Redox processes and occurrence of arsenic in a volcanic aquifer system of Kumamoto Area, Japan

Sources, distributions, and controlling factors for mobilization of arsenic (As) in Kumamoto basin were investigated relating to the determination of redox processes of the study area. Groundwater and sediment core samples were analyzed. Nitric acid digestion and sequential leaching experiment of sediment core samples revealed that the source of As in the groundwater is geogenic and leached primarily due to the sediment–water interactions. Unlikely, similar relations of total As with total Fe, Mn, and Al in acid extracts, leaching experiments showed a positive relation of Asₜₒₜₐₗ with Feₜₒₜₐₗ and Alₜₒₜₐₗ indicated that Fe and Al oxides/hydroxides are abundant and may be the major adsorbent of As in low pH condition. Arsenic concentrations in groundwater ranges from 0.1 to 60.6 μg/l. High As concentrations occurred in anaerobic stagnant groundwaters from Kumamoto plain area with high dissolved Feₜₒₜₐₗ, Mnₜₒₜₐₗ, and Alₜₒₜₐₗ, moderately dissolved HCO₃ ⁻, PO₄ ³⁻, SO₄ ²⁻, and low concentrations of NO₃ ⁻ and DOC suggesting the reducing condition of subsurface aquifer. There is a range of As(III)/As(T) ratios from mostly arsenate to mostly arsenite. Groundwater pH was relatively high, and high As occurred at higher pH range. It is assumed that desorption of As from metal oxide surfaces was facilitated by the elevated pH, which is considered as an important process for As mobilization. In addition to this, a wide range of δ³⁴SSO₄ values (8.3–57.6 ‰) indicates that sulfur is undergoing redox cycling mediated by microbial activities. Following δ³⁴SSO₄ results, it is anticipated that dissolved As is coprecipitated as sulfides in the presence of dissolved Fe(II) in some places, though at this moment, there is no direct evidence of coprecipitation or sequester of As with Fe and sulfide ion. Finally, a combination of the following three variables are considered potentially important causes for the high dissolved As concentrations in groundwater of Kumamoto area—(1) high groundwater pH, (2) anoxic redox conditions, and (3) stagnant groundwater in younger age sediments, which have not been well flushed since burial.