The Aquatic Geochemistry of the Particle-Reactive Radionuclides PO-210, PB-210, and BE-7
Integrated monthly wet-only and bulk precipitation samples for Be-7 and Pb-210 analysis were collected from November 1982 through October 1983 at two sites in lower Chesapeake Bay. Atmospheric deposition fluxes for Be-7 in wet-only and bulk precipitation averaged 0.013 and 0.014 pCi/cm2/d, respectively, and for Pb-210 averaged 0.36 and 0.40 pCi/cm2/yr, respectively, suggesting that dry deposition makes only a minor contribution to the total fluxes. A seasonal trend was observed in the depositional flux of Be-7, with a pronounced maximum in the Spring. Approximately 40 percent of the annual Be-7 deposition occurred between February and April, with a maximum in March. This is consistent with the injection of Be-7 of statospheric origin into the tropopause during the vernal tropopause break. Pb-210 exhibited a slightly weaker seasonal trend, with ca. 35 percent delivered between February and April, which appears to be related to the rate at which its parent, Rn-222, emanates from the earth. The average deposition velocities for Be-7 and Pb-210 in bulk precipitation were 1.5 and 0.8 cm/s, respectively. The removal behaviors of two atmospherically-derived radionuclides, Pb-210 and Be-7, have been examined in real-time following their deposition to a shallow coastal bay by four precipitation events. In general, there was a marked elevation in the water column concentrations following a precipitation event and the concentrations decreased with time. The removal residence times of these radionuclides were on the order of several days. Po-210 exhibited similar behavior. Since the atmospheric flux of Po-210 is small, the elevated concentrations of Po-210 following a precipitation event may result from the mobilization of Po-210, in the dissolved form, from sediments. Vertical profiles of dissolved Po-210, Pb-210, and Ra-226 were obtained from the Orca Basin, an anoxic, hypersaline and yet non-sulfide bearing basin. At the seawater-brine interface at 2230 m, a sharp maximum was observed in the profiles of Po-210 and Pb-210, reaching concentrations of 56.0 and 28.3 dpm/l00 kg, respectively. These are some of the highest values reported for the open ocean. These sharp maxima may be caused by a combination of the dissolution of hydrous manganese oxides and the decomposition of biogenic particulate material. Below the interface, concentrations decreased to ca. 9 and 6 dpm/100 kg, respectively. These concentrations are much higher than those observed in sulfide-bearing anoxic basins. The residence time of Pb-210 in the brine is estimated to be ca. 1 year.
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