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

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. 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 2+ - 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 under saturated with calcite. The δ ¹⁵N values of nitrate and δ³⁴S values of sulfate in the waters ranging mainly from + 11.2 to + 15.4%o and from + 4.0 to + 7.0%o, 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 super saturation of pyrite confirmed by the chemical equilibrium calculations. In the pore waters of the Marsh sediment, the NO₃ ^- and SO₄ ^2- 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 2+ + sulfate redactors. 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 Ca ²⁺ in smectite of the sediment and Na⁺ in the pore water that originated from the river water.