Factors affecting arsenic content of unconsolidated sediments and its mobilization in the Ishikari Plain, Hokkaido, Japan

Arsenic (As) contamination of groundwater is often caused by As leaching from geological formations. This study focused on factors affecting As content in unconsolidated sediments by using three sediment cores in the Ishikari plain, Hokkaido, Japan. The geochemical properties of the sediments were characterized by the chemical and mineral compositions as well as leaching experiments and sequential extractions of As. The results showed that higher As content of 4-9 mg/kg was observed in peat layers with organic matter content > 10 wt%, and that higher As contents deeper than 8 m below the surface were also observed. The deeper sediments were likely to be formed in brackish water conditions due to lower weight ratios of organic carbon content to sulfur content (C/S < 10). The results of the leaching experiments showed that As was mobilized at slightly alkaline region. These indicate that the distribution of As depended on the organic content in the sediments and sedimentation condition, and that the mobility of As in the sediment was affected by pH

The solid-phase partitioning of arsenic in unconsolidated sediments of the Mekong Delta, Vietnam and its modes of release under various conditions

Arsenic (As) contamination of the groundwater in the Mekong Delta is a serious problem affecting millions of people who rely on this important resource for drinking and agriculture. In this study, borehole cores up to a depth of 40 m were collected in the Vietnamese-side of the delta, and the solid-phase partitioning of As with depth was investigated to understand the factors and processes controlling the release of this toxic element under oxic, acidic and reducing conditions. The results showed that in most of the sediments, substantial amounts of As are partitioned with exchangeable phases that are easily released into solution. Two borehole cores obtained between the Hau and Tien Rivers also had significantly high As partitioned with organic/sulfide phases and one of these cores had abundant As-bearing pyrite in 1-m thick peat layers. Leaching experiments in deionized (DI) water coupled with principal component analysis suggest that As release was controlled by sorption-desorption reactions with clays/phyllosilicates (i.e., kaolinite, muscovite and clinochlore), proton-promoted dissolution of iron-oxyhydroxides, and oxidation of pyrite/organic matter. The mobility of As was further promoted under acidic conditions in the presence of chloride (Cl-), which suggests that seasonal drying/flooding episodes generating acid sulfate soils, as well as salt water intrusion due to excessive groundwater abstraction may exacerbate this problem in the future

Geological and geochemical characterizations of sediments in six borehole cores from the arsenic-contaminated aquifer of the Mekong Delta, Vietnam

The Mekong Delta, situated between Cambodia and Vietnam, is one of the most productive aquifer systems in the region. In recent years, however, several studies have shown that groundwater in several areas of the delta is highly contaminated with arsenic (As). Although more than 80% of the total area of the Mekong Delta is situated in Vietnam, most of the studies have been conducted on the Cambodian-side of the delta. In this study, borehole core samples were collected around the Tien and Hau Rivers, the two main branches of the Mekong River as it enters Vietnam. We present a raw data collection of the chemical and mineralogical composition of distinct lithological features from six borehole core samples drilled up to a depth of 40 m. The data also include the pH, Eh, EC, As, Si, Al, DOC, dissolved heavy metals (Fe and Mn) and major coexisting ions of leachates obtained by leaching the 34 selected sediment samples in deionized water. The information provided in this paper would be useful as a baseline for reactive transport or geochemical modeling to understand and predict As migration in naturally contaminated aquifers under various conditions. For more insights, the reader is referred to our paper entitled "The solid-phase partitioning of arsenic in unconsolidated sediments of the Mekong Delta, Vietnam and its modes of release under various conditions" Huyen et al., 2019