Chemical evolution of river water infiltrating the bottom sediment at the Sugao Wealth nourishing Marsh

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  • MURAMATSU Yoichi
    Department of Liberal Arts, Faculty of Science and Technology, Tokyo University of Science
  • OSHIRO Eri
    Graduate School of Science, Tokyo University of Science
  • CHIBA Hitoshi
    Department of Earth Science, Faculty of Science, Okayama University

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  • 富栄養化が進む菅生沼河川水の地下浸透に伴う水質組成の変化とその要因
  • フエイヨウカ ガ ススム スガオヌマ カセンスイ ノ チカ シントウ ニ トモナウ スイシツ ソセイ ノ ヘンカ ト ソノ ヨウイン

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Abstract

We investigated the major chemical and stable isotopic compositions of the Sugao Marsh and pore waters in the sediment under the Marsh in Ibaraki Prefecture, central Japan. We also examined the mineral assemblage of the sediment to verify anthropogenic pollutant sources and the formation mechanism of the water quality.<BR> The high concentrations of T-P, T-N and COD in the river waters indicate its ongoing eutrophication. The increase in Na+ concentrations together with arise in the concentrations of Cl- in the waters can be attributed to sodium chloride from domestic sewage. A linear correlation between Ca2+ and HCO3- concentrations in the waters can be explained by the dissolution of calcite being supported by the chemical equilibrium calculation results showing that the water is undersaturated with calcite. The δ15N values of nitrate and δ34S values of sulfate in the waters ranging mainly from + 11.2 to + 15.4 ‰ and from + 4.0 to + 7.0 ‰, respectively, confirm that both the nitrogen and sulfate contamination originated from domestic sewage. Additionally, the oxidation of pyrite in the sediment acts as a source of sulfate and iron in the Upper Marsh. This interpretation is in close agreement with supersaturation of pyrite confirmed by the chemical equilibrium calculations.<BR> In the pore waters of the Marsh sediment, the NO3- and SO42- contents are rapidly reduced, while the Fe content rapidly increases from the bottom of the Marsh up to a depth of 24cm. This appears to be due to the organic matter decomposition processes by which once oxygen has been depleted in order of energy efficiency by nitrate, iron and sulfate reductors. Moreover, the rapid increase of Ca2+ content and the equally rapid decrease of Na+ content in the pore waters between 24 and 100cm in depth can be explained by the cation exchange between Ca2+ in smectite of the sediment and Na+ in the pore water that originated from the river water.

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