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[SCG43-P01] Effect of active faults on the water quality of the Ado river in Shiga Prefecture
Keywords:Ado river, Active fault, Water quality, Spring water
1.Introduction and Method
Along a fault plain, there is a fractured damage zone, which has high permeability. This zone is often become groundwater path from deep to shallow (Faulkner et al., 2010). As a result, the fault-related groundwater stably supplies heat and chemical materials including water itself, which make some effect on the environment in and around the fault zone. For example, hot and mineral springs tends to distribute in and around fault zones(Koizumi et al., 1985; Mino et al.,1985). Since river has a base flow from groundwater, an active fault along or across the river also can have some effects on the water quality of the river by the base flow. The active fault causes a large earthquake at a very low frequency such as once every several thousand to tens of thousands years and destroys the surrounding environment. However, during the period when the large earthquake or the fault movement does not occur, the active fault forms and maintains the environment through the groundwater.
The length of the Ado river, which is located in the western part of Shiga Prefecture, is 58km and the second longest in the rivers which flow into Lake Biwa, which is the largest lake in Japan. The catchment area of the Ado river is 300 km2 and the third largest in the rivers which flow into Lake Biwa (Shiga Prefecture, 2018). In the part of the upstream and most of the midstream, the Ado river runs along the Hanaore fault. In the downstream the Ado river crosses the Biwako-seigan fault(Fig.1). Both of the faults are the main active faults in the western part of the Shiga Prefecture. In addition the ado river basin is not well developped (Biwa basin study group, 2003). Therefore the Ado river is suitable for investigating the effect of active faults on the river water quality. There are some spring waters in the end of the alluvial fan of the Ado river. Hereafter they are called the fan spring water.
During the period from 1996 to 1998, the water quality of the downstream of the Ado river was firstly Ca-HCO3 type and later turned mainly into NaHCO3 type or rarely into Na-Cl type (Koizumi et al., 2019). Water quality of the hot springs along the Hanaore fault and Biwako-seigan fault around the Ado river is Na-HCO3 type and its concentration is several times to 10 times larger than that of river water (Koizumi et al., 2019). Therefore it is considered that the major cation should change from Ca2+ to Na+ when the effect of the hot spring waters through the faults increases.
We surveyed the Ado river during the period from August 12 to August 14 in 2018. Before this period there had been no precipitation for thirteen days. Therefore it is considered that the river water was mainly composed of the base flow. The results showed that the water quality of the Ado river was
Ca-HCO3 type from the upstream to the downstream at the period (Koizumi et al., 2019).
Considering these situations, we have regularly surveyed the Ado river and the fan springs once every 1-2 months since April or June, 2018 (Fig.1). We examine the spatio-temporal change in water quality of the Ado river and investigate the effect of the Hanaore and Biwako-seigan faults on it.
2.Results and Discussion
From August to September 2018, all ion concentrations decreased in the lower reaches of the Ado River. In particular, the Ca2 + concentration decreased significantly. As a result, the water quality type changed from Ca-HCO3 type to Na-HCO3 type.
In February 2020, we discovered that there were landslides and sediment deposits in the upper reaches of the Ado River (NKH-AD7 in Fig.1). We found that they were caused by Typhoon No. 18 on September 15-16, 2013 and Typhoon No. 21 on September 4-5, 2018. The place where there was a landslide or sedimentation was also the place where almost all the ion concentrations increased sharply in August 2018 (Koizumi et al.,2019).
The change in ion concentration in the lower reaches of the Ado River from August to September 2018 is considered to have been caused by heavy rainfall and landslides and sediment accumulation in the upper reaches of the Ado River caused by the Typhoon No.21 in 2018. In addition, the rapid increase in almost all the ion concentrations at the same location in August 2018 before the Typhoon No.21 may be due to the landslide and sediment accumulation caused by the Typhoon 18 in 2013.
These results show the effect of the typhoon on the water quality of the Ado River. However we did not obtained the result that the active faults had some effects on the water quality of the Ado River this time.
Fig.1: Study Area and sampling points. Solid circles and open squares show river water and fan spring water, respectively. Background map is from the active faults database of AIST(AIST, 2020).
Along a fault plain, there is a fractured damage zone, which has high permeability. This zone is often become groundwater path from deep to shallow (Faulkner et al., 2010). As a result, the fault-related groundwater stably supplies heat and chemical materials including water itself, which make some effect on the environment in and around the fault zone. For example, hot and mineral springs tends to distribute in and around fault zones(Koizumi et al., 1985; Mino et al.,1985). Since river has a base flow from groundwater, an active fault along or across the river also can have some effects on the water quality of the river by the base flow. The active fault causes a large earthquake at a very low frequency such as once every several thousand to tens of thousands years and destroys the surrounding environment. However, during the period when the large earthquake or the fault movement does not occur, the active fault forms and maintains the environment through the groundwater.
The length of the Ado river, which is located in the western part of Shiga Prefecture, is 58km and the second longest in the rivers which flow into Lake Biwa, which is the largest lake in Japan. The catchment area of the Ado river is 300 km2 and the third largest in the rivers which flow into Lake Biwa (Shiga Prefecture, 2018). In the part of the upstream and most of the midstream, the Ado river runs along the Hanaore fault. In the downstream the Ado river crosses the Biwako-seigan fault(Fig.1). Both of the faults are the main active faults in the western part of the Shiga Prefecture. In addition the ado river basin is not well developped (Biwa basin study group, 2003). Therefore the Ado river is suitable for investigating the effect of active faults on the river water quality. There are some spring waters in the end of the alluvial fan of the Ado river. Hereafter they are called the fan spring water.
During the period from 1996 to 1998, the water quality of the downstream of the Ado river was firstly Ca-HCO3 type and later turned mainly into NaHCO3 type or rarely into Na-Cl type (Koizumi et al., 2019). Water quality of the hot springs along the Hanaore fault and Biwako-seigan fault around the Ado river is Na-HCO3 type and its concentration is several times to 10 times larger than that of river water (Koizumi et al., 2019). Therefore it is considered that the major cation should change from Ca2+ to Na+ when the effect of the hot spring waters through the faults increases.
We surveyed the Ado river during the period from August 12 to August 14 in 2018. Before this period there had been no precipitation for thirteen days. Therefore it is considered that the river water was mainly composed of the base flow. The results showed that the water quality of the Ado river was
Ca-HCO3 type from the upstream to the downstream at the period (Koizumi et al., 2019).
Considering these situations, we have regularly surveyed the Ado river and the fan springs once every 1-2 months since April or June, 2018 (Fig.1). We examine the spatio-temporal change in water quality of the Ado river and investigate the effect of the Hanaore and Biwako-seigan faults on it.
2.Results and Discussion
From August to September 2018, all ion concentrations decreased in the lower reaches of the Ado River. In particular, the Ca2 + concentration decreased significantly. As a result, the water quality type changed from Ca-HCO3 type to Na-HCO3 type.
In February 2020, we discovered that there were landslides and sediment deposits in the upper reaches of the Ado River (NKH-AD7 in Fig.1). We found that they were caused by Typhoon No. 18 on September 15-16, 2013 and Typhoon No. 21 on September 4-5, 2018. The place where there was a landslide or sedimentation was also the place where almost all the ion concentrations increased sharply in August 2018 (Koizumi et al.,2019).
The change in ion concentration in the lower reaches of the Ado River from August to September 2018 is considered to have been caused by heavy rainfall and landslides and sediment accumulation in the upper reaches of the Ado River caused by the Typhoon No.21 in 2018. In addition, the rapid increase in almost all the ion concentrations at the same location in August 2018 before the Typhoon No.21 may be due to the landslide and sediment accumulation caused by the Typhoon 18 in 2013.
These results show the effect of the typhoon on the water quality of the Ado River. However we did not obtained the result that the active faults had some effects on the water quality of the Ado River this time.
Fig.1: Study Area and sampling points. Solid circles and open squares show river water and fan spring water, respectively. Background map is from the active faults database of AIST(AIST, 2020).