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[HCG21-P08] Sensitivity Analysis of Influencing Factors in Regional Groundwater Flow Analysis Based on Paleohydrogeological Transition
Keywords:groundwater flow analysis, fossil seawater
Background
One of the safety functions of the geological environment for the geological disposal of high-level radioactive waste is to be a slow hydraulic field, which inhibits the migration of radioactive elements due to a small hydraulic gradient or low permeability. In the deep subsurface of thick marine sedimentary formations, fossil seawater is often present, which is thought to be seawater in the pore spaces during the sedimentation and altered during the burial and diagenesis processes. The presence of fossil seawater suggests that groundwater in the site has not been washed out by recharge flow even after uplift with a high hydraulic gradient, suggesting that such a site is a long-term low-flow area.
The deeper part of the Wakkanai F. of Neogene marine sediments in Horonobe, Hokkaido, Japan, has low fracture connectivity, and fossil seawater is often found there (Ishii, 2018). There, fossil seawater shows older groundwater ages, suggesting that there may have been little movement, at least since the uplift (Nakata et al., 2018). Therefore, using fossil seawater as an indicator, multiple investigations have been developed to determine low-flow areas' distribution efficiently. It is important to compare the estimation using fossil seawater with the results of groundwater flow analysis to understand groundwater flow conditions comprehensively. This study focused on the inland area around the Horonobe Underground Research Laboratory (HURL), where much hydrological information on the deeper part of the Wakkanai F. has been obtained. Sensitivity analyses were conducted to understand the effects of various factors affecting long-term groundwater flow, such as topography, hydraulic conductivity, density flow, abnormally high pressure, and sea-level/recharge rate changes, by steady-state groundwater flow analysis.
Method
The analysis area is about 30 km NS, 90 km EW, and 10 km vertically from the hills of Horonobe to the sea. The focus area is a 5 km EW and 1.5 km vertical section around the HURL. Dtransu3D/EL was used for the analysis. Three models were used based on the previous studies: a model that reconstructed the topography and geological structure at about 1 Ma and 0.33 Ma and a current model. The hydrogeological classification is seven strata of the Quaternary to Neogene, Paleogene, and Cretaceous, and seven major faults. The hydraulic parameters were given based on the previous studies.
Results and discussion
The results confirmed that topography, low permeability below 10−9 m s−1, and abnormal pressure significantly influence assessing the low-flow areas in the inland area. Regarding the influence of topography, it was found that as the hills were formed by uplift, different groundwater flow patterns were formed in the shallow and deep subsurface areas. It was confirmed that a localized groundwater flow system was formed in the shallow part, which takes hundreds of thousands of years from recharge to discharge, and a more sluggish flow system was formed in the deep part. Such deep subsurface flow is slow even when compared to the uplift rate.
Here, the analytical model reflects the findings that highly permeable fractures form and develop in the shallower part of the Wakkanai F. due to reduced sealing pressure caused by uplift and erosion (Ishii, 2015), which have contributed to the separation of the shallow and deep flow systems. A hydraulic conductivity of 1.6 × 10−8 and 5.9 × 10−10 m s−1 were respectively given for the shallower and deeper part of the Wakkanai formations. A change of one order of magnitude in the hydraulic conductivity of the deeper part of the Wakkanai F. resulted in a significant change in the boundary position between the shallow and deep flow systems, which indicates that the giving of a low hydraulic conductivity has a significant impact in terms of evaluating the slow hydraulic field deep in the subsurface.
When abnormal pressure of + 140 m above the hydrostatic pressure in the maximum was fixed in the deeper part of the Wakkanai F. based on the observed data, the actual flow velocity and hydraulic gradient were about one order of magnitude higher in the shallow Wakkanai F. than when the abnormal pressure was not set. On the other hand, the effects of density flow and reduced sea level and recharge rate on the actual flow velocity and hydraulic gradient were minor.
This study was carried out as a part of R&D supporting program titled "Development and Improvement on Groundwater Flow Evaluation Technique in Rock" under the contract with Ministry of Economy, Trade and Industry (METI) (Grant Number: JPJ007597).
One of the safety functions of the geological environment for the geological disposal of high-level radioactive waste is to be a slow hydraulic field, which inhibits the migration of radioactive elements due to a small hydraulic gradient or low permeability. In the deep subsurface of thick marine sedimentary formations, fossil seawater is often present, which is thought to be seawater in the pore spaces during the sedimentation and altered during the burial and diagenesis processes. The presence of fossil seawater suggests that groundwater in the site has not been washed out by recharge flow even after uplift with a high hydraulic gradient, suggesting that such a site is a long-term low-flow area.
The deeper part of the Wakkanai F. of Neogene marine sediments in Horonobe, Hokkaido, Japan, has low fracture connectivity, and fossil seawater is often found there (Ishii, 2018). There, fossil seawater shows older groundwater ages, suggesting that there may have been little movement, at least since the uplift (Nakata et al., 2018). Therefore, using fossil seawater as an indicator, multiple investigations have been developed to determine low-flow areas' distribution efficiently. It is important to compare the estimation using fossil seawater with the results of groundwater flow analysis to understand groundwater flow conditions comprehensively. This study focused on the inland area around the Horonobe Underground Research Laboratory (HURL), where much hydrological information on the deeper part of the Wakkanai F. has been obtained. Sensitivity analyses were conducted to understand the effects of various factors affecting long-term groundwater flow, such as topography, hydraulic conductivity, density flow, abnormally high pressure, and sea-level/recharge rate changes, by steady-state groundwater flow analysis.
Method
The analysis area is about 30 km NS, 90 km EW, and 10 km vertically from the hills of Horonobe to the sea. The focus area is a 5 km EW and 1.5 km vertical section around the HURL. Dtransu3D/EL was used for the analysis. Three models were used based on the previous studies: a model that reconstructed the topography and geological structure at about 1 Ma and 0.33 Ma and a current model. The hydrogeological classification is seven strata of the Quaternary to Neogene, Paleogene, and Cretaceous, and seven major faults. The hydraulic parameters were given based on the previous studies.
Results and discussion
The results confirmed that topography, low permeability below 10−9 m s−1, and abnormal pressure significantly influence assessing the low-flow areas in the inland area. Regarding the influence of topography, it was found that as the hills were formed by uplift, different groundwater flow patterns were formed in the shallow and deep subsurface areas. It was confirmed that a localized groundwater flow system was formed in the shallow part, which takes hundreds of thousands of years from recharge to discharge, and a more sluggish flow system was formed in the deep part. Such deep subsurface flow is slow even when compared to the uplift rate.
Here, the analytical model reflects the findings that highly permeable fractures form and develop in the shallower part of the Wakkanai F. due to reduced sealing pressure caused by uplift and erosion (Ishii, 2015), which have contributed to the separation of the shallow and deep flow systems. A hydraulic conductivity of 1.6 × 10−8 and 5.9 × 10−10 m s−1 were respectively given for the shallower and deeper part of the Wakkanai formations. A change of one order of magnitude in the hydraulic conductivity of the deeper part of the Wakkanai F. resulted in a significant change in the boundary position between the shallow and deep flow systems, which indicates that the giving of a low hydraulic conductivity has a significant impact in terms of evaluating the slow hydraulic field deep in the subsurface.
When abnormal pressure of + 140 m above the hydrostatic pressure in the maximum was fixed in the deeper part of the Wakkanai F. based on the observed data, the actual flow velocity and hydraulic gradient were about one order of magnitude higher in the shallow Wakkanai F. than when the abnormal pressure was not set. On the other hand, the effects of density flow and reduced sea level and recharge rate on the actual flow velocity and hydraulic gradient were minor.
This study was carried out as a part of R&D supporting program titled "Development and Improvement on Groundwater Flow Evaluation Technique in Rock" under the contract with Ministry of Economy, Trade and Industry (METI) (Grant Number: JPJ007597).