2:30 PM - 2:45 PM
[AHW28-15] Groundwater aquifer types and possible sources in the Hondo-Shiromawari area, Misato Town, Akita Prefecture, Japan
Keywords:groundwater, Akita, source, chemical composition, aquifer type, Rokugo area
The study area, the Hondo-Shiromawari area, is located in the northeast of the Rokugo district in Misato Township, Akita Prefecture. The area is surrounded by four main rivers: the Mahiru River, Akakura River, Kawaguchi River, and Maruko River. Furthermore, the study area is divided into three major domains (hilly, alluvial fan, and plain domains). The hilly domain has high elevations, while the plain domain has low elevations. The alluvial fan domain occurs between the Mahiru and Akakura Rivers. There is the Tazawa Irrigation Canal for agriculture, which runs from north to south between the hilly and alluvial fan domains. The hilly domain is mainly covered by vegetable fields, whereas rice paddy fields widely cover the alluvial fan and plain domains. The water samples were collected from the following a total of 11 sites: Hoidosuzu Well and Hayasaka Spring (plain domain), Oo-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu Springs (pipe & pond) (alluvial fan domain), Kogane-shimizu Spring (hilly domain), two river water (Akakura and Mahiru Rivers), Tazawa Irrigation Canal, and precipitation by using a collected monthly from spring to fall in tank.
This study aims to clarify the types of aquifers of each groundwater of the three domains, and the sources of groundwater in those domains in the Hondo-Shiromawari area.
Kogane-shimizu Spring (hilly domain) fed by an large unconfined aquifer based on a quick response to flow and precipitation changes. Its major recharge source is Akakura River with small contribution of Mahiru River. The Tazawa Irrigation Canal has no effect due to the high elevation of this spring in hilly domain.
In the alluvial fan domain, the aquifer types of groundwater are divided into confined and unconfined aquifers. Furuyashiki-shimizu Spring (pipe) is an artesian well from the confined aquifer. Oo-shimizu Spring is also thought to be derived from a confined aquifer based on the small variations of chemical compositions and water level. Nogiwa-shimizu Spring and Furuyashiki-shimizu Spring (pond), in contrast, show large variation in temperature, water level, and chemical compositions, suggesting an unconfined aquifer. Based on the chemical compositions, the groundwater from the Oo-shimizu and Furuyashiki-shimizu (pond) mainly recharged from the Mahiru River. The groundwater from Oo-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu (pipe & pond) have similar (Ca+Mg)/(Na+K) ratios. However, the groundwater from the Nogiwa-shimizu has highest HCO3- concentration, while the groundwater from the Furuyashiki-shimizu (pipe) has the lowest HCO3- concentration. The groundwater from the Nogiwa-shimizu and Furuyashiki-shimizu (pipe) may be the two different end members of groundwater in the alluvial fan domain.
The groundwater at the Hayasaka Spring and Hoidosuzu Well in the plain domain are Na-Cl-SO4 type, which is similar to the chemical composition of local precipitation in this domain. These groundwaters are the different from groundwater in the alluvial fan domain, which is Ca-Mg-Cl type. In the plain domain, the main source of groundwater is precipitation, indicating recharge from an unconfined aquifer, replenished by precipitation.
According to the hydrogen and oxygen isotopic ratios of water, the Mahiru River shows a high d-excess value (21-26), close to winter precipitation (25) in Akita Prefecture, whereas local precipitation has a relatively low d-excess value (11-16). The groundwater from Furuyashiki-shimizu (pipe) shows a lower d-excess value (15–17) compared to other groundwaters in the alluvial fan domain. In contrast, the d-excess value of the groundwater from Oo-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu (pond) are close to the d-excess value of the Mahiru River, suggesting that the groundwaters in the alluvial fan domain were effected from Mahiru River. The characteristics is consistent with the idea that source of Oo-shimizu is mainly the Mahiru River based on chemical compositions.
This study aims to clarify the types of aquifers of each groundwater of the three domains, and the sources of groundwater in those domains in the Hondo-Shiromawari area.
Kogane-shimizu Spring (hilly domain) fed by an large unconfined aquifer based on a quick response to flow and precipitation changes. Its major recharge source is Akakura River with small contribution of Mahiru River. The Tazawa Irrigation Canal has no effect due to the high elevation of this spring in hilly domain.
In the alluvial fan domain, the aquifer types of groundwater are divided into confined and unconfined aquifers. Furuyashiki-shimizu Spring (pipe) is an artesian well from the confined aquifer. Oo-shimizu Spring is also thought to be derived from a confined aquifer based on the small variations of chemical compositions and water level. Nogiwa-shimizu Spring and Furuyashiki-shimizu Spring (pond), in contrast, show large variation in temperature, water level, and chemical compositions, suggesting an unconfined aquifer. Based on the chemical compositions, the groundwater from the Oo-shimizu and Furuyashiki-shimizu (pond) mainly recharged from the Mahiru River. The groundwater from Oo-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu (pipe & pond) have similar (Ca+Mg)/(Na+K) ratios. However, the groundwater from the Nogiwa-shimizu has highest HCO3- concentration, while the groundwater from the Furuyashiki-shimizu (pipe) has the lowest HCO3- concentration. The groundwater from the Nogiwa-shimizu and Furuyashiki-shimizu (pipe) may be the two different end members of groundwater in the alluvial fan domain.
The groundwater at the Hayasaka Spring and Hoidosuzu Well in the plain domain are Na-Cl-SO4 type, which is similar to the chemical composition of local precipitation in this domain. These groundwaters are the different from groundwater in the alluvial fan domain, which is Ca-Mg-Cl type. In the plain domain, the main source of groundwater is precipitation, indicating recharge from an unconfined aquifer, replenished by precipitation.
According to the hydrogen and oxygen isotopic ratios of water, the Mahiru River shows a high d-excess value (21-26), close to winter precipitation (25) in Akita Prefecture, whereas local precipitation has a relatively low d-excess value (11-16). The groundwater from Furuyashiki-shimizu (pipe) shows a lower d-excess value (15–17) compared to other groundwaters in the alluvial fan domain. In contrast, the d-excess value of the groundwater from Oo-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu (pond) are close to the d-excess value of the Mahiru River, suggesting that the groundwaters in the alluvial fan domain were effected from Mahiru River. The characteristics is consistent with the idea that source of Oo-shimizu is mainly the Mahiru River based on chemical compositions.