Role of unsaturated soil above a heavily contaminated aquifer in the natural attenuation of arsenic

Large attenuation of arsenic is observed in a shallow aquifer in central Mexico, where the concentration decreases from 91.5 to 11.3 mg/L, over 1.3 km horizontal distance. To investigate possible mechanisms of attenuation of this pollutant, we dug a pit between the surface and the saturated zone. We then described the soil profile and determined arsenic and iron concentrations in soil samples as a function of depth. Next, we determined particle size distribution, bulk density, particle density, soil moisture and porosity of the soil material. We also analyzed arsenic concentration in the groundwater. The 2.2 m deep profile intersected 4 soil horizons (A, AB, B1 and B2). We found arsenic accumulation in the B2 horizon, directly overlying the saturated zone. These accumulations coincide with the thickness of the capillary fringe, revealing that arsenic is drawn up in the soil profile by capillary rise of arsenic-contaminated groundwater. Furthermore, arsenic accumulation showed a direct relationship with iron, likely due to arsenic adsorption on iron oxides. Results from this study contribute to understanding a capillary-driven natural attenuation mechanism that removes contaminants from groundwater by sinking them in the capillary fringe.