Influence of Wetting/Drying Cycles on the Vadose Zone Dynamics and the Consequent Redox-Facilitated Arsenic Transport

Abstract:

Given long-term health threat, arsenic (As) contamination has been a major concern in soil and groundwater. Vadose zone is known as “natural buffer” to protect groundwater resources by blocking or/and degrading contaminants through biogeochemical processes. However, behavior of arsenic in vadose zone subjected to metrological dynamics remains largely unknown. This study aimed to elucidate the fate and transport of arsenic from the As-containing topsoil through vadose zone subjected to repeated wetting/drying cycles and consequently related geochemical processes.

A particularly designed soil column was subjected to recursive wetting/drying cycles at the top, representing rainfall hysteresis with As(V)-contaminated topsoil. Water content and oxygen concentration were measured regularly by non-invasive sensors. Pore-water and soil samples were collected at different depths, and then analyzed with As(III) and As(V) concentrations and sequentially extracted to evaluate the binding forms to As, respectively.

Results showed oxygen concentration distribution along the depth was lower during wetting followed by replenishment in drying period. Most of As existed in the solid phase, while 5-13% of As was in aqueous phase throughout the depth. At redox in range of 200-500 mV and pH of 7.6-8.2, As(V) was the main inorganic form in pore-water, which is consistent with the previous Pourbaix diagram. A large fraction of solid-phase As exists as the second fraction in Wenzel’s method (i.e. up to 70% at upper parts), which further propagates downwards through wetting-drying cycles. Arsenic bounded with crystalline and amorphous iron (hydr)oxides remained relatively stable throughout wetting/drying cycles, indicating iron content plays a key parameter in natural attenuation of As in vadose zone under transient redox condition. Currently, changes in Fe oxides composition and consequent attenuation mechanism of As is under investigation using XANES analysis.

Acknowledgement: The authors acknowledge support from the Korea Environmental Industry & Technology Institute (KEITI) through Subsurface Environment Management (SEM) Project (2018002440006) funded by the Korea Ministry of Environment (MOE).