Analysis of Spatial Characteristics for Groundwater Flow in Okimi Landslide Area

This paper deals with the study on hydrological origins of groundwater runoff that were measured at two small basins in Okimi landslide in Southern Niigata Prefecture, Japan. The study shows the appreciable contribution of high groundwater runoff to the landslide movement that has been depicted by the runoff observation at the two small basins A and B. The observation of groundwater runoff was carried out for over one year. The following results were obtained from the study: 1) Chemical analysis on components of groundwater shows significant difference of chemical composition of groundwater between the two small basins A and B. Further, there are a wide variety of chemical compositions among waters from the drainage well, drainage bore holes and small torrents. 2) Origins of groundwater in Okimi landslide area were estimated by using “principal component method”. Results of the analysis shows that there are two types of water from different origins. One is originated from surface water which has low ion density. And the other is originated from groundwater which has high ion density. As a result of separation of hydrograph components, in A basin 69% of the total groundwater runoff consists of groundwater and in B basin 64% of the total groundwater runoff consists of groundwater. 3) As a result of separation of hydrograph components by using ∂^<18>O method, it was estimated that in A basin 62% of the total runoff consists of groundwater and in B basin 54% of the total runoff consists of groundwater. 4) As a result of separation of hydrograph components by using electric conductivity method, it was estimated that in A basin 74% of the total runoff consists of groundwater and in B basin 64% of the total runoff consists of groundwater during snow melt season. Electric conductivity method is a simple and useful method to separate the hydrograph components of groundwater runoff. 5) As a result of separation of hydrograph components by using electric conductivity method, it was estimated that in both A and B basins contribution of runoff to groundwater during summer rainy season is higher than that during snow melt season. In summer rainy season 95-96% of the runoff consists of groundwater. During snow melt season almost constant amount of groundwater runoff was observed continuously. On the other hand, during summer rainy season significantly high amount of runoff was observed. The different characteristics of landslide movement between snow melt season and summer rainy season might be due to the different behaviour of deep groundwater between the two seasons