Why arsenic is everywhere

Arsenic is a naturally occurring element in the Earth's crust, with an average concentration of about 5 mg/kg. It concentrates in specific rock types: organic-rich black shales, volcanic rocks, and sulfide deposits associated with gold and polymetallic mineralization. In these contexts, its mobilization into groundwater can occur naturally, without any human intervention.

Simultaneously, numerous anthropogenic activities release arsenic into the environment: mining (arsenic is frequently associated with gold and copper), wood treatments using arsenical compounds (CCA, widely used until the 2000s), historic arsenical pesticides, and smelter emissions.

The stakes of the distinction

Distinguishing between natural and anthropogenic arsenic has direct implications for management decisions. A naturally arseniferous aquifer cannot be "cleaned up" since the source is the rock itself; the solution lies in water treatment at distribution or finding alternative resources. Conversely, industrial arsenic contamination can be subject to source remediation, liability claims, and the containment or excavation of contaminated materials.

An emblematic example: Lebanon

In the Bekaa Plain of Lebanon, high concentrations of arsenic were detected in groundwater. Research utilized dissolved oxygen isotopes and other geochemical tracers to demonstrate that this contamination was primarily geological, linked to the dissolution of arseniferous minerals in clayey aquifers under reducing conditions. This conclusion directly influenced drinking water management policy in the region.

What isotopes bring to the table

Several isotopic systems are relevant for characterizing environmental arsenic, even though arsenic itself has only one stable isotope (⁷⁵As, which precludes a direct isotopic approach on arsenic).

  • Sulfur Isotopes (³²S, ³⁴S): Arsenic is often associated with sulfides. The isotopic signature of sulfur helps distinguish natural sulfide sources (magmatic or sedimentary pyrite) from anthropogenic sources (industrial sulfides).
  • Iron Isotopes (⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe, ⁵⁸Fe): Arsenic in aqueous environments is strongly linked to the iron cycle. Iron oxides adsorb arsenic, and their dissolution releases it into the water. Iron isotopic signatures provide information on the redox processes controlling this release.
  • Oxygen and Hydrogen Isotopes in Water: Tracers of origin and residence times of water masses, useful for distinguishing deep geological source waters from shallow waters influenced by recent activities.
  • Lead (Pb): In contexts where arsenic is a co-contaminant with lead (polymetallic mine sites), the isotopic signature of lead allows for the identification of the sources of mixed contamination.
  • Antimony Isotopes (¹²¹Sb/¹²³Sb): A powerful indirect proxy in mining environments, detailed below.

Antimony isotopes as a proxy for tracing arsenic

Since arsenic has only one stable isotope (⁷⁵As), its isotopic signature cannot be measured directly. In gold mining environments, this has long been a major obstacle to isotopic investigation of arsenic contamination.

Antimony offers an elegant solution. As and Sb share very similar geochemistry: the same mineralogical associations (arsenopyrite and stibnite frequently coexist in gold deposits), the same behavior in solution (oxyanionic metalloids), and the same transport and adsorption mechanisms on iron oxides. In environments where As and Sb are co-contaminants, the source of one is, in the vast majority of cases, the same as the other.

Antimony has two stable isotopes (¹²¹Sb and ¹²³Sb) whose ratios vary measurably according to geological sources and transformation processes. Its isotopic signature can therefore be used as an indirect tracer of the source and fate of arsenic in these contexts.

Scientific Reference

This approach is developed in the invited review: Ferrari C., White K.B., Ptacek C.J., Blowes D.W. - Tracing arsenic and antimony in mining-impacted environments: New insights from antimony isotopes, Chemical Geology, vol. 707, 2026.

The article demonstrates that Sb isotopes allow for the distinction of As-Sb contamination sources (a review written after feedback from the Giant Mine site in Canadian sub-Arctic) and helps reconstruct transport processes from tailings piles to Great Slave Lake.

This proxy approach is particularly relevant for gold mine sites, sulfide ore processing sites, and environments where arsenic and antimony coexist in water and sediments. It opens the door to complete isotopic investigation of arsenic contamination in contexts where it was previously impossible.

Geochemical processes releasing natural arsenic

In many Asian aquifers, the natural release of arsenic is linked to the reduction of iron oxides under anaerobic conditions. At depth, in the absence of oxygen, bacteria reduce ferric oxides into soluble ferrous ions. This process releases the arsenic adsorbed on these oxides, which then enters the groundwater in concentrations that can be very high.

Iron isotopes allow for the tracing of this reductive dissolution process and distinguish it from anthropogenic arsenic release, which follows different geochemical pathways with characteristic isotopic signatures.

Key Takeaways
  • Natural arsenic is released through geological and geochemical processes independent of human activity.
  • Anthropogenic arsenic originates from mining, industrial processing, and historic agricultural use.
  • The two types have radically different management implications.
  • Sulfur, iron, and lead isotopes help discriminate sources in mixed contexts.
  • As arsenic is monoisotopic, antimony isotopes (geochemically similar) act as an indirect proxy for tracing As in gold mining environments.