²³⁴Th and ⁷Be as tracers for the sources of particles to the turbidity maximum of the Hudson River estuary

The natural radionuclides ²³⁴Th (half-life=24·1 days) and ⁷Be (half-life=53·3 days) were measured in suspended particles of the turbidity maximum zone of the lower Hudson River estuary during several tidal cycles in August 1995 in an effort to understand the dynamics and sources of particles to this zone. The relatively short half-lives and high particle reactivity of ²³⁴Th and ⁷Be, coupled with the spatially variable production of ²³⁴Th (from ²³⁸U, which varies conservatively with salinity in the Hudson), make these radionuclides useful tracers of particle dynamics. The activity ratio of ²³⁴Th(xs)/⁷Be reduces the effects of particle type, concentration and size on the individual activities and serves as an indicator of where in the estuary the particles were labelled with these radionuclides. Detailed sampling at three stations in the turbidity maximum during the course of a tidal cycle shows an average ²³⁴Th(xs)/⁷Be activity ratio of 1.9 ± 0.7. Comparable ²³⁴Th(xs)/⁷Be ratios in suspended particles from surface and bottom water with different salinities indicate that particles are mixed throughout the water column. Comparison of observed ²³⁴Th(xs)/⁷Be activity ratios with those calculated from models assuming that local processes dominate the tracer distributions suggests that particle residence times are short. However, the observed ratios are greater on average than those calculated, indicating the import of particles with higher ratios from the more saline reaches of the estuary. Estimates of the particle fluxes necessary to account for the observed ²³⁴Th/⁷Be activity ratios suggest that about 30% of the flux could arise from advective transport of particles into the turbidity maximum zone in association with the tidal circulation and the remainder is supplied from local resuspension. A sensitivity analysis shows that these estimates are dependent on the activity ratio in the suspended sediment reservoir and fluxes of radionuclides supplied by local resuspension. Variability in these factors gives rise to estimates of the advected flux that can vary from 24% to 59%.