Evaluation of sedimentation rates by 210Pb, in water bodies of semiarid and arid areas

Appl. Radiat. lsot. Vol. 46, No. 6/7, pp. 617-618, 1995

Pergamon

0969-8043(95)00108-5

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-8043/95 $9.50 + 0.00

EVALUATION OF SEDIMENTATION RATES BY 210pb, IN WATER BODIES OF SEMIARID AND ARID AREAS H. AFIF and S. BOUHLASSA Laboratoire De Radiochemie, Faculte Des Sciences, B.P. 1014, Rabat, Morocco.

Moroccan reservoirs suffer froln severe siltation problems resulting from high erosion rate. The dating method based on an excess of 21°pb (Appleby, 1978) in reservoir sediments, successfully used in areas of low erosion rate, cannot be extended to our case owing to the dilution of a low concentration of atmospheric 21°pb in high sediment flux. Whenever used, this method leads to evaluate the siltation rate, which is a very important parameter in dam water management. An alternative to this method has been developed, based on the variation of 210pb in sediment cores. EXPERIMENTS Four sediment cores, l m depth and 5 cm large, (PS9, PS 11, PS 13, P 14) have been taken with a remotely-operated pneulnatic corer, from the Bouregreg reservoir, located in the north-west of Morocco, near Rabat (figure 1). Cores were split longitudinally and serially analyzed at 2 cm intervals for the bulk density and radionuclide content. Radiometric data have been acquired on an n-type hyper-pure gerlnaniuln detector. Operational software for 3, radiometric data processing, at very l o w background, was developed within the framework of this study, to establish the activity variation profiles. RESULTS AND DISCUSSION The non-linear variation of unsupported 210pb (figure 2) suggests non-constant sedimentation rates. The profile indicates, a high value of unsupported 21°pb concentration, close to organic layers (dark stratum in figure 2). This behaviour, related to colloidal form of unsupported 21°pb in water, constitutes an indication of annual deposition limits even in the absence of eutrophication. The application of this finding, leads to the distribution of the sedimentation rates in the reservoir area sampled (table 1).

617

618

H. Afif and S. Bouhlassa

P p

$

1

~

~

~-~

S

..

Fig. I. Location of cores in the map of Bouregreg reservoir.

Activity (Bq/Kg) 20 40 60

0 0

~

2o'. ~

80 -...,

'~ 'i)!!!:

60

core

80 100 Fig. 2. Unsupported 21°pb versus depth profile in PS9 core. Tab. I. Distribution of sedimentation rates (r) (r in kg/m2) in Bouregreg reservoir for various cores.

............... Core~

PS9

PSll

PS13

P14

349.37 443.64 382.82

60.74 111.71 30.63

4.83 36.26 26.88

17.86 100.59 43.72

r "'"'"'----..

1987-88 1986-87 198~86 REFERENCES

Appleby, P.G., Oldfield, F., (1978) The calculation of 21°pb dates assuming a constant rate of supply of unsupported 21°pb to the sediment Catena, 5: 1-8.