Characteristics of discharge and water chemistry in a sediment disaster area in Hofu, Yamaguchi prefecture, western Japan
Masamitsu Fujimoto1,2*, Takahisa Mizuyama2and Maki Sekiguchi3
1Colleage of Science and Civil Engineering, Ritsumekan University, Japan 2Graduate School of Agriculture, Kyoto University, Japan
3Department of Agriculture, Kyoto University Japan
*Corresponding author. E-mail: m-fuji@fc.ritsumei.ac.jp
INTRODUCTION
Studies have found that bedrock groundwater exfiltration affects the occurrence of landslides.
Several studies have reported the existence of bedrock springs at landslide scars. By evaluating the spatial characteristics of discharge and stream water chemistry (mainly bedrock groundwater), it should be possible to identify hazardous locations. We evaluated the spatial characteristics of discharge and stream water chemistry in sediment disaster and non-disaster areas.
METHODS
The investigation was conducted in the Sawagawa catchment, in Hofu, Yamaguchi Prefecture, western Japan. Forty-seven sites were investigated: 36 in the sediment discharge area, nine at non-disaster locations, one spring on a collapsed slope, and one spring on a natural slope. We sampled spring and stream water for chemical analysis and measured the discharge rate during base-flow conditions at sampling points with different catchment sizes.
Stream flow velocity was measured with an electromagnetic velocity meter. Discharge was calculated by multiplying the cross-sectional area by stream velocity. At low-discharge points (<1 L/s), discharge was measured directly by collecting water using a measuring cylinder and stopwatch. The Na+ and Ca2+ concentrations were measured by ion chromatography (DIONEX, ICS-90), while SiO2 concentrations were determined using the molybdenum yellow method on a Hitachi U-1000.
RESULTS AND DISCUSSIONS
There was no clear relationship between catchment area and specific discharge (Fig. 1).
The specific discharge in disaster catchments tended to be higher than at non-disaster catchments.
As the Na+ concentration of spring and streamwater increased, so did the SiO2
concentration (Fig. 2). Although the Ca2+ concentration of spring and bedrock groundwater also increased with the SiO2 concentration, the relationship between the concentrations was highly variable. Generally, Na+, Ca2+, and SiO2 are produced by the chemical weathering of bedrock. Although the ratio of Na+ to SiO2 did not differ markedly among catchments, the variability in the Ca2+ concentration reflected differences in the chemical components or weathering of bedrock. In addition, the concentrations were lower in non-disaster catchments than in disaster catchments, indicating that bedrock weathering was greater in the disaster catchments.
The Na+, Ca2+, and SiO2 concentrations of streamwater clearly increased with the specific discharge (Fig. 3). These results indicated that the high specific discharge of catchments was largely due to groundwater that had infiltrated deep into bedrock. The difference in specific discharge and chemistry between disaster and non-disaster catchments suggested that it is possible to evaluate the contributions of bedrock groundwater and weathering conditions in a catchment.
Catchment area (ha)
0.1 1 10 100 1000
Specific discharge (mm/day)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Disaster point Non-disaster point
Spring from landslide hillslope
Na+(mg/L)
4 5 6 7 8 9 10
Si(mg/L)
4 5 6 7 8 9 10 11 12
Ca2+(mg/L)
0 1 2 3 4 5 6 7 8
Si(mg/L)
4 5 6 7 8 9 10 11 12
Fig. 1 Relationship between catchment Fig. 2 Characteristics of streamwater and spring chemistry area and specific discharge (the legend is the same as in Fig. 1)
Specific discharge (mm/day) 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Si (mg/L)
4 6 8 10 12
Specific discharge (mm/day) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Na+ (mg/L)
4 5 6 7 8 9 10
Specific discharge (mm/day) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Ca2+ (mg/L)
0 2 4 6 8
Fig. 3 Relationships between the discharge and chemistryof streamwater and spring (the legend is the same as in Fig. 1)
CONCLUSIONS
Hydrological and hydrochemical observations were conducted in Hofu, Yamaguchi Prefecture where a landslide and debris flow occurred in July 2009. The specific discharge was larger in landslide catchments than in non-landslide catchments. The chemical characteristics of springwater and streamwater in the observed catchments varied due to differences in bedrock weathering. Na+ and Ca2+
concentrations were lower in landslide catchments than in non-landslide catchments, suggesting that the concentrations of these chemical components are good indicators of the level of risk of collapse.
Our study suggests the possibility of predicting areas that are likely to collapse based on hydrological and hydrochemical investigations.
Keywords: Sediment discharge, specific discharge, water chemistry, bedrock groundwater, weathering
