A global three‐dimensional model is used to investigate the transport and tropospheric residence time of 210Pb, an aerosol tracer produced in the atmosphere by radioactive decay of 222Rn emitted from soils. The model uses meteorological input with 4°×5° horizontal resolution and 4‐hour temporal resolution from the Goddard Institute for Space Studies general circulation model (GCM). It computes aerosol scavenging by convective precipitation as part of the wet convective mass transport operator in order to capture the coupling between vertical transport and rainout. Scavenging in convective precipitation accounts for 74% of the global 210Pb sink in the model; scavenging in large‐scale precipitation accounts for 12%, and scavenging in dry deposition accounts for 14%. The model captures 63% of the variance of yearly mean 210Pb concentrations measured at 85 sites around the world with negligible mean bias, lending support to the computation of aerosol scavenging. There are, however, a number of regional and seasonal discrepancies that reflect in part anomalies in GCM precipitation. Computed residence times with respect to deposition for 210Pb aerosol in the tropospheric column are about 5 days at southern midlatitudes and 10–15 days in the tropics; values at northern midlatitudes vary from about 5 days in winter to 10 days in summer. The residence time of 210Pb produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of 210Pb produced in the upper atmosphere. Both model and observations indicate a weaker decrease of 210Pb concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that 222Rn is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus 210Pb is not simply a tracer of aerosols produced in the continental boundary layer, but also of aerosols derived from insoluble precursors emitted from the surface of continents. One may draw an analogy between 210Pb and nitrate, whose precursor NOx is sparingly soluble, and explain in this manner the strong correlation observed between nitrate and 210Pb concentrations over the oceans.

Five groundwater samples taken from different hydrogeologic settings in Connecticut were analyzed for major cation chemistry and the concentration of U and Th decay series nuclides 238U, 234Th, 226Ra, 222Rn, 2•øpb, 2•øpo, 232Th, 228Ra, 228Th, and 224Ra. The concentration of 222Rn in the waters ranged between 103 and 104 dpm 1-• and was three to four orders of magnitude greater than that of the shortlived alpha daughters 224Ra, 228Ra, and 234Th, even though the rates of supply of these four nuclides to solution are expected to be similar. We infer that sorption removes radium and thorium from these groundwaters on a time scale of 3 minutes or less. The (224Ra/228Ra)•.and (234Th/228Th) activity ratios in these waters indicate that desorption of these nuclides occurs on a time scale of a week or less and that equilibrium between solution and surface phases is established. In situ retardation factors for radium, thorium, and lead may therefore be calculated directly from the isotopic data; values range from 4,500 to 200,000. Neither sorption time scales nor retardation factors are, strongly dependent on the nuclide or on hydrogeology of the aquifer. Since our study includes nuclides with diverse chemical properties, we suggest that other uncomplexed heavy metals and transuranic elements will also behave in a manner similar to those measured here. The approach presented here should therefore find application in developing site-specific models of the transport of radioactive or stable elemental waste through water-saturated media. 1. geochemical properties, their behaviors may serve as indica-1On leave from the Physical Research Laboratory, Ahmadabad, India. tors of the in situ chemical behavior of other nuclides injected into the system. With this aim we have analyzed several groundwaters from Connecticut for virtually every long-lived (half-life > 1 day) member of the 238U and 232Thseries in order to understand the processes controlling their concentration in solution and thereby to estimate the rates of those processes. The U and Th series nuclides are uniquely suited to this study because several isotopes of the same element are continuously introduced into groundwaters and because the supply rates of many of these nuclides can be estimated with adequate accuracy. Measurements of the distribution of these natural decay series nuclides yield site-specific empirical data on the in situ sorption processes. Parameters such as distribution coefficients or retardation factors derived from these measurements may be used in models of transport of both radioactive and stable nuclides that show analogous chemical behaviors. METHODSFive groundwater samples from the major aquifer types of Connecticut were collected for this study. The sampling locations, aquifer descriptions, and water chemistries are given in Table 1. Samples GW-3 and GW-7 were both taken from municipal water supply production wells in glacial drift aquifers. The large volumes of water pumped from these wells (Table 1) suggests that recharge is induced from n...

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Whewellite and weddellite, calcium salts of oxalic acid, have been found in the litter layer of several different soils, indicating that oxalate is a major metabolic product of fungi in natural environments. The presence of oxalate in soil solution speeds weathering of soil minerals and increases the availability of nutrients to vegetation.

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Strontium-87/strontium-86 ratios indicate the sources of strontium in samples of natural waters, vegetation, and soil material taken from watersheds in the Sangre de Cristo Mountains of New Mexico. More than 75 percent of the strontium in the vegetation is ultimately derived from atmospheric transport and less than 25 percent from the weathering of the underlying rock. Much of the airborne strontium enters the watersheds by impacting on coniferous foliage, but deciduous foliage apparently traps little, if any, strontium-bearing aerosol. The strontium and presumably other nutrients are continuously recycled in a nearly closed system consisting of upper soil horizons, forest litter, and the standing crop of vegetation.

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