Patterns of arsenic speciation and associated potentially toxic elements at sites impacted by mining and mine waste

The ability to find, mine, extract and smelt metallic ores has been central to technological development throughout history. Demand for metals is projected to increase in the coming decades and the supply of metals such as lithium, copper, indium, neodymium and tantalum have become strategically important in the manufacture of energy-efficient 'green' products and renewable energy generators. However, mining is accompanied by an environmental cost; mining accounts for almost 34% of total worldwide solid waste generated. Historically poor disposal practices in the industry have created a legacy of tailings piles, polluted rivers and health impacts that have the potential to impact surface water quality for centuries after the cessation of mining, primarily through the continued interaction of meteoric and groundwater with mine waste and mill tailings dumped on-site. Arsenic is a toxic metalloid that is commonly present in trace to minor amounts in ores of gold, mercury, lead and zinc as the mineral arsenopyrite (FeAsS) or as substitutions within other sulphide minerals. Consequently, arsenic (As), a known carcinogen, is a contaminant of concern in many mining districts and a useful indicator of water quality. Arsenic exists in natural waters as both As(III) and As(V) oxyanions, whose relative abundance is controlled by pH and redox conditions. The toxicity and mobility of dissolved As in water draining mine tailings is directly related to the inorganic arsenic species present. However, at many sites arsenic contamination of surface water is accompanied by other toxic metals, which are released with arsenic from the weathering and dissolution of sulphide minerals. To investigate the effect of other metal species, on arsenic speciation, field measurements of As(III) /As(V) ratios in surface water were made at selected historical mining sites with elevated levels of dissolved arsenic and of one or more toxic metals. Data are presented from nine contaminated mine sites in Scotland, Alaska and Colombia, representing different types of ore deposits and ore processing sites that have elevated As signatures. At these sites, antimony, mercury or cadmium is present as a secondary contaminant in surface water at levels near or above the USEPA MCL. In addition to arsenic species separation, major and trace elements, vii anions and stable isotopes were determined at each sample point, along with a full suite of in-situ water parameters. Local site variables, including bedrock geology, ore mineralogy, groundwater geochemistry and soil chemistry, contribute to the complexity of As speciation at each study location. It was determined that As(III) is the dominant species in water draining mine adits and tailing piles, instead of the more thermodynamically favoured As(V) species. As(III) is more mobile than As(V) and persists up to 300 metres downstream, before transformation to the more oxidised form. Better understanding of As mobility has the potential to improve remediation strategies at other contaminated mine sites.The ability to find, mine, extract and smelt metallic ores has been central to technological development throughout history. Demand for metals is projected to increase in the coming decades and the supply of metals such as lithium, copper, indium, neodymium and tantalum have become strategically important in the manufacture of energy-efficient 'green' products and renewable energy generators. However, mining is accompanied by an environmental cost; mining accounts for almost 34% of total worldwide solid waste generated. Historically poor disposal practices in the industry have created a legacy of tailings piles, polluted rivers and health impacts that have the potential to impact surface water quality for centuries after the cessation of mining, primarily through the continued interaction of meteoric and groundwater with mine waste and mill tailings dumped on-site. Arsenic is a toxic metalloid that is commonly present in trace to minor amounts in ores of gold, mercury, lead and zinc as the mineral arsenopyrite (FeAsS) or as substitutions within other sulphide minerals. Consequently, arsenic (As), a known carcinogen, is a contaminant of concern in many mining districts and a useful indicator of water quality. Arsenic exists in natural waters as both As(III) and As(V) oxyanions, whose relative abundance is controlled by pH and redox conditions. The toxicity and mobility of dissolved As in water draining mine tailings is directly related to the inorganic arsenic species present. However, at many sites arsenic contamination of surface water is accompanied by other toxic metals, which are released with arsenic from the weathering and dissolution of sulphide minerals. To investigate the effect of other metal species, on arsenic speciation, field measurements of As(III) /As(V) ratios in surface water were made at selected historical mining sites with elevated levels of dissolved arsenic and of one or more toxic metals. Data are presented from nine contaminated mine sites in Scotland, Alaska and Colombia, representing different types of ore deposits and ore processing sites that have elevated As signatures. At these sites, antimony, mercury or cadmium is present as a secondary contaminant in surface water at levels near or above the USEPA MCL. In addition to arsenic species separation, major and trace elements, vii anions and stable isotopes were determined at each sample point, along with a full suite of in-situ water parameters. Local site variables, including bedrock geology, ore mineralogy, groundwater geochemistry and soil chemistry, contribute to the complexity of As speciation at each study location. It was determined that As(III) is the dominant species in water draining mine adits and tailing piles, instead of the more thermodynamically favoured As(V) species. As(III) is more mobile than As(V) and persists up to 300 metres downstream, before transformation to the more oxidised form. Better understanding of As mobility has the potential to improve remediation strategies at other contaminated mine sites