Cycling of7Be and210Pb 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 of210Pb and239,240Pu 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,7Be and210Pb 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 of7Be and210Pb were generally higher than in most other coastal waters. The dissolved residence times of7Be and210Pb (accounting for riverine input) varied between 0•6 and 9•6 days and 1•7 and 9•8 days, respectively. Distribution coefficients (Kd)ranged between 1500 and 87 100 cm3 g−1for7Be and 2600 and 37 000 cm3 g−1for210Pb. TheseKds are lower than those reported for most coastal waters. There was no significant correlation between suspended particle concentration andKdof either7Be and210Pb; 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 2 days. The relatively lowKdvalues, longer dissolved residence times of7Be and210Pb, 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.