Changes in chemical speciation and mobility of arsenic during the mixing of arsenic-bearing "snow-melting" system effluent and river water in the Ishikari Plain, Japan.

An effective and ingenious method called "snow-melting" system was widely implemented for snow management in the Ishikari Plain, Japan. In this system, groundwater is pumped up, mixed with snow, and discharged into a nearby river. Since the groundwater in the Ishikari Plain is contaminated with arsenic (As), the impacts of directly discharging As-bearing groundwater into the river were assessed and monitored. In-situ monitoring data collected between 2013 and 2015 showed that As concentrations were higher in the groundwater (23-95 μg/L) than in the river water (2-71 μg/L). The major As speciation in the groundwater and river water were dissolved arsenite (As(III)) and As in suspended iron (Fe)-bearing solids, respectively. Precipitation of dissolved Fe when "snow-melting" system effluent mixes with the river water could be attributed to more oxic and oxidizing conditions of the resulting fluid mixture. Dissolved iron (Fe), mainly as ferrous ion (Fe), coexisted with dissolved As(III) in groundwater, so after mixing with the river water, Fe was oxidized to ferric ion (Fe) and then precipitated as amorphous Fe oxyhydroxide phases that also sequestered dissolved arsenate (As(V)) via adsorption and coprecipitation. A strong correlation between As and Fe contents in river sediments was also observed, suggesting that Fe-bearing phases play an essential role in As immobilization. The results also showed a strong interaction between groundwater and river water that affected the chemical speciation and mobility of As and Fe. In addition, it was found that discharging As-bearing groundwater did not have profound impact on river water quality. Based on the results, dissolved Fe, Fe-bearing solid phases, and geochemical conditions strongly influenced how As speciates and migrates in a system where two fluids are mixed. This study could provide significant insights concerning the impacts of As on surrounding environments where As-bearing groundwaters are used and discharged without treatment.