2:00 PM - 2:15 PM
[HCG20-02] The influence of sea-level changes on coastal groundwater
Keywords:radioactive waste disposal, sea- and fresh-water interface, sea level changes, groundwater
The nationwide map of “Scientific Features” relevant for geological disposal (METI, 2017) shows coastal areas within approximately 20 km of the coastal line are favorable for safe waste transportation. In addition, literature survey reports for Suttu Town and Kamoenai Village show the continental shelf within 15 km of the coastline as a candidate area for a preliminary investigation. On site selection in coastal areas, mobility and quality of groundwater are important in evaluating groundwater scenarios.
The mobility and quality of groundwater in coastal areas will be influenced by long-term sea level changes. Sea level changes occur in cycles of about 120,000 years with regression and transgression for 100,000 years and 20,000 years, respectively. The minimum sea level will be more than 100 m below the present sea level.
The Ghyben-Herzberg law is used to estimate the position of the salt-freshwater interface by focusing on the difference in density between seawater and freshwater. When a sea level is 100 m lower than the present sea level, the sea and freshwater interface will be 4000 m lower, and a large area of subseabed in continent shelf will be replaced by freshwater.
Conversely, as sea level rises to the present sea level, and the area will be covered by present seawater. When a geological formation has low permeability, it may be unrealistic to be replaced by seawater in less than 20,000 years during transgression. For example, hydraulic conductivity k=1×10-8 m/s (≒0.3 m/y) like dense sand and silt, the Darcy velocity will be about 0.01 m/y because hydraulic gradient will be 3% due to density difference between sea and freshwater. Assuming a porosity of 30%, the real velocity will be about 0.03 m/y. Therefore, over the time scale of several thousand years of submergence, seawater will penetrate only 100 m in depth. If the hydraulic conductivity is 10 or 1/10 times higher (k=1×10-7 or 1×10-9 m/s), seawater will penetrate to 1000 or 10 m in depth, respectively.
As shown above, groundwater in coastal areas will be affected by sea level changes. If k= 1×10-7 m/s, the groundwater will be replaced by the present seawater, if 1×10-8 m/s the glacial precipitation will remain. If 1×10-9 m/s, the groundwater flow is too low to be replaced, and fossil seawater will remain.
As sea-level changes can be considered a natural tracer test using geological events, it is important to identify the long-term stability and quality of groundwater in candidate geological formation focusing on present seawater, glacial precipitation or fossil seawater.
A part of this work was supported by the Ministry of Economy, Trade, and Industry (METI) under the R&D support program entitled “The project for validating assessment methodology of geological disposal system in coastal region (Development of Technology to Evaluate and Validate Coastal Disposal Systems)” [grant number JPJ007597, Fiscal year 2022].
The mobility and quality of groundwater in coastal areas will be influenced by long-term sea level changes. Sea level changes occur in cycles of about 120,000 years with regression and transgression for 100,000 years and 20,000 years, respectively. The minimum sea level will be more than 100 m below the present sea level.
The Ghyben-Herzberg law is used to estimate the position of the salt-freshwater interface by focusing on the difference in density between seawater and freshwater. When a sea level is 100 m lower than the present sea level, the sea and freshwater interface will be 4000 m lower, and a large area of subseabed in continent shelf will be replaced by freshwater.
Conversely, as sea level rises to the present sea level, and the area will be covered by present seawater. When a geological formation has low permeability, it may be unrealistic to be replaced by seawater in less than 20,000 years during transgression. For example, hydraulic conductivity k=1×10-8 m/s (≒0.3 m/y) like dense sand and silt, the Darcy velocity will be about 0.01 m/y because hydraulic gradient will be 3% due to density difference between sea and freshwater. Assuming a porosity of 30%, the real velocity will be about 0.03 m/y. Therefore, over the time scale of several thousand years of submergence, seawater will penetrate only 100 m in depth. If the hydraulic conductivity is 10 or 1/10 times higher (k=1×10-7 or 1×10-9 m/s), seawater will penetrate to 1000 or 10 m in depth, respectively.
As shown above, groundwater in coastal areas will be affected by sea level changes. If k= 1×10-7 m/s, the groundwater will be replaced by the present seawater, if 1×10-8 m/s the glacial precipitation will remain. If 1×10-9 m/s, the groundwater flow is too low to be replaced, and fossil seawater will remain.
As sea-level changes can be considered a natural tracer test using geological events, it is important to identify the long-term stability and quality of groundwater in candidate geological formation focusing on present seawater, glacial precipitation or fossil seawater.
A part of this work was supported by the Ministry of Economy, Trade, and Industry (METI) under the R&D support program entitled “The project for validating assessment methodology of geological disposal system in coastal region (Development of Technology to Evaluate and Validate Coastal Disposal Systems)” [grant number JPJ007597, Fiscal year 2022].