Unraveling the Role of Capillarity in Arsenic Mobility: Insights from a Sedimentary–Karstic Aquifer in Semiarid Soil

Arsenic (As) contamination in soil and groundwater poses significant environmental and human health concerns. While chemical mechanisms like solubility equilibria, oxidation–reduction, and ionic exchange reactions have been studied to understand As retention in soil, the influence of capillarity on As transport remains poorly understood, particularly in semiarid soils with broader capillary fringes. This research aims to shed light on the capillary contribution to As attenuation and mobilization in the groundwater, focusing on degraded soil in the northeast of San Luis Potosí, Mexico. Groundwater surveys revealed a remarkable depletion of As concentrations from 91.50 to 11.27 mg L⁻¹, indicating potential As sorption by the underlying shallow aquifer. We examined soil samples collected from the topsoil to the saturated zone using advanced analytical techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and wet chemical analyses. Our findings unveiled the presence of three distinct zones in the soil column: (1) the A horizon with heavy metals, (2) dispersed calcium sulfate dihydrate crystals and stratified gypsum, and (3) a higher concentration of arsenic in the capillary fringe. Notably, the capillary fringe exhibited a significant accumulation of As, constituting 40% (169.22 mg kg⁻¹) of the total arsenic proportion accumulated (359.27 mg kg⁻¹). The arsenic behavior in the capillary fringe solid phase correlated with total iron behavior, but they were distributed among different mineral fractions. The labile fraction, rich in arsenic, contrasted with the more recalcitrant fractions, which exhibited higher iron content. Further, thermodynamic stability assessments using the geochemical code PHREEQC revealed the critical role of Ca₅H₂(AsO₄)₄:9H₂O in controlling HAsO₄²⁻ and the formation of HAsO₄:2H₂O and CaHAsO₄:H₂O. During experimentation, we observed arsenate dissolution, indicating the potential mobilization of As in aqueous species. This mobilization was found to vary depending on redox conditions and may become labile during flooding events or water table variations, especially when As concentrations are low compared to metal cations, as demonstrated in our experiments. Our research underscores the significance of developing accurate geochemical conceptual models that incorporate capillarity to predict As leaching and remobilization accurately. This study presents novel insights into the understanding of As transport mechanisms and suggests the necessity of considering capillarity in geochemical models. By comprehending the capillary contribution to As attenuation, we can develop effective strategies to mitigate As contamination in semiarid soils and safeguard groundwater quality, thereby addressing crucial environmental and public health concerns.