Arsenic is a widely dispersed element in the Earth's crust and exists at an average concentration of approximately 5 mg/kg. There are many possible routes of human exposure to arsenic from both natural and anthropogenic sources. Arsenic occurs as a constituent in more than 200 minerals, although it primarily exists as arsenopyrite and as a constituent in several other sulfide minerals. The introduction of arsenic into drinking water can occur as a result of its natural geological presence in local bedrock. Arsenic-containing bedrock formations of this sort are known in Bangladesh, West Bengal (India), and regions of China, and many cases of endemic contamination by arsenic with serious consequences to human health are known from these areas. Significant natural contamination of surface waters and soil can arise when arsenic-rich geothermal fluids come into contact with surface waters. When humans are implicated in causing or exacerbating arsenic pollution, the cause can almost always be traced to mining or mining-related activities. Arsenic exists in many oxidation states, with arsenic (III) and (V) being the most common forms. Similar to many metalloids, the prevalence of particular species of arsenic depends greatly on the pH and redox conditions of the matrix in which it exists. Speciation is also important in determining the toxicity of arsenic. Arsenic minerals exist in the environment principally as sulfides, oxides, and phosphates. In igneous rocks, only those of volcanic origin are implicated in high aqueous arsenic concentrations. Sedimentary rocks tend not to bear high arsenic loads, and common matrices such as sands and sandstones contain lower concentrations owing to the dominance of quartz and feldspars. Groundwater contamination by arsenic arises from sources of arsenopyrite, base metal sulfides, realgar and orpiment, arsenic-rich pyrite, and iron oxyhydroxide. Mechanisms by which arsenic is released from minerals are varied and are accounted for by many (bio)geochemical processes: oxidation of arsenic-bearing sulfides, desorption from oxides and hydroxides, reductive dissolution, evaporative concentration, leaching from sulfides by carbonate, and microbial mobilization. Arsenic enrichment also takes place in geothermally active areas; surface waters are more susceptible than groundwater to contamination in the vicinity of such geothermal systems, and evidence suggests that increased use of geothermal power may elevate risks of arsenic exposure in affected areas. Past and current mining activities continue to provide sources of environmental contamination by arsenic. Because gold- and arsenic-bearing minerals coexist, there is a hazard of mobilizing arsenic during gold mining activities. The Ashanti region of central Ghana currently faces this as a real risk. Historical arsenic contamination exists in Cornwall, UK; an example of a recent arsenic pollution event is that of Ron Phibun town in southern Thailand, where arsenic-related human health effects have been reported. Other important sources of arsenic exposure include coal burning in Slovakia, Turkey, and the Guizhou Province of China; use of arsenic as pesticides in Australia, New Zealand, and the US; and consumption of contaminated foodstuffs (China) and exposure to wood preserving arsenicals (Europe and North America).