Wetland is one of the most precious environments. Recently, the natural environment of wetland deteriorates rapidly, due to mainly human activities. The hydrogeological information, however, has not been obtained sufficiently. Therefore, the objective of this study is to clarify the physicochemical and hydrological properties of peat in South Sarobetsu Mire, northern Hokkaido.
Peat samples were taken by means of a peat sampler and a boring. The boring c ore showed that the peat had a thickness of almost 5 m at this point, and that silt and sand layers were overlain by the peat layer. Anisotoropic characteristics of hydraulic conductivity of peat, ranging from 10^-52 to 10^-2 cm/s, were not observed, whereas the horizontal hydraulic coductivity were larger than the vertical one for the underlain silt and sand layers. Little difference was found in the properties of peat sampled by the above-mentioned two methods. The average sedimentation and degradation rates of the organic carbon in the peat were estimated at 1.2 mm/y and 1.2 x 10^-4y^-1 respectively, using the ¹⁴ C age and vertical profiles of the organic carbon content. This suggests that the age of peat and organic carbon content do not affect its hydrogeological properties in the study area. Therefore, the peat layer can be treated as an aquifer with a representative hydraulic conductivity in the groundwater flow analysis.

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Recently, the interface between freshwate r and seawater in sub-seabed has been discussed actively by using a numerical simulation for construction of huge underground buildings and/or for various wastes disposal in the coastal area. Generally, groundwater flow rate will possibly be small in seawater side behind the interface, and solute transport will be controlled by diffusion phenomena. However, salt-water found in the sub-seabed and a deep stratum has not yet been considerably characterized. Furthermore, there are no criteria and procedures to verify that saltwater behind the interface is not only stagnant but the fossil seawater. In this study, we propose the methodology to characterize it based on information on groundwater residence time using a ³⁶Cl dating method and on groundwater origin from stable isotopes.

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