12:00 PM - 12:15 PM
[HGM02-06] Volume of Late Pleistocene Sand Dunes Underground at Tottori University Koyama Campus-Estimation of Groundwater Reserve for BCP-
Keywords:the Tottori University Koyama Campus, Buisiness continuity programe, boreing, sand dunes, groundwater recharge, groundwater reserve
Introduction
Tottori University is implementing a well drilling project as part of BCP. Old sand dunes spread under the Tottori University Koyama Campus (Akagi, 1972), and the sand dunes serve as a storage for groundwater. The purpose of this study is to i) investigate the spread of old sand dunes from the boring logs on the Tottori University Koyama Campus, ii) find the groundwater reserve from the volume of the old sand dunes and determine the amount of groundwater recharge by annual precipitation at the Tottori University Koyama Campus.
Survey method
Old sand dune was determined from a total of 62 boring log charts published in 26 boring reports on the Koyama Campus of Tottori University. The certification method was to certify uniform fine to medium sand layers that are covered with pumice or volcanic ash soil as old dune sand. Most of these old dune sands had an N value of 25 or more. Therefore, the uniform fine to medium sand layer not covered by the volcanic ash layer was recognized as old dune sand with an N value of ≧ 25. Next, using Esri's Arc GIS Pro, the volume of the old sand dunes was calculated by surface-difference between the top and bottom of the old sand dunes. Furthermore, the groundwater reserve was estimated in consideration of the water table extrapolated from the underground water level in the wells. Then, using the effective porosity of the sand dunes 0.2 (JSCE, 1971), the amount of groundwater in the old dunes was calculated.
Results and Discussion
1) Spread of old sand dunes
From the north-south projection boring cross section, one ridge of the old sand dunes could be recognized. The height of the dunes reaches 25-30 m above sea level, followed by sand dunes at 15-5 m above sea level to the south. The main part of the Tottori University Koyama Campus is located on the old sand dunes, and no Holocene sand dunes were found to be covered.
2) Groundwater reserve and groundwater recharge due to precipitation
The bottom of the old sand dunes was unknown in many borehole logs. Therefore, it was found that the volume of the old dunes was at least 8.0 × 106 m3. Except for the northern dune ridge, the old dunes are filled with groundwater at many points. Therefore, the volume of the old sand dunes sandwiched between the bottom of the strainer of the BCP well and the water level in the hole was calculated to be 6.8 × 106 m3, and if the effective porosity is 0.2 (JSCE, 1971), the amount of groundwater reserve of 1.4× 106 m3 was calculated. The average annual rainfall over the past 19 years at the Tottori Meteorological Observatory at Koyama was 1,653 mm. The area of the Tottori University Koyama Campus located on the old sand dunes is about 50 ha, and if the outflow coefficient, which is the ratio of rainwater that evaporates and drains without penetrating underground, is 0.9 (Ministry of Land, Infrastructure, Transport and Tourism, 2004), the amount of groundwater recharge is 8.3 x 106 m3 / yr. On the other hand, the average water usage for the past nine years (2011-2019) at Tottori University Koyama Campus was 87,786 m3. There is sufficient groundwater reserve for the use of BCP wells in emergencies such as disasters. However, in order to use sustainable groundwater, it may be necessary to increase the penetration rate of rainfall. In any case, continuous observation of groundwater level and water quality by monitoring wells is indispensable.
Conclusion
It was revealed that the old sand dunes in the basement of the Tottori University Koyama Campus have the role of a groundwater tank, and the amount is at least 1.4 million m3. Therefore, it can be said that the amount of groundwater is sufficient in an emergency such as a disaster. However, the amount of groundwater recharge may be slightly insufficient for that of water used. In order to consider the sustainable use of groundwater in the future, discussions based on continuous observation and monitoring of groundwater level and water quality will be necessary.
Tottori University is implementing a well drilling project as part of BCP. Old sand dunes spread under the Tottori University Koyama Campus (Akagi, 1972), and the sand dunes serve as a storage for groundwater. The purpose of this study is to i) investigate the spread of old sand dunes from the boring logs on the Tottori University Koyama Campus, ii) find the groundwater reserve from the volume of the old sand dunes and determine the amount of groundwater recharge by annual precipitation at the Tottori University Koyama Campus.
Survey method
Old sand dune was determined from a total of 62 boring log charts published in 26 boring reports on the Koyama Campus of Tottori University. The certification method was to certify uniform fine to medium sand layers that are covered with pumice or volcanic ash soil as old dune sand. Most of these old dune sands had an N value of 25 or more. Therefore, the uniform fine to medium sand layer not covered by the volcanic ash layer was recognized as old dune sand with an N value of ≧ 25. Next, using Esri's Arc GIS Pro, the volume of the old sand dunes was calculated by surface-difference between the top and bottom of the old sand dunes. Furthermore, the groundwater reserve was estimated in consideration of the water table extrapolated from the underground water level in the wells. Then, using the effective porosity of the sand dunes 0.2 (JSCE, 1971), the amount of groundwater in the old dunes was calculated.
Results and Discussion
1) Spread of old sand dunes
From the north-south projection boring cross section, one ridge of the old sand dunes could be recognized. The height of the dunes reaches 25-30 m above sea level, followed by sand dunes at 15-5 m above sea level to the south. The main part of the Tottori University Koyama Campus is located on the old sand dunes, and no Holocene sand dunes were found to be covered.
2) Groundwater reserve and groundwater recharge due to precipitation
The bottom of the old sand dunes was unknown in many borehole logs. Therefore, it was found that the volume of the old dunes was at least 8.0 × 106 m3. Except for the northern dune ridge, the old dunes are filled with groundwater at many points. Therefore, the volume of the old sand dunes sandwiched between the bottom of the strainer of the BCP well and the water level in the hole was calculated to be 6.8 × 106 m3, and if the effective porosity is 0.2 (JSCE, 1971), the amount of groundwater reserve of 1.4× 106 m3 was calculated. The average annual rainfall over the past 19 years at the Tottori Meteorological Observatory at Koyama was 1,653 mm. The area of the Tottori University Koyama Campus located on the old sand dunes is about 50 ha, and if the outflow coefficient, which is the ratio of rainwater that evaporates and drains without penetrating underground, is 0.9 (Ministry of Land, Infrastructure, Transport and Tourism, 2004), the amount of groundwater recharge is 8.3 x 106 m3 / yr. On the other hand, the average water usage for the past nine years (2011-2019) at Tottori University Koyama Campus was 87,786 m3. There is sufficient groundwater reserve for the use of BCP wells in emergencies such as disasters. However, in order to use sustainable groundwater, it may be necessary to increase the penetration rate of rainfall. In any case, continuous observation of groundwater level and water quality by monitoring wells is indispensable.
Conclusion
It was revealed that the old sand dunes in the basement of the Tottori University Koyama Campus have the role of a groundwater tank, and the amount is at least 1.4 million m3. Therefore, it can be said that the amount of groundwater is sufficient in an emergency such as a disaster. However, the amount of groundwater recharge may be slightly insufficient for that of water used. In order to consider the sustainable use of groundwater in the future, discussions based on continuous observation and monitoring of groundwater level and water quality will be necessary.