ABSTRACT
ABSTRACT: To investigate the regional scale groundwater flow system in low permeability siliceous Tertiary sedimentary rocks, analyses for electric conductivity (EC), hydrogen and oxygen stable isotopes (D and 18O) have been done on porewaters of drilled cores. Porewaters were extracted by the compression method from eleven drillhole boreholes (HDB1-11) drilled at Horonobe, Japan. 18O, D and EC data show that the origin of groundwater can be divided into three groups; present rainwater, paleo rainwater and fossil sea water. The estimated percolation depth of paleo rainwater (rainwater from during the ice age) is about 400m below present sea level, while present rainwater has percolated only down to 160m. The relationship between the percolation depth of rainwaters and hydraulic gradients are also investigated. The regional groundwater flow pattern estimated from the hydraulic potential distribution considerably accords with the percolation of fresh water revealed by isotopic information in porewater. These results show paleo records of regional groundwater flow would be preserved in bound water in low permeability sedimentary rock cores.
