That of the deeper layer, 30 cm to 80 cm in depth, was slightly greater than the shallower layer, 2.3 to 4.5 mm/h. Saturated hydraulic conductivity of undisturbed sample from shallow layer, 5 to 30 cm in depth, was 2.0 mm/h. The additional hardpan was observed at 4 sites out of 8 survey points in the paddy field. This shallow hardpan was suspected to form due to load of traffic of heavy machinery during decontamination. Cone penetrometer test at a paddy field in Iitate suggested in addition to the common hardpan an alternative one occurred at shallower layer, 5 to 15 cm in depth. Specific feature of each hardpan is variable depending on soil physical property and history of agricultural practices. Hardpan has a role to keep ponding water depth. Paddy fields typically have a dense layer that we called hard pan at a depth of 20cm to 30cm. Texture of the original soil was mostly heavy clay, and dressed soil had light clay texture. Besides, disturbed and undisturbed soil samples were taken and brought back to the laboratory for soil physical analysis such as saturated hydraulic conductivity, dry bulk density, porosity, and texture of the soil. In the field, cone penetrometer test and infiltration test using disk permeameter were conducted. Thus, in this study, extent of soil compaction at decontaminated paddy fields in Iitate village in Fukushima was surveyed for near future resume of agricultural activity. The soil compaction may be an obstacle for farming resume after the power plant accident. During the decontamination activity, traffics of machinery could have caused soil compaction. The leveling was the end of decontamination procedure. After the skimming clean, radioactive material free, subsoil was dressed and leveled. Stripping surface soil was conducted by using backhoes or a turf stripper and a skimmer attachment for tractors. Area of surface soil stripping was approximately 8300 ha in Fukushima prefecture. In this process, skimming the surface 5 cm thick soil layer was employed for agricultural lands which had radioactive materials in surface soil greater than 5000 Bq/kg-soil. In Fukushima, 20000 ha of agricultural lands have been decontaminated after the accident of Fukushima Dai-Ichi nuclear power station on March 2011. (2015) discussed cost for mitigation of arable soil compaction. Then, increase in runoff during rainfall event, water logging during and after the precipitation as well as depression of germination of seeds occurs.Later, Chamen et al. Rise of dry bulk density may cause decrease in hydraulic conductivity, and increase in soil hardness. The stress makes dense layer at a depth of arable land. Stress caused by traffic of agricultural machinery is a typical cause of soil compaction. low to moderate risk, susceptibility to compaction, while coarse and medium textured soils especially those are under lightly packed condition have high risk to compaction. (2003) reported fine and very fine textured soils are less, i.e. Batey (2009) reviewed reasons of soil compaction at agricultural lands. Soil compaction has been a serious problem in agricultural lands.
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