Seasonal variation in efflorescence minerals in a mining-impacted arid region: implications for human health risk

Efflorescent salts, indicators of acid mine drainage (AMD), accumulate metals and metalloids in arid environments, with their formation influenced by surface temperature. This study, the first of its kind, analyzes seasonal variations in the mineralogy, metal content, bioaccessibility, and airborne dispersion of efflorescent salts at Mexico's largest copper mine. We collected 94 samples over four seasons from five sites, including areas with artisanal miners. Mineral and element concentrations were analyzed using XRD, SEM, and XRF, while bioaccessibility was evaluated with Gamble's and artificial lysosomal fluids (ALF). Dispersion modeling and health risk assessments were also conducted. Seasonal mineral variations included jarosite and chalcanthite in winter, 7 sulfate salts in spring, antlerite and magnesiocopiapite in summer, and 5 sulfate salts in fall. Metal content varied seasonally, with Cu ranging from 57,709 mg·kg^-1 in winter to 75,495 mg·kg^-1 in summer, and reaching peak levels of 655,568 mg·kg^-1. Bioaccessibility in Gamble solution was low for most elements, but ALF bioaccessibility was highly variable. Over 80 % of particles were < 2.5 μm (PM_2.5), with 5 % <0.5 μm, raising concerns about deep respiratory penetration of low soluble particles. Risk assessment showed no carcinogenic risk after exposure to metal(loid)s. Dispersion modeling revealed northward transport toward the U.S.-Mexico border. Efflorescent salts also concentrated critical elements like Ni, Co, and Mn, suggesting opportunities for secondary resource recovery. These findings underline the environmental risk and resource potential of efflorescent salts, emphasizing the need for seasonal monitoring and sustainable recovery.