Inventory and spatial pattern of ¹³⁷Cs in a pond: A comparison of two survey methods

The sediments of Pond A, a former Savannah River Site cooling pond for R-reactor, were contaminated with 137Cs between 1954 and 1964. Pond A is unique because it is very shallow and contains an extremely high density of aquatic vegetation and old, undecomposed tree stumps which modify normal sedimentation processes and cause special radiological characterization challenges. To determine the most efficient technique for estimating inventory and spatial patterns of 137Cs, we measured exposure rates at 124 sediment surface locations with two types of thermoluminescent dosimeters (TLDs) and compared them to estimates calculated from 137Cs measured in 58 extracted sediment cores. The mean net exposure rate (± 1SEM) measured at the sediment water interface with a UD-802 multi-element TLD (differentially shielded lithium borate and calcium sulfate) was 40 ± 4 μR h-1, while the corresponding value measured with a CaF2 TLD was 64 ± 10μR h-1. Both sets of TLD measurements were found to correlate well with each other (R2 = 0.88,p < 0.001), and moderately well with theoretical calculations derived from 137Cs activity concentrations measured in sediment cores (R2 > 0.50). The corresponding mean exposure rate calculated from the sediment data, 69 ± 10μR h-1, was likely an over-estimate resulting from the core sampling bias created by the large number of tree stumps. Overall, peak 137Cs activity occurred at ~2-4 cm depth in the sediment cores, with > 99% in the top 20 cm of sediment. The total 137Cs inventory of Pond A was estimated as 4.1 ± 0.5 x 1010 Bq, with most activity located in the deeper portions. Approximately 1% of the 137Cs activity thought to have been released by R-Reactor can be accounted for in Pond A, with an additional 53% estimated from other work to be in the much larger Pond B, and Par Pond, located further down the drainage. However, the mean deposition in Pond A (7.9 x 109 Bq ha-1) was higher than either Pond B (4.0 x 109 Bq ha-1), or Par Pond (1.4 x 109 Bq ha-1). It was concluded that, although the TLD method was more efficient and could employ more sampling locations to estimate spatial pattern, a reasonable amount of coring was essential to determine depth distribution, radionuclide composition, and to interpret the TLD data. Optimal estimation and characterization efficiency can benefit from simultaneous application of both techniques. (C) Published by Elsevier Science Ltd.