Ti rIO Zt)* For Private Circulation o 400 THE INSTITUTION OF WATER ENGINEERS

(INCORPORATED 1911)

RADIOACTIVE STRONTIUM IN THE

BY

N. T. MITCHELL, B.Sc., Ph.D., A.R.I.C., and G. E. EDEN, B.Sc., F.R.I.C.

[Reprinted from the Journal of the Institution of Water Engineers, Vol. 16, No. 2, March, 1962.)

1962

PUBLISHED AT THE OFFICES OF THE INSTITUTION, PARLIAMENT MANSIONS. ABBEY ORCHARD STREET, , S.W.1 175

RADIOACTIVE STRONTIUM IN THE RIVER THAMES BY N. T. MITCHELL, B.Sc., PH.D., A.R.I.C.,* AND G. E. EDEN, B.Sc., F.R.I.C.*

CONTENTS Page Page Introduction .. 175 Treatment of the Experimental Sources of Radioactive Con- Data .. 181 tamination .. 176 Discussion of Results 182 Design of the Survey .. 177 Conclusion .. 186 Examination of the Samples .. 179 Acknowledgments 187 References .. 187

INTRODUCTION THE LEVEL OF RADIOACTIVITY in the River Thames is of particular interest, since this river not only serves as a source of a large proportion of the water supply of London but also provides a means of disposal of effluent from several establishments of the United Kingdom Atomic Energy Authority. Fall-out from the world-wide dispersion of activity resulting from nuclear weapons tests also contributes some radioactivity to the river. The investigation described below was undertaken at the request of the Ministry of Housing and Local Government and was designed primarily to establish, if possible, the extent to which radioactive material discharged from the Atomic Energy Research Establishment ("A.E.R.E.") at Harwell persisted during passage of the river water downstream. It had been suggested, for example, that much of this activity was retained on bottom deposits in the river, and was thereby localized near the point of discharge. Because of limited facilities, the investigation was confined to a study of the two most important isotopes of strontium, namely Sr 89 (half- life 51 days) and Sr 90 (half-life 28 years). An investigation of the behaviour of Cs 137, the principal radioactive constituent of the Harwell effluent, was undertaken by A.E.R.E. at about the same time (Morgan and Arkell 1961)t. Both isotopes of strontium are formed in the fission of heavy nuclei, such as U 235, and are therefore present in the products of nuclear explosions as well as in the wastes from operations concerned with the peaceful exploitation of nuclear power. Though as a result of the process of fission both radioisotopes are formed in approxi- mately equal weights, the disintegration rates differ widely. Thus in terms of the number of disintegrations per second resulting from one * Water Pollution Research Laboratory, Stevenage, Herts. t An alphabetical list of references is given on p. 187. 176 MITCHELL AND EDEN ON

100

tn 0 f.7) (I) Sr 89 (half -life 51 days) '0 10

Sr 90 (half - life 28 years)

tn z*\ Ratio Sr 89\ u, Sr 90 \ 6 < cc

6 \ 0 B 2 3 TIME AFTER FISSION (years) Fig. Relative activities due to Sr 89 and Sr 90 in fission products million fissions, the disintegration rates of Sr 89 and Sr 90 as calculated at the time of fission are 650 and 4.0 respectively, a ratio of 162/1. However, as radioactive decay proceeds (Fig. 1), these disintegration rates are progressively reduced, the Sr 89 decaying more rapidly than the Sr 90. For example, at one year after fission the ratio is 1.2/1. Thus the ratio of Sr 89 to Sr 90 is an indication of the time which has elapsed since fission and may, therefore, serve as a means of differen- tiating different sources of fission-product activity. It may be added here that in a mixture containing both Sr 89 and Sr 90 the radioisotopes are chemically indistinguishable, and cannot be separated by normal chemical and biological mechanisms. There is, of course, a special interest in these radioisotopes since they are potentially the most dangerous of all the fl-emitters, as is recognized by the extremely low concentrations which are permissible in drinking waters.

SOURCES OF RADIOACTIVE CONTAMINATION An exhaustive study of the occurrence of radiostrontium in the Thames requires consideration of the following sources:— RADIOACTIVE STRONTIUM IN THE RIVER THAMES 177 (a) FALL-OUT Activity falling in rain enters the main stream of the river through tributaries, by direct fall-out on the water surface, and in surface water from built-up areas. Some of this surface water enters sewerage systems and eventually reaches the river via sewage-disposal works. (b) CONTROLLED DISCHARGES The Atomic Energy Research Establishment at Harwell and the Atomic Weapons Research Establishment ("A.W.R.E.") at Alder- maston are authorized by the Minister of Housing and Local Govern- ment and the Minister of Agriculture, Fisheries, and Food, under the Atomic Energy Authority Act, 1954, to discharge controlled amounts of activity in effluent directly to the River Thames. Information made available by the Ministry of Housing and Local Government showed that the amount of /3-activity discharged from A.W.R.E. was very small and this discharge was therefore disregarded. The Radiochemical Centre of the Atomic Energy Authority at Amersham likewise discharges active effluent in authorized amounts to the local sewer. The sewage from this area is treated in the works of the West Hertfordshire Main Drainage Authority at Maple Lodge, the effluent from which is discharged to the River Colne, a tributary of the Thames. (c) OTHER DISCHARGES It was thought that small quantities of radioactivity might perhaps enter the Thames from laboratories in which radioactive tracers are used. For example, laboratory wastes discharged to the sewers in Oxford would appear at the sewage-treatment works of the Oxford Corporation at Sandford-on-Thames and might lead to contamination of the final effluent discharged to the river. No other major source of radioactive effluent in the Thames basin was known to the Ministry. DESIGN OF THE SURVEY It was apparent at the outset that the quantitative examination of this extensive and complicated system presented considerable diffi- culties, and that a satisfactory outcome could be expected only if reasonably steady conditions prevailed. In this connexion it was fortunate that much of the survey was carried out in a period during which the rainfall was very low and the rate of flow of the river com- paratively steady. It was not possible to determine directly the input of radioactivity from tributaries, since it was not possible to monitor these continuously. The following programme of gauging and sampling was operated for a period of 19 weeks beginning on 17th July 1959. RIVER WATER Five locks, including those at Eynsham and , were selected on the advice of the Thames Conservancy as being suitably placed and best equipped for accurate gauging and sampling. The 178 MITCHELL AND EDEN ON

EYNSHAM LOCK 5

m,L'ES

MAPLE LCOGE SEWAGE WORKS AMERSHAM

ALLINGFORO ARWELL

TEDOINGTON LOCK

Fig. 2 Sketch map of River Thames showing sampling points for radiostrontium survey marked X. Water abstracted for domestic supply at points marked W three intermediate locks were those at Day's Weir (6 miles below the point of discharge of the Harwell effluent at Sutton Courtenay), at Caversham (Reading), and at Bray. The positions of the various sampling points are shown in Fig. 2. Arrangements were made for the lock-keeper at each of the five locks to take a 1 gal. sample of water at 9 a.m. each morning and, except at Teddington, to send to the office of the Chief Engineer of the Thames Conservancy, at Reading, data from which the flow of the river at the time of sampling could be calculated. The time 9 a.m. was chosen as being most suitable for sampling, since the weir-gate settings would not normally have been altered for at least 12 hours, and locking would usually not have begun. At weekly intervals the samples were brought to the laboratory for examination and the rate of flow at the time of collection of each sample was calculated by the Thames Conservancy. For samples collected at Teddington a pro- visional value estimated by the for the "natural flow of the river" was taken. This is the rate of flow which would have been observed had there been no withdrawal of water for public supply. In addition, snap samples were taken from the major tributaries of the Thames on three occasions. RAIN WATER Two large collecting funnels draining into a Polythene bottle were set up at the sewage-disposal works at Sandford. The sample collected in each weekly period was taken for analysis, and was assumed to be representative of the composition of rain water falling over the whole catchment area. Figures for the daily rainfall for the catchment areas above the weir at Eynsham, above Day's Weir, and above Teddington Weir were provided by the Meteorological Office. RADIOACTIVE STRONTIUM IN THE RIVER THAMES 179 SEWAGE EFFLUENTS Only two sewage works, those at Sandford and at Maple Lodge, were examined, since it was assumed that other works would be con- taminated to a lesser extent. Examination of samples ultimately showed that the amount of activity discharged to the river from both works was negligible, and that the neglect of other sewage effluents was justified. At each works a 1 gal. composite sample of final effluent was collected by the staff each day, and was returned to the Laboratory weekly for examination. Rates of flow were obtained from the flow meters at the works. HARWELL EFFLUENT The Harwell effluent is discharged in monitored batches, and enters the Thames at Sutton Courtenay. Arrangements were made for a composite sample representative of the total weekly discharge to the river to be prepared at Harwell. This sample was collected weekly, together with a note of the total volume discharged.

EXAMINATION OF THE SAMPLES For each lock a composite weekly sample was prepared by mixing the seven sub-samples in volumes proportional to the rate of flow at the time of sampling. Usually about 30 litres of composite sample were obtained. After addition of 30 mg. stable strontium carrier, the sample was filtered through a Metafilter type C charged with kieselguhr (Metasil B). This filter was believed to retain quantitatively particles larger than 0.5 micron diameter. The activities due to Sr 89 and Sr 90 in both suspended matter and filtrate were then determined by methods based on standard radiochemical procedures. Cations in the filtrate were concentrated, first by cation exchange and then by evaporation, and the strontium was separated from calcium by the usual nitric acid method (Willard and Goodspeed 1936). After further purification stages the strontium was set aside for several days to allow growth of the daughter Y 90 from the Sr 90. The Y 90 was separated by extraction with a solution of 2-thenoyl-trifluoracetone ("T.T.A.") in benzene, and was precipitated as the oxalate. The activity of the precipitate was determined in an anti-coincidence counter based on the A.E.R.E. type 1224 A. The activity due to Sr 90 in the original sample was then calculated, making corrections for counter background, chemical yield, and for growth and decay of Y 90. The strontium activity was precipitated as the oxalate, which was similarly counted. In this case additional corrections were applied for the stable strontium content of the original sample, and for the presence of calcium in the final precipitate, the latter estimated from a volumetric determination of the equivalent weight of the calcined strontium oxalate. The activities due to Sr 89 and Sr 90 in the original sample were thus determined. All measurements of Sr 89 (and of Sr 89/Sr 90 ratios) were expressed in terms of the notional activities on 20th July 1959.

▪

WEEK No.(Week No.1 began on 17th July 1959) WEEK No. (Week WI began on (7th July 1959) 100 6.713 ,9R1 y2,13,14)5,16 ,17,18,19 , ./ HARWELL EFFLUENT UNWELL EFFLUENT 50 ' .11Z:""."1' • KA' 'IN.' IAANT... SS.A. KT..... A'S.,St. VAS" :IS SS. ; t s's " 1 A ■NNASSAISsw,A NNSJIIMess.. .. MA skkkA ' E NSHA EYNSHAM 400 '.... 2 )-- ■ ■ 11 ...4. -•••■.1 A.- AS, ...,.; N . ..\N■\ L...... e,h.. 1....■..;/..W.St.:.0..v. Ntls..V 1 \ \ )....I \ 111 \\ \,\ \ \ \ \ DAY'S 80 I \ 1 \II \ \ \\ I I 1 I I I\ \ \ \\ I \ 1 \ \ 400 )— 1 I ‘ $ \\ \\ \ N \ \ \1 1\ I I III; 1 I \\ N .. 41 A • \ \ \ \ \ ) N & .E., SA s.,, .wilIW A MA `• -.: ...... 0 ‘ ■ \ A \\ I I I I 1 \ \ I C AVERSH AM V\ \\ \ C AVER SHAM 1 I % 1000 ,, \\ \\ \ \ \ I1 I - , • " '' \ \ \ % \ \\\ \ \ I N-A NI 14.a. s \ \ \ \ 1 N, \ I \ ,, \\ , 500 s•A' ,, ■•■ • • \ v, I N , • k... k1 N . k;RN, .,NI' Rklh ' ,,s, N1/4.,ot \ ,, Ui 1 4 . UI .... ,tVgt,...... k...... f ,. 1 • 0 0 1 , I 1 1 1 . 1 , \ 1 4000 1 i 1 I 1 I \ I I ) I I 1 „. 2. 1- 1 \ I BRAY cr 1 1 1 1 1 1 UI 1 , , 0 1 i1 1 i1 t 1 , 0. .9.4; \ , 1 , I ' \ " 1 N I. 1 2000 I N' :N ', , , I z N 1 t , Z ` IS'iVIS's: - A; .... •-• '11 , ..,, .'-`, 1 ' 1 \I" ■ \ .., N vQk.q..%1N , , u- 0 ■ •, •,N 1k.!. ktk. N cc 00 • . ,`.&1111111,\,&'&, ‘,, 0 _ 1 i 1 TECOINGTON ▪ 4000 6 4. I 1 I 1 1 1 i T EWING - 1 1 I 30 0 TON I I 1 1 1 1 1 1 1 1 1 1 1 1 I I I .1 •1- ° 6 - I I I I 2000 u 2.0 I I I I I I I I ; i i ■V 1 , I 1 1 I . 1 1000 N s NI ‘ L 00 1 2 3 4 5 6 7 8 9 101112 13141516r7 1819 1 2 3 4 5 67 8 9 101112 13141516171819 WEEK No. (Week No.1 began on 17th July 1959) WEEK No (Week No.1 began on 17th July 1959 Fig. 3 Fig. 4 Rate of flow of Harwell effluent and of River Concentration of Sr 90 at five sampling points Thames at five sampling points on River Thames and in Harwell effluent RADIOACTIVE STRONTIUM IN THE RIVER THAMES 181 The suspended matter was leached with an acid solution of strontium carrier; this solution was then examined in a rather similar way. However, after a few weeks it was shown that the activity in suspended matter was negligible, and regular determinations were discontinued. Samples of Harwell effluent were similarly treated, except that sintered glass filters were employed. Rain-water samples were filtered through bacteriological membrane filters and the ion-exchange con- centration step was omitted. All the analyses, with the exception of those of the Harwell samples, were carried out in a special laboratory remote from those in which radioactive tracers were used. It was hoped, in this way, to minimize the possibility of contamination at the very low levels of activity which were encountered. TREATMENT OF THE EXPERIMENTAL DATA Since, at the rates of flow measured during the survey, the time of passage of an element of water from Eynsham to Teddington was estimated at about 4 weeks, it was necessary, before comparing levels of activity at various points on the river, to make allowance for this delay. The method of calculation may be clarified by reference to Fig. 3, in which the weekly flows at the five locks are shown graphically. Taking the mid-point of each weekly period at Teddington, the time delay between Bray and Teddington was roughly computed using measurements of volume provided by the Thames Conservancy, and an estimated mean flow between Teddington and Bray. From this tentative estimate of the time delay a better estimate of the flow at Bray could be made, and hence by an iterative procedure the delay was calculated. This procedure was then repeated using the calculated time at Bray, to calculate the time delay between Bray and Caversham. Thus, each mid-point of a weekly period at Teddington was extrapolated back as far as Eynsham if possible. Since weekly periods before Week 4.5* could not be extrapolated back as far as Eynsham, complete extrapola- tions were available only for Weeks 4.5 to 18.5 at Teddington corre- sponding to Weeks 0.9 to 14.3 at Eynsham. These extrapolations are indicated on the figure by broken lines. In Fig. 4 these extrapolations are superimposed on graphs of con- centration of Sr 90 at the five locks and in the Harwell effluent. Except for the Harwell discharge of Week 11, which was detected at Day's and Caversham Weirs, it was not possible to observe the passage of peaks of activity downstream, because of the combined effects of longitudinal mixing, dilution, and the bulking of samples into weekly composites. Estimates were therefore made of the total amounts of Sr 89 and Sr 90 passing the various sampling points during the periods corre- sponding to Weeks 4.5 to 18.5 at Teddington and shown in Figs. 3 * The 19 weeks of the survey are denoted serially by number, Week 1 beginning on 17th July 1959. For computations the weeks are further subdivided: thus the mid-point of the 8th week is designated Week 7.5. 182 MITCHELL AND EDEN ON and 4 bounded by the heavy broken lines. These periods are referred to subsequently as "comparable periods". The results are given in Table I. Several assumptions are implicit in these calculations and, in particular, no allowance has been made for longitudinal mixing during passage downstream. There is, in fact, no basis on which such an allowance could be made. However, in view of the relatively steady conditions prevailing during the survey it seems probable that errors due to this mixing at the start of a comparable period would be approximately balanced by similar errors occurring at the end.

TABLE I AMOUNTS OF SR 89 and SR 90 PASSING SAMPLING POINTS DURING PERIODS OF ABOUT 14 WEEKS COMPARABLE WITH THE PERIOD 19TH AUGUST-23RD NOVEMBER 1959 AT TEDDINGTON

Sampling Activity, Volume, Mean Ratio point mc. m.g. concentration, Sr 89/Sr 90* p.pc./1.

Sr 89* Sr 90 Sr 89* Sr 90

Locks:- Eynsham 14.0 6.1 3,092 1.0 0.4 2.3 Day's .. 69.8 64.0 5,258 2.9 2.7 1.1 Caversham 77-6 67.2 7,967 2.1 1.9 1.2 Bray .. 152.6 109.2 24,260 1.4 1.0 1-4 Teddington . . 294-5 163.5 42,360 1.5 0.9 1.8 Harwell effluent . . 31.4 45.9 45-2 153 224 0.68 Sewage works effluents:- Sandford 2-4 1.1 474 1.1 0-5 2.2 Maple Lodge 8.3 4.6 1,590 1.2 0.6 1.8

* As on 20th July 1959. The amounts of activity entering in the two sewage effluents were comparatively small and were therefore not further considered.

DISCUSSION OF RESULTS The figures in Table I show that the ratio of Sr 89 to Sr 90 in the Harwell effluent was markedly different from that in the river water to which it was discharged, as exemplified by the water at Eynsham. Furthermore, the amount of Sr 89 discharged in the effluent was similar in magnitude to the amount reaching the discharge point in river water during the period in question. In the case of Sr 90 over seven times as much was present in the Harwell effluent as in the river water passing Eynsham. As a result the ratio of Sr 89 to Sr 90 measured at Day's Weir was appreciably different from that observed at Eynsham. On passage downstream the mean Sr 89 to Sr 90 ratio progressively rose; though at Teddington the mean value had not reached that observed at Eynsham, it is doubtful whether statistically the difference between these two points is truly significant. RADIOACTIVE STRONTIUM IN THE RIVER THAMES 183 The interpretation of the data is complicated by the entry into the Thames of tributary water containing Sr 89 and Sr 90. Thus, during the comparable periods the Sr 90 activity passing Day's Weir was 64.0 mc., whilst that passing Teddington was 163.5 mc. The most probable explanation of the progressive increase in the ratio of Sr 89 to Sr 90 on passage from Day's Weir to Teddington is that radiostrontium is not removed by bottom deposits, or by any other means, and that any activity discharged from Harwell will appear at Teddington after a lapse of time determined entirely by the hydraulic characteristics of the river. On its passage downstream the Harwell effluent is progressively diluted by tributary water containing fall-out activity, and possibly also by comparatively uncontaminated spring water. Assuming, therefore, that the ratio of Sr 89 to Sr 90 is approxi- mately the same for all tributary water, and that this ratio is higher than that in the Harwell effluent, the ratio in the main river will increase progressively on passage downstream. The assumption that there is no permanent uptake by bottom deposits is supported by subsidiary experiments in which it was shown that uptake by bottom deposit material is readily reversible and is small. For example, in river water containing 100 p.p.m. of re- suspended bottom deposit radiostrontium is divided between solution and suspension in the ratio of 99.4 to 0.6.

TABLE II AMOUNTS OF SR 89 AND SR 90 PASSING SAMPLING POINTS DURING COMPARABLE PERIODS, AFTER SUBTRACTING HARWELL CONTRIBUTION (ASSUMING NO LOSSES)

Sampling Activity, Ratio Percentage of point mc. Sr 89/Sr 90* original activity attributable to Harwell

Sr 89* Sr 90 Sr 89* Sr 90 Eynshamt ...... 14-0 6-1 2-3 — — Day's ...... 38-4 18-1 21 45 72 Caversham ...... 46-2 21-3 2-2 40 68 Bray ...... 121-2 63-3 1-9 1 21 42 Teddington ...... 263-1 117-6 2-2 11 28

* As on 20th July 1959. t Above Harwell discharge. If this explanation is accepted, it is permissible to subtract the amounts of Sr 89 and Sr 90 discharged from Harwell, from the activities passing sampling points downstream (Table II) and to calculate the ratios of Sr 89 to Sr 90 in the residual activity. The ratios obtained in this way are reasonably constant and are in good agreement with the value observed in the Thames at Eynsham above the Harwell discharge. These results, therefore, support the assumption that uptake by bottom deposits was negligible. The proportions of the total activity attributable at each point to the Harwell discharge are also given. 184 MITCHELL AND EDEN ON These figures are probably high compared with the average for the year, since they were obtained during a very dry period when the flow of the river tended to be small. In Table III the ratios of Sr 89 to Sr 90 at the various sampling points, after subtraction of the Harwell contribution, are compared with the value in rain water and with values obtained for snap samples taken at various times from some of the major tributaries of the Thames. In general, the values obtained for the snap samples are similar to those determined over a longer period in the main river. The value for rain water was appreciably higher, and the higher values observed in some of the snap samples of tributary water may therefore indicate the presence of recent rainfall. TABLE III RATIO OF SR 89 TO SR 90 IN WATER FROM VARIOUS PARTS OF THE THAMES SYSTEM

Sampling point Date or period of Ratio sampling Sr 89/Sr 90$ Main river:— Eynsham 2.3 Day's .. 2.1-1- Caversham 2.2t Bray .. Teddington 2.2t Harwell effluent 0.68 Rain water .. 17th July-26th Nov. 3.8 Tributaries:— Evenlode .. 6th Nov. 2.1 Cherwell .. 7th Sept. 2.8 Ock 7th Sept. 2.0 Thame 7th Sept. 3.5 Thame 29th Sept. 2.7 Thame 6th Nov. 2.5 Kennet .. 7th Sept. 2.3 Loddon 7th Sept. 2.6 Loddon 6th Nov. 4.7 Colne 7th Sept. 2.6 Colne 6th Nov. 2-5 Wey 7th Sept. 2.7 Wey 29th Sept. 3.9 Wey 6th Nov. 3.7

* Periods comparable with 19th August to 23rd November 1959 at Teddington. t Corrected for effect of Harwell discharge. As on 20th July 1959. The marked differences between the average ratio of Sr 89/Sr 90 in rain water, and the ratios observed in most of the snap samples from tributaries, require further consideration. Results of examination of the individual weekly samples of rain water show that when corrected for radioactive decay from 20th July 1959, and within the limits of error of the determinations, the ratio of Sr 89 to Sr 90 remained constant. That is, if the ratio had been measured at the time of sampling, the value would have fallen progressively throughout the RADIOACTIVE STRONTIUM IN THE RIVER THAMES 185 period of the survey according to the half-life of Sr 89. (The half-life of Sr 90 is sufficiently long to be negligible in this connexion.) This implies that the activity present in the rain water was derived from a uniform reservoir of fission products which was not supplemented by new material during the course of the survey. Under these conditions the reduced ratio of Sr 89 to Sr 90 in river waters cannot be explained by retention of radioactive strontium in the catchment area, since the proportion of Sr 89 lost by radioactive decay is the same whether the decay occurs before or after deposition. A more probable explanation is that the ratio is reduced by the slow leaching-out of "old" Sr 90 originating in explosions earlier than those which gave rise to the Sr 89 observed in this survey. This "old" Sr 90 may reside in bottom deposits in ditches or in minor tributaries. It seems unlikely that much Sr 90 is leached from arable land in view of the observations that radiostrontium is usually retained by the upper layers of the soil (Russell and Milbourne 1957). Morgan and Stanbury (1959) in their radiological survey of certain tributaries of the Thames in 1958 also observed a discrepancy between ratios of Sr 89 to Sr 90 in rain water and river waters. At this time, however, the atmospheric reservoir of fission products was being frequently replenished with material from weapons tests, with the result that the ratio of Sr 89 to Sr 90 at the time of sampling remained approximately constant. Under these conditions the discrepancy may be accounted for, as these authors have done, as the result of a time delay between fall-out and sampling. However, there may well have been some contribution of "old" Sr 90 by the mechanism suggested above, which under the conditions of 1958 would have remained undetected. In Table IV the amounts of Sr 89 and Sr 90 falling on the catchment area during the first 14 weeks of the sampling period are compared with the average amounts passing the sampling points during the

TABLE IV RELATION BETWEEN FALL-OUT AND ACTIVITY IN RIVER WATER IN APPROXIMATELY COMPARABLE PERIODS OF ABOUT 14 WEEKS

Sampling Fall-out on Activity leaving Percentage point catchment catchment in run-off area inc. river water,t mc. Sr 89* Sr 90 Sr 89* Sr 90 Sr 89* Sr 90 Eynsham .. .. 1,510 430 14 6 0-9 1-4 Day's ...... 3,140 860 38 18 F2 21 Teddington .. .. 8,230 2,200 263 118 3-2 5-4

* As on 20th July 1959. t Harwell contribution deducted. Direct fall-out on the surface of the main river was:— Between Eynsham and Day's Locks 1-6 mc. Sr 89 0-5 mc. Sr 90. Between Day's and Teddington Locks 7-6 mc. Sr 89 + 2-0 mc. Sr 90. 186 MITCHELL AND EDEN ON comparable period. In view of the comments made in the preceding paragraphs, and because of the impossibility of making proper allowance for the time delay between rainfall and passage past the sampling points on the river, no great importance can be attached to these results, which are presented mainly to illustrate the very low proportion of fall-out activity which finds its way into a river.

CONCLUSION The survey demonstrated that at the time the measurements were made there existed a progressive change in the ratio of Sr 89 to Sr 90 between Sutton Courtenay, the point of discharge of the Harwell effluent, and Teddington. This result could be explained in several ways, of which the most probable is that the radiostrontium passes downstream virtually unaffected by contact with mud and that all radiostrontium entering the river, whether it originates in the Harwell effluent or in fall-out, passes Teddington Weir (or enters the water- works intakes above the weir) after a time delay determined by purely hydraulic considerations. However, the actual concentrations of Sr 89 and Sr 90 found in in the regions where water is abstracted for public supply were at all times well below the accepted limits for lifetime consumption by a large population, even though the survey was carried out during a period when the rate of flow in the river was exceptionally low. The authorization of the Harwell discharge specifies that the con- centration of radioisotopes of calcium* and strontium in the river at the point of discharge shall not exceed 20 ,uktc. per litre (Kenny 1956) provided that no other isotopes are present. The concentrations quoted in Table I are clearly well below this figure. It may also be noted that since all figures for Sr 89 activity were corrected for radioactive decay as from a date very early in the sampling programme, levels of Sr 89 activity determined later in the programme were multiplied by a very large correction factor—as much as 5 in some cases. Activities at the time of sampling therefore were often considerably lower than the values quoted in the Tables. The conclusion that radioactive strontium is not retained on the bed of the Thames may be compared with the findings of Morgan and Arkell (1961), namely, that of all the Cs 137 discharged from Harwell during the past 12 years the proportion detected in the river mud within 25 miles of the outfall was about 3 per cent, though it was considered that this figure might be an underestimate. Furthermore, they obtained evidence that Cs 137 discharged from Harwell was detectable in the London drinking water. In view of the much greater affinity of river mud for Cs 137 as compared with Sr 90 (Water Pollution Research 1958) it is reasonable to expect that the proportion of Sr 90 retained on the bed of the Thames will be very small. * Radioactive calcium is not a fission product and is not likely to be present in appreciable amounts. RADIOACTIVE STRONTIUM IN THE RIVER THAMES 187 Additional points which have been brought out by the investigation are that:— (a) Radiostrontium in the Thames and in Harwell effluent was almost completely in solution; and (b) The amounts of radiostrontium entering the Thames in sewage effluents from Maple Lodge and Sandford-on-Thames were negligible in comparison with the amounts entering in tributary water and in Harwell effluent.

ACKNOWLEDGMENTS This investigation was made possible only by the active co-operation of many members of the staff of the Thames Conservancy, the Atomic Energy Authority, the Metropolitan Water Board, the City of Oxford, and the West Hertfordshire Main Drainage Authority. K. R. Brendish, Mrs. M. Gameson, B. R. Harvey, and A. P. Hopwood were responsible for the radiochemical analysis of the several hundred composite samples which were examined in the course of the survey. F. S. Davies was responsible for the detailed design and the main- tenance of the counting equipment, which was built in the Laboratory's workshops by F. Mundy. The paper is published by permission of the Department of Scientific and Industrial Research.

REFERENCES Kenny, A. W. 1956 Bulletin, Vorld Health Organization, vol. 14, 1007, "The Safe Disposal of Radioactive Wastes". P. Morgan, A., and Arkell, Mrs. G. M. 1961 A.E.R.E.-R3555, "Radioactive Effluent Discharged from A.E.R.E. Harwell into the River Thames. (1) Pre- liminary Survey". Morgan, A., and Stanbury, D. G. 1959 A.E.R.E.-M539, "The Contamination of Rivers with Fission Products from Fall-out". Russell, R. S., and Milboume, G. M. 1957 Nature (London), vol. 180, p. 322, "Rate of Entry of Radioactive Strontium into Plants from Soil". Water Pollution Research 1958, p. 93. H.M. Stationery Office, 1959. Willard, H. H., and Goodspeed, E. W. 1936 Industrial and Engineering Chemistry (Anal.), vol. 8, p. 414.