A combination of chemical and carbon stable isotopic compositions has been used to examine controlling processes of groundwater chemistry along the flow paths in the Ohmama Alluvial Fan (OAF) in Gunma Prefecture. The groundwater modifies its chemical compositions from Ca-HCO₃ type to Ca-(SO₄+NO₃) type along the paths; from upper part to lower part of the OAF. In addition, increases of NO₃^－ and SO₄^2－ concentrations, a decrease of dissolved inorganic carbon (DIC) concentrations, and a rise of δ¹³C_DIC values in the groundwater are observed. These observations can be attributed to three processes; one was oxidation process of ammonium sulfate fertilizer in vegetable fields distributing widely in the OAF and the others were processes of CO₂ degas and mineral dissolution associated with the oxidation process. Furthermore, it is noted that EC values in the groundwater drastically decrease at the lowest part of the OAF where paddy fields distribute. The most plausible cause of this EC decrease is infiltration of paddy water. Model calculations based on Cl^－ concentrations indicate that paddy water occupied high ratio, such as approximately 80%, in groundwater at the lower part of the OAF.

View full abstract

Wood chips are considered as one of important recyclable materials for nitrate nitrogen attenuation in groundwater. Column experiments were conducted to study the processes and removal efficiency for denitrification by wood chips (bamboo and Lithocarpus eduilis). It was found that wood chips could reduce DO in groundwater at first and make denitrification available downstream. Comparing the concentrations of nitrate nitrogen, nitrite nitrogen and bicarbonate between the influent and effluent, it was believed that nitrate nitrogen was removed mainly by denitrification within the columns. As a result, the rate of nitrate nitrogen removal is 21%, 56% and 73% for granular processed Lithocarpus eduilis, granular processed bamboo chips and unprocessed bamboo powder, respectively. Also, the rate of nitrate nitrogen removal was up in the column compacted by bamboo chips when phosphate had been added, suggesting that phosphate-P would be the limit element for nitrate attenuation in groundwater.

View full abstract

In large and/or deep excavations of soil with a high groundwater level, the three-dimensional concentration of seepage flow is a problem. Boiling (or blow out of soil particles) occurred during the construction of caisson type piles under the K-line viaduct pier in Kitakyushu City. A ground surface　area of 10.8×10.8 m was surrounded by sheet piles (III type) 11.0 m long, and a working platform was constructed at 5 m below the surface. Four caisson type piles (consisting of rings of 0.5 m in height, and　2.0 m inner and 2.5 m outer diameters) were driven manually to a depth of 13.5 m. Soil particles blew out at the 4.5 to 6.0 m level under the working platform. In this paper, analyses of the FEM seepage flow　and stability against seepage failure are conducted, and the characteristics of seepage flow and seepage failure stability are discussed.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract