Some Oxygen Studies in the River Lark

Reprinted from the Proceedings of the Society for Water Treat n1 11/''' ‘X ‘:.(-71/ and Examination, Vol. 12, 1963. SOME OXYGEN STUDIES IN THE RIVER LARK

By WA Z. M. OWENS, B.Sc., M.I.Biol. RES 7 REF, ,,NT c1 and 4 ! 9 R. W. EDWARDS, B.Sc., M.I.Biol. No. (Water Pollution Research Laboratory, Stevenage)

INTRODUCTION It has long been accepted that the dissolved oxygen concentration in natural waters is an important index of water quality. Its distribution within a river represents the degree of unbalance between processes of supply and demand; only by a study of these processes will it be possible to assess the organic load which any river could receive without becoming seriously depleted of oxygen. In deep rivers it is sometimes possible to predict the distribution of oxygen from the biochemical oxygen demand of the water and the degree of surface aerationl. In shallower rivers the effects of bottom deposits, attached algae, and rooted plants are of much greater significance, particularly in lowland streams where water veloci- ties are low enough to allow fine organic material to settle. This paper describes studies, carried out by the Water Pollution Research Laboratory with the co-operation of the Great Ouse River Board, of the oxygen balance of two stretches of the river Lark, Suffolk. It is said that the first stretch (Fig. la) once provided good fishing but it has deteriorated of recent years and attempts to stock it with trout have failed. Until January, 1962, effluent from the old Bury St. Edmunds sewage works, which employed primary settlement followed by short retention lagoon treatment, was discharged into the head of the reach, but this works has now been replaced by a new works giving full biological treatment in percolating filters and discharging effluent much further upstream. Studies were made before and after the closure of the old works. The second stretch (Fig. lb) was sited about a mile below the effluent outfall of the new sewage works and was studied shortly after they had been opened.

OXYGEN BALANCE STUDIES

REACH 1. LACKFORD BRIDGE—MARSTON'S MILL On 9th and 10th August, 1961, samples of river water taken at two stations Fl and F2 (Fig. la) were analysed for dissolved oxygen, ammonia, nitrite and nitrate. Rainbow trout were transferred from well- aerated water to floating cages moored in the river and the numbers dead were observed at intervals; results are shown in Fig. 2. Very little sewage effluent was discharged into the river during the daytime, the lagoons being emptied at night. The first river water to contain high concentrations of sewage effluent (about 26 per cent) reached Station Fl at 2230 hours and Station F2 at 0430 hours; its arrival is clearly defined by the abrupt changes in the nitrate and ammonia concentrations that occurred at those times. It was further notable that the dissolved oxygen concen,-- tration of the river water had fallen to relatively low values—less than 5 ppm at Station Fl and about 1 ppm at Station F2—before water containing high concentrations of effluent had reached the stations. 126 (a) ..r.t.,,, 11":" ,,,-=..,.-- --0,...,-.... ,..... MARSTON'S E7 16 15 U,14 13 MILL ICKLING 12 WEIR ALACKFORD 17 FARTHINdk, • .BRIDGE BRIDGE V. 2 (14 A) 3

CAVENNAM STREAM

0 500 IC C." 1 . 1 k t i . T

CHIMNEY NENGRAVE BRIDGE - TO LAC,31=z24, . DUCKSLUICE FARM IR A EFFLUENT ,4e OUTFALL FOR „....- Au sAitas A TO BURY ST / EDMUNDS

Fig. 1. Reaches of river Lark in which experiments were made (a) Reach 1, below old sewage works discharge (b) Reach 2, below new sewage works discharge Arrows indicate direction of flow

(b) (a) — — -

--. .■•• 3 - -• NO ' 6 s -- -/— ■ .' — s. - s . • X .." : ...... o 4 — - - I o 2 • En

I I I I I I I I I I i l

- I I

ARROWS 910W TIME AT WHICH FISH REP PLACED IN CAGE

Z1. 22 24 2 4 6 8 10 2 20 22 24 2 4 6 10 12 9.8;61 10.8.61 9.8 ,61 10.8.61 Fig. 2. Survival of rainbow trout and variations in concentration of dissolved respiratory gases, nitrate and ammonia in the river Lark at (a) Station Fl, and (b) Station F2, 9th-10th August

127 No deaths of fish occurred while the dissolved oxygen concentration was above 2 ppm, although fish died rapidly when it fell below 1.5 ppm. This is in accord with laboratory studies of the susceptibility of this species to low dissolved oxygen concentrations2. The results also show that trout put into cages before the oxygen content of the river fell very low, and which were thus given an opportunity to become acclimatised to low but not lethal concentrations of dissolved oxygen, survived for a longer period after the oxygen content had fallen to lethal levels than did fish transferred directly to very low oxygen levels from well-aerated water. This has been observed during similar studies with rainbow trout in another stream containing an almost undiluted sewage effluent where low dissolved oxygen concentration was the principal lethal factor'. Any trout in the river Lark could not have endured the low levels of dissolved oxygen prevailing at night in this stretch. Further observations made on 9-10th August included the taking of samples of river water from a boat moving downstream at a rate approxi- mately equal to the average water velocity. This was achieved by labelling a body of water with NH4Br-82 and moving the boat downstream on its peak of activity. Water samples were analysed for concentrations of dissolved oxygen (see Fig. 6), ammonia, nitrite and nitrate. Although the body of water analysed contained very little sewage effluent, the main discharge of effluent being released after this sampling run had started, the dissolved oxygen concentration fell from 11-6 ppm at Station 3 to under 4 ppm at Station 11. It was felt that this remarkable oxygen demand warranted further investigation. On 19th and 20th September, 1961, concentrations of dissolved oxygen (Fig. 3), ammonia, nitrite and nitrate were again determined in samples of river water taken from a boat moving downstream at a rate approxi- mately equal to that of the river water. Two such runs were made; during the first the body of water analysed contained very little sewage effluent; the second was carried out during the main effluent discharge period. The effluent flow was monitored and samples were analysed for their organic carbon (Fig. 4) and oxygen contents (Fig. 3). It will be seen from Fig. 4 that not only does the flow of effluent increase markedly at night but the quality as reflected in the organic carbon concentration is then considerably poorer. Throughout the period of observation, samples of river water for dissolved oxygen analysis were taken above the effluent discharge, 200 yd below the effluent discharge, where effluent and river water were well mixed (Station 1), and 2 200 yd below the effluent discharge, immediately upstream of a tributary (Station 11). As during the earlier observation period the dissolved oxygen concentration of the river water had fallen to a very low value before the main discharge of the sewage effluent, though above the effluent discharge point the value did not fall below 6.6 ppm (Fig. 3). To estimate the oxygen demand of the river water, samples were sealed in dark bottles and suspended just below the water surface; every hour a sample was removed and its dissolved oxygen concentration determined. The rates of oxygen consumption of the organic deposits were estimated by measuring the oxygen consumption of mud cores, taken from this reach, in the laboratory using a mud respirometer4. The effect of changes in dissolved oxygen concentrations on the respiratory demand of the muds was determined. 128 • •

10 STATIONARY SAMPLING STATIONS - - - SAMPLES TAKEN FROM BOAT MOVING 03 DOWNSTREAM

RIVER ABOVE EFFLUENT 1 X

)35 "06 \ STATION 1 'OZ X,x X X X x \ X - - - -1 -X-X ' 3 11--K X X - \Ck8 I X X 7C 6 I .t \ STATION I I 3'k 1.11:1 2 x \ > - • ...3 . "-'2L - - ,- o EFF1-110 ,-_ 0 r *r"X 1 r rl-X-X-X-•x-X4X-X 1.0UT I - sx 6 -- x-- " - --,12. 1 1 1 1 a 6 • L . I. • I. ;; le #1. 1900 2100 2300 0100 0300 0500 0700 0900 TIME (B.s.T.) Fig. 3. Changes in concentrations of dissolved oxygen in the river Lark, 19th-20th September, 1961 Numbers on curves are sampling stations

X—X EFFLUENT FLOW 1,--• TOTAL ORGANIC CARBON LOAD — 9 0 0 ORGANIC CARBON CONCENTRATION 90

— 8 80 g

t — 2 ..g 7 70 FC E — — --- 6 60 — I- 0 — 50 E • z Z 0 — ° 40 u cn Z š ) — 30 21

a 2-

— ° '' 20 t.; -u cco -10 i 0 1 0 2100 2300 0100 0300 0500 0700 0900 TIME (B. S. T.) Fig. 4. Discharge of organic carbon with effluent from Bury St. Edmunds old sewage works, 19th-20th September, 1961 129 If a stretch of river receives no tributaries or run-off water, the change in the concentration of dissolved oxygen between an upstream station and a downstream station depends on: the release of oxygen into the water as a result of photosynthesis by plants; the uptake of oxygen from the water as a result of respiration by plants, animals, and aerobic bacteria and of direct chemical oxida- tions; the exchange of oxygen with the air in a direction which depends upon whether the water is super-saturated or sub-saturated with oxygen. These processes can be expressed in the equation

1 (C2-C4=P±D-R ...... ( )

Q where (C2—C1)— is the rate of gain or loss of oxygen per unit surface area between stations, P is the rate of photosynthesis per unit area, R is the rate of oxygen consumed per unit area, D is the rate of oxygen uptake or loss by diffusion per unit area, C1 is the dissolved oxygen concentration at the upstream station at time T1, C2 is the dissolved oxygen concentration at the downstream station at time T2, (T2 —T1) is the average retention time of water between the two stations, Q is the flow (volume per unit time), and S is the surface area between stations. During the hours of darkness, the rate of change of the dissolved oxygen content of the water is determined by the rates of respiration and diffusion through the water surface

(C2-C1) = D-R ...... (2)

The rate of diffusion depends on the degree of saturation of the water

3 D=f(Cs-C) ...... ( ) where f is the exchange coefficient of the reach, es is the average saturation concentration of a body of water within the reach, and C its average dissolved oxygen concentration. Thus during the hours of darkness

(C2-C1)Q g —ffes -e)-R (4)

It is possible to calculate the exchange coefficient f from equation 4 by measuring the rate of change of oxygen concentration at different times during the night at different saturation deficits (Cs—e) if it is assumed that respiration during this time does not vary° or that it varies in a predictable manner with temperature and oxygen changes°. This method can be used only when there is a large change in the saturation deficit during the night. Where no such large change occurs, the oxygen concentration may be reduced by the addition of sodium sulphite. The rate of change of the oxygen content between stations is measured before and during the passage of a body of water deoxygenated by the controlled , 7 addition of sodium sulphite and cobalt catalyst° ' 8. Typical profiles of the dissolved oxygen concentrations at several stations during sulphite 130 711 , PERIOD OF ADDITION 4• OF SULPHITE SOLUTION

1 0 —%,..5 Z E`X*.LIC O '0. A _ F 13..cl I ix: 9 °-.°' 6 \ CC 1 I— x Z ‘k) La 8_ 1 u 1 z 1 o 1 u 7 — 1 z ,)1 x t _ 1 t1.4>- 6 1 I t..) 5— I CI Iii LaJ \ I > A I I kJ I t/) o■ 3 o 0 U) \ I ., 63- ■0

2 2100 2200 2300 2400 0100 0200 0300 TIME (B.S.T.) Fig. 5. Oxygen content of river Lark at Stations 5, 8 and 11 during sulphite run runs are shown in Fig. 5. In the calculation of exchange coefficients° it has been assumed that the relation between oxygen consumption of the river community and oxygen concentration is the same as that of the mud studied on 13th September and shown in Fig. 7. Rates of flow were determined radiochemically using Br-82 as ammonium bromide. The "total count" or " gulp " method was employed°. Tracer with a known total activity was added to the river water, and at stations downstream, at which mixing was complete, the activity/time curves were determined. Under these conditions A F= 00 fo c.dT

where F= flow rate, A= total activity added, and c= specific activity of sample taken at time T. The retention times between stations were determined by measuring the times of transit of activity from the centres of gravity of the concentration/time curves at each station. It has already been stated that in large rivers the distribution of dissolved oxygen may be predicted with some accuracy from the biochemical 131 oxygen demands of the polluting discharge and stream water, and the rate of reaeration from the atmosphere. In equation 4, R then becomes the rate of oxygen demand of the river water only. When oxygen profiles of Reaches 1-11 were calculated from the biochemical oxygen demand of the river water (0-21 ppm/h), measured in darkened bottles immersed in the river, and the surface aeration rate (3.4 cm/h ppm deficit) for 9th-10th August and 19th-20th September, 1961, they differed markedly from those actually occurring in the river (Fig. 6). The concentration of dissolved oxygen in the water passing downstream fell at a much greater rate than that calculated. It would seem then that this method of calcu- lating the oxygen profile, although successful for large systems such as the Thames estuary, would not predict the oxygen distribution in the river Lark, which has an average depth of only 2-3 ft, and that further information on other processes which remove oxygen from the river water was necessary. Similar discrepancies between observed oxygen profiles of the river Colne, Hertfordshire, and the B.O.D. of the river water are discussed by Southgate and Gameson1°. From laboratory experiments on the oxygen consumption of aquatic plants" and field studies on their quantitative distribution12, it has been

9 August 1961 12 (a) 19 - 20 September 1961 II 0— 9 78 ci. 7 —6 5

\a‘ 4 STATIONS I-II STATIONS 3-11 x \\ 3 - 3 3 - FLOW 27 FT /SEC FLOW 21 FT /SEC .NN 2 - RETENTION TIME 4½h _RETENTION TIME 5h x■-,t 0 I I 10 9-10 May 1962 (b) 12- 13 9 September 1962 DISSOLVED OXYGEN 8 B • o o C

- 7 STATIONS I -11 .0 gTATIONS 0 -I1 3 * 0 — FLOW 36 FT /SEC -FLOW 30.5 FT 3/SEC 6 3 RETENTION TIME, 3h RETENTION TIME 13 /4h 5 I I I I 1000 2000 2200 1000 2000 2200 DISTANCE DOWNSTREAM FROM FORMER EFFLUENT DISCHARGE (yd) Fig. 6. Oxygen profiles of the river Lark at night below point of discharge of effluent from old sewage works (a) before, (b) after closure of works A, Calculated from B.O.D. only B, Calculated from B.O.D. and surface aeration C, Calculated from B.O.D., surface aeration and mud respiration 0, Observed 132 I 0

0, z-c0.8 0,, E SEPT. 19 1961 ...— D

Z 0 0cc O4 Uc.) Z D0.2 0 >- 0 I 0 2 4 6 8 10 DISSOLVED OXYGEN IN SUPER- NATANT WATER (p.p.m.) 100 t° 90 0 7.; x z o 80

0 z) 70 oc a,. /360

0 a-50 CF) E =t7)40c9 c

LU z o LU '" 20 0 > 2 10 0 2 0 I 2 3 4 5 6 7 8 9 10 DEPTH OF WATER (ft)

Fig. 7. Oxygen consumption by river muds (a) Observations on muds from river Lark. Temperature 20°C (b) Calculated effect of depth of water on proportion of total oxygen consumption of a river due to river mud, using rates of consumption by mud found in river Lark on 19th September, 1961, and two values for B.O.D. of the river water corresponding to maximum and minimum flows of sewage 133 demonstrated that in some rivers rooted plants may utilise appreciable quantities of oxygen for respiration13. However, there were no large growths of plants in the river Lark at the time of these observations and the discrepancy between observed and calculated oxygen profiles cannot be accounted for in this way. On the night of 19th-20th September, 1961, the oxygen consumption rates of representative samples of the mud deposits were determined (Fig. 7a), and were found to be very high (about 0.95 g oxygen/m2 of mud surface per h at 20°C). About 140 000 midge larvae (Chironomus riparius) per m2 of mud surface were present and from previous work on their oxygen consumption14 it seems likely that these animals alone would consume about 0.3 g oxygen/m2 of mud surface per h at 20°C. When this oxygen consumption by the mud deposits, corrected for changes in the consumption rate brought about by changes in the oxygen concentration and temperature of the river water, was included in the calculation of the oxygen profile much closer agreement was obtained (Fig. 6). It is perhaps of some interest to calculate the relative importance of oxygen consumption by mud and by water in a hypothetical river of varying depth, using consumption rates found in the river Lark. Fig. 7b shows that even if the biochemical oxygen demand of the water were 1-25 ppm/h, mud could still be responsible for more than 50 per cent of the total oxygen consumption in a river less than 2.5 ft deep and for about 20 per cent in a river 10 ft deep. 15, 16 Other authors ' 17 have also shown that oxygen consumption in a river is frequently the result of benthic processes and not simply of the biochemical oxygen demand of the river water; equation 4 may therefore be re-written

(C2-c1)-s=f(CS-)-R.B.O.D.-RPLANTS-RMUD ...... (5)

An attempt was made to account for the remaining small discrepancy by examining the results of analyses of samples of river water for ammoniacal, nitrate and nitrite nitrogen to see if nitrification was responsible for the additional oxygen demand. The results seem to suggest that no nitrification occurred in this stretch of the river Lark; on the contrary there was a considerable loss of inorganic nitrogen (about 0.3 ppm/h) in the body of water as it passed downstream. Although, in the absence of mud deposits, reduction of nitrates has not been observed in river water or sewage effluent until the dissolved oxygen has fallen to less than about 1 ppmis, nitrate was lost from the river Lark when the water above the mud deposits contained several ppm dissolved oxygen, and it seemed likely that denitification was occurring in the mud. In laboratory experiments loss of nitrate (240 mg nitrate N/m2 day at 10°C) occurred in water, saturated with oxygen and containing about 10 ppm nitrate nitrogen, covering a 5 cm layer of settled activated sludge. Losses of nitrate nitrogen of this order were also observed in water, saturated with oxygen and containing between 9 and .20 ppm nitrate nitrogen, covering muds from the Thames estuary at Tilbury and North- ern Outfall and from the river Ive11°. Loss of nitrate nitrogen has also been observed in aerated water standing over pond2° and lake muds21. Reasons which might account for the remaining discrepancy between observed and calculated profiles are: 134 (o) (b)

A X X ) —Period of transit of sewage effluent A ( r\ X X 5, X X k xx x l ' x . St * x x el4.6X x x 1' x _ N _ x x x x x x X x x xx X x x x . X x x z _ .. — X. X X X X x . ,' i( - . _ X X X X XX X x x % x x x X x \ % - xYlk X 1-., x 2 - - xxd . . . - _ , x x . 27 4 62 28 4 62 29 4 62 x I - x , x . x x --- X x x _ x x x x x - k x X x x _ x x x x - X x x *-- 1 x x )k x K )t. X )5. X w 2 X Xi( 2 k ); X - X k X 0 — . . x . 1 V29 7 61 I, 30 7 - 61 x 31 . 7 .61 - I I t I 3°r 1 I I I 1 I 1)S')15555 1< 1 1 1 1 1 1 1 1 1 1 1 1 1

9400 1200 2400 1200 2400 1200 2400 2400 1200 2400 1200 2400 1 200 2400

HOURS Fig. 8. Oxygen concentrations about 2 miles below effluent from old sewage works (a) Works in operation (b) Three months after cessation of discharge With some muds, especially those containing high numbers of midge larvae, it is difficult to make an exact measurement of the total volume of water from which oxygen is consumed in respirometer studies', ". These muds are not compact and contain considerable quantities of interstitial water. Heterogeneity of mud deposits in the river must also be taken into account. The exchange coefficient used in these calculations was determined in a later exercise and might be slightly different for the night on which oxygen profiles were studied. Similar observations were made on 9th-10th May and 12th-13th September, 1962, about six and nine months after the discharge from the old works had ceased and when much of the settled organic matter had been scoured from the reach by high winter flows. The oxygen profiles observed show considerable improvement over those previously found (Fig. 6). The oxygen consumption rates of representative mud samples also showed a marked decline, from 0.95 to 0-14 g/m2 mud surface per h (Fig. 7a). No measurable B.O.D. of the river water was detected in the short-term determinations (B.O.D., was 2.6 ppm). There were marked discrepancies between calculated and observed oxygen profiles even when allowance was made for oxygen consumption by mud. At present no adequate explanation can be given for these discrepancies other than the reasons already listed above. The great improvement in the river was most striking and was also confirmed by records from a dissolved oxygen recorder installed about 2 miles downstream of the old effluent outfall. Typical traces are shown in Fig. 8.

PRODUCTIVITY OF REACH FARTHING BRIDGE-MARSTON'S MILL, ICKLINGHAM (STATIONS 14-17) From the changes in dissolved oxygen concentrations which occur between two stations throughout 24 hours, and knowing the flow of water, retention time and surface area between stations, it is possible to calculate the relative contributions of photosynthesis, respiration and diffusion to the oxygen balance by means of equation 1 5, 13. Before rates of oxygen production by photosynthesis can be determined from oxygen concentrations during the hours of daylight, the community respiration rate must be calculated over this period; this is done by correcting its night-time value for temperature and oxygen changes occurring during the hours of daylight. Fig. 9 shows typical curves of oxygen concentration at Stations 14 and 17 over 24 hours on 8th June and 5th July, 1961. The curves for the downstream station are displaced to the left a distance corresponding to the retention time of water flowing between the upstream and downstream stations. The difference in oxygen content between the curves at a given time thus represents the change in concentration occurring in a given body of water flowing between the stations. The cumulative values for diffusion, community respiration, and photosynthesis calculated from these curves are also shown in Fig. 9. Between 8th June and 5th July weeds were cut by the River Board; this apparently reduced the total respiratory demand from 24.4 to 19.6 g/m2 day. The small effect of weed-cutting on the oxygen demand is largely due to the presence of large mud deposits which consume the greater proportion of oxygen within this reach. The oxygen consumptions of plants before and after cutting were 136

_ (a) June 7-8 1961 July 4-5 1961 , A '',\ _ - X X i 'I\

— 4 x 9 1 _ '7 STATION 14 .0 7

, \ STATION 14 29 ' / ll ' 'a ,01 q \ ,?'/4 - STATION 1 ;b 6 'a, — 'A \ 2 .. TATION --o, s _ "„: ‘..:,.Z.'" 0 I I I I I 1 i I 1 I. I I I , I . I (b) June 7-8 1961 July 4-5 1961 I 2- s.6 8-

- DIFFUSION 4 DIFFUSION

0 \,.

4- N N N N ..foo

'42 S,, .fSg, 54, ■ sio I

20-

24- I I 1 1 1 1 1 1 1 1 1 1 2200 0200 0600 1000 1400 1800 2200 2400 0400 0800 1200 1600 2000 2400 TIME (B S. T.) Fig. 9. Oxygen balance of reach between Stations 14 and 17 (Fig. I A) (a) Diurnal oxygen curves (Station 17 has been displaced backwards 80 min. on 7th-8th June and 110 min. on 4th-5th July) (b) Cumulative values of photosynthesis, community respiration and diffusion

9 and 6 g oxygen/m2 day cespectively. The values for photosynthetic oxygen production of about 14 and 13 g/m2 day are very similar. This may be explained by the presence of sufficient plant growth after cutting to maintain an effective light-absorbing canopy, and by an increase in solar radiation from 498 cal/cm2 on 8th June to 580 cal/cm2 on 5th July. Throughout the period of observations, samples of river water were sealed in light and dark bottles and suspended just below the water surface for 2 hours; there was no significant change in the oxygen concentration in either light or dark bottles. This suggests that the biochemical oxygen demand of the water and oxygen production by plank- 137 z 10 30-31 MAY 1962 27-28 JUNE 1962 _ 9 • 8 >< ,1!; C • 13 8 oC e- E 6 N i7i a.7 x . > 6_4 STATIONS A-D x 0 STATIONS A- 1 -1) --- .-- 'FLOW 16 FT3 /SEC TLOW II FT3/SEC x" 17') 0. R ETENTION TIME 21(2h x0 1 RETENTI9N TIME 30 2 5 0 0 500 1000 1500 0 500 1000 1500 a DISTANCE DOWNSTREAM OF DUCKSLUICE WEIR (yd)

Fig. 10. Oxygen profiles of river Lark at night below point of discharge of effluent from new sewage works (profiles start 1,300 yd below point of discharge)

B, Calculated from B.O.D. and surface aeration C, Calculated from B.O.D., surface aeration and mud respiration

0, Observed tonic algae are small and can be neglected in calculations of oxygen balance within the reach at this time. In the river Ivel, an unpolluted chalk stream which has recently been studied, values for community respiration were generally lower than those found in the river Lark, the summer average being about 14 g oxygen/m2 day, whereas the photosynthetic values were similar, between 10 and 17 g oxygen/m2 dayi3.

REACH 2. DUCKSLUICE FARM WEIR-LOCK WEIR (STATIONS A-D) Similar observations to those already described for Reach 1 were carried out on Reach 2 during 30th-31st May and 27th-28th June, 1962. This reach starts about 1 300 yd below the effluent outfall from the new Bury St. Edmunds sewage works. The observations were made at night when critical conditions are most likely to prevail owing to the absence of photosynthesis. The observed oxygen profiles of the river are shown in Fig. 10. Determinations of the short-term B.O.D. (about 0.1 ppm/h) and exchange coefficient (6 cm/h) were made in the usual manner and the oxygen consumption of mud deposits was determined in the laboratory. The values, shown in Fig. 7a, are very similar to those recorded in Reach 1 after the closure of the old works, with an average oxygen consumption of 0.14 g oxygen/m2 h at 20°C. Attempts to calculate oxygen profiles of this reach on these two occasions from the B.O.D., surface aeration, and respiration of mud deposits were unsuccessful (Fig. 10). The reasons for these discrepancies are not yet understood.

SOME AERATION STUDIES AT THREE WEIR SYSTEMS The importance of weirs in the aeration of rivers and the effect of temperature on this aeration have been discussed by previous authors23, 24, 25. During the course of experiments on the river Lark a number of observations were made of the dissolved oxygen content immediately above and below three weir systems. The observed loss or gain of dissolved oxygen could then be compared with that predicted from the aeration equation25, r =1+0.11 ab (1+0•046T)h ...... (6) where rT is the deficit ratio at the water temperature T°C, h the height (ft) through which the water falls, and a and b are parameters which depend upon the quality of the water and the form of the weir respectively; a is equal to unity in polluted river water, and b is equal to unity for a weir with a free fall. The defict ratio r is given by

Cc-CA r— (7) Cs-C B where Cs is the saturation value, and CA and C, are the concentrations of dissolved oxygen in the water above and below the weir respectively.

RIVER LARK, MARSTON'S MILL, ICKLINGHAM (STATION 17, FIG. la) • This weir takes the form of a movable board which may be raised or lowered to control the level of the river. At the time of the study, 4th- 5th July, 1961, the water level upstream was about 6i ft above the water

139 1,4. surface downstreamoftheweir.Thisweirwascomplicatedbyfact Fig. taken aboveandbelowtheweiroveraperiodof that somewaterflowedbeneaththemovableboardandfellfreelyfor Fig. 11.

ft. 11a, Thevariationsinthedissolvedoxygenconcentrationofsamples (b) (a)

Variation inconcentrationswith timeofday Relation betweendeparturesfrom saturationaboveandbelowweir

DISSOLVED where thesaturationvaluesarealsoshown. Dissolved oxygenimmediately above andbelowweiratMarston's OXYGEN(p.p.m.) 10 12 11 8 7 6 9 5 4 3 2

DEPARTURE FROMSATURATIONABOVE

— — — _ _ 2400 04000800120016002000

(b) x 9 8765432 1

1 x SUB-SATU

Icklingham, 4th-5thJuly,1961 x 1 r

TIME B.S.T. = 3 1 .05 RAT ION 1 140 WEIR X x

1 1 (p.p.m.) x 1 BE LOWWEIR AVERAGE SATURATION ABOVE WEIR 1 CONCENTRATION 1 x 0 24 hours areshownin SUPER-SATURATION I I 23 I I

Mill,

Departures from saturation above and below the weir are plotted in Fig. 1 lb; the reciprocal of the slope of the line, fitted by eye, gave the mean deficit ratio as 3.05. The value of the deficit ratio given by equation 6 was 2.02. This discrepancy may be accounted for by the fact that just under the water surface, below the weir, there is a concrete sill on which a certain amount of splashing occurs.

RIVER LARK, LOCK WEIR (STATION D, FIG. lb) This was a single free fall weir with a fall of 2 ft. Observations were made on two occasions, on 30th-31st May and 27th-28th June, when the fall was 2 ft but the flow of water was different; the flows were 14.9 f t3/s and 10.9 ft3/s respectively. Departures from saturation above and below the weir are plotted in Fig. 12a. The reciprocal of the slopes of the straight lines gave mean deficit ratios of 1.25 and 1.33 respectively. The value of the deficit ratio given by equation 6 was 1.35, which is in good agreement with the observed values.

3 gz 30-31 May 1962 x (a) 27-28 June 1962 2 - E Wg _ r =1 25 _ r =133 o r - _ WEIR

_ BELOW m - 1 1 1 I 1 1 1 I I I ! i l l l i l l I I I 1

(b) _ 30-31 May 1962 •--- 27-28 June 1962 /2k1W _ r = 1 49 x _ r=1.25

_ -

6 - _ Ft 3 _ 4 - 1 [ 1 i I I 1 1 1 1 1 1 /11 1 1 1 1 7 6 5 4 3 2 1 0 1 2 3 4 8 7 6 5 4 3 2 0 1 2 3 4 DEPARTURE SUB- SATURATION SUPER-SATURATION SUB- SATURATION SUPER-SATURATION DEPARTURE FROM SATURATION ABOVE WEIR P- P.m) Fig. 12. Relation between departures from saturation above and below weirs (a) Lock Weir (Station D), (b) Ducksluice Farm Weir (Station A)

RIVER LARK, DUCKSLUICE FARM WEIR (STATION A, FIG. lb) This again was a single free fall weir with a fall of 2 ft 2 in. Observa- tions were made on 30th-31st May and 27th-28th June, 1962. In each case the fall was similar but during the second run considerable loss of water occurred between the boards, so that the average height of fall would be rather less than the 26 in. Departures from saturation above 141 and below the weir are plotted in Fig. 12b. The reciprocals of the slopes of the straight lines gave mean deficit ratios of 1.49 and 1-25. The value of the deficit ratio given by equation 6 was 1.37, which is again in good agreement with the observed values. These studies of the oxygen exchange at the weir systems discussed in this paper support the view2' that generally the change in oxygen content of water passing over a weir system is predictable in a manner described by equation 6. Difficulties however do arise in the substitution of numerical values for the constants a and b in equation 6. It is advisable wherever possible to determine the combined constant ab for each weir system as the quality of water will vary from river to river and many variations in the design of weirs are found in practice. Where large variations in the amount of pollution in the river water occur and where leaks are present in weir structures such that b varies with rate of discharge, the value of ab may itself be somewhat variable for a given weir system.

COMPARISON OF kERATION AT A WEIR WITH AERATION THROUGH THE SURFACE OF FLOWING WATER It may be of interest to compare the rate of uptake of oxygen at a weir with uptake in a flowing river. Consider a stretch such as that between Ducksluice Farm Weir and Lock Weir (Stations A and D) having a surface area of 10 220 yd2 and the following characteristics: Flow =20 ft'is Exchange coefficient=6 cm/h

C/ =initial oxygen concentration =3.00 ppm cs= initial saturation concentration =10.00 ppm T= temperature —13.9°C At the end of this stretch, the concentration of dissolved oxygen will have risen to 4.4 ppm, 1.4 ppm having been supplied by aeration through the surface. Now if the river, with the same initial saturation deficit of 7 ppm, were to flow over a weir with a deficit ratio of 1.25 (that is, similar to the deficit ratios of both Ducksluice Farm and Lock Weirs), the concentration of dissolved oxygen would also be raised to 4.4 ppm, while if it flowed over a weir with a deficit ratio of 3.05 (Marston's Mill), the concentration of dissolved oxygen would be raised to 7.7 ppm. These represent gains of dissolved oxygen of 1.4 and 4.7 ppm respectively, clearly demonstrating the importance of the effect of a weir on re-aeration. It should however be pointed out that, generally, upstream of a weir the velocity of water is considerably reduced so that conditions are made more favourable for the accumulation of mud deposits, a factor which it has already been demonstrated is an important one in the consideration of the oxygen balance of a river.

ACKNOWLEDGMENTS The authors wish to thank Dr. H. Clay and Mr. W. Summers (Great Ouse River Board), the riparian owners, for the use of the selected sites, and Lord Iveagh and Mr. C. Marston for special facilities, such as power supply and the siting of the dissolved oxygen recorder. Mr. D. W. M. Herbert and Mr. R. Lloyd (Water Pollution Research Laboratory) sup- 142 plied data on fish kills and Mr. H. L. J. Rolley (Water Pollution Research Laboratory) on the respiration rates of muds. Many members of staff of the Laboratory helped in the field sampling programmes. The paper is published by permission of the Department of Scientific and Industrial Research. RE1-ERENCES 1. Streeter, H. W., and Phelps, E. B. Bull. U.S. Pub!. Hlth Serv., No. 146, 1925 (reprinted 1958). 2. Alabaster, J. S., Herbert, D. W. M., and Hemens, J. Ann. app!. Biol., 1957, 45, 177. 3. Allan, I. R. H., Herbert, D. W. M., and Alabaster, J. S. Fish. Invest., Ser. 1, 1958, 6, No. 2. 4. Knowles, G., Edwards, R. W., and Briggs, R. Limnol. & Oceanogr., 1962, 7, 481. 5. Odum, H. T. Limnol. & Oceanogr., 1956, 1, 102. 6. Edwards, R. W., Owens, M., and Gibbs, J. W. J. lnstn Wat. Engrs, 1961, 15, 395. 7. Gameson, A. L. H., Truesdale, G. A., and Downing, A. L. J. lnstn Wat. Engrs, 1955, 9, 571. 8. Gameson, A. L. H., and Truesdale, G. A. J. Instn Wat. Engrs, 1959, 13, 175. 9. Eden, G. E. J. Inst. Sew. Purif., 1959, 522. 10. Southgate, B. A., and Gameson, A. L. H. Surveyor, Land., 1956, 115, 349. 11. Owens, M., and Marls, P. J. Hydrobiologia (in press). 12. Edwards, R. W., and Owens, M. J. Ecol., 1960, 48, 151. 13. Edwards, R. W., and Owens, M. J. Ecol., 1962, 50, 207. 14. Edwards, R. W. J. exp. Biol., 1958, 35, 383. 15. Streeter, H. W. Industr. Engng Chem., 1930, 22, 1343. 16. Fair, G. M., Moore, E. W., and Thomas, H. A. Sewage Wks J., 1941, 13, 1209. 17. Velz, C. J. Sewage Wks J., 1949, 21, 309. 18. Wheatland, A. B., Barrett, M. J., and Bruce, A. M. J. Inst. Sew. Purif., 1959, 149. 19. Department of Scientific and Industrial Research. "Water Pollu- tion Research 1962." H.M. Stationery Office, London, 1963. 20. Lind, E. M. J. Ecol., 1940, 28, 484. 21. Mortimer, C. H. J. Ecol., 1941, 29, 280. 22. Edwards, R. W., and Rolley, H. L. J. "Oxygen consumption of aquatic muds" (in preparation). 23. Gameson, A. L. H. J. Instn Wat. Engrs, 1957, 11, 477. 24. Gameson, A. L. H., Vandyke, K. G., and Ogden, C. G. Wat. & Wat. Engng, 1958, 62, 489. 25. Barrett, M. J., Gameson, A. L. H., and Ogden, C. G. Wat. & Wat. Engng, 1960, 64, 407. 143 DISCUSSION Mr. Owens, introducing the paper, said that at the Water Pollution Research Laboratory they were investigating the magnitude of some of the processes of self-purification of streams and that the paper described the marked effect of some of these processes on the oxygen regime of the river Lark. The practice of discharging the effluent from the sewage works at night was an unusual one; unfortunately it ensured that the discharged effluent entered the head of the reach of the river Lark under observation, at a time when the dissolved oxygen content of the incoming stream-water was at its lowest. The authors thought that their paper emphasised the inadequacy of taking spot samples of river water to determine the sanitary condition of a stream, for while samples taken during daylight would have indicated a river water which contained more than adequate oxygen resources, somples taken at night would have revealed a far from satisfactory situation. This demonstrated the great need for adequate field sampling surveys and automatic sampling and recording equipment. Though the downstream oxygen profiles of some large rivers had been calculated with some success from consideration of the B.O.D. of effluent and river water and stream reaeration rates, the present paper demonstrated yet again that that form of mathematical treament was an over-simplification of the processes which occurred in shallow streams and that deposits on the stream bed could have effects of considerable magniture on the oxygen balance of a stream. Professor Odum (" Analysis of Diurnal Oxygen Curves for the Essay of Reaeration Rates and Metabolism in Polluted Marine Bays," in "Waste Disposal in the Marine En- vironment," Pergamon Press, 1960) concluded, from many marine and fresh-water studies, that in systems less than 3 metres deep, the oxygen consumption of the bottom was usually greater than that in the water. The magnitude of the effect that the stream bed and its associated community of living organisms—plant, animal and bacteria—could exert on the oxygen balance of streams, made it essential to have some knowledge of the rates of oxygen consumption of the mud and its associated organisms. Together with oxygen, nutrients could be removed from the overlying water by organisms living on and within the mud. Changes had been studied in the inorganic nitrogen concentrations of the overlying water because they were important when considering the overall oxygen balance of a stream. Mr. Owens said that though they had tried to demonstrate the importance of the effect of a weir on the reaeration of streams, it was not yet possible in general to answer the question whether or not it was desirable to have a weir in a river. The placing of a weir would alter the hydraulic parameters, such as slope, average depth, velocity, and even perhaps surface area. It would also increase the retention time of water within a particular reach and would facilitate the deposition of fine organic materials. Fishery and water abstraction interests should also be taken into account when considering the problem.

Mr. F. T. K. Pentelow congratulated the authors of an admirably written and presented paper and noted that they had solved the thirty-five year mystery of the diurnal variation in the ammonia content of the water of the river Lark; it had no relation to the variation in oxygen content but had been found to be due to the factor that the sewage effluent was discharged at night. He asked whether the authors in their calculations had taken into account the microscopic algae which grew on the bed of the stream and on higher plants; he thought that they might affect the calculations and it was not clear that they had been included in the reckoning. Mr. Pentelow suggested that the importance of the authors' work lay in bringing back into consideration of water quality the nature of the stream bed; the Royal Commission on Sewage Disposal had paid a good deal of attention to that factor but in the intervening years it had come to be neglected and it was only recently, through the work of the Water Pollution Research Board, that its importance was again being realised.

Dr. B. A. Southgate said that the large uptake of oxygen from organic deposits on a river bed, mentioned by Mr. Owens, was an important factor in deciding on the best treatment for polluting effluents. The effect of discharging organic suspended matter in an effluent was to compress the zone of deoxygenation, which might therefore be very intense for a short distance below the point of discharge, where the insoluble matter formed banks of sludge. For that reason methods of removing suspended solids from sewage effluents were being examined by the Water Pollution Research Laboratory; one of the most efficient, and he thought 144 cheapest, of these was the passage of the effluent over grass, which did not require any intensive system of management and which brought about a large reduction in concentration of suspended matter.

Dr. G. U. Houghton referred to the loss of nitrate nitrogen, which he had also observed in standing resorvoirs; it occurred without the corresponding appearance of ammonia and was presumably due to denitrification by biochemical processes. It seemed that bacterial reduction of nitrate might have a different effect on the ultimate oxygen budget of the water according to whether nitrogen or ammonia resulted. Mr. A. B. Wheatland remarked that the loss of nitrate during storage of water in reservoirs mentioned by Dr. Houghton was almost certainly a result ot denitrification, nitrate being reduced to nitrogen. In a paper to the Institute of Sewage Purification (Wheatland, A. B., Barrett, M. J., and Bruce, A. M. "Some observations on denitrification in rivers and estuaries." J. Inst. Sew. Punt., 1959, Part 2, 149) Mr. Wheatland had postulated that a loss of nitrate would occur, and in a contribution to the discussion of the paper Mr. Hammerton had presented data on the Metropolitan Water Board's reservoirs to show that a loss of nitrate occurred with no corresponding increase in ammonia. Mr. Wheatland thought that the nitrate probably diffused into the mud on the bed of the reservoir, was reduced there under substantially anaerobic conditions and that no significant amount of nitrate was reduced in the main body of water. Mr. T. W. Brandon wrote to say that the loss of inorganic nitrogen observed by the authors recalled an investigation made some years earlier by himself and Dr. J. Grindley (J. & Proc. Inst. Sew. Punt., 1944, Part 2, 175) into the effect of nitrates on the rising of sludge in sedimentation tanks; it was found that where the concentration of nitrate nitrogen was 20 to 30 ppm, and particularly in the warmer months when algae were abundant, sludge was buoyed up in sedimenta- tions tanks by gas containing 92 .1 per cent nitrogen, 4.9 per cent carbon dioxide and small concentrations of other constituents. Perhaps a similar conversion of nitrates to nitrogen accounted for the loss of nitrogen observed by the authors.

AUTHORS' REPLY Mr. Owens, replying to Mr. Pentelow, said that benthic and epiphytic algae were clearly of great importance in the oxygen balance of many shallow streams. particularly in spring-time, and that he had found 3 .8 x 10" algal cells/m2 of river bed growing on Hippuris in the river Ivel during a spring bloom. More recently he had recorded 2.6 x 10" and 1.6 x 10" cellsirre of mud surface during similar blooms in the rivers Ivel and Gade. In the present paper the respiration rate of mud had included that of algae on the mud surface but during the periods of measurement large numbers of algae were not present. The values quoted for the photosynthesis and respiration of the Farthing Bridge-Marston's Mill Reach included those of algae, and the small effect of weed-cutting on the oxygen production of that reach might be due in part to the growth of algae on the mud surface as a response to the thinning of the canopy of rooted plants. Mr. Edwards, replying to Dr. Houghton's question about the loss of nitrate nitrogen from standing reservoirs, said that while agreeing with Mr. Wheatland's suggestion that it could be the result of denitrification, nitrate being reduced to nitrogen, he thought that that was only partly true, for some nitrate would be assimilated by micro-organisms and utilised in the synthesis of proteins. He had recently shown that one river mud covered by algae during a spring bloom removed about 1 g nitric nitrogen/m2 day at 20°C with 10 ppm nitric nitrogen in the overlying water. No equivalent ammonia or nitrite was produced. Much of the nitrate was probably removed by algae as a nitrogen source in protein synthesis.

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