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

The sediments of Pond A, a former Savannah River Site cooling pond for R-re actor, were contaminated with Cs-137 between 1954 and 1964. Pond A is uniqu e because it is very shallow and contains an extremely high density of aqua tic vegetation and old, undecomposed tree stumps which modify normal sedime ntation processes and cause special radiological characterization challenge s. To determine the most efficient technique for estimating inventory and s patial patterns of Cs-137, We measured exposure rates at 124 sediment surfa ce locations with two types of thermoluminescent dosimeters (TLDs) and comp ared them to estimates calculated from Cs-137 measured in 58 extracted sedi ment cores. The mean net exposure rate(+/- 1SEM) measured at the sediment w ater interface with a UD-802 multi-element TLD (differentially shielded lit hium berate and calcium sulfate) was 40 +/- 4 mu R h(-1), while the corresp onding value measured with a CaF2, TLD was 64 +/- 10 mu R h(-1). Both sets of TLD measurements were found to correlate well with each other (R-2 = 0.8 8, p < 0.001), and moderately well with theoretical calculations derived fr om Cs-137 activity concentrations measured in sediment cores (R-2 > 0.50). The corresponding mean exposure rate calculated from the sediment data, 69 +/- 10 mu R h(-1), was likely an over-estimate resulting from the core samp ling bias created by the large number of tree stumps. Overall, peak Cs-137 activity occurred at similar to 2-4 cm depth in the sediment cores, with > 99% in the top 20 cm of sediment. The total Cs-137 inventory of Pond A was estimated as 4.1 +/- 0.5 x 10(10) Bq, with most activity located in the dee per portions. Approximately 1% of the Cs-137 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 10(9) Bq ha(-1)) was higher than either Pond B (4.0 x 10(9) Bq ha(-1) ), or Par Pond (1.4 x 10(9) 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 de termine depth distribution, radionuclide composition, and to interpret the TLD data. Optimal estimation and characterization efficiency can benefit fr om simultaneous application of both techniques. Published by Elsevier Scien ce Ltd.