Sedimentation

Calculation of Particle Removal Velocities in the Mecklenburg Bight Based on the Thorium Deficit

This paper is similar to the one titled "Calculation of Particle Removal Velocities in the Mecklenburg Bight Based on the Thorium Deficit", in: Final Report, Project MOST, Bundesministerium für Forschung und Technologie, Bonn, Germany, 1996.

Joachim Gruber

Address at the time this paper was finished:

Arbeitsbereich Umweltschutztechnik
TU Harburg
P.O.Box 901052
D 21071 Hamburg
E-Mail: Joachim.Gruber[at]acamedia.info

Abstract

The temporal development of the activity (concentration) of thorium formed by decay of the natural uranium in sea water was modeled with a dynamical model derived from Kershaw and Young ["Scavenging of Th 234 in the Eastern Irish Sea" J. Environ. Radioactivity 6, 1 - 23, 1988]. It is a system of 6 ordinary linear first order differential equations. The independent variable is the time t, the dependent variables are the soluble and adsorbed thorium and uranium activities (concentrations) in the water column and the sediment.

Consistently with the experimental errors, the system of equations can be simplified without loss of accuracy. This way all system parameters become experimentally accessible.

In the end, the sedimentation rate has to be estimated from a single equation in which we have the following three unknowns:

  1. the time t after the perturbation of the settling velocity v of the suspended matter,
  2. the ratio of the thorium and the uranium activities immediately before the perturbation,
  3. the settling velocity v.
Since the system is indeterminate (three unknowns in a single equation) we have to enter probable values for some of the unknowns.

The simplification and the use of probable values exceeds the mentioned model of Kershaw and Young.

In addition to the the system being indeterminate the following errors contribute to the error of the here calculated sedimentation rates:

  1. the systematic and statistical experimental errors and
  2. the uncertainties resulting from not having experimentally resolved the spatial and temporal variability of the thorium and uranium activities (concentrations).
The sedimentation velocities v thus calculated are up to two orders of magnitude smaller than the settling velocities calculated after Aldredge and Gotschalk ["In-situ settling behaviour of marine snow" Limnol. Oceanog., 33, 339-351, 1988] in stagnant water without resuspension based on the density and size of the particulate matter.

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