Cycling of Be-7 and Pb-210 in a high DOC, shallow, turbid estuary of south-east Texas

The Sabine-Neches estuary is a shallow, turbid estuary in south-east Texas with high concentrations of dissolved organic carbon (DOC). The sediment inventory of Pb-210 and Pu-239,Pu-240 indicates that only a fraction of the particle-associated nuclides that reach the estuary were retained in the sediment. To better understand the cause for this low-sediment inventory of particle-reactive nuclides, Be-7 and Pb-210 concentrations have been measured in the dissolved and particulate phases, in addition to the DOC and suspended particle concentrations. The ratios of dissolved to particulate concentrations of Be-7 and Pb-210 were generally higher than in most other coastal waters. The dissolved residence times of Be-7 and Pb-210 (accounting for riverine input) varied between 0.6 and 9.6 days and 1.7 and 9.8 days, respectively. Distribution coefficients (K-d) ranged between 1500 and 87 100 cm(3) g(-1) for Be-7 and 2600 and 37 000 cm(3) g(-1) for Pb-210. These K(d)s are lower than those reported for most coastal waters. There was no significant correlation between suspended particle concentration and K-d of either Be-7 and Pb-210; this has been observed for many other particle-reactive nuclides, suggesting that particle concentration is not the primary controlling variable for the removal of particle-reactive nuclides in these high DOC waters. The average particle residence time in this estuary is similar to 2 days. The relatively low K-d values, longer dissolved residence times of Be-7 and Pb-210, longer particle residence times and shorter hydraulic residence times compared to other coastal areas, result in only a partial removal of particle-reactive radionuclides in this estuary. (C) 1997 Academic Press Limited.