Arsenic mobility controlled by solid calcium arsenates: a case study in Mexico showcasing a potentially widespread...

Area of Science:

• Environmental Science
• Hydrogeology
• Geochemistry

Background:

• Urban areas affected by mining can experience significant groundwater contamination.
• Arsenic (As) contamination in aquifers is a major environmental and health concern.
• Understanding arsenic mobilization mechanisms is crucial for effective remediation.

Purpose of the Study:

• To identify the source and species of arsenic contamination in a perched aquifer.
• To elucidate the primary mechanism of arsenic mobilization and retention.
• To investigate the role of calcium in controlling arsenic mobility.

Main Methods:

• A year-long study of an arsenic-contaminated perched aquifer.
• Analysis of water samples to determine dissolved arsenic levels and species.
• Investigation of mineral precipitation and dissolution processes in aquifer residues.

Main Results:

• Dissolution of calcium arsenates in smelter residues caused high arsenic levels (up to 158 mg/L) in a 6-km aquifer.
• Approximately 7.5 tons of arsenic were released annually.
• Free calcium ions controlled arsenic mobility by precipitating calcium arsenates, thus limiting further mobilization.

Conclusions:

• Calcium arsenate dissolution is a significant source of arsenic pollution from mining residues.
• Calcium ion availability plays a critical role in retaining arsenic through precipitation in calcium-rich environments.
• This calcium-mediated retention mechanism is underreported and important in calcareous settings, contrasting with iron-adsorption dominance.