2:00 PM - 2:15 PM
[HTT18-02] Understanding of bedrock groundwater dynamics using stable isotope ratios of strontium in mountain headwaters -Comparison between granitic and accretionary sedimentary rocks catchments-
Keywords:Bedrock groundwater, Granite, Accretionary sedimentary rock, Strontium stable isotope ratio, Water resources, Deep-seated slope failure
It is necessary to clarify the dynamics of bedrock groundwater because it is a water resource, but it also triggers deep-seated slope failure in forested headwaters. We will examine the groundwater dynamics by applying tracer observations using the stable isotope ratio of strontium (87Sr/86Sr), which reflects the difference of geology, in catchments where bedrock groundwater levels have been observed by many boreholes.
The observations were carried out in two catchments, one in the granite and the other in the accretionary sedimentary rocks. The granitic mountain area is the Fudoji in Otsu City, Shiga Prefecture, located south of Lake Biwa. The site consists of six adjacent subcatchments (F1 to F6), and the existence of a one-layered groundwater zone has been confirmed within each subcatchment down to about 30 m below ground level. Groundwater samples were collected from 34 boreholes installed in each catchment to the bedrock layer. We also collected streamwater from each subcatchment and rock samples from one borehole core. On the other hand, the accretionary sedimentary rock mountain is the Katsuragawa site in Otsu City, Shiga Prefecture, located to the west of Lake Biwa. The site belongs to the Tamba Belt, and two groundwater layers (a shallow layer and a deep layer) have been identified up to about 60 m below the surface. In this site, groundwater is intercepted by gauge clay generated by the activity of an associated fault that runs parallel to the nearby Hanaore fault zone, and there are several water spring points along this associated fault. Groundwater was collected from 45 boreholes at 28 locations within the bedrock, and groundwater level was observed at the same time. Three spring samples and 34 rock samples from 16 boreholes were collected. The 87Sr/86Sr values of the above collected water and rock samples were measured.
The rock samples from the Fudoji showed larger 87Sr/86Sr values in the more weathered layer. According to the columnar map of the drilling core, the subcatchments with lower mean surface elevation were more weathered, and at the same time, the 87Sr/86Sr of the bedrock groundwater tended to be larger in those subcatchment areas. It is thought that the contact with highly weathered rocks resulted in the higher values. The 87Sr/86Sr of stream water tended to be larger in the subcatchments with lower mean surface elevations, as in the case of the bedrock groundwater, but in one of the subcatchments located in the center (F3), the 87Sr/86Sr was the largest, deviating from this relationship. The distribution of 87Sr/86Sr shows that the groundwater in the F2 and F4 catchments near the boundary of the F3 catchment has a large 87Sr/86Sr value, indicating that the groundwater in the F2 and F4 catchments contributes to the stream water in the F3 catchment beyond the catchment boundary at the ground surface. This indicates the extent of groundwater contributing to stream water in the F3 catchment beyond the catchment boundary.
At the Katsuragawa, groundwater had different 87Sr/86Sr at different depths even at the same location. It was also found that 87Sr/86Sr of rocks differed depending on the rock type. The relationship between 87Sr/86Sr of groundwater and of exchangeable Sr on the rock surface are plotted on an approximate 1:1 line, suggesting that the 87Sr/86Sr of groundwater reflects the 87Sr/86Sr of the rock surface. The groundwater level contours, the two-component mixing analysis, and the 87Sr/86Sr distribution map were used to estimate the groundwater flow, and it was found that the C1 spring water, which has a high flow rate, is likely to be recharged from a large groundwater zone at depth. On the other hand, the M1 spring, which is located about 90 m away from the C1 spring and has a low flow rate, is recharged from a shallow, small groundwater zone. The complexity of groundwater flow is considered to be a factor that causes the diversity in the mechanism of deep-seated slope failure.
The observations were carried out in two catchments, one in the granite and the other in the accretionary sedimentary rocks. The granitic mountain area is the Fudoji in Otsu City, Shiga Prefecture, located south of Lake Biwa. The site consists of six adjacent subcatchments (F1 to F6), and the existence of a one-layered groundwater zone has been confirmed within each subcatchment down to about 30 m below ground level. Groundwater samples were collected from 34 boreholes installed in each catchment to the bedrock layer. We also collected streamwater from each subcatchment and rock samples from one borehole core. On the other hand, the accretionary sedimentary rock mountain is the Katsuragawa site in Otsu City, Shiga Prefecture, located to the west of Lake Biwa. The site belongs to the Tamba Belt, and two groundwater layers (a shallow layer and a deep layer) have been identified up to about 60 m below the surface. In this site, groundwater is intercepted by gauge clay generated by the activity of an associated fault that runs parallel to the nearby Hanaore fault zone, and there are several water spring points along this associated fault. Groundwater was collected from 45 boreholes at 28 locations within the bedrock, and groundwater level was observed at the same time. Three spring samples and 34 rock samples from 16 boreholes were collected. The 87Sr/86Sr values of the above collected water and rock samples were measured.
The rock samples from the Fudoji showed larger 87Sr/86Sr values in the more weathered layer. According to the columnar map of the drilling core, the subcatchments with lower mean surface elevation were more weathered, and at the same time, the 87Sr/86Sr of the bedrock groundwater tended to be larger in those subcatchment areas. It is thought that the contact with highly weathered rocks resulted in the higher values. The 87Sr/86Sr of stream water tended to be larger in the subcatchments with lower mean surface elevations, as in the case of the bedrock groundwater, but in one of the subcatchments located in the center (F3), the 87Sr/86Sr was the largest, deviating from this relationship. The distribution of 87Sr/86Sr shows that the groundwater in the F2 and F4 catchments near the boundary of the F3 catchment has a large 87Sr/86Sr value, indicating that the groundwater in the F2 and F4 catchments contributes to the stream water in the F3 catchment beyond the catchment boundary at the ground surface. This indicates the extent of groundwater contributing to stream water in the F3 catchment beyond the catchment boundary.
At the Katsuragawa, groundwater had different 87Sr/86Sr at different depths even at the same location. It was also found that 87Sr/86Sr of rocks differed depending on the rock type. The relationship between 87Sr/86Sr of groundwater and of exchangeable Sr on the rock surface are plotted on an approximate 1:1 line, suggesting that the 87Sr/86Sr of groundwater reflects the 87Sr/86Sr of the rock surface. The groundwater level contours, the two-component mixing analysis, and the 87Sr/86Sr distribution map were used to estimate the groundwater flow, and it was found that the C1 spring water, which has a high flow rate, is likely to be recharged from a large groundwater zone at depth. On the other hand, the M1 spring, which is located about 90 m away from the C1 spring and has a low flow rate, is recharged from a shallow, small groundwater zone. The complexity of groundwater flow is considered to be a factor that causes the diversity in the mechanism of deep-seated slope failure.