Arsenic Behaviors in the Hyporheic Zone - Impact of Rainfall Events

Arsenic (As) is well-known as a human carcinogen and could be released into water, soil and ecosystems by oxidation of As-bearing sulfide minerals such as arsenopyrite (FeAsS). Arsenic behavior depends on pH and reduction-oxidation potential (ORP) in the aquatic system. The pH and ORP can change according to rainfall events (e.g., drought, heavy rainfall), affected by the global climate change. Therefore, it is essential to understand arsenic behavior in water resources in areas with potential sources such as abandoned mines.

In this study, field measurements (pH, EC, ORP, DO, Temperature, water level) and water sampling were performed from May to July in 2022 for groundwater, hyporheic zone water and stream water. In addition, sediments on the bottom of piezometers were sampled to observe the precipitants or minerals with respect to arsenic. In the laboratory, cations, anions, and water stable isotopes of water samples were analyzed, and hydrogeochemical modeling was performed for arsenic speciation modeling and saturation index calculations using PHREEQC interactive.

Before a heavy rainfall event (90 mm for one day), groundwater discharged into the stream from one side of the riparian zone, and stream water recharged into the other side. After the rainfall event, groundwater discharged from both side, and arsenic concentration in stream increased before-and-after from 7.7 to 16.2 μg/L, respectively. Arsenic species included acid species (HAsO42-, H2AsO4- and H3AsO3) and species bound with Fe, Ca, and Zn. Also, Fe-oxides and co-precipitants (amorphous FeAsO4·2H2O; pharmacolite, CaHAsO4·2H2O; koritnigite, ZnHAsO4·2H2O) were in equilibrium states in some of the water samples.

These results suggest that arsenic concentrations in the stream could be increased following rainfalls, which could drive flow directions between groundwater and stream. This change could cause water chemistry of stream in pH and ORP, and subsequent increase in the mobile arsenic species through desorption and re-dissolution of previously immobile species on stream sediments. To confirm these phenomena, SEM-EDX analysis for sediments will be conducted. Detailed results and interpretation will be presented in the Poster and discussions will be welcomed.