title: Distribution and hosts of arsenic in a sediment core from the Chianan Plain in SW Taiwan: Implications on arsenic primary source and release mechanisms.
abstract: High arsenic abundance of 50-700μg/L in the groundwater from the Chianan Plain in southwestern Taiwan is a well-known environmental hazard. The groundwater-associated sediments, however, have not been geochemically characterized, thus hindering a comprehensive understanding of arsenic cycling in this region. In this study, samples collected from a 250m sediment core at the centre of the Chianan Plain were analyzed for arsenic and TOC concentrations (N=158), constituent minerals (N=25), major element abundances (N=105), and sequential arsenic extraction (N=23). The arsenic data show a prevalence of >10mg/kg with higher concentrations of 20-50mg/kg concentrated at 60-80 and 195-210m. Arsenic was extracted mainly as an adsorbate on clay minerals, as a co-precipitate in amorphous iron oxyhydroxide, and as a structural component in clay minerals. Since the sediments consist mainly of quartz, chlorite, and illite, the correlations between arsenic concentration and abundances of K2O and MgO pinpoint illite and chlorite as the major arsenic hosts. The arsenic-total iron correlation reflects the role of chlorite along with the contribution from amorphous iron oxyhydroxide as indicated by arsenic extraction data. Organic matter is not the dominant arsenic host for low TOC content, low arsenic abundance extracted from it, and a relatively low R(2) of the arsenic-TOC correlation. The major constituent minerals in the sediments are the same as those of the upriver metapelites, establishing a sink-source relationship. Composition data from two deep groundwater samples near the sediment core show Eh values and As(V)/As(III) ratios of reducing environments and high arsenic, K, Mg, and Fe contents necessary for deriving arsenic from sediments by desorption from clay and dissolution of iron oxyhydroxide. Therefore, groundwater arsenic was mainly derived from groundwater-associated sediments with limited contributions from other sources, such as mud volcanoes.
authors.affiliation: Department of Earth Sciences, National Cheng-Kung University, 1, University Road, Tainan 70101, Taiwan, ROC. Electronic address: hjyang@mail.ncku.edu.tw.
authors.forename: Huai-Jen
authors.initials: HJ
authors.lastname: Yang
authors.affiliation: Department of Geosciences, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, ROC.
authors.forename: Chi-Yu
authors.initials: CY
authors.lastname: Lee
authors.affiliation: Department of Earth Sciences, National Cheng-Kung University, 1, University Road, Tainan 70101, Taiwan, ROC.
authors.forename: Yu-Ju
authors.initials: YJ
authors.lastname: Chiang
authors.affiliation: Department of Earth Sciences, National Cheng-Kung University, 1, University Road, Tainan 70101, Taiwan, ROC.
authors.forename: Jiin-Shuh
authors.initials: JS
authors.lastname: Jean
authors.affiliation: Department of Marine Biotechnology and Resources, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, ROC.
authors.forename: Yen-Hong
authors.initials: YH
authors.lastname: Shau
authors.affiliation: Department of Geology, Niigata University, 8050, Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan.
authors.forename: Eiichi
authors.initials: E
authors.lastname: Takazawa
authors.affiliation: Department of Earth Sciences, National Cheng-Kung University, 1, University Road, Tainan 70101, Taiwan, ROC.
authors.forename: Wei-Teh
authors.initials: WT
authors.lastname: Jiang
mesh_terms: D001151:Arsenic / Q000032:analysis*; D002244:Carbon / Q000032:analysis*; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000032:analysis*; D060587:Groundwater / Q000737:chemistry; D045483:Rivers / Q000737:chemistry; D013624:Taiwan; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D002244:Carbon; D001151:Arsenic
keywords: Arsenic; Chianan; Major element; Sediment; Sequential extraction; Toc