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Studies of estuarine sediment dynamics using 137Cs from the Tjernobyl accident as a tracer
Estuarine,
Coastal
and She&Science
(1989)
28,249-259
Studies of Estuarine Sediment Using 13’Cs from the Tjernobyl a Tracer
Lars
Brydsten
Department Sweden Received
and Mats
of Physical
4January
Keywords:
Geography,
1988 and in
Dynamics Accident
as
Jansson University
of Umei,
revisedform 30 November
S-901
87 Umeh,
1988
sedimentdynamics;estuaries;137-G; tracer
Sedimentdynamicsin the 6re River estuaryin northern Swedenwerestudiedby monitoring the turnover of particles associated with 13’Csin surfacesediment and sedimenttraps during the period following the Tjernobyl accidentin late April until lateNovember 1986.River transportedmaterialwasdepositedin the estuary and then frequently redistributed due to resuspension-redeposition processes during the ice-freeperiod. There wasa slownet transportof particles out of the estuary which wasdelayedby at leastone event with a significant particle redistribution to the inner part of the estuary.Wind andwave induced water dynamicsareresponsiblefor resuspension andtransportof particles.
Introduction
In late April 1986the whole catchment area of the River ore (Figure 1) in the province of Vlsterbotten, Northern Sweden, was affected by precipitation contaminated with radionuclides from the nuclear power plant accident in Tjernobyl. The mean fallout in the catchment was approximately 40 kBq mP2 (137Cs).The radioactive fallout coincided with the rapid melting of a 30-50 cm deep snow cover in the drainage area. This situation led to the assumption that the majority of the radionuclides would be drained into the river by surface runoff. Furthermore, these radionuclides should, for the most part, be associatedwith fine grained particles (McHenry et al., 1973). A considerable quantity of the fallout should therefore be carried by the river spring flow to the estuary, ijrefjarden, outside the river mouth. If this had happened, radioactively labelled particles would be incorporated in the sediment, thus providing a unique opportunity to study sediment dynamics by following the turnover of radionuclides in the estuary. A sediment dynamic study was therefore initiated in June 1986 and 137Cswas chosen as the tracer nuclide. The study area, &efjGden, is a semi-closedwater-body, partly isolated from the outer sea,Kvarken, by a densearchipelago (Figure 1). The salinity is low, about 3 per ml, and so the water circulation driven by the salinity difference between inland and coastal water is of relatively low importance. There are no tides in the area, and large scalecurrents exist 0272-7714/89/030249+
11 $03.00/O
@1989 AcademicPressLimited
250
L. Brvdsten
Figure
&M.
1. River
Bansson
6re catchment
area and location
of the investigated
area
only outside the estuary. Therefore, sediment dynamics in the estuary are determined by wind-induced waves, wind-driven currents and fluvial action. The sediment source is mainly particles carried by the River Ore. A rough calculation (Brydsten, unpublished) indicates that about 95% of the total input of fine-grained particles to the estuary emanates from fluvial input. Of this, the major part (809,)) is supplied during the spring flow in May-June. Since the area is still influenced by the Holocene uplift, new unwashed till are continuously being exposed to wave-washing. Fine-grained particles, suspended by the waves and transported from the shore, are thus another, albeit less important, sediment source. Extensive areas of continuous accumulation exist on bottoms below a water depth of about 65 m, i.e. not within the estuary. Smaller locations with accumulation occur in the estuary up to about 5 m water depth. Therefore, the main part of the suspended particle input from the river is transported through the estuary and accumulated in the open sea. The fresh-water plume does not reach the mouth of the estuary, so the transport of finegrained particles from the river-mouth to the deeper waters is not controlled by river action alone, but is a result of several processes. The hypotheses to describe sediment dynamics in this type of estuary are as follows: * River input is the main source of particles. *The suspended material supplied to the estuary is primarily deposited within the estuary. * Deposited sediment particles are resuspended due to wave action. * The major portion of the fine-grained deposited particles are then transported through the estuary by a series of resuspension and transport processes, so that only a minor part is permanently accumulated within the system. *The transport of resuspended particles is controlled mainly by wind-driven nearbottom currents. * The distribution of sites with continuous accumulation within the estuary is controlled by bottom morphometry plus the wave-induced near-bottom water dynamics or by the wind-driven near-bottom currents.
Estuarine
sediment
dynamics
251
t
Figure
2. Morphometry
of the &efjirden
estuary
and locations
of sample
stations.
The use of 137Cs as a particle tracer was, in the first year of this study, used mainly to study the following questions in the complex patterns of estuarine sediment dynamics: * Where are the river transported suspended particles deposited? * Where, to what extent and under what conditions are sediment particles resuspended? * Where are resuspended particles redeposited? * Are there any correlations between climatic conditions and transport directions?
Material
and methods
Sample stations were located according to Figure 2. Station 1 was positioned in a shallow area (8 m) near the river mouth, with expected strong fluvial power and low salinity. Stations 4, 6 and 7 were placed along the expected fluvial plume and at relatively great depths (25 m, 35 m and 45 m, respectively) on transport bottoms with comparatively long periods of accumulation. Station 7 was situated outside the mouth of the estuary. Stations 2 and 3 were in positions lateral to the fluvial plume centre-line and were expected to reveal non-fluvial sediment dynamics. Station 2 is sheltered from open-water waves by a shallow
252
L. Brydsren & M. Jansson
sub-marine moraine, while Station 3 is well exposed to the open-water. Both stations are situated at local depressions(25 and 30 m, respectively). The suspendedmaterial in river water was sampled with a depth-integrating sampler near the river mouth (Figure 1). The samplewas dried (105 “C) and the 137Cs activity was measuredon the solid residue. Suspendedmaterial in the estuarine water column was collected with sediment traps. The traps were put out on 16 June at 6 stations spread over the estuary. At each station traps were placed at 3 depths, 1 and 2 m over the bottom and 2 m below the water surface, and were drained on four occasions(16 June, 25 July, 2 October and 4 November). Surface sediment wascollected with a grab sampler on 16 June, 25 July, 2 October and 4 November. Sub-sampleswere taken from the grab sampler (50 cm’, O-l cm, l-2 cm, 2-5 cm). All sediment and trap sampleswere dried (105 “C) and the radioactivity was measured on the dried samples. The 137Csand 134Csradioactivity in suspendedparticles from the river and estuarine waters and in surface sediment sampleswas measuredwith a high-resolution germanium detector by the National Defence Research Institute in Umea. In order to try to explain variations in the 137Csdistribution in sediment and settling material the following environmental factors, which are crucial for the sediment dynamics have been utilized. River discharge is an approximation for the fluvial influence. The River are is of nival low-land regime with a maximum in May and a minimum in February. The maximum is determined by the snow melting and about 65O;”of the annual discharge occurs during May and the first half of June (Figure 3a). Therefore, the annual influence of fluvial action on the sediment dynamics in the estuary is limited to a short time during the spring. Wind-speed can be used as a rough approximation for near-bottom wind-induced currents (Gedney, 1971). The wind-speed at Sydostbrotten, situated near the mouth of the estuary, during the period under study is shown in Figure 3b. During the period investigated there were generally low wind speedsfrom early May to mid September, never exceeding 15 m SK*. Thereafter, three deep low pressures with strong winds occurred at about two weeks intervals. Thus, the relative magnitude of wind-induced currents washigher at the end of the ice-free period. The wave-induced resuspensionof fine-grained sediment situated below 20 m depths requires waves generated in seaswith fetches longer than about 30 km (wind-speed 20 m s-l) (Silvester, 1974; Komar StMiller, 1973) (Figure 4). Therefore, the sediment in the deeper parts of the estuary can only be resuspendedby waves entering from the open sea,e.g. from SSW-SE. Waves from all other directions break at the archipelago. Windspeedsfrom SSW-SE in the southern Gulf of Bothnia (Figure 3c) were therefore used to estimate the occurrence of wave-induced resuspension. Results
The major input (> 809,) of contaminated particles to the estuary occurred during the spring flow period in May and the first half of June (Figure 3a). The 137Cs activities of the surface sediment are presented in Table 1. The ratio 137Cs/134Cs wasapproximately 2.2 in all samplesshowing that all measured137Cscome from the Tjernobyl reactor. Deposited radioactive particles were found only in the O-l cm layer during the Summer of 1986. 137Csactivity in the layer l-2 cm or 2-5 cm never exceeded the ‘ background ’ value of
Estuarine sediment dynamics
253
June
July
June
July
August
September
October
ib)
20 f8167 m 14L D IF?8 :: IOI E 8 3 6
. August
September
October
22 ,
May
June
Figure 3. Time distributions directions at Sydostbrotten
JL ly
august
September
October
of discharge in the River 8re (a), wind-speeds from all (b) and wind-speeds from SSW to SE at Sydostbrotten (c).
254
L. Brydsren &3 M. Jamson
Depth
Figure
4. Transport
of ‘)‘Cs in the outlet
(ml
of the River
ijre.
1. l”Cs activity in surface sediment (kBq kg ’ dry weight). only on 16 June due to loss of sediment traps on this location
TABLE
Station
5 was sampled
Station Date 16 June 25 July 2 October 4 November
1
2
3
4
8.4 7.0 5.9 4.9
6.3 11.0 14.0 29.0
3.4 13.0 9.6 13.0
9.4 10.0 6.5 20.0
5 9.8 NA NA NA
6
7
10.0 32.0 2-5 16.0
3.8 4.6 6.6 8.2
0.2 kBq kg-‘. Activities are generally very high in the central part of the estuary, over 30 kBq kgg’ dried substance. These values are well above those measured in sediment samplestaken near brook outlets in lakes in extremely contaminated areasin northern Sweden (Holmgren & Jansson,unpubl.). Background activities, as measured on several samples taken from depth of 5-10 cm below sediment surface in the estuary, were ~0.2 kBq kg-’ Dw-’ (detection limit). The assumption that a large part of the fallout on the River ore catchment region should be transported to and deposited in the river mouth area was thus correct. The net sedimentation rates (g day- ’ mP2)calculated from the amounts accumulated in the traps are presented in Table 2. A general increasein the amount of material collected in the near-bottom traps occurs with time and in a direction towards the deep sea. The greatest net sedimentation is observed outside the mouth of the estuary between 25 September and 4 November and reachesa maximum of 322 g day-’ mP2. For the nearsurface traps, the trend is also an increase with time, but a decrease in the deep-sea direction.
Estuarine
sediment
dynamics
255
TABLE 2. Net sedimentation rate in sediment traps (g day-’ mm*). (Top-l m below water surface, Middle-2 m above the sediment surface and Bottom-l m above the sediment surface)
Station
1
3i
4I
7
Date 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November
Position
of sediment
traps
Top
Middle
Bottom
12.5 22.4 48.6 1.9 3.3 22.8 1.7 2.4 17.0 1.3 1.3 18.6
7.8 23.9 58.9 7.2 6.6 67.5 7.6 6.0 51.2 10.1 15.5 107.9
9.3 28.9 90.5 13.6 7.3 96.9 12.0 8.0 72.6 18.8 19.8 322.8
3.l”Cs activity in material collected in sediment traps and surface sediment (kBq kg-’ dry weight). (Top-l m below water surface, Middle-2 m above the sediment surface and Bottom-l m above the sediment surface) TABLE
Station
1
3I
41
7
Date 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November
Position
of sediment
Top
Middle
Bottom
Surface sediment
47.5 9.2 11.2 63.0 29.0 15.5 91.6 30.5 20.3 62.7 19.7 15.0
53.7 9.5 11.4 51.0 27.0 16.0 71.6 28.7 23.0 36.6 15.3 10.1
7.7 6.5 5.4 8.2 11.3 11.3 9.7 8.3 13.2 4.2 5.6 7.4
20.4 1.9 1.9 13.3 17.8 7.6 29.4 35.7 11 .o 45.4 98.3 8.4
traps
The 137Cs activities in the sediment trap material and the mean values for the surface sediment over the period, are shown in Table 3. It can be seen that the 137Cs activity in the trap material was high compared with the surface sediment, and that the highest activity was generally found in the middle trap. There was also a general decrease in 137Cs activity with time, an effect most pronounced in the material in the lowest trap. This is to be expected, since settling labelled particles are diluted with non-labelled particles on reaching the surface sediment. Since the radioactive material in the bottom trap contains material resuspended from the surface sediment, the difference in radioactivity between them will tend to decrease with time.
256
L. Brydsten
& M. Jansson
Great differences between the quantities of material collected in the lower and middle traps indicates resuspension (Hakansson et al., 1987). All stations in the current investigation display significant resuspension (Table 2) and are thus defined as transport bottoms, i.e. bottoms with discontinuous accumulation. This indicates that most particles discharged by the river are frequently resuspended before reaching the accumulation bottoms in the estuary and the deep sea. The degree of sediment contamination at Station 1 shows a continuous decrease (Table 1). Station 1 is placed near the river mouth where the fluvial power prevents transportation back from the central part of the estuary, and therefore sediment deposition in this area is governed by fluvial discharge. The results tell us that a considerable sedimentation of particles occurs in this area during the spring flow, i.e. during May and June. Since activities decrease during the summer, deposited particles could be either eroded or diluted by the deposition of non-radioactive sediment. However, as the fluvial input of particles during the summer is negligible, erosion is the likely explanation. Station 2 is located in a local depression, probably preventing transport out of the area and causing it to act as a sediment sink. This is verified by the fact that the 137Cs activity continuously increases in the surface sediment during the summer. Stations 3, 4 and 6 show variations in the degree of contamination with time, which indicates rapid changes from accumulation to erosion, with a positive net sedimentation balance during some periods and a negative effect during others. They, thus, represent typical transport bottoms. Station 7 is placed in relatively deep water outside the estuary mouth. The contamination levels slowly increased during the whole study period, which indicates a slow net transport of particles out of the estuary throughout the investigation. Table 2 shows that the bottom is of the transport type, with frequent significant resuspensions occurring during the latter part of the study. The continuously increasing degree of contamination indicates that a unidirectional transport mechanism, from the estuary mouth to deeper bottoms, dominates.
Discussion The results obtained in this study demonstrate that river transported material ‘ labelled ’ the surface sediment of the &efjHrden estuary with radioactive particles. Transport and deposition of such labelled particles took place during the spring flow, whence any further supply of radioactivity was insignificant. Table 1 clearly shows that a significant redistribution of primarily deposited radioactive material occurs during the remainder of the icefree period. The net transport of particles is towards the sea, but significant inward transport and deposition on local sediment area within the estuary also occurs. This means that the Tjernobyl accident has provided an excellent opportunity to follow the sediment dynamics of this estuary. As mentioned in the introduction, it is supposed that fluvial action, wave-induced resuspension and near-bottom currents are the main processes which govern the sediment dynamics. There are, as yet, no measurements on these different processes in the orefjlrden estuary, but various environmental factors in combination with our results can be used to estimate the causal relationships in the sediment dynamics process. Figure 3 indicates a relatively high magnitude of fluvial action and a low frequency of resuspension due to wind-driven currents or waves, for the first period of investigation.
Estuarine sediment dynamics
257
The distribution of contaminated surface sediment after the spring flow is shown in Table 1 (16 June). A high 13?Csactivity occurs along the centre-line of the fresh water plume (Stations 1,4 and 6), with the highest activity near its front end (Station 6), about 10 km from the river mouth. The activity is seento rapidly decreasein directions lateral to the centre-line (Stations 2 and 3), and forward of the front of the plume (Station 7). This distribution indicates that fluvial action is the strongest controlling factor for the deposition of suspendedparticles discharged into the estuary by the river. During the secondmeasurementperiod (July to September), the input of contaminated particles was drastically decreased. Therefore, the change in the distribution of contaminated surface sediment which was observed is a result of redistribution within, and possibly also a transport out of, the estuary. Table 1 showsa general increase in the degree of contamination during these months. With either a low or non-existent input of contaminated particles during the secondperiod, the observed increasemust be causedby transportation of radioactive material from shallow areas(i.e. those without measurement stations) to deeper parts of the estuary, where the measurementstations are situated. The increasein contamination at Stations 2 and 3 indicates transport directions which can not be explained by fluvial action, since these stations are situated outside the fluvial plume’s zone of influence. Non-fluvial processestherefore had a greater relative importance than in the first period. The extent of resuspensioncan be estimated by an analysis of the results from sediment traps, since deviations from a linear increase of trap material vs. depth indicate the magnitude of the resuspension(Hakanson et al., 1987). Trapped material showsthat only a weak resuspensionoccurred at the deeper parts of the estuary during period 2. From the data in Figure 2 it is alsounlikely that resuspensionshould occur to any great extent from sediment deposited in deeper water during this period, except at high wind speedsfrom NW-N during mid July. An explanation for the observed increase in contamination at deeper parts of the estuary could then be that radioactive particles from shallow sediment are resuspendedby waves and transported by local wind-driven currents. In this respect, it should be observed that sediment situated in shallow water can be resuspendedby waves of relatively low amplitude. Figure 4 shows the relationship between wind-speed, fetch and maximum depth for the resuspensionof particles larger than 4.5 phi. The figure is based on the empirical relationship derived by Komar and Miller (1973) and the wave generation formulae evaluated by Silvester (1974). Resuspensioncausedby waves generated within the estuary should occur at water depths above 4 m, and waves generated in open seahave sufficient power to instigate resuspensionof fine-grained sediment down to about 20 m depth at the wind speedsoccurring during the investigated period (Figure 3~). Thus, a substantial transport of fine-grained particles might have occurred from shallow bottoms to the deeper parts of the estuary. A general decreasein the contamination of the surface sediment occurs between July and October. This can not be explained by an incresed output from the estuary (see Station 7 in Table l), since the November sampling showed high contamination within the estuary. On the other hand, it can be seenfrom Figure 2c that high wind-speeds from the SW occurred immediately before the October sampling. Therefore, it is probable that the particles were still in suspensionon the sampling date asa direct result of the preceding storm. The difference in the contamination degree for Station 6 showsthat even sediment at great depths (35 m) may be eroded on occasion. During the last period (October-November), there was a transformation of contaminated sediment within the deeper parts of the estuary, with a significant net transport of
258
L. Brydsten & M.Jansson
contaminated particles from outer (decreased radioactivity at Station 6) to inner (increased radioactivity at Stations 2,3 and 4) estuarine regions. This period is characterized by frequent high wind speedsfrom different directions. Table 2 also shows a great increasein the resuspensiondegree, particularly at the mouth of the estuary (Station 7). Resuspensionat such great depths (30-45 m) can not be induced by wind-driven currents, and so must be causedby waves. Wave-induced resuspensionof sediment at depths of more than 30 m requires waves generated in seaswith relatively long fetches (Figure 4). Fine-grained sediment on bottoms down to 30 m depth can only be resuspendedby waves generated by winds having a duration of 6 hours and a speed of 12 m s-‘. Such circumstanceswere prevalent only during the last period (Figure 2~). It therefore seemsthat waves generated by low wind-speeds (lessthan about 10 m s-l) only causeresuspensionof sediment on shallow areasabout 20 m, but waves generated by higher wind-speeds and longer fetches alsocauseresuspensionof sediment in the deeper parts of the estuary. Particles resuspended from shallow bottoms are transported to greater depths, whereas resuspended particles from bottoms below 20m depth are redeposited at the samelocations. The net transport towards the inner part of the estuary during late autumn (Table 1) is difficult to explain, but may be causedby the variation in wind-direction during the lowpressure passage.Before the warm-front, the wind-direction is generally from the south, i.e. the direction which will generate high waves within the estuary. This situation causes wave-induced resuspensionand residual near-bottom currents towards the mouth of the estuary. The wind-direction thereafter changesclockwise to N and NW, with consequent changesin the residual current directions. The residual current will be from the mouth towards the inner part of the estuary, which probably causesa net transportation of resuspendedparticles in the samedirection. Ifthis explanation isvalid, then the generalexposureofthe orefjarden to prevailing hardwind meteorological conditions is of great importance for the sediment dynamics within the estuary. If a wind, which causeswave-induced resuspension,is followed by one which generates near-bottom currents directed against the inner part of the estuary, then the redistribution of sediment particles is directed towards the land, thus delaying the net transport of particles through the estuary to the sea.The results of the 137Csstudy tell us that this happened during the Summer of 1986.
Conclusions * The spring flow in the River are carries a dominant shareof the particle loading on the estuary. * River transported material is primarily deposited within the influence areaof the river plume and within the estuary. * A net flow of particles occurs towards the estuary mouth. The main part of the annual river input of particles is still situated within the estuary at the end of the ice-free period. The mean receding time is at least several years. * Quantitatively important resuspension of the surface sediment occurs several times during the ice-free period mainly due to the erosive power of wind induced waves. Resuspendedmaterial from shallow (< 20 m) bottoms is transported to deeper areas while material resuspendedfrom deeper lying sediment ( > 20 m) seemsto be redeposited at approximately the samedepth.
Estuarine
sediment
dynamics
259
* Following a storm in late autumn, a significant redistribution of particles towards the inner part of the estuary occurred. If it is a regular phenomenon, this type of event will increase the residence time of particles within the estuary. * The internal sediment dynamics, i.e. resuspension-redistribution within the estuary are much more intense compared with the rates of input and output of particles. Acknowledgements This work was carried out with financial support from National Institute of Radiation Protection. We thank Lennart Johansson, Torbjorn Nyle’n and Ronnie Bergman at the National Defence Research Institute in Umea for help with measurement of 137Cs. Bjorn Jonsson and Rolf Zale provided help with sampling and preparation of sediment from orefjarden. References McHenry, J. R., Ritchie, J. C. & Gill, A. C. 1973 Accumulation of fallout cesium-137 in soils and sediments in selected watersheds. Water Resource Research 9,676-686. Gedney, R. T. 1971 Numerical calculations of the wind-driven currents in Lake Eire. Case Western Reserve University, Cleveland, Ohio. Hakanson, L., Floderus, S. & Wallin, M. 1989 Sediment trap swarms-a methodological description. Hydrobiologia (submitted). Komar, P. D. & Miller, M. C. 1973 The threshold of sediment movement under oscillatory water waves. Journal Sedimentary Petrolology 43, 1101-l 110. Silvester, R. 1974 Coastal Engineering, Vol. 1. Amsterdam: Elsevier.
Coastal
and She&Science
(1989)
28,249-259
Studies of Estuarine Sediment Using 13’Cs from the Tjernobyl a Tracer
Lars
Brydsten
Department Sweden Received
and Mats
of Physical
4January
Keywords:
Geography,
1988 and in
Dynamics Accident
as
Jansson University
of Umei,
revisedform 30 November
S-901
87 Umeh,
1988
sedimentdynamics;estuaries;137-G; tracer
Sedimentdynamicsin the 6re River estuaryin northern Swedenwerestudiedby monitoring the turnover of particles associated with 13’Csin surfacesediment and sedimenttraps during the period following the Tjernobyl accidentin late April until lateNovember 1986.River transportedmaterialwasdepositedin the estuary and then frequently redistributed due to resuspension-redeposition processes during the ice-freeperiod. There wasa slownet transportof particles out of the estuary which wasdelayedby at leastone event with a significant particle redistribution to the inner part of the estuary.Wind andwave induced water dynamicsareresponsiblefor resuspension andtransportof particles.
Introduction
In late April 1986the whole catchment area of the River ore (Figure 1) in the province of Vlsterbotten, Northern Sweden, was affected by precipitation contaminated with radionuclides from the nuclear power plant accident in Tjernobyl. The mean fallout in the catchment was approximately 40 kBq mP2 (137Cs).The radioactive fallout coincided with the rapid melting of a 30-50 cm deep snow cover in the drainage area. This situation led to the assumption that the majority of the radionuclides would be drained into the river by surface runoff. Furthermore, these radionuclides should, for the most part, be associatedwith fine grained particles (McHenry et al., 1973). A considerable quantity of the fallout should therefore be carried by the river spring flow to the estuary, ijrefjarden, outside the river mouth. If this had happened, radioactively labelled particles would be incorporated in the sediment, thus providing a unique opportunity to study sediment dynamics by following the turnover of radionuclides in the estuary. A sediment dynamic study was therefore initiated in June 1986 and 137Cswas chosen as the tracer nuclide. The study area, &efjGden, is a semi-closedwater-body, partly isolated from the outer sea,Kvarken, by a densearchipelago (Figure 1). The salinity is low, about 3 per ml, and so the water circulation driven by the salinity difference between inland and coastal water is of relatively low importance. There are no tides in the area, and large scalecurrents exist 0272-7714/89/030249+
11 $03.00/O
@1989 AcademicPressLimited
250
L. Brvdsten
Figure
&M.
1. River
Bansson
6re catchment
area and location
of the investigated
area
only outside the estuary. Therefore, sediment dynamics in the estuary are determined by wind-induced waves, wind-driven currents and fluvial action. The sediment source is mainly particles carried by the River Ore. A rough calculation (Brydsten, unpublished) indicates that about 95% of the total input of fine-grained particles to the estuary emanates from fluvial input. Of this, the major part (809,)) is supplied during the spring flow in May-June. Since the area is still influenced by the Holocene uplift, new unwashed till are continuously being exposed to wave-washing. Fine-grained particles, suspended by the waves and transported from the shore, are thus another, albeit less important, sediment source. Extensive areas of continuous accumulation exist on bottoms below a water depth of about 65 m, i.e. not within the estuary. Smaller locations with accumulation occur in the estuary up to about 5 m water depth. Therefore, the main part of the suspended particle input from the river is transported through the estuary and accumulated in the open sea. The fresh-water plume does not reach the mouth of the estuary, so the transport of finegrained particles from the river-mouth to the deeper waters is not controlled by river action alone, but is a result of several processes. The hypotheses to describe sediment dynamics in this type of estuary are as follows: * River input is the main source of particles. *The suspended material supplied to the estuary is primarily deposited within the estuary. * Deposited sediment particles are resuspended due to wave action. * The major portion of the fine-grained deposited particles are then transported through the estuary by a series of resuspension and transport processes, so that only a minor part is permanently accumulated within the system. *The transport of resuspended particles is controlled mainly by wind-driven nearbottom currents. * The distribution of sites with continuous accumulation within the estuary is controlled by bottom morphometry plus the wave-induced near-bottom water dynamics or by the wind-driven near-bottom currents.
Estuarine
sediment
dynamics
251
t
Figure
2. Morphometry
of the &efjirden
estuary
and locations
of sample
stations.
The use of 137Cs as a particle tracer was, in the first year of this study, used mainly to study the following questions in the complex patterns of estuarine sediment dynamics: * Where are the river transported suspended particles deposited? * Where, to what extent and under what conditions are sediment particles resuspended? * Where are resuspended particles redeposited? * Are there any correlations between climatic conditions and transport directions?
Material
and methods
Sample stations were located according to Figure 2. Station 1 was positioned in a shallow area (8 m) near the river mouth, with expected strong fluvial power and low salinity. Stations 4, 6 and 7 were placed along the expected fluvial plume and at relatively great depths (25 m, 35 m and 45 m, respectively) on transport bottoms with comparatively long periods of accumulation. Station 7 was situated outside the mouth of the estuary. Stations 2 and 3 were in positions lateral to the fluvial plume centre-line and were expected to reveal non-fluvial sediment dynamics. Station 2 is sheltered from open-water waves by a shallow
252
L. Brydsren & M. Jansson
sub-marine moraine, while Station 3 is well exposed to the open-water. Both stations are situated at local depressions(25 and 30 m, respectively). The suspendedmaterial in river water was sampled with a depth-integrating sampler near the river mouth (Figure 1). The samplewas dried (105 “C) and the 137Cs activity was measuredon the solid residue. Suspendedmaterial in the estuarine water column was collected with sediment traps. The traps were put out on 16 June at 6 stations spread over the estuary. At each station traps were placed at 3 depths, 1 and 2 m over the bottom and 2 m below the water surface, and were drained on four occasions(16 June, 25 July, 2 October and 4 November). Surface sediment wascollected with a grab sampler on 16 June, 25 July, 2 October and 4 November. Sub-sampleswere taken from the grab sampler (50 cm’, O-l cm, l-2 cm, 2-5 cm). All sediment and trap sampleswere dried (105 “C) and the radioactivity was measured on the dried samples. The 137Csand 134Csradioactivity in suspendedparticles from the river and estuarine waters and in surface sediment sampleswas measuredwith a high-resolution germanium detector by the National Defence Research Institute in Umea. In order to try to explain variations in the 137Csdistribution in sediment and settling material the following environmental factors, which are crucial for the sediment dynamics have been utilized. River discharge is an approximation for the fluvial influence. The River are is of nival low-land regime with a maximum in May and a minimum in February. The maximum is determined by the snow melting and about 65O;”of the annual discharge occurs during May and the first half of June (Figure 3a). Therefore, the annual influence of fluvial action on the sediment dynamics in the estuary is limited to a short time during the spring. Wind-speed can be used as a rough approximation for near-bottom wind-induced currents (Gedney, 1971). The wind-speed at Sydostbrotten, situated near the mouth of the estuary, during the period under study is shown in Figure 3b. During the period investigated there were generally low wind speedsfrom early May to mid September, never exceeding 15 m SK*. Thereafter, three deep low pressures with strong winds occurred at about two weeks intervals. Thus, the relative magnitude of wind-induced currents washigher at the end of the ice-free period. The wave-induced resuspensionof fine-grained sediment situated below 20 m depths requires waves generated in seaswith fetches longer than about 30 km (wind-speed 20 m s-l) (Silvester, 1974; Komar StMiller, 1973) (Figure 4). Therefore, the sediment in the deeper parts of the estuary can only be resuspendedby waves entering from the open sea,e.g. from SSW-SE. Waves from all other directions break at the archipelago. Windspeedsfrom SSW-SE in the southern Gulf of Bothnia (Figure 3c) were therefore used to estimate the occurrence of wave-induced resuspension. Results
The major input (> 809,) of contaminated particles to the estuary occurred during the spring flow period in May and the first half of June (Figure 3a). The 137Cs activities of the surface sediment are presented in Table 1. The ratio 137Cs/134Cs wasapproximately 2.2 in all samplesshowing that all measured137Cscome from the Tjernobyl reactor. Deposited radioactive particles were found only in the O-l cm layer during the Summer of 1986. 137Csactivity in the layer l-2 cm or 2-5 cm never exceeded the ‘ background ’ value of
Estuarine sediment dynamics
253
June
July
June
July
August
September
October
ib)
20 f8167 m 14L D IF?8 :: IOI E 8 3 6
. August
September
October
22 ,
May
June
Figure 3. Time distributions directions at Sydostbrotten
JL ly
august
September
October
of discharge in the River 8re (a), wind-speeds from all (b) and wind-speeds from SSW to SE at Sydostbrotten (c).
254
L. Brydsren &3 M. Jamson
Depth
Figure
4. Transport
of ‘)‘Cs in the outlet
(ml
of the River
ijre.
1. l”Cs activity in surface sediment (kBq kg ’ dry weight). only on 16 June due to loss of sediment traps on this location
TABLE
Station
5 was sampled
Station Date 16 June 25 July 2 October 4 November
1
2
3
4
8.4 7.0 5.9 4.9
6.3 11.0 14.0 29.0
3.4 13.0 9.6 13.0
9.4 10.0 6.5 20.0
5 9.8 NA NA NA
6
7
10.0 32.0 2-5 16.0
3.8 4.6 6.6 8.2
0.2 kBq kg-‘. Activities are generally very high in the central part of the estuary, over 30 kBq kgg’ dried substance. These values are well above those measured in sediment samplestaken near brook outlets in lakes in extremely contaminated areasin northern Sweden (Holmgren & Jansson,unpubl.). Background activities, as measured on several samples taken from depth of 5-10 cm below sediment surface in the estuary, were ~0.2 kBq kg-’ Dw-’ (detection limit). The assumption that a large part of the fallout on the River ore catchment region should be transported to and deposited in the river mouth area was thus correct. The net sedimentation rates (g day- ’ mP2)calculated from the amounts accumulated in the traps are presented in Table 2. A general increasein the amount of material collected in the near-bottom traps occurs with time and in a direction towards the deep sea. The greatest net sedimentation is observed outside the mouth of the estuary between 25 September and 4 November and reachesa maximum of 322 g day-’ mP2. For the nearsurface traps, the trend is also an increase with time, but a decrease in the deep-sea direction.
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sediment
dynamics
255
TABLE 2. Net sedimentation rate in sediment traps (g day-’ mm*). (Top-l m below water surface, Middle-2 m above the sediment surface and Bottom-l m above the sediment surface)
Station
1
3i
4I
7
Date 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November
Position
of sediment
traps
Top
Middle
Bottom
12.5 22.4 48.6 1.9 3.3 22.8 1.7 2.4 17.0 1.3 1.3 18.6
7.8 23.9 58.9 7.2 6.6 67.5 7.6 6.0 51.2 10.1 15.5 107.9
9.3 28.9 90.5 13.6 7.3 96.9 12.0 8.0 72.6 18.8 19.8 322.8
3.l”Cs activity in material collected in sediment traps and surface sediment (kBq kg-’ dry weight). (Top-l m below water surface, Middle-2 m above the sediment surface and Bottom-l m above the sediment surface) TABLE
Station
1
3I
41
7
Date 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November 25 July 2 October 4 November
Position
of sediment
Top
Middle
Bottom
Surface sediment
47.5 9.2 11.2 63.0 29.0 15.5 91.6 30.5 20.3 62.7 19.7 15.0
53.7 9.5 11.4 51.0 27.0 16.0 71.6 28.7 23.0 36.6 15.3 10.1
7.7 6.5 5.4 8.2 11.3 11.3 9.7 8.3 13.2 4.2 5.6 7.4
20.4 1.9 1.9 13.3 17.8 7.6 29.4 35.7 11 .o 45.4 98.3 8.4
traps
The 137Cs activities in the sediment trap material and the mean values for the surface sediment over the period, are shown in Table 3. It can be seen that the 137Cs activity in the trap material was high compared with the surface sediment, and that the highest activity was generally found in the middle trap. There was also a general decrease in 137Cs activity with time, an effect most pronounced in the material in the lowest trap. This is to be expected, since settling labelled particles are diluted with non-labelled particles on reaching the surface sediment. Since the radioactive material in the bottom trap contains material resuspended from the surface sediment, the difference in radioactivity between them will tend to decrease with time.
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L. Brydsten
& M. Jansson
Great differences between the quantities of material collected in the lower and middle traps indicates resuspension (Hakansson et al., 1987). All stations in the current investigation display significant resuspension (Table 2) and are thus defined as transport bottoms, i.e. bottoms with discontinuous accumulation. This indicates that most particles discharged by the river are frequently resuspended before reaching the accumulation bottoms in the estuary and the deep sea. The degree of sediment contamination at Station 1 shows a continuous decrease (Table 1). Station 1 is placed near the river mouth where the fluvial power prevents transportation back from the central part of the estuary, and therefore sediment deposition in this area is governed by fluvial discharge. The results tell us that a considerable sedimentation of particles occurs in this area during the spring flow, i.e. during May and June. Since activities decrease during the summer, deposited particles could be either eroded or diluted by the deposition of non-radioactive sediment. However, as the fluvial input of particles during the summer is negligible, erosion is the likely explanation. Station 2 is located in a local depression, probably preventing transport out of the area and causing it to act as a sediment sink. This is verified by the fact that the 137Cs activity continuously increases in the surface sediment during the summer. Stations 3, 4 and 6 show variations in the degree of contamination with time, which indicates rapid changes from accumulation to erosion, with a positive net sedimentation balance during some periods and a negative effect during others. They, thus, represent typical transport bottoms. Station 7 is placed in relatively deep water outside the estuary mouth. The contamination levels slowly increased during the whole study period, which indicates a slow net transport of particles out of the estuary throughout the investigation. Table 2 shows that the bottom is of the transport type, with frequent significant resuspensions occurring during the latter part of the study. The continuously increasing degree of contamination indicates that a unidirectional transport mechanism, from the estuary mouth to deeper bottoms, dominates.
Discussion The results obtained in this study demonstrate that river transported material ‘ labelled ’ the surface sediment of the &efjHrden estuary with radioactive particles. Transport and deposition of such labelled particles took place during the spring flow, whence any further supply of radioactivity was insignificant. Table 1 clearly shows that a significant redistribution of primarily deposited radioactive material occurs during the remainder of the icefree period. The net transport of particles is towards the sea, but significant inward transport and deposition on local sediment area within the estuary also occurs. This means that the Tjernobyl accident has provided an excellent opportunity to follow the sediment dynamics of this estuary. As mentioned in the introduction, it is supposed that fluvial action, wave-induced resuspension and near-bottom currents are the main processes which govern the sediment dynamics. There are, as yet, no measurements on these different processes in the orefjlrden estuary, but various environmental factors in combination with our results can be used to estimate the causal relationships in the sediment dynamics process. Figure 3 indicates a relatively high magnitude of fluvial action and a low frequency of resuspension due to wind-driven currents or waves, for the first period of investigation.
Estuarine sediment dynamics
257
The distribution of contaminated surface sediment after the spring flow is shown in Table 1 (16 June). A high 13?Csactivity occurs along the centre-line of the fresh water plume (Stations 1,4 and 6), with the highest activity near its front end (Station 6), about 10 km from the river mouth. The activity is seento rapidly decreasein directions lateral to the centre-line (Stations 2 and 3), and forward of the front of the plume (Station 7). This distribution indicates that fluvial action is the strongest controlling factor for the deposition of suspendedparticles discharged into the estuary by the river. During the secondmeasurementperiod (July to September), the input of contaminated particles was drastically decreased. Therefore, the change in the distribution of contaminated surface sediment which was observed is a result of redistribution within, and possibly also a transport out of, the estuary. Table 1 showsa general increase in the degree of contamination during these months. With either a low or non-existent input of contaminated particles during the secondperiod, the observed increasemust be causedby transportation of radioactive material from shallow areas(i.e. those without measurement stations) to deeper parts of the estuary, where the measurementstations are situated. The increasein contamination at Stations 2 and 3 indicates transport directions which can not be explained by fluvial action, since these stations are situated outside the fluvial plume’s zone of influence. Non-fluvial processestherefore had a greater relative importance than in the first period. The extent of resuspensioncan be estimated by an analysis of the results from sediment traps, since deviations from a linear increase of trap material vs. depth indicate the magnitude of the resuspension(Hakanson et al., 1987). Trapped material showsthat only a weak resuspensionoccurred at the deeper parts of the estuary during period 2. From the data in Figure 2 it is alsounlikely that resuspensionshould occur to any great extent from sediment deposited in deeper water during this period, except at high wind speedsfrom NW-N during mid July. An explanation for the observed increase in contamination at deeper parts of the estuary could then be that radioactive particles from shallow sediment are resuspendedby waves and transported by local wind-driven currents. In this respect, it should be observed that sediment situated in shallow water can be resuspendedby waves of relatively low amplitude. Figure 4 shows the relationship between wind-speed, fetch and maximum depth for the resuspensionof particles larger than 4.5 phi. The figure is based on the empirical relationship derived by Komar and Miller (1973) and the wave generation formulae evaluated by Silvester (1974). Resuspensioncausedby waves generated within the estuary should occur at water depths above 4 m, and waves generated in open seahave sufficient power to instigate resuspensionof fine-grained sediment down to about 20 m depth at the wind speedsoccurring during the investigated period (Figure 3~). Thus, a substantial transport of fine-grained particles might have occurred from shallow bottoms to the deeper parts of the estuary. A general decreasein the contamination of the surface sediment occurs between July and October. This can not be explained by an incresed output from the estuary (see Station 7 in Table l), since the November sampling showed high contamination within the estuary. On the other hand, it can be seenfrom Figure 2c that high wind-speeds from the SW occurred immediately before the October sampling. Therefore, it is probable that the particles were still in suspensionon the sampling date asa direct result of the preceding storm. The difference in the contamination degree for Station 6 showsthat even sediment at great depths (35 m) may be eroded on occasion. During the last period (October-November), there was a transformation of contaminated sediment within the deeper parts of the estuary, with a significant net transport of
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L. Brydsten & M.Jansson
contaminated particles from outer (decreased radioactivity at Station 6) to inner (increased radioactivity at Stations 2,3 and 4) estuarine regions. This period is characterized by frequent high wind speedsfrom different directions. Table 2 also shows a great increasein the resuspensiondegree, particularly at the mouth of the estuary (Station 7). Resuspensionat such great depths (30-45 m) can not be induced by wind-driven currents, and so must be causedby waves. Wave-induced resuspensionof sediment at depths of more than 30 m requires waves generated in seaswith relatively long fetches (Figure 4). Fine-grained sediment on bottoms down to 30 m depth can only be resuspendedby waves generated by winds having a duration of 6 hours and a speed of 12 m s-‘. Such circumstanceswere prevalent only during the last period (Figure 2~). It therefore seemsthat waves generated by low wind-speeds (lessthan about 10 m s-l) only causeresuspensionof sediment on shallow areasabout 20 m, but waves generated by higher wind-speeds and longer fetches alsocauseresuspensionof sediment in the deeper parts of the estuary. Particles resuspended from shallow bottoms are transported to greater depths, whereas resuspended particles from bottoms below 20m depth are redeposited at the samelocations. The net transport towards the inner part of the estuary during late autumn (Table 1) is difficult to explain, but may be causedby the variation in wind-direction during the lowpressure passage.Before the warm-front, the wind-direction is generally from the south, i.e. the direction which will generate high waves within the estuary. This situation causes wave-induced resuspensionand residual near-bottom currents towards the mouth of the estuary. The wind-direction thereafter changesclockwise to N and NW, with consequent changesin the residual current directions. The residual current will be from the mouth towards the inner part of the estuary, which probably causesa net transportation of resuspendedparticles in the samedirection. Ifthis explanation isvalid, then the generalexposureofthe orefjarden to prevailing hardwind meteorological conditions is of great importance for the sediment dynamics within the estuary. If a wind, which causeswave-induced resuspension,is followed by one which generates near-bottom currents directed against the inner part of the estuary, then the redistribution of sediment particles is directed towards the land, thus delaying the net transport of particles through the estuary to the sea.The results of the 137Csstudy tell us that this happened during the Summer of 1986.
Conclusions * The spring flow in the River are carries a dominant shareof the particle loading on the estuary. * River transported material is primarily deposited within the influence areaof the river plume and within the estuary. * A net flow of particles occurs towards the estuary mouth. The main part of the annual river input of particles is still situated within the estuary at the end of the ice-free period. The mean receding time is at least several years. * Quantitatively important resuspension of the surface sediment occurs several times during the ice-free period mainly due to the erosive power of wind induced waves. Resuspendedmaterial from shallow (< 20 m) bottoms is transported to deeper areas while material resuspendedfrom deeper lying sediment ( > 20 m) seemsto be redeposited at approximately the samedepth.
Estuarine
sediment
dynamics
259
* Following a storm in late autumn, a significant redistribution of particles towards the inner part of the estuary occurred. If it is a regular phenomenon, this type of event will increase the residence time of particles within the estuary. * The internal sediment dynamics, i.e. resuspension-redistribution within the estuary are much more intense compared with the rates of input and output of particles. Acknowledgements This work was carried out with financial support from National Institute of Radiation Protection. We thank Lennart Johansson, Torbjorn Nyle’n and Ronnie Bergman at the National Defence Research Institute in Umea for help with measurement of 137Cs. Bjorn Jonsson and Rolf Zale provided help with sampling and preparation of sediment from orefjarden. References McHenry, J. R., Ritchie, J. C. & Gill, A. C. 1973 Accumulation of fallout cesium-137 in soils and sediments in selected watersheds. Water Resource Research 9,676-686. Gedney, R. T. 1971 Numerical calculations of the wind-driven currents in Lake Eire. Case Western Reserve University, Cleveland, Ohio. Hakanson, L., Floderus, S. & Wallin, M. 1989 Sediment trap swarms-a methodological description. Hydrobiologia (submitted). Komar, P. D. & Miller, M. C. 1973 The threshold of sediment movement under oscillatory water waves. Journal Sedimentary Petrolology 43, 1101-l 110. Silvester, R. 1974 Coastal Engineering, Vol. 1. Amsterdam: Elsevier.