²³⁴Th and ⁷Be as tracers for the transport and dynamics of suspended particles in a partially mixed estuary

The sources of the natural radionuclides ²³⁴Th (half-life = 24 d) and ⁷Be (half-life = 53 d) to the partially mixed Hudson River estuary are distinctly different: ²³⁴Th is produced from decay of dissolved ²³⁸U that varies conservatively as a function of salinity, and ⁷Be is added directly from the atmosphere. ²³⁴Th and ⁷Be activities were measured in suspended and bottom sediments under varying conditions of river flow to evaluate the relative importance of local sediment resuspension and advection of suspended sediments through the estuary. Both radionuclides are removed from the estuarine water column on time scales of from <1 to 13 d. The ²³⁴Th(xs)/⁷Be activity ratio is used to eliminate variations in the specific activities caused by grain size or sediment compositional changes. A simple steady-state model is used to set limits on the activity ratios expected if local processes (in situ production of ²³⁴Th, atmospheric supply of ⁷Be, scavenging, particle settling, and resuspension) dominate the activity ratio. During low river discharge, observed ²³⁴Th(xs)/⁷Be ratios exceed the model upper limits in the higher salinity portions of the estuary, suggesting that particles labeled with Th and Be in higher salinities are transported to lower salinities with the estuarine circulation. Distance scales of particle transport are estimated to be from 10 to 20 km, comparable to the tidal excursion. Sampling during a high-flow event of short duration shows that the activity ratios tend toward the lower limit. Such a pattern could be produced by down-river transport of particles or by enhanced resuspension. Similarities between the ²³⁴Th(xs)/⁷Be activity ratios of suspended and bottom sediments suggest that increases in resuspension during high flow events dominate the activity ratio in the suspended sediment reservoir.