently, arsenic was often not on the list of constituents in drinking water routinely analysed by national laboratories, water utilities and NGOs and so the body of information about the distribution of arsenic in drinking water is not as well known as for many other drinking-water constituents. In recent years, it has become apparent that both the WHO guideline value and current national standards are quite frequently exceeded in drinking-water sources, and often unexpectedly so. Indeed, arsenic and fluoride are now recognised as the most serious inorganic contaminants in drinking water on a worldwide basis. In areas of high arsenic concentrations, drinking water provides a potentially major source of arsenic in the diet and so its early detection is of great importance. 1.2 Arsenic in natural waters 1.2.1 Aqueous speciation Arsenic is perhaps unique among the heavy metalloids and oxyanion-forming elements (e.g. arsenic, selenium, antimony, molybdenum, vanadium, chromium, uranium, rhenium) in its sensitivity to mobilisation at the pH values typically found in groundwaters (pH 6.5–8.5) and under both oxidising and reducing conditions. Arsenic can occur in the environment in several oxidation states 2 (-3, 0, +3 and +5) but in natural waters is mostly found in inorganic form as oxyanions of trivalent arsenite (As(III)) or pentavalent arsenate (As(V)). Organic arsenic forms may be produced by biological activity, mostly in surface waters, but are rarely quantitatively important. Organic forms may however occur where waters are significantly impacted by industrial pollution. Most toxic trace metals occur in solution as cations (e.g. Pb2+, Cu2+, Ni2+, Cd2+, Co2+, Zn 2+) which generally become increasingly insoluble as the pH increases. At the near-neutral pH typical of most groundwaters, the solubility of most trace-metal cations is severely limited by precipitation as, or coprecipitation with, an oxide, hydroxide, carbonate or phosphate mineral, or more likely by their strong adsorption to hydrous metal oxides, clay or organic matter. In contrast, most oxyanions including arsenate tend
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