Radioactive tracer techniques for sand and silt movements under water

International Journal of Applied Radiation and Isotopes,1956, Vol. 1, pp. 24-32. PergamonPressLtd., London

Radioactive Tracer Techniques for Sand and Silt Movements under Water j. L. P U T M A N AND D. B. S M I T H Atomic E n e r g y Research Establishment, Harwell, Didcot, Berks

(Received 3 February 1956; in final form 10 February 1956)

T h e choice of p o w d e r e d glass, containing 46Sc as a physical tracer for following the u n d e r w a t e r m o v e m e n t of sand a n d silt, is discussed. T w o experiments are described to show its application to the study of river siltation a n d of the offshore m o v e m e n t of sand on the sea-bed. T h e results o b t a i n e d give information a b o u t the a m o u n t of radioactivity required in such experiments, a n d methods of depositing it a n d tracing its movement.

Techniques du traceur radioactif appliqu5es a l'etude des m o u v e m e n t s du sable et de la vase sous les eaux O n discute d u choix d ' u n verre en p o u d r e c o n t e n a n t du 46Sc c o m m e traceur physique p o u r suivre le m o u v e m e n t d u sable et de la vase sous les eaux. Deux expSriences sont prSsentSes, m o n t r a n t son utilisation ~ l'6tude de l'envasement de riviSre et au m o u v e m e n t de reflux d u sable sur le fond de la met. Les rSsultats obtenus d o n n e n t des informations sur la quantit6 de radioactivit6 nScessaire dans de telles exp6riences, sur la maniSre de rSpartir le verre et de suivre son dSplacement.

IIpn~enenHe pa~14oaH'rnBnLix 14n~nHaTop0B ~an Hccae~OBaHI4~ n e p e ~ e m e n n g necHa u n a a non Bo;Ioi~. ~ a n nay,~eHnn nepeMemennA necKa n u s a n o r BO~Oi~ B Ha,~ecTBe nH~nHaTopa npeaaomeHo nopomKoo6paanoe CTeHao, co~Iepmamee *"Sc. O n ~ c a n u ~Ba O1]bITa, Ii3 HOTOp~IXcae~yeT npn~eHnMOCT~ aToro MeTo~a H HccaeT~oBann~o aanaeH~n pen ~ ~IBnmen~4~ necna Ha MopcHoM ~He. IIoJ~yqenHble peuyabTaT~I ~amT yRa3aHne Ha TO, RaHau Be~nqnHa aI~TI4BHOCTI~InpenapaTa Heo6xo~HMa ~ TaHnx Hcc~e~oBaHni~, a TaHme Ha CI~OC06bI BBe~IeHn~ nH~nHaTopa H saSa~o~Iennu aa ero nepe~BnmenneM.

Radioaktive I n d i k a t o r m e t h o d e n zur Bestimmung der U n t e r w a s s e r b e w e g u n g von S c h l a m m u n d Sand Die V e r w e n d u n g von 46Sc-markiertem Glas als Indikator bei der U n t e r s u c h u n g der U n t e r wasserbewegung von Sand u n d S c h l a m m wird besprochen. Zwei Versuche w e r d e n beschrieben, welche die A n w e n d u n g bei der U n t e r s u c h u n g der Flussverschlammung u n d des Sandtransports a m M e e r e s b o d e n in der R i c h t u n g v o n d e r Kfiste zeigen. Die erhaltenen Resultate geben Aufschluss fiber die Aktivit~itsmengen, die ffir solche Versuche ben6tigt w e r d e n u n d fiber die M e t h o d e n eine radioaktive M a r k i e r u n g durchzuffihren u n d ihre Bewegung zu verfolgen. 24

25

Radioactive tracer techniquesfor sand and silt movements under water

The underwater movement of sand and silt has generally been followed in the past by introducing some material not normally present in the sand and by subsequent sampling and tedious analysis. The use of radioactive tracers offers the possibility of detecting and measuring the tracer material whilst it is on the river- or sea-bed by using underwater gamma-ray detectors operated from a boat. Obviously this is a tremendous advantage, particularly when surveying a large area. Choice of Tracer

In order that the movements of sand or silt shall be faithfully followed by the tracer material, it is desirable that: (a) It should behave exactly as the material being traced. (b) The label, whether radioactive or otherwise, should remain firmly bound to the tracer material, and it should be impossible for this to wash or chip off the labelled material. (c) For radioactive labelling, the activity of a particle should be proportional to its mass, indicating a bulk labelling rather than a surface layer of activity. (d) The tracer should be detected on the sea- or river-bed, and the detector should be capable of recording over a reasonably large area to avoid statistical inaccuracies which would result from the detection of dispersed single particles. The choice of a radioactive tracer is principally controlled by the requirements that the isotope should emit energetic gamma rays (preferably over 1 MeV) with a half-life suitable for the particular investigation, and that it should be possible to induce a high specific activity in the element. Table 1 gives a list of isotopes which might be useful for such tracer work. The methods to be described relate essentially to the movement of sand particles of various sizes. Although the problems involved in these two investigations are different and are treated separately, they have in common the requirement of pro-

ducing an insoluble tracer which will simulate the movement of s a n d . The direct irradiation of sand generally yields only short-lived 3asi and 24Na, although some sands ~a) have been found which contain sufficient phosphorus for this to be activated as rap. This is not an ideal tracer, since the absence of gamma radiation means that samples have to be taken. As long ago as 1951, Dr. W. J. ARROL proposed the use of glass containing 46Sc as a label to check the movement of beach sand, C2)but the project was abandoned at the time. He showed that at least 5% by weight of S%O 3 can be dissolved in soda glass and is distributed uniformly through the glass. This was the method used for the investigations to be described: a boron-free glass was chosen, to avoid unnecessary neutron absorption in the reactor. Thus, when the glass is ground to the required particle size distribution, a tracer material is obtained which is physically similar to sand yet can be activated with 46Sc at high specific activity by neutron irradiation. 46Sc emits two gamma rays per disintegration with energies of 0-89 M e V and 1.12 MeV. The half-life of eighty-five days makes it ideal for use in problems where it is TABLE 1

Element

Scandium 46Sc Cobalt

Half-life

Spee. activity in a flux of 1011 n/see/cm 2 for one week

85 days

39 mc]gm

0-89, 1.12

5"3 years

1'8 mc/gm

1.17, 1-33

250 days

100/~c/gm

1.12

Gamma rays (MeV)

6oCo

Zinc 65Zn

Rubidium S6Rb Silver lXOAg Antimony

19.5 days 1-9 me/gm

1.08 (20%)

270 days

320/~c]gm

0'17 to 1.52

60 days

0.95 me/gin

0"21 to 2"0

124Sb

Tantalum 111 days 6.9mc/gm aS2Ta i BariumLanthanum 12'8 days i Fission product 140Ba/X*0La

0.07 to 1-22 1.6 etc.

26

j . L. Putman and D. B. Smith

desirable to follow the activity for several months, and also permits a similar experiment to be carried out in the same area after the lapse of about a year. When such a tracer material is widely dispersed, the limit of detection may occur when the general level of activity is too low for accurate measurement, or alternatively when the statistical probability of finding one or more particles in the region assayed by the detector becomes small. It is desirable that the latter limit shall not be reached, since it will give rise to misleading results, and also artificially limit the use of the radioactive tracer. Ideally, the number of particles should be such that both limits are approached simultaneously, and this will be a function of the sensitivity and volume o f assay of the detector. When using large multiple Geiger counter detectors, it is found that these ideal conditions can be approached if 109 equally sized particles are associated with about 10 curies of ~6Sc activity. In practice, experimental restrictions on the weight of tracer may cause this figure to be modified. 1. T H A M E S

The specific problems investigated* were: 1. The investigation of silt movements on the bed of the river in the Thames estuary: In order to keep the port of London open to shipping, about 3 million tons of mud is dredged from the river annually. This is several times the estimated quantity of silt carried into the estuary by upland waters and effluent. About [ million tons is dumped on mud flats in the estuary, and the rest is taken out to sea. M u d thus deposited is known to disperse, and it was required to confirm a significant upstream movement of mud on the bed of the river, as expected from flow measurements and model experiments made by the Hydraulics Research Station. 2. The investigation of sand dispersion under the sea: For this an area of flat sandy sea-bed off the south coast of England was used. It was hoped that the experiment would show the use and limitations of the method, and also the general direction and extent of sand movements off shore. These two problems, although similar, require different techniques of radioactive handling and detecting, and will be considered separately.

SILTATION

(a) Choice of Tracer The choice of tracer material for the experiment in the Thames Estuary was governed by the four principles mentioned earlier. It was first necessary to find a physical form of material which would behave in a similar manner to the mud, which exists in two forms. One of these is fluid mud, a thin watery mud easily transported by currents or tide, and generally localized in mobile patches which can be identified from the river surface with echosounding apparatus. Where deposition is predominant, fluid mud is continuously settling to form consolidated mud, which is not moved by flowing water except under conditions of severe erosion. It is this mud which has to be dredged from the channel. A sample of fluid mud was examined and

INVESTIGATION

found to consist of aggregates in which particles of sand were enmeshed with hair and fibrous organic material. The organic material was removed with hydrogen peroxide, leaving the particles of sand, and the size distribution of these was measured by settling tests. The median diameter was about 40 #. A sample of glass was ground to a similar size distribution, and comparative settling tests were carried out with irradiated glass mixed with mud (31Si tracer, half-life 2.62 hours), and with irradiated m u d (31Si tracer in the sand). It was established that the settling rates were very similar. (b) Injection Equipment It was required that the tracer material should behave exactly as ordinary mud from

* Both experiments were m a d e in collaboration with the Hydraulics Research Station, Walllngford, Berkshire.

27

Radioactive tracer techniquesfor sand and silt movements under water

the moment of injection, and that it should be released directly on the river-bed. In order to ensure that sufficient organic material was present to support the glass particles, the tracer was therefore mixed with about 50 kg of mud before injection. Because of the difficulty of screening the

chamber via the small lid shown in the figure. Several red and white table-tennis balls were also placed in the chamber to give surface indication of its opening under water. Finally, the radioactive glass was added. The active glass was stored in aluminium cans in two large lead pots until required. RELEASE MECHANISM

FILLING APERTURE

FIG 1.--Apparatus for mixing and injecting radioactive mud onto the river bed. (Two notes on diagram showing (a) release mechanism, (b) filling aperture.)

gamma rays from this large volume of radioactive material, the mixing was done on a boat immediately prior to injection, using remote control to reduce the exposure of personnel to a minimum. Fig. 1 shows the injection apparatus for the remote handling of the mixing operation at 8 ft distance. The apparatus consisted of a cylindrical chamber closed by lids at each end. These were secured by springs and could be opened when on the river bed by pulling the release wire. The water then had easy access to the mud in the chamber and could wash it out quite freely. When filling the apparatus, the two main lids were sealed with rubber gaskets and a sealing compound, and 30 to 50kg of inactive mud and water were tipped into the

The lids of the pots were lifted with long rods and the can lids were unscrewed remotely. A tubular windshield was placed over the top of each can before pouring its contents into the chamber. After closing the chamber, it was rocked for about 1 min, and then swung outboard on the ship's derrick and lowered to the river-bed. The release cable was pulled and, after the appearance of the table-tennis balls, the chamber was tipped by further pulling the release cable to clear any remaining mud. A few minutes later it was pulled up and hosed clean. (c) The Detection Apparatus Since the labelled mud was expected to move and settle close to the river-bed, some form of detector that could be dragged along

28

J. L. Putman and D. B. Smith

the bottom was required. Gamma-ray Geiger counters were used in preference to scintillation counters because of their greater robustness, and the ease of constructing a detector which is sensitive over a large volume. Three detectors were constructed, each consisting of three low-voltage gamma-ray Geiger counters (type G24H), working at about 400 V into a cold cathode-valve circuit. This operated a portable ratemeter (type 1292B), which gave a nearly linear response over a wide counting range. In order to extend this range further, it was arranged that two of the Geiger counters could be switched out of circuit, thus reducing the counting rate approximately by a factor 3. About 100 ft of screened multicore cable connected the detector to the ratemeter, and this was enclosed in rubber hose-pipe for waterproofing. Mechanical strength was provided by a length of Bowden cable in the hose-pipe, although this was not normally used for raising and lowering the detector. The counters were housed in a strong brass case with rubber ‘0’ ring seals, and were weighted with lead. For measurements, a detector was lowered on a cable from a davit amidships, and little strain was taken on the recording cable. When working in the current, a thin wire was led forward to the bows of the boat to keep the main lowering cable as nearly vertical as possible. (d) Experimental Procedure in the Preliminary Investigation The initial experimentt3) was carried out in July 1954, with the object of establishing the validity of the method for tracing mud movements and obtaining as much information as possible on the movement of the mud, the “dilution” experienced by the tracer, and the technique of handling and detecting the labelled mud. In order to minimize the risk of losing the tracer completely, the injection was timed for slack low water, SO that mud would be initially swept upstream by the flood tide. The tracer material was prepared by incorporating 5 g of scandium oxide in 100 g

of soda glass, which was ground until the particle-size distribution was the same as that of sand in the mud. 85 g of the ground glass was irradiated in the Harwell pile for a period of four weeks in a neutron flux of 1.2 x 1012 n/sec/cm2, and was allowed to decay for two weeks to eliminate the shortlived radioactive products. The glass was examined with mirrors in a remote handling cell, and it was confirmed that it had not sintered. The total activity was then about 4 curies. On the boat used for injection, the derrick was attached to the mixing chamber so that the latter was left free to be rocked for mixing, and could be operated without anyone approaching nearer than 8 ft to the unshielded activity. The boat was anchored at the point chosen for injection, and the mixing chamber, loaded as described earlier, was swung outboard and lowered below water. The whole operation from lifting the lids of the lead pots took only 24 min. No measurable radiological dose was recorded by either the films or pocket ionization chambers of the operatives. These were capable of measuring doses down to 0.01 rontgens. The activity was released on the river-bed, and the floating table-tennis balls recovered. The chamber was raised and hosed down and found to be only slightly active. There was no contamination on the boat except for a trace on the tarpaulin used under the mixing apparatus and lead storage pots. As soon as the injection had been completed, a detector was lowered from another boat to trace the activity. On the previous day, background measurements had been made over a wide area and at various depths and showed only small variations of the river. The at different points background decreased with increasing depth, and had fallen at 30 ft to one third of its level on deck. The method of searching was to lower the detector to the river-bed with the boat drifting with the current. The instrument was dragged along the river-bed and timed readings were taken, and correlated with the depth and nature of the bed, as interpreted from the echo-sounding equipment.

Radioactive tracer techniquesfor sand and silt movements under water Positions were fixed by Decca navigator and by sextant. Little activity was located immediately after injection, b u t some hours later the main concentration was found to be lying in a newly dredged cut situated about half a mile upstream of the injection point. It was associated with a patch of fluid mud in the area. Further searches showed that fluid mud found upstream showed slightly higher readings than that of hard bottom, but as no background had been taken specifically in this mud, no conclusions could be drawn. No tracer was found downstream. The activity continued to stay in the newly dredged cut, and after about four days it was found to be consolidating. Measurements were discontinued after about two weeks, and at this time the activity in the area still gave readings well above background. (e) Large-scale Experiment The results and experience gained by this preliminary experiment could now be employed in the design of a large-scale experiment from which it was hoped to determine the movement of mud in the Gravesend area of the estuary, and to investigate whether mud from the lower reaches of the estuary could be carried up to points of accretion in the upper reaches near Barking. 18 g of Sc203 was dissolved in 845 g of glass, which was again ground and carefully graded to size-match the sand in the mud. This weight of glass increased the number of particles to a total of about 101° with sizes above 10/,, which ensured that there would be no complications arising from statistical sampling by the detectors. This tracer material was irradiated for ten weeks in a neutron flux of 1.2 × 1012 n/sec/cm 2, and the total ~nSc activity produced was 29 curies. The initial experiment had shown thgt there should be no areas of rapid accretion near the injection point, or much of the activity would stick there. Hence, dredging in Gravesend Reach was discontinued about one month before the experiment was carried out. I f traces of activity were to move

29

far upstream, it was desirable to concentrate them so that identification might be more certain, and a cut was therefore dredged in Barking Reach, f o u r t e e n m i l e s upstream from the injection point. The activity was to be injected off Tilbury Docks in mid-stream just after high water, and would initially be swept downstream. U n d e r these conditions, significant amounts of radioactivity would be found above the injection point only if a landward drift of mud occurred. An extensive background survey was made first, and showed that liquid mud gave readings slightly higher than average, and that some coaling jetties were almost up to twice normal background. Readings were taken at all points of interest and were subsequently used when intei'preting the results. The injection of the activity took place in J u l y 1955, using the same procedure as for the preliminary experiment. The highest radiological dose received by any of the operatives was 0.07 r6ntgens. Tracing was carried out initially from two boats, one covering the downstream search, and the other the upstream. The activity moved initially downstream, and was traced on the ebb tide for a distance of six miles. It was not possible to investigate beyond this point owing to the hazards of working in darkness in the estuary. At the second high tide following injection, activity was found associated with fluid mud as far as four miles upstream. The bulk of the activity settled in Gravesend Reach, as might be expected, since this is an area of accretion. As the search continued, the amount of, activity found upstream gradually increased, and after seven days meter-readings up to twice background were found at the dredged cut at Barking, fourteen miles upstream. During the following month, the activity continued to increase upstream of the injection point, and to decrease below it, and readings of twice background were found over a wide area at Barking. Later surveys showed a steady decrease of levels of radioactivity, but four months later, local activity

30

J . L. Putman and D. B. Smith

at Barking was still almost twice background, of activity were found at different times, although by this time much of the labelled and clearly shows the initial movement material must have been buried deeply in of the material upstream, and the final the consolidated mud. The steady level of distribution of all the detectable material high readings obtained at Barking may show in the two mud reaches at Gravesend and that the labelled mud was still being brought Barking. upstream from its original bed in Gravesend • A fuller discussion of the hydraulic Reach. aspects of these results will be found in the Fig. 2 shows the distance above and below report on the experiment published by the the injection point where significant amounts Hydraulics Research Station. ~4~

15

.j.~

g

z6

~-I0 /

~0

o

° G ®

f /o

o

/

o

o

®

~

f

.i

o

O

o

o

i

MUD REACHES AT BAI~KINC t

°

° o// o / e

/

5 e ~// °/ O Xl--

~o

oo

ooo~

I

i °

~

~ ° ~ ~°

°o

~SHOAL AREA AI' ~ t GRAVESEND

o

®

o

o

o

o Q

a..v. O~

..a---u-

o

O

o

,

,

,

,

,

.

i

i

t

i

7 TiME

t

i

i

|

i

i

t

,

i

i

I

i

i

14 21 (DAYS AFTER INJECTION )

i

i

i

~

i

,

,

,

i

i

28

Fro. 2. M o v e m e n t of radioactive tracer in the Thames Estuary. Points represent positions at which significant quantities of radioactivity were detected during the m o n t h after injection. (Notes on the figure as in accompanying sketch.)

2. T R A C I N G U N D E R W A T E R

The study of the large-scale movement of sand offshore has long been restricted from the lack of a suitable tracer material. 46Sc-labelled ground glass appeared to be suitable for such studies, and a preliminary small-scale experiment was carried out to obtain experience which could be applied to more general sand-movement problems. The area selected for the experiment was off the south coast of England where the sea was shallow and a flat sandy bed extended well out from the coast. The activity was to be placed on the sea-bed some 2000ft from the shore. The particle sizes of natural sand in this

SAND MOVEMENTS

area were up to 800 #, with a median diameter of 180 #. 84% by weight of the sand consisted of particles with diameters between 100 # and 250 #. 12~o had diameters above 250/~. For the tracer, about 2 curies of 46Sc were used in 130 g of glass ground to simulate the sand-particle size distribution up to 250 F. Larger particles were excluded because of the possibility (considered significant before the experiment) that the tracer material might be partially deposited on the beach. If this were to happen, the general activity would be spread over an area too large to constitute a general radiation hazard, but a large

Radioactive tracer techniquesfor sand and silt movements under water

particle remaining attached for some weeks to the human skin might have resulted in local erythema. It was considered that the exclusion of the 12% fraction of particles from 250/~ to 800/~ would not greatly affect the validity of the tracer. The tracer thus consisted of about 2 × 10v, particles with diameters between 100/~ and 250/~, and from the statistical considerations

31

of the sand occurs principally along the surface of the bed, and it was desirable to spread the tracer over an area which could be mapped by the detector, but which was still small compared to the migration of the sand. In view of the larger particle sizes, release on the bed was unnecessary and even undesirable. Release at a few feet above the bed was preferred, to allow the sinking

FIG. 3. Sledge and waterproof detector, cut away to show the three Geiger counters.

discussed above, a large detector was required, to measure over a large area of sea-bed. Low-voltage Geiger counters, type 60H, 6 0 c m long, were available, and a waterproof container housing six of these was made to operate through 60 ft of hosepipe-covered cable into a ratemeter type 1292B. A switch was incorporated so that .either one or all the counters could be used, depending upon the level of activity being measured. The use of six counters proved to give unnecessarily high counting rates, and a second detector was made up using three counters. This detector was spring mounted on a heavy sledge (see Fig. 3), so that there was about 2 in. clearance between the sand and the detector casing. The sledge could be towed on 50 ft of chain in depths of up to 20 ft of water at speeds up to about 2 knots without leaving the sea-bed. Injection Preliminary settling tests showed that the ground glass behaved similarly to natural sand. It was therefore unnecessary to mix the tracer with natural sand before injection, which simplified the operation. Movement

particles to disperse and spread thinly over the bed. Injection was made just before low water, when the depth was 19 ft. It was done by breaking a glass ampoule, into which the tracer material had been sealed before irradiation, under water 8 ft from the sea-bed. On a boat over the injection point the ampoule was removed from a lead container and rapidly slipped into a long aluminium tube held vertically over the side of the boat. A sharp blow with a plunger crushed the ampoule against a steel bolt running across the lower end of the tube, releasing the tracer material. Owing to a slight current, estimated to between ½ and 1 knot, the radioactive glass particles were spread over an area about 500 ft long by 100 ft wide. There is no doubt that size-sorting of the particles occurred during deposition, and for later tests release nearer to the sea-bed would be recommended. Survey A survey of background in the area prior to injection showed it to be constant. After injection, the activity was mapped

32

j . L. Putman and D. B. Smith

out by running the boat along a series of traverses set out by a shore-based theodolite, and fixing the position of the boat along the line by sextant angles. Subsequent surveys were carried out for some days, and showed little movement of the sand. One month later, and again four months later, surveys

Conclusions It is established that glass, ground to conform with natural particle-size distribution, provides a reliable tracer for movements of natural sand of a large range of particle sizes and under widely differing conditions.

N

...:....'"......... --

+

."

INJECTION POINT

-..-

O

i

i

IOO METRES

200

FIG. 4. Distribution of radioactive tracer before and after south-east gales (the contours represent readings of twice background). -Distribution of activity after one day. . . . . Distribution of activity after four months.

were carried out after the coast had been subject to gales and heavy wave action. The results show that the movement of the sand was very limited. Fig. 4 shows contours which represent readings of twice background on the day of injection, and after four months. The latter readings were corrected for decay, and represent the limit of certain detection. The amount of activity rises rapidly inside the contours, and even after four months, actual instrument readings of up to five times background were obtained. Both the amount of activity and the number of particles were quite adequate.

Acknowledgements--The authors wish to express their thanks to their colleagues at Hydraulics Research Station, Wallingford, in collaboration with whom these experiments were carried out. They are also indebted to the Port of London Authority for their support of the investigation into the silt movement in the Thames Estuary, and to Messrs. R. M. WELLS and A. J. STONE for assistance in the experimental investigations. Thanks are due to Dr. PARTRIDGE of the General Electric Company, who dissolved the S%Oa in the large mass of glass required for the principal experiment in the Thames Estuary.

REFERENCES 1. GOLDBERG E, D.

a n d INMAN D. L. Bull. Geol.

Soc. Arner.66, 611 (1955). 2. ARROLW. J., private communications. 3. PUTMANJ . L., SMITH D. B., WELLSR. M., ALLEN

F., a n d ROWAN G., Report A.E.R.E. I/R.1576

(1954). 4. Hydraulics Research Station, Wallingford, Berks. Report HRS[PLA 20.