19 Key: USEV, POSEIDON, THREETOX, NRPA. FIG. 9. Calculated and measured mean 137Cs activity concentrations for bed sediments in several basins. Error bars are 1? standard deviations of the measured mean values. Only one sample was measured if error bars do not appear. FIG. 10. Calculated 137Cs activity concentrations for January 1991 in water (Bq/m3) and sediments (Bq/kg) by hydrodynamic models THREETOX and USEV. Although THREETOX domain is slightly larger, maps are drawn over the same area (USEV domain) for a better comparison.