electrical-conductivity- distribution measurement using
ELECTRICAL-CONDUCTIVITYDISTRIBUTION MEASUREMENT USING AN ELECTROMAGNETIC SURVEY OF PADDY FIELDS DAMAGED BY TSUNAMI K. YAMAMOTO*, A. KOBAYASHI**, K. HARASHINA*, Y. MUTO*, E. KURASHIMA* *Faculty of Agriculture, Iwate University, Ueda 3-18-8, Morioka, Iwate, 020-8550 Japan **Faculty of Environmental and Urban Engineering, Kansai University, Yamate-machi 33-35, Suita, Osaka, 564-8680 Japan
SUMMARY: A large portion of coastal farmland was damaged by the 2011 Tohoku earthquake and tsunami. This study details the electrical-conductivity distributions measured in an electromagnetic survey of paddy fields conducted in Rikuzentakata City, Iwate Prefecture, Japan, from 2012 to 2016. Desalinization of paddy fields was considered by comparing the measurement results obtained before and after the construction of an embankment. In addition, variations in electrical conductivity were studied to investigate the soundness of paddy fields after the construction of the embankment. The following results were obtained: (1) before the construction of the embankment, the salinity concentration of the surface soil decreased between September 2012 and May 2013 over the whole study area; (2) after the construction of the embankment, salinity remained relatively high in the areas near the drainage canals at a depth of 2.25 m, corresponding to the bottom of the embankment; and (3) after the construction of the embankment, the electrical conductivity of the surface soil in 2015 tended to be lower than that measured in 2014. It was inferred that this phenomenon is related to the decrease in crop yield.
1. INTRODUCTION In the coastal farmland damaged by the 2011 Tohoku earthquake and tsunami, seawater may flow owing to ground subsidence during high tide. In such regions, embankments have been constructed in order to improve drainage. The largest area of damaged farmland in Iwate Prefecture was near Rikuzentakata city. The purpose of this study is to investigate the changes in this region’s salinity concentration over a long period. When the salinity concentration in the soil is higher than that inside root cells, the soil will draw water from the root; consequently, the plant will wilt and die (FAO, 2005). Therefore, it is important to evaluate the salinity concentration of soil water in order to understand the effect of desalinization on agricultural production. In recent years, electromagnetic surveys have been widely used to evaluate farmland (Herrero 2014; Li 2013; McLeod 2010; Mohammad 2011; Somura 2015). This technique
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
involves geophysical exploration for noncontact measurement of apparent electrical conductivity (EC), which increases with salinity. The salinity concentration of underground pore water in the Kujukurihama Plain was estimated based on the results of an electromagnetic survey (Mitsuhata 2006a). Moreover, it has been reported that investigations of salt damage can be conducted via electromagnetic surveys using a soil EC sensor in tsunami-inundated farmland (Kanmuri 2012). In this study, the distributions of EC were measured on the basis of an electromagnetic survey of paddy fields conducted from 2012 to 2016. Desalinization of a paddy field was considered by comparing the measurement results obtained before and after the construction of an embankment. Changes in the EC were also studied to investigate the soundness of the paddy field after the construction of the embankment.
2. MATERIALS AND METHODS 2.1 Study area The study area is located at the base of the Hirota Peninsula near Rikuzentakata City, Iwate Prefecture. Figure 1 shows a schematic of the paddy fields. Hereafter, the position of the fields will be indicated by the coordinates in Figure 1. Figure 2 shows a location map of the study area. This area was submerged in seawater owing to the 2011 Tohoku tsunami and ground subsidence. Then, rubble and sediment piled up. Rainwater temporarily covered the paddy fields, before being drained through the canal. Moreover, seawater flowed into some of the paddy fields from the sea at high tide through the drainage canal, and locally, the salinity concentration of the soil in such areas was inferred to be high. Repair work was started in 2013 by constructing an embankment. The embankment soil and surface soil were recycled from the rubble and sediment of the 2011 Tohoku tsunami. Figure 3(a) shows the grading curve of the surface soil before the construction of the embankment, and Figure 3(b) shows the grading curve of the surface soil after the construction of the embankment. The heights of the embankment obtained using global positioning system (GPS) measurements are shown Table 1. Farming was resumed in 2014; rice was produced in the paddy fields, and the crop yield in 2015 was lesser than that in 2014. 2.2 Field measurements and data processing EC was measured by dead reckoning using GEM-2 (Figure 4; Geophex Ltd., USA), a device commonly used in electromagnetic surveys. The measurement lines have a length of 100 m, （342m,113m）
（262m,113m） （250m,100m）
Drainage canal （x=85m）
（0m,100m）
y x
Field A Field B
Drainage canal（ x=345m） （342m, 13m） （262m,13m）
（250m,0m） Drainage canal（x=255m）
Figure 1. Schematic view of the paddy fields.
（0m,0m）
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Study area (Paddy field)
(km)
1.7
0.7
1.1
0.52 0.52
2.1
0.57
1.3
Study area (Paddy field)
0.60
1.4
0.59 0.60
6.2
1.16
3.8
7.4
1.3
0.75 0.63
0.70
0.79 0.49
1.21
0.67
0.7
0.76
1.5 0.79
1.1
1.8
1.6
0.75
1.9 1.09
1.11
1.23
2.8 2.1 1.65
28.3
2.6
1.2 1.08
1.98 2.10
2.5 1.56 44.3 2.59 2.7
8.7
10.3
11.1
5.4
3.3 4.3
24.7
Elevation (m) before the construction of the embankment
28.6 40.7
12.2 0
23.6
4.3
5.5
100m
3.0
200m
100 80
(0m, 40m) (250m, 40m) (343m, 10m)
Pecentage of passing (%)
Pecentage of passing (%)
Figure 2. Map of the study area. (Image date: June 27, 2012, Google Earth)
60 40 20 0 0,001
(a) 0,01
0,1 1 Perticle size (mm)
10
100 80
(0m, 40m) (240m, 40m) (342m, 40m)
60 40 20 0 0,001
(b) 0,01
0,1 1 Perticle size (mm)
10
Figure 3. Grading curves of (a) the surface soil before the construction of the embankment and (b) the surface soil after the construction of the embankment.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Height of the embankment Position Height x y (m) (m) (m) 0 40 1.76 83 40 1.82 167 40 2.39 254 40 1.81 0 0 1.62 100 0 2.83 200 0 2.92 262 13 2.42 344 13 2.34
GEM-2 Line length Line spacing
Measurement line
Figure 4. The paddy field during measurement with GEM-2
and the spacing between them is 10 m. There are nine measurement lines in Field A and 26 in Field B. The data were recorded using GEM-2 at seven discrete frequencies (2,025 Hz, 3,675 Hz, 6,525 Hz, 11,625 Hz, 20,625 Hz, 36,625 Hz, and 65,025 Hz) and processed using an analytical program (Mitsuhata 2006b) to obtain a one-dimensional vertical EC distribution. The EC data at a depth of 0.025 m (surface soil) and 2.25 m (bottom of the embankment) were extracted from the vertical distribution and reconstructed to find the EC distribution at each depth. Here, values ≥1,000 mS/m were excluded to remove noise from the data.
3. RESULTS AND DISCUSSION 3.1 Surface soil before the construction of the embankment The EC distributions in the surface soil before the construction of the embankment are shown in Figures 5 and 6. The area in which EC is above 240 mS/m is large in Field A in 2012 (Figure 5). On the other hand, by 2013, the area in which EC exceeds 120 mS/m locally in Field A has become much smaller (Figure 6). The high-EC area is considered to have decreased from 2012 to 2013 because the inflow of seawater during high tide was suppressed more in 2013 than in 2012 because of repair work on the drainage canals. Hence, we infer that the salinity concentration of Field A decreased from 2012 to 2013; however, a high salinity concentration was observed in 2013 locally. In Field B of Figure 5, there is a high-EC area near x = 70 m. It is inferred that the salinity concentration increased owing to an inflow of seawater from drainage canals and leaching of the water from high-salinity soil upstream. The EC distributions are above 90 mS/m in most areas of Field B (Figure 5). In Field B of Figure 6, EC decreased over the whole area; however, EC remained high (30–90 mS/m) in areas where it was high in 2012. From the above results, it can be inferred that the salinity concentrations of Fields A and B decreased from 2012 to 2013.. 3.2 Bottom of the embankment The EC distributions at a depth of 2.25 m after the construction of the embankment are shown in Figures 7–9. Although there are areas where EC is more than 35 mS/m in Field A of 2014 (Figure 7), there are many areas where EC is less than 20 mS/m in this field in 2015 and
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
30
60
90
113
100
93
80 y（m）
y（m）
0
73 53
150
180
210
(mS/m)
240
60 40 20
33 13 342
120
0 250
262 302 x（m）
200
150
50
100 x（m）
0
Figure 5. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, September 2012)
30
60
90
113
100
93
80 y（m）
y（m）
0
73 53
120
150
180
210
240
(mS/m)
60 40 20
33
0 250 200 150 100 50 262 302 x（m） x（m） Figure 6. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, May 2013) 13 342
10
20
113
100
93
80
73 53 33
y（m）
y（m）
0
30
40
50
60
(mS/m)
60 40 20
0 250 200 150 100 50 262 302 x（m） x（m） Figure 7. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 2.24 m, November 2014) 13 342
0
0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2016 (Figures 8 and 9). EC decreased from 2014 to 2016 in Field B, except at x = 80 m (Figures 7–9). There were high-EC areas near x = 80 m in 2014 and 2015. However, these areas disappeared by 2016. This behavior is considered to have been affected by the old and new drainage canals located at x = 85 m. 3.3 Surface soil after the construction of the embankment The EC distributions at a depth 0.025 m after the construction of the embankment are shown in Figures 10–12. The high-EC areas of the surface soil in Fields A and B decreased from 2014 to 2016. The crop yield was reported to decrease from 2014 to 2015 in districts that managed irrigation, including the study area. The ion concentration in the study area is considered to have decreased from 2014 to 2015 because the ion concentration of soil water is related to EC. It is inferred that this phenomenon is related to the decrease in crop yield.
4. CONCLUSIONS EC distributions were evaluated via an electromagnetic survey of paddy fields conducted in
10
20
113
100
93
80 y（m）
y（m）
0
73 53
30
40
50
60
(mS/m)
60 40 20
33
0 250 200 150 100 50 262 302 x（m） x（m） Figure 8. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 2.24m, November 2015) 13 342
10
20
113
100
93
80
73 53 33
y（m）
y（m）
0
30
40
50
60
(mS/m)
60 40 20
0 250 200 150 100 50 262 302 x（m） x（m） Figure 9. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 2.24 m, November 2016) 13 342
0
0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
10
20
113
100
93
80 y（m）
y（m）
0
73 53
30
40
50
60
(mS/m)
60 40 20
33
0 250 50 0 262 200 150 100 302 x（m） x（m） Figure 10. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, November 2014) 13 342
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20
113
100
93
80 y（m）
y（m）
0
73 53
40
50
60
(mS/m)
60 40 20
33 13 342
30
0
262 302 x（m）
250
200
150
100 x（m）
50
0
Figure 11. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, November 2015)
10
20
113
100
93
80
73 53 33
y（m）
y（m）
0
30
40
50
60
(mS/m)
60 40 20
0 250 200 150 100 50 0 262 302 x（m） x（m） Figure 12. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, November 2016) 13 342
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Rikuzentakata City, Iwate Prefecture, Japan, between 2012 and 2016. These paddy fields were damaged by the 2011 Tohoku earthquake and tsunami. Based on the relationship between EC and salinity concentration, desalinization of the paddy fields was considered by comparing the measurement results obtained before and after the construction of an embankment. Changes in EC were also analyzed to investigate the soundness of paddy fields after the construction of the embankment. The following conclusions were drawn. 1) Before the construction of the embankment, the salinity of surface soil decreased between September 2012 and May 2013 over the whole study area. However, certain areas with a high salinity concentration were present locally. 2) After the construction of the embankment, salinity remained relatively high in the areas near the old and new drainage canals, as indicated by the EC measured at a depth of 2.25 m, corresponding to the bottom of the embankment near the canal. 3) After the construction of the embankment, the EC of the surface soil in 2015 tended to be lower than that in 2014. This phenomenon is considered to be related to the decrease in crop yield.
ACKNOWLEDGEMENTS This study was partially supported by the JFE 21st Century Foundation.
REFERENCES FAO (United Nations Food and Agriculture Organization) (2005). The impact of salt water on agricultural land in Aceh Province. FAO Field guide Herrero, J. and Hudnall, W. H. (2014). Measurement of soil salinity using electromagnetic induction in a paddy with a densic pan and shallow water table. Paddy and Water Environment, 12, 263–274 Kanmuri, H., Sekiya H., Yusa, T., and Otani, R. (2012). Rapid survey method for soil electrical conductivity in tsunami‒inundated farmlands using electromagnetic measurements. Journal of the Japanese Society of Soil Physics, 121, 19–28 (in Japanese with English abstract) Li., H.Y., Shi, Z., Webster, R. and Triantafilis, J. (2013). Mapping the three-dimensional variation of soil salinity in a rice-paddy soil. Geoderma, 195–196, 31–41 McLeod, M.K., Slavich, P.G., Irhas,Y., Moore,N., Rachman,A., Ali. N., Iskandar, T., Hunt, C. and Caniago, C. (2010). Soil salinity in Aceh after the December 2004 Indian Ocean tsunami. Agricultural Water Management, 97, 605–613 Mitsuhata, Y. (2006a). Electromagnetic Investigation of High-salinity Groundwater Zones in Coastal Plains -A Case Study of the Kujukuri Coastal Plain-. Journal of Geography (Chigaku Zasshi), 115, 416–424 (in Japanese with English abstract) Mitsuhata, Y., Uchida, T., Matsuo, K., Marui, A. and Kusunose, K. (2006b). Various-scale electromagnetic investigations of high-salinity zones in a coastal plain. Geophysics, 71, B167B173 Mohammad Monirul Islam, Liesbet Cockx, Eef Meerschman, Philippe De Smedt, Fun Meeuws and Marc Van Meirvenne (2011). A floating sensing system to evaluate soil and crop variability within flooded paddy rice fields. Precision Agriculture, 12, 850–859 Somura, H., Masunaga, T., Mori, Y. Takeda, I., Ide, J. and Sato, H. (2015). Estimation of nutrient input by a migratory bird, the Tundra Swan (Cygnus columbianus), to winter-flooded paddy fields. Agriculture, Ecosystems and Environment, 199, 1–9
SUMMARY: A large portion of coastal farmland was damaged by the 2011 Tohoku earthquake and tsunami. This study details the electrical-conductivity distributions measured in an electromagnetic survey of paddy fields conducted in Rikuzentakata City, Iwate Prefecture, Japan, from 2012 to 2016. Desalinization of paddy fields was considered by comparing the measurement results obtained before and after the construction of an embankment. In addition, variations in electrical conductivity were studied to investigate the soundness of paddy fields after the construction of the embankment. The following results were obtained: (1) before the construction of the embankment, the salinity concentration of the surface soil decreased between September 2012 and May 2013 over the whole study area; (2) after the construction of the embankment, salinity remained relatively high in the areas near the drainage canals at a depth of 2.25 m, corresponding to the bottom of the embankment; and (3) after the construction of the embankment, the electrical conductivity of the surface soil in 2015 tended to be lower than that measured in 2014. It was inferred that this phenomenon is related to the decrease in crop yield.
1. INTRODUCTION In the coastal farmland damaged by the 2011 Tohoku earthquake and tsunami, seawater may flow owing to ground subsidence during high tide. In such regions, embankments have been constructed in order to improve drainage. The largest area of damaged farmland in Iwate Prefecture was near Rikuzentakata city. The purpose of this study is to investigate the changes in this region’s salinity concentration over a long period. When the salinity concentration in the soil is higher than that inside root cells, the soil will draw water from the root; consequently, the plant will wilt and die (FAO, 2005). Therefore, it is important to evaluate the salinity concentration of soil water in order to understand the effect of desalinization on agricultural production. In recent years, electromagnetic surveys have been widely used to evaluate farmland (Herrero 2014; Li 2013; McLeod 2010; Mohammad 2011; Somura 2015). This technique
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
involves geophysical exploration for noncontact measurement of apparent electrical conductivity (EC), which increases with salinity. The salinity concentration of underground pore water in the Kujukurihama Plain was estimated based on the results of an electromagnetic survey (Mitsuhata 2006a). Moreover, it has been reported that investigations of salt damage can be conducted via electromagnetic surveys using a soil EC sensor in tsunami-inundated farmland (Kanmuri 2012). In this study, the distributions of EC were measured on the basis of an electromagnetic survey of paddy fields conducted from 2012 to 2016. Desalinization of a paddy field was considered by comparing the measurement results obtained before and after the construction of an embankment. Changes in the EC were also studied to investigate the soundness of the paddy field after the construction of the embankment.
2. MATERIALS AND METHODS 2.1 Study area The study area is located at the base of the Hirota Peninsula near Rikuzentakata City, Iwate Prefecture. Figure 1 shows a schematic of the paddy fields. Hereafter, the position of the fields will be indicated by the coordinates in Figure 1. Figure 2 shows a location map of the study area. This area was submerged in seawater owing to the 2011 Tohoku tsunami and ground subsidence. Then, rubble and sediment piled up. Rainwater temporarily covered the paddy fields, before being drained through the canal. Moreover, seawater flowed into some of the paddy fields from the sea at high tide through the drainage canal, and locally, the salinity concentration of the soil in such areas was inferred to be high. Repair work was started in 2013 by constructing an embankment. The embankment soil and surface soil were recycled from the rubble and sediment of the 2011 Tohoku tsunami. Figure 3(a) shows the grading curve of the surface soil before the construction of the embankment, and Figure 3(b) shows the grading curve of the surface soil after the construction of the embankment. The heights of the embankment obtained using global positioning system (GPS) measurements are shown Table 1. Farming was resumed in 2014; rice was produced in the paddy fields, and the crop yield in 2015 was lesser than that in 2014. 2.2 Field measurements and data processing EC was measured by dead reckoning using GEM-2 (Figure 4; Geophex Ltd., USA), a device commonly used in electromagnetic surveys. The measurement lines have a length of 100 m, （342m,113m）
（262m,113m） （250m,100m）
Drainage canal （x=85m）
（0m,100m）
y x
Field A Field B
Drainage canal（ x=345m） （342m, 13m） （262m,13m）
（250m,0m） Drainage canal（x=255m）
Figure 1. Schematic view of the paddy fields.
（0m,0m）
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Study area (Paddy field)
(km)
1.7
0.7
1.1
0.52 0.52
2.1
0.57
1.3
Study area (Paddy field)
0.60
1.4
0.59 0.60
6.2
1.16
3.8
7.4
1.3
0.75 0.63
0.70
0.79 0.49
1.21
0.67
0.7
0.76
1.5 0.79
1.1
1.8
1.6
0.75
1.9 1.09
1.11
1.23
2.8 2.1 1.65
28.3
2.6
1.2 1.08
1.98 2.10
2.5 1.56 44.3 2.59 2.7
8.7
10.3
11.1
5.4
3.3 4.3
24.7
Elevation (m) before the construction of the embankment
28.6 40.7
12.2 0
23.6
4.3
5.5
100m
3.0
200m
100 80
(0m, 40m) (250m, 40m) (343m, 10m)
Pecentage of passing (%)
Pecentage of passing (%)
Figure 2. Map of the study area. (Image date: June 27, 2012, Google Earth)
60 40 20 0 0,001
(a) 0,01
0,1 1 Perticle size (mm)
10
100 80
(0m, 40m) (240m, 40m) (342m, 40m)
60 40 20 0 0,001
(b) 0,01
0,1 1 Perticle size (mm)
10
Figure 3. Grading curves of (a) the surface soil before the construction of the embankment and (b) the surface soil after the construction of the embankment.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Height of the embankment Position Height x y (m) (m) (m) 0 40 1.76 83 40 1.82 167 40 2.39 254 40 1.81 0 0 1.62 100 0 2.83 200 0 2.92 262 13 2.42 344 13 2.34
GEM-2 Line length Line spacing
Measurement line
Figure 4. The paddy field during measurement with GEM-2
and the spacing between them is 10 m. There are nine measurement lines in Field A and 26 in Field B. The data were recorded using GEM-2 at seven discrete frequencies (2,025 Hz, 3,675 Hz, 6,525 Hz, 11,625 Hz, 20,625 Hz, 36,625 Hz, and 65,025 Hz) and processed using an analytical program (Mitsuhata 2006b) to obtain a one-dimensional vertical EC distribution. The EC data at a depth of 0.025 m (surface soil) and 2.25 m (bottom of the embankment) were extracted from the vertical distribution and reconstructed to find the EC distribution at each depth. Here, values ≥1,000 mS/m were excluded to remove noise from the data.
3. RESULTS AND DISCUSSION 3.1 Surface soil before the construction of the embankment The EC distributions in the surface soil before the construction of the embankment are shown in Figures 5 and 6. The area in which EC is above 240 mS/m is large in Field A in 2012 (Figure 5). On the other hand, by 2013, the area in which EC exceeds 120 mS/m locally in Field A has become much smaller (Figure 6). The high-EC area is considered to have decreased from 2012 to 2013 because the inflow of seawater during high tide was suppressed more in 2013 than in 2012 because of repair work on the drainage canals. Hence, we infer that the salinity concentration of Field A decreased from 2012 to 2013; however, a high salinity concentration was observed in 2013 locally. In Field B of Figure 5, there is a high-EC area near x = 70 m. It is inferred that the salinity concentration increased owing to an inflow of seawater from drainage canals and leaching of the water from high-salinity soil upstream. The EC distributions are above 90 mS/m in most areas of Field B (Figure 5). In Field B of Figure 6, EC decreased over the whole area; however, EC remained high (30–90 mS/m) in areas where it was high in 2012. From the above results, it can be inferred that the salinity concentrations of Fields A and B decreased from 2012 to 2013.. 3.2 Bottom of the embankment The EC distributions at a depth of 2.25 m after the construction of the embankment are shown in Figures 7–9. Although there are areas where EC is more than 35 mS/m in Field A of 2014 (Figure 7), there are many areas where EC is less than 20 mS/m in this field in 2015 and
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
30
60
90
113
100
93
80 y（m）
y（m）
0
73 53
150
180
210
(mS/m)
240
60 40 20
33 13 342
120
0 250
262 302 x（m）
200
150
50
100 x（m）
0
Figure 5. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, September 2012)
30
60
90
113
100
93
80 y（m）
y（m）
0
73 53
120
150
180
210
240
(mS/m)
60 40 20
33
0 250 200 150 100 50 262 302 x（m） x（m） Figure 6. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, May 2013) 13 342
10
20
113
100
93
80
73 53 33
y（m）
y（m）
0
30
40
50
60
(mS/m)
60 40 20
0 250 200 150 100 50 262 302 x（m） x（m） Figure 7. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 2.24 m, November 2014) 13 342
0
0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2016 (Figures 8 and 9). EC decreased from 2014 to 2016 in Field B, except at x = 80 m (Figures 7–9). There were high-EC areas near x = 80 m in 2014 and 2015. However, these areas disappeared by 2016. This behavior is considered to have been affected by the old and new drainage canals located at x = 85 m. 3.3 Surface soil after the construction of the embankment The EC distributions at a depth 0.025 m after the construction of the embankment are shown in Figures 10–12. The high-EC areas of the surface soil in Fields A and B decreased from 2014 to 2016. The crop yield was reported to decrease from 2014 to 2015 in districts that managed irrigation, including the study area. The ion concentration in the study area is considered to have decreased from 2014 to 2015 because the ion concentration of soil water is related to EC. It is inferred that this phenomenon is related to the decrease in crop yield.
4. CONCLUSIONS EC distributions were evaluated via an electromagnetic survey of paddy fields conducted in
10
20
113
100
93
80 y（m）
y（m）
0
73 53
30
40
50
60
(mS/m)
60 40 20
33
0 250 200 150 100 50 262 302 x（m） x（m） Figure 8. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 2.24m, November 2015) 13 342
10
20
113
100
93
80
73 53 33
y（m）
y（m）
0
30
40
50
60
(mS/m)
60 40 20
0 250 200 150 100 50 262 302 x（m） x（m） Figure 9. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 2.24 m, November 2016) 13 342
0
0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
10
20
113
100
93
80 y（m）
y（m）
0
73 53
30
40
50
60
(mS/m)
60 40 20
33
0 250 50 0 262 200 150 100 302 x（m） x（m） Figure 10. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, November 2014) 13 342
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20
113
100
93
80 y（m）
y（m）
0
73 53
40
50
60
(mS/m)
60 40 20
33 13 342
30
0
262 302 x（m）
250
200
150
100 x（m）
50
0
Figure 11. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, November 2015)
10
20
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100
93
80
73 53 33
y（m）
y（m）
0
30
40
50
60
(mS/m)
60 40 20
0 250 200 150 100 50 0 262 302 x（m） x（m） Figure 12. Distributions of electrical conductivity obtained via an electromagnetic survey. (Depth = 0.025 m, November 2016) 13 342
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Rikuzentakata City, Iwate Prefecture, Japan, between 2012 and 2016. These paddy fields were damaged by the 2011 Tohoku earthquake and tsunami. Based on the relationship between EC and salinity concentration, desalinization of the paddy fields was considered by comparing the measurement results obtained before and after the construction of an embankment. Changes in EC were also analyzed to investigate the soundness of paddy fields after the construction of the embankment. The following conclusions were drawn. 1) Before the construction of the embankment, the salinity of surface soil decreased between September 2012 and May 2013 over the whole study area. However, certain areas with a high salinity concentration were present locally. 2) After the construction of the embankment, salinity remained relatively high in the areas near the old and new drainage canals, as indicated by the EC measured at a depth of 2.25 m, corresponding to the bottom of the embankment near the canal. 3) After the construction of the embankment, the EC of the surface soil in 2015 tended to be lower than that in 2014. This phenomenon is considered to be related to the decrease in crop yield.
ACKNOWLEDGEMENTS This study was partially supported by the JFE 21st Century Foundation.
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