Capillarity as a hydrologic control in the unsaturated zone drives arsenic transport from groundwater to soil solid phase in Matehuala (Mexico)

Recent studies showed an arsenic concentration decrease from 91.5 to 11.3 mg/L in Matehuala (Mexico) groundwater, a semi-arid environment where calcite and gypsum predominate. Arsenic retention has been widely studied and explained in iron-rich systems, however, little is known about hydrological controls over arsenic attenuation in semi-arid soils rich in calcite and gypsum. In this study, mineralogy, water-rock interaction modeling soil fractionation have been used to explain arsenic transport from groundwater to soil solid phase and arsenic distribution in a low iron oxides content soil. In the alluvial-evaporitic geologic setting of the study area, arsenic wa distributed mainly in soluble fraction and adsorbed in iron oxy-hydroxides and bonded to no-crystalline pyrite Capillarity plays a key role in soluble arsenic retention from groundwater to soil solid phase, which under reducing conditions typical of aquifer systems, would spread due to As(III) high mobility.