H. Feng et al., Th-234 and Be-7 as tracers for the sources of particles to the turbidity maximum of the Hudson River estuary, EST COAST S, 49(5), 1999, pp. 629-645
The natural radionuclides Th-234 (half-life=24.1 days) and Be-7 (half-life=
53.3 days) were measured in suspended particles of the turbidity maximum zo
ne 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 th
is zone. The relatively short half-lives and high particle reactivity of Th
-234 and Be-7, coupled with the spatially variable production of Th-234 (fr
om U-238, which varies conservatively with salinity in the Hudson), make th
ese radionuclides useful tracers of particle dynamics. The activity ratio o
f Th-234(xs)/Be-7 reduces the effects of particle type, concentration and s
ize on the individual activities and serves as an indicator of where in the
estuary the particles were labelled with these radionuclides. Detailed sam
pling at three stations in the turbidity maximum during the course of a tid
al cycle shows an average Th-234(xs)/Be-7 activity ratio of 1.9 +/- 0.7. Co
mparable T-234(xs)/Be-7 ratios in suspended particles from surface and bott
om water with different salinities indicate that particles are mixed throug
hout the water column. Comparison of observed Th-234(xs)/Be-7 activity rati
os 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 acco
unt for the observed Th-234/Be-7 activity ratios suggest that about 30% of
the flux could arise from advective transport of particles into the turbidi
ty maximum zone in association with the tidal circulation and the remainder
is supplied from local resuspension. A sensitivity analysis shows that the
se estimates are dependent on the activity ratio in the suspended sediment
reservoir and fluxes of radionuclides supplied by local resuspension. Varia
bility in these factors gives rise to estimates of the advected flux that c
an vary hom 24% to 59%. (C) 1999 Academic Press.