[en] A simple inverse relationship between trace metal residence time and particle flux through the water column is tested on the trace metals Zn²⁺ in fresh water and Th(IV) in ocean water systems with available data on the vertical flux and concentration of particulate matter and Zn²⁺ in lakes or on the disequilibrium of U/Th nuclides in the ocean. Natural or model aquatic ecosystems with different chemical compositions and particle fluxes confirm the concept that the particle flux through the water column is the major factor regulating trace metal concentrations in natural waters

[en] Activities of radionuclides in the ²³⁸U (²³⁰Th, ²²⁶Ra, ²¹⁰Pb) and ²³²Th (²³²Th, ²²⁸Th, ²²⁸Ra) decay series were determined in sediments from an east Texas watershed and examined with isotope ratios and compared to particulate organic carbon (POC), % fines (< 63 μm) and total concentrations of Al, Fe and Mn. The objective was to elucidate the presence or absence of relationships affecting the effectiveness of these radionuclides in modeling sediment transport. Strong positive correlations were observed between radionuclides and Mn (Th) and % fines (Ra and Th). Isotope ratios effectively reduce these influences, supporting the contention that isotope ratios offset extrinsic variability in terrestrial sediments. Strong associations of ²¹⁰Pb_xs (excess ²¹⁰Pb) and ²²⁶Ra/²³⁰Th with POC agree with data from marine and terrestrial settings, indicating that the role of POC in isotope fractionation, transport and sequestration merits further investigation

[en] Anthropogenic sources from nuclear reprocessing discharges and bomb test fallout have completely overwhelmed the natural signal on the surface of the earth in the last 50 years. However, the transfer functions in and out of environmental compartments are not well known due to temporal variations in the sources of ¹²⁹I and to a lack of knowledge regarding the forms of iodine. From a vertical profile of ¹²⁹I/¹²⁷I ratios in sediments located in the Mississippi Delta region in approximately 60 meters water depth, the ¹²⁹I input function to this region was reconstructed. Dates in the core were assigned based on the plutonium peak at 20 cm depth (assumed to have been deposited in 1963) and the excess ²¹⁰Pb profile in the same depth interval, and below that, based on the steadily decreasing ²⁴⁰Pu/²³⁹Pu ratios from a ratio of 0.18 at 22 cm to 0.05 at 57 cm depth, the 1953 horizon. Atom ratios of ¹²⁹I/¹³⁷I Cs, decay corrected to 1962, the year of maximum radionuclide production, are about 0.3, very close to the production ratios of about 0.2 during atomic bomb tests. This evidence, combined with other observations, strongly suggests that ¹²⁹I in Mississippi River Delta sediments originates from atomic bomb fallout eroded from soils of the Mississippi River drainage basin, with little alteration of the isotopic ratios during transport from watershed to coastal deposits. Based on these observations and on laboratory evidence, the authors propose a conceptual model which explains this correspondence and the low ¹²⁹I/¹²⁷I ratios. Differences in mobilities of the different chemical forms of ¹²⁹I and ¹²⁷I, as well as the variances in chemical forms of ¹²⁹I from nuclear bomb fallout versus nuclear fuel reprocessing, are proposed to have created such a correspondence between I-isotope ratios and bomb fallout nuclides, without revealing recent inputs from nuclear fuel reprocessing releases to the northern hemisphere observed in watersheds of the USA and Europe

[en] The half removal time (t/sub c/) of Th from the surface waters of the New York Bight was almost independent of season suggesting that variable biological activity does not affect the removal rate of Th. The decreasing trend of t/sub c/ from the continental slope (t/sub c/ = 70 +/- 10 days) toward the shelf (t/sub c/ = 20 +/- 10 days) correlated with the general trend of increasing concentrations of suspended non-living particles toward the shore throughout the year. Incorporation in zooplankton fecal pellets was not a significant removal mechanism of Th in the surface waters of the continental shelf of the New York Bight. The same conclusion was reached for Pu whose removal was related to that of Th. The most important process affecting the removal of Th in the ocean is the total particle flux through the system

[en] This report outlines the results of work performed at the Colorado School of Mines, Brookhaven National Laboratory and Texas A and M University during the second reporting phase of this project. The sub-projects focused on this year include: (1) Biotransformation of Pu-contaminated soil; (2) Environmental colloids at the Rocky Flats Environmental Technology Site; (3) Production, isolation and characterization of EPS (exopolymeric substances, or exopolysaccharides); (4) Colloid trapping; (5) Determination of stability constants of complexes of Pu(IV) with organic ligands; and (6) The role of bacterial EPS in the transport of Pu through saturated porous media

[en] The accidental release of a tritium pulse of about 500 Ci into a river in northern Switzerland allowed the measurement of travel times at a site where the river recharges a granular aquifer almost perpendicularly to the ground water flow. A series of sampling wells had been previously installed in the aquifer. Most wells give access to shallow freshly infiltrated water. The infiltrated water flows along in the top layer of the aquifer, thus allowing for a one-dimensional analysis of the spreading process from the river to the ground water in the wells. The linear ground water flow velocities calculated from the tritium responses at distances from the river of up to 500 m confirm the velocities determined from an earlier local-scale uranine experiment at the same site. The scarce data used to describe tracer transport in the aquifer allowed for an assessment of dispersivity with the method of moments. A linearly scale-dependent dispersivity is evaluated from the mode of increase of the temporal variance of the tracer concentration distribution. Some wells revealed bimodal responses; here the method of moments is applied in combination with a two-layer model

[en] Apparent sorption rate constants on oceanic particles were measured in batch experiments for a series of soluble and particle-reactive radionuclides. The authors have found that radionuclide uptake by natural particles is slow compared to adsorption rates onto defined oxide surfaces. Equilibrium is attained only at time scales of days. One reason for slow scavenging reactions is the coagulation of radioactively tagged colloidal particles onto larger (filterable) particles. This interpretation explains both the slow sorption (scavenging) kinetics and the dependence of the sorption rate and the distribution ratio (K/sub d/) on the particle concentration. The sorption rate constants for Th isotopes calculated by this method are in good agreement with the rates determined by in situ measurements of the U/Th disequilibrium. In areas where particle residence times are shorter than the time scales of the sorption reactions, such as in shallow coastal marine ecosystems, radioisotope removal from the water column can become limited by the kinetics of sorption onto resuspended particles. If the authors use a simple box model, typical particle residence times, and rate constants for the sorption reactions of Th isotopes that depend on the particle concentration, they can predict residence times of Th isotopes for environments with particle fluxes varying over six orders of magnitude. The fact that these estimates are consistent with those measured directly by the U/Th disequilibrium method implies that Th isotopes might be natural coagulometers; that is, they might be good indicators of coagulation reactions of colloidal-sized particles in natural waters

No abstract available

[en] The uptake of radioactive trace metals by suspended particles in natural aquatic systems is often slow and the time constants for scavenging are of the same order of magnitude as the residence times of particles in the water column. Therefore, models simulating removal of radioactive trace metals from water to sediments in shallow aquatic systems with short particle residence times need to include sorption (i.e. scavenging) kinetics. The authors present a numerical kinetic transport model designed to simulate tracer movements from a well mixed water column where particles are generated by both primary production and sediment resuspension processes. Tracers are allowed to penetrate into underlying sediments by diffusion and particle and pore water mixing. All model parameters can be experimentally determined either in the ecosystem itself or in separate laboratory experiments. Limited sensitivity analyses and simulations of actual experiments indicate that under conditions of particle cycling in the sediments and the water, tracer removal can be affected and even be limited by the slow kinetics of tracer uptake by suspended particles. The model is also useful in identifying and quantifying critical pathways in the complicated web of interrelated processes of an aquatic ecosystem

[en] The concentrations of radionuclides of the U-Th series (²³⁸U, ²³⁴Th, ²³⁴U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, and ²³²Th, ²²⁸Ra, ²²⁸Th) in the water of Narragansett Bay are reported. Analysis of the total, particulate, dissolved and colloidal forms of Th isotopes reveal a consistent removal behavior which is controlled mainly by the particulate matter concentration and the sediment resuspension rate. Half-removal times of Th from solution onto particles range from 1.5 to 15 days, and settling velocities of Th containing particles range generally between 1 and 11 m/day. ²¹⁰Pb and ²¹⁰Po concentrations are seasonally dependent, with higher concentrations and slower removal during the early summer (half-removal times from solution onto particles of 1-5 days in winter and up to 2 months in early summer). (Auth.)