(a) The contribution ratio of direct gamma rays emitted from radiocesium was high when radiocesium was emitted from the surface-layer sediment. (b) The contribution ratio of scattered gamma rays was high when radiocesium was emitted from deeper-layer sediment because the emitted gamma rays were shielded by soil particles. (c) Relationship between RPC obtained by in-situ measurement and β_eff obtained by sediment sampling.