Arsenic Dynamics in Groundwater and Soils

Arsenic is a ubiquitous metalloid in the Earth’s crust, occurring primarily as pentavalent arsenate [As(V)] and trivalent arsenite [As(III)]. Its mobilisation in aquifers and soils is governed by redox transformations, pH, competing ions and sorption onto mineral surfaces—particularly iron oxyhydroxides. Under reducing conditions, the reductive dissolution of iron phases liberates arsenic into pore waters, where As(III) predominates and exhibits greater toxicity and mobility than As(V). In oxidising environments, arsenate is retained through inner-sphere complexation and co-precipitation with ferric minerals. Organic matter, microbial activity and hydraulic perturbations such as groundwater pumping further modulate arsenic speciation and transport. Global hotspots in South Asia, North America and parts of China illustrate the intersection between natural geochemistry and anthropogenic stress. Emerging research highlights atmospheric deposition of methylated arsenic species and the role of engineered amendments and microbial pathways in controlling arsenic bioavailability, with direct implications for human health, agriculture and water security.

Research from Nature Portfolio

Recent studies have demonstrated that atmospheric cycling contributes significantly to soil and plant exposure by delivering methylated arsenic species via wet and dry deposition. Analyses under free-tropospheric conditions reveal long-range transport of methylated arsenicals, with fluxes comparable to soil methylation rates and potential impacts on crop uptake. In parallel, investigations into intensive groundwater extraction show that overpumping induces subsidence of clay strata, promoting the release of arsenic from pore waters into aquifers. Quantitative models link land-surface subsidence to elevated arsenic concentrations, underscoring the need to couple water-quantity management with contaminant risk assessment. Together, these insights broaden the conceptual framework of arsenic dynamics beyond local hydrogeology to encompass atmospheric inputs and subsurface mechanical processes.

Arsenic Dynamics in Groundwater and Soils publication trend

The graph below shows the total number of articles in arsenic dynamics in groundwater and soils across all publications each year (not limited to Nature Index journals).

Technical terms

Arsenite (As(III)): The trivalent form of arsenic, more mobile and toxic than arsenate under reducing conditions.

Arsenate (As(V)): The pentavalent form of arsenic, which tends to adsorb strongly to iron oxyhydroxides in oxidising soils.

Reductive dissolution: The microbially mediated process by which ferric minerals are reduced, releasing adsorbed arsenic into pore water.

Zero-valent iron (ZVI): Elemental iron used in soil amendments to promote reductive precipitation and immobilisation of arsenic.

Bioavailability: The fraction of arsenic that is accessible for uptake by organisms, influenced by speciation and soil chemistry.

Methylation: Addition of methyl groups to inorganic arsenic by microbial or abiotic pathways, yielding organic arsenic species.

Demethylation: Removal of methyl groups from organic arsenic, converting it back to inorganic forms.

Subsidence: The downward displacement of the ground surface, often induced by groundwater withdrawal and linked to contaminant release.

Aerosol deposition: Transport of fine airborne particles that carry arsenic species, depositing them onto terrestrial surfaces.

References

1. Marine and terrestrial contributions to atmospheric deposition fluxes of methylated arsenic species. Nature Communications (2024).
2. Overpumping leads to California groundwater arsenic threat. Nature Communications (2018).
3. Biochar-supported zero-valent iron enhanced arsenic immobilization in a paddy soil: the role of soil organic matter. Biochar (2024).
4. Sulfate-reducing bacteria and methanogens are involved in arsenic methylation and demethylation in paddy soils. The ISME Journal: Multidisciplinary Journal of Microbial Ecology (2019).
5. Arsenic in tube well water in Bangladesh: health and economic impacts and implications for arsenic mitigation. Bulletin of the World Health Organization (2012).