Arsenic is a naturally occurring metalloid, possessing properties of both metals and nonmetals, and is widely distributed across the globe. While it exists in various forms, the inorganic compounds found in water are highly toxic to human health. Arsenic in water is odorless, colorless, and nearly tasteless, making it impossible to detect without specialized testing. This widespread contamination results from a combination of the planet’s natural geological processes and various human activities.
Arsenic’s Natural Presence in Earth’s Crust
Arsenic is a ubiquitous component of the Earth’s crust, typically present at an average concentration of 1.5 to 2 parts per million (ppm) in continental rock formations. The element is a constituent in over 245 different mineral species, establishing a vast reservoir from which it can be mobilized. The highest natural concentrations are found in rocks associated with hydrothermal and volcanic activity.
The primary geological host for arsenic is sulfide minerals, such as arsenopyrite and pyrite, where it is chemically bound to iron and sulfur. When these mineral-rich rocks weather, the arsenic becomes incorporated into surrounding sediments and soil. Sedimentary basins, particularly those containing shales, clays, and high amounts of organic matter, represent another significant storage location. The presence of arsenic in these subsurface materials provides the source for contamination when favorable chemical conditions arise in the groundwater.
Mobilization Mechanisms: How Arsenic Enters Groundwater
The release of arsenic from rock and sediment into groundwater is driven primarily by two distinct geochemical processes. The most significant mechanism is reductive dissolution, which occurs in anoxic, or low-oxygen, underground environments. Here, organic carbon provides a food source for microorganisms deep within the aquifer.
These microbes utilize iron oxyhydroxides, the minerals to which arsenic is strongly bound, as a substitute for oxygen in their metabolic processes. This microbial action reduces the solid iron (Fe(III)) into a soluble form (Fe(II)), dissolving the iron-containing minerals. As the iron oxyhydroxides dissolve, the attached arsenic is simultaneously released directly into the surrounding groundwater, often leading to the highest observed concentrations.
A second mechanism is competitive adsorption and desorption, which is highly influenced by changes in water chemistry. Arsenic naturally adheres to the surface of minerals like iron and aluminum oxides through adsorption. If the water’s pH increases significantly, the mineral surfaces lose their positive electrical charge, weakening the bond with the arsenic ions.
This change in surface charge causes the previously adsorbed arsenic to desorb, or release, into the water column. Furthermore, the introduction of other dissolved ions, such as phosphate or bicarbonate, can competitively displace arsenic from the mineral surfaces. These competing ions bind more strongly to attachment sites, pushing the arsenic into the groundwater.
Human Activities That Introduce or Accelerate Contamination
Human activities can either introduce new sources of arsenic or accelerate the natural mobilization from geological sources. The mining and smelting of metal ores, particularly gold, copper, and lead, are major anthropogenic sources of contamination. Waste products, known as tailings, often contain concentrated sulfide minerals like arsenopyrite. When these tailings are exposed to air and water, the sulfide minerals oxidize, rapidly releasing high levels of arsenic into local surface and groundwater supplies.
Agricultural Practices
Agricultural practices have contributed to the lasting contamination of soil and water sources. Historically, arsenic compounds were widely used in pesticides, herbicides, and defoliants to protect crops. The residues from these applications persist in the upper soil layers for decades and can leach into underlying aquifers during periods of rainfall. Additionally, some phosphate fertilizers contain trace amounts of arsenic as a contaminant, which is introduced directly into the soil over time.
Industrial Waste Disposal
Industrial waste disposal provides a third pathway for contamination, introducing arsenic-containing byproducts directly into the environment. Materials such as coal ash, a residue from coal-fired power plants, often contain elevated levels of the metalloid. Improper disposal of this ash can allow arsenic to leach into local water bodies. The use of chromated copper arsenate (CCA) as a wood preservative, though now restricted, has also left a legacy of arsenic-contaminated soils around treated structures, which continues to pose a risk to groundwater quality.