Paleo-hydrology information from the lake bottom deposit core and its pore water, Lake Kasumigaura, Japan

To predict the hydrological condition in the future, the paleo-hydrology information is important. Because we can apply the prediction model by using the paleo-hydrology information as an input to estimate the present. In Japan, where is characterized by relatively small catchments scale with steep relief topography under the humid temperate climatic condition, the groundwater flow residence time is relatively short. Thus the paleo- hydrological information preserved in the groundwater aquifer is thought to be limited. However, in the low- permeable deposit mainly composed by clay and silt, the groundwater flow should be very slow than the conventional aquifers. There is a possibility to conserve the paleo-hydrology information in those formations after their deposition. The Holocene clay and silt deposits in the Lake Kasumigaura were core drilled to retrieve the past 10,000 year's paleo-hydrology information. Borehole with the depth of 29.7m (SiteK-2) was drilled in the central part of the lake under the 5m water depth and we analyzed paleo-hydrology information from deposits and its pore water. Lake Kasumigaura is second big fresh-water lake in Japan, located in the southern part of Ibaragi Prefecture, 50 km Northeast of Tokyo with the area in of 172km2, maximum depth of 7.3m, lake water elevation of 0.26- 0.46 m above present mean sea level. The bottom formations of Lake Kasumigaura were created by fluvial incision during the marine regression stage in the last glacial maximum. Afterwards, it becomes an inner bay condition along with the sea level rise in the post-ice age, and it becomes a costal lake caused by fluvial filling of the bay entrance. Lake Kasumigaura will strongly affected by the change of the sea level in about 10,000 years. It consists of both marine and land facies which were affected during transgression and regression stages. The pore water is collected from the sampled core deposit by using centrifuging methods. The EC, inorganic chemistry content, D and 18O in the water were analyzed. The C14 age analyses were done for the organic materials from the different depths to confirm the formation age. The diatom fossil analyses of core were also done to confirm the salinity condition during the deposition time. By composing those informations, we will characterize the paleo-hydrological condition of the study area.