Modeling of 137Cs migration in cores of marine sediments of Mumbai Harbor Bay

The vertical concentration profile of 137Cs in cores of marine sediment of Mumbai Harbor Bay has been studied by the compartment and the diffusion-convection models. Based on the measured concentrations of 137Cs in the sediment layers, the various transport parameters such as sedimentation rate, residence half- time, effective migration velocity, diffusion coefficient and the convective velocities were determined. The sedimentation rate was determined to be 1.61, 1.03 0.69 and 1.25 cm year−1 from the slope of lines obtained from the depth profile of 137Cs in cores using a least-square fitting method at site 1, 2, 3 and 4 respectively. The mean residence half-times, ranging from 11 to 35 years were observed to be the highest at the upper layers (up to 8 cm) of all sites and decreased with sediment depth. Subsequently, the ranges of mean value of effective vertical migration velocity in the same layers were between 0.15 and 0.46 cm year−1. As expected, the vertical migration in the upper sediment layers was very slow and thereafter increased slowly in the succeeding layers (12 cm onwards) of all sites with a mean ranging from 1.11 to 4.13 cm year−1. The obtained migration velocities were quite higher than those reported in literatures for global fallout. The convective velocity and diffusion coefficient at each site were assumed to be constant in the whole depth and calculated under the assumption of steady state. Using a depth-zoned bioturbational mixing model, the estimated biological diffusion coefficients ranged from 7 × 10−7 to 3.8 × 10−6 cm2 s−1 which were within the literature values reported for shallow coastal environments and deep sea.