This dataset holds measurements of Cesium-137 (137Cs) activity sampled from sites in the Atchafalaya and Terrebonne Basins on the southern coast of Louisiana, USA. Sediment cores were taken to validate modeled estimates of soil accretion rates. The NUMAR model was designed to predict soil properties and corresponding accretion rates in marsh environments. This study explored whether the probabilistic outcomes generated by the model align with accretion rates determined through field-based measurements using 137Cs activity. The model's probabilistic simulations were conducted for active and inactive basins, encompassing fresh, brackish, and saline sites. Aliquots of approximately 5-9 g of finely ground dry sediment were placed into vials of known geometry for direct gamma counting of 137Cs using high-purity Germanium (Ge) well detectors. The 137Cs dating technique is based on detecting the specific core section in which the radioactive isotope 137Cs peak activity occurs across a vertical soil profile. This peak corresponds to the peak fallout in 1963 deposited during the atmospheric nuclear testing and provides a timeline reference for estimating soil accumulation rates, sediment deposition, and marsh accretion. Therefore, to validate the NUMAR accretion results, 137Cs activity was measured at different core sections across a vertical soil profile to determine the peak corresponding to the peak fallout in 1963 due to atmospheric nuclear testing and estimate soil accretion rates. The data are provided in comma separated values (CSV) format.