10:45 〜 12:15
[AHW19-P12] The geochemical characteristics of groundwater in the Hondo-Shiromawari area, Misato Town, Akita, Japan
キーワード：groundwater, source, contamination, chemical composition, Holocene, Akita
The Hondo-Shiromawari area is located north of the Rokugo area in Akita Prefecture and has many springs, as well as the Rokugo area. The study area consists of a hilly area, an alluvial fan area, and a plain area from west to east. The elevation of these areas also decreases from the hilly area to the plain area. The Mahiru River and Akakura River are the boundaries of this area. In addition to these features, the Tazawa irrigation canal from the Tama River runs north to south near the top of the alluvial fan. The main agricultural activity in the hilly area is the production of vegetables, while the main activity in the alluvial fan and the plain area is rice production in rice paddies. The cultivated area in the hilly area is much smaller than that in the alluvial fan and plain area. The population of the hilly area is also smaller than the alluvial fan and plain areas.
This study aims to clarify the source of groundwater and assess the present status of groundwater contamination based on the geochemical characteristics of groundwater and surface water in the Hondo-Shiromawari area.
Field surveys were carried out seven times in the area from May to November of 2022, and a total of 11 samples (seven spring waters (hilly area: Kogane-shimizu, alluvial fan area: Oh-shimizu, Nogiwa-shimizu, Furuyashiki-shimizu (pipe & pond), plain area: Hoidosuzu well, Hayasaka spring), two river waters (the Akakura River and the Mahiru River), irrigation water (Tazawa irrigation canal), and precipitation) were collected each time.
Based on the stiff diagram and piper diagram, Kogane-shimizu in the hilly area and Oh-shimizu and Furuyashiki-shimizu in the alluvial fan area were classified into Ca-Mg-Cl type. On the other hand, Nogiwa-shimizu is classified into Ca-HCO3 type, even though Nogiwa-shimizu is located between Oh-shimizu and Furuyashiki-shimizu at a short distance. The chemical compositions of groundwater of the Oh-shimizu and Furuyashiki-shimizu are plotted along the mixing line between the Mahiru River and Akakura River, mainly close to the chemical composition of the Mahiru River. Therefore, the major source of Oh-shimizu and Furuyashiki-shimizu is considered to be river water of the Akakura River and the Mahiru River.
The geochemical composition of the Hoidosuzu well in the plain area is classified into Na-Cl-SO4 type and is similar to that of precipitation. The groundwater of Hayasaka has a relatively high bicarbonate concentration, but the Na/Cl ratio is similar to that of precipitation in the Hondo-Shiromawari area. Based on these facts, the source of groundwater of the Hoidosuzu well and Hayasaka spring is considered to be local precipitation.
The effect of agricultural activities was evaluated based on the NO3 concentrations. The NO3 concentrations of groundwater of the Oh-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu in the alluvial fan area (0.09 to 0.13 mmoL) and Kogane-shimizu in the hilly area (0.06-0.12 mmoL) are higher than that of river water of the Mahiru River and Akakura River (0.01 to 0.03 mmoL). On the other hand, The NO3 concentrations of groundwater of Hoidosuzu well and Hayasaka spring (0.01 to 0.03 mmoL) are similar to river water. The high NO3 concentrations in the groundwater in the alluvial fan area show no seasonal variation. In contrast, the groundwater of Kogane-shimizu in the hilly area decreases from the busy farming season (0.09 to 0.12 mmoL) to the agricultural off-season (0.06 mmoL). Based on the decreases in NO3 concentrations at the Kogane-shimizu, the high NO3 concentrations in the groundwater in the alluvial fan area seems to be affected by agricultural activities.
This study aims to clarify the source of groundwater and assess the present status of groundwater contamination based on the geochemical characteristics of groundwater and surface water in the Hondo-Shiromawari area.
Field surveys were carried out seven times in the area from May to November of 2022, and a total of 11 samples (seven spring waters (hilly area: Kogane-shimizu, alluvial fan area: Oh-shimizu, Nogiwa-shimizu, Furuyashiki-shimizu (pipe & pond), plain area: Hoidosuzu well, Hayasaka spring), two river waters (the Akakura River and the Mahiru River), irrigation water (Tazawa irrigation canal), and precipitation) were collected each time.
Based on the stiff diagram and piper diagram, Kogane-shimizu in the hilly area and Oh-shimizu and Furuyashiki-shimizu in the alluvial fan area were classified into Ca-Mg-Cl type. On the other hand, Nogiwa-shimizu is classified into Ca-HCO3 type, even though Nogiwa-shimizu is located between Oh-shimizu and Furuyashiki-shimizu at a short distance. The chemical compositions of groundwater of the Oh-shimizu and Furuyashiki-shimizu are plotted along the mixing line between the Mahiru River and Akakura River, mainly close to the chemical composition of the Mahiru River. Therefore, the major source of Oh-shimizu and Furuyashiki-shimizu is considered to be river water of the Akakura River and the Mahiru River.
The geochemical composition of the Hoidosuzu well in the plain area is classified into Na-Cl-SO4 type and is similar to that of precipitation. The groundwater of Hayasaka has a relatively high bicarbonate concentration, but the Na/Cl ratio is similar to that of precipitation in the Hondo-Shiromawari area. Based on these facts, the source of groundwater of the Hoidosuzu well and Hayasaka spring is considered to be local precipitation.
The effect of agricultural activities was evaluated based on the NO3 concentrations. The NO3 concentrations of groundwater of the Oh-shimizu, Nogiwa-shimizu, and Furuyashiki-shimizu in the alluvial fan area (0.09 to 0.13 mmoL) and Kogane-shimizu in the hilly area (0.06-0.12 mmoL) are higher than that of river water of the Mahiru River and Akakura River (0.01 to 0.03 mmoL). On the other hand, The NO3 concentrations of groundwater of Hoidosuzu well and Hayasaka spring (0.01 to 0.03 mmoL) are similar to river water. The high NO3 concentrations in the groundwater in the alluvial fan area show no seasonal variation. In contrast, the groundwater of Kogane-shimizu in the hilly area decreases from the busy farming season (0.09 to 0.12 mmoL) to the agricultural off-season (0.06 mmoL). Based on the decreases in NO3 concentrations at the Kogane-shimizu, the high NO3 concentrations in the groundwater in the alluvial fan area seems to be affected by agricultural activities.