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Natural tracers for sediment transport studies
ContinentalShelfResearch,Vol. 7, Nos. 11/12,pp. 1333-1343, 1987.
0278~.343/87 $3.00 + 0.(~1 © 1987PergamonJournalsLtd.
Printedin Great Britain.
Natural tracers for sediment transport studies W. SALOMONS* and W. G. MOOK, (Received 5 September 1986; in revised form 29 May 1987; accepted 10 June 1987) Abstract--The use of natural differences in composition between marine and fluvial sediments makes it possible to determine their mixing ratio in estuarine deposits. Differences in chemical, mineralogical and isotope geochemical composition can be used as tracers, provided that three criteria are met: conservative behaviour during transport, with time, and after deposition. Examples of the utility of natural tracers are presented, including their use for the interpretation of pollutant patterns in estuaries. The application of natural tracers has shown that marine sediments may be transported past the freshwater boundary in estuaries, and thus contribute to the sedimentation in freshwater tidal areas.
INTRODUCTION
PARTICLES in estuaries have different origins. Rivers are supplying particles to the estuary; suspended matter may also be introduced into the estuary from the marine environment. Apart from these two primary sources, internal recycling through erosion and deposition takes place. Studies .of the fate and behaviour of trace metals and other pollutants in estuaries benefit from methods which distinguish between marine and fluvial sediment origins. Basically, two approaches are used to study processes affecting pollutants in the estuarine system, and to determine whether estuaries act as sinks or sources for contaminants. The first approach studies the changes in water chemistry, and the second the changes in composition of the sediments and suspended particles. In most estuarine studies, information on the behaviour of trace constituents has been derived from mixing curves, which depict the relationship between salinity and the dissolved constituents. In principle, these observations should be confirmed by analyses of the particulate components (i.e. losses from the dissolved phase should show up as elevated levels in the particulate fraction). However, due to lack of suitable tracers this approach has seldom been used (SALOMONS and MooK, 1977; SALOMONSand EYS1NK, 1981). If harbours are located within an estuary, accumulations of sediments in the harbour areas and navigation channels may have to be removed by dredging. Understanding the origin of the dredged material (fluvial or marine derived) is of great importance for dredging practices. Furthermore, if man makes changes in the morphology of estuaries * Delft Hydraulics Laboratory, c/o Institute for Soil Fertility, P.O. Box 30003, 9750 RA Haren (Gr), The Netherlands. t Centre for Isotope Research, University of Groningen, Westersingel 34, 9718 CM Groningen, The Netherlands. 1333
1334
w . SALOMONSand W. G. MOOK
(barrages, dams, harbours etc.), the changing sedimentation patterns may necessitate increased dredging operations. For predicting these changes, knowledge of the origins of sediments is indispensable. In this article, tracers are discussed which can be used to distinguish between different sediment sources in an estuary. The tracers are natural differences in composition between marine and fluvial sediments. The use of natural tracers has some advantages over the use of artificial tracers (radio-nuclide labelling or labelling with Ir or Ta). The method is far less expensive, the field work is relatively simple, and sampling can be repeated as often as is necessary (e.g. to detect changes during isolated events such as major storms). Artificial tracers have the advantage that transport pathways in an estuary can be traced. However, information can be obtained only over a relatively short time period. The choice between natural or artificial tracer methods is not an either/or case; the two approaches are complementary and provide their own solutions to sediment transport and sedimentation problems in estuaries. TRACERS FOR SEDIMENT TRANSPORT
If simple mixing of marine and fluvial suspended matter took place in an estuary, the relationship between suspended matter concentration and salinity would be a straight line; the mixing ratio of riverwater and seawater would determine the suspended matter concentration. If this were the case, salinity would be a good tracer for suspended matter. Unfortunately this is not so. A linear relationship is not observed; the relationship sometimes shows a turbidity maximum which may extend into the freshwater tidal area. This indicates that the movement of suspended matter is different from that of water. A conservative index of mixing should be based not on properties of the water, but on properties of the sediments (e.g. the composition of the fluvial and marine endmembers). It is convenient to distinguish between tracer methods in which the sediment sample as taken from the field is analysed ("whole" sediment tracer) and those in which, through a chemical or mechanical treatment, individual mineral components are isolated and subsequently analysed. Whole sediment tracers
The two "whole" sediment tracers are the chemical and the mineralogical composition. In using these two properties one should be aware that the composition of sediments depends on their grain-size distribution. Sandy samples from the same area have a different mineralogical composition from that of the clay-rich samples (Fig. 1). Also, the chemical composition depends on the grain-size distribution. A common method to correct for grain-size differences is to determine, in samples from the same locality, both the mineralogy and a variable which is representative of the grain-size distribution. Several methods are available (FORSTNER and SALOMONS, 1980; SALOMONS and FORSTNER, 1984). An example of the variation in mineralogical and chemical composition of sediment samples from one locality (Wadden Sea, The Netherlands) with the percentage of particles with diameter <16 gm (a variable which is representative of the grain-size distribution) is presented in Fig. 1. From these curves the particle composition at a certain grain-size composition (e.g. 50% < 16 gm) can be derived and used for comparison purposes.
Natural tracers for sediment transport studies
1335
(a) % 100
oi|i
iliii iiili
fill! ~i!iii!
;-;iii i~!!ii i
iilii
|
6O
Clays ~i~'J'~ Feldspars
40
Quartz
'
Org Matter m
i
Carbonates
2O
oiii | 17
21
24
29
32
34
37
43
51
57
62
68
% < 16 ~Jm
b) Jg/g 300
250
i
L f L
200
jxf
J
;z~ 4' ; J
I 150
f
100
0
i J
J 50
/
t J
0 0
10
20
30
40
50
60
70
80
Fig. 1. (a) The mineralogical composition of sediment samples from the Dutch Wadden Sea. (b) Zinc, copper and lead concentrations in sediments from the Dutch Wadden Sea as functions of the percentage of particles with diameters less than 16 lam (% < 16 I-tm).
B y c o m p a r i n g s e d i m e n t s a m p l e s corrected for different grain-size distributions, it is possible to d e t e r m i n e w h e t h e r significant differences exist which are potentially of use for tracer studies. E x a m p l e s of the differences in c h e m i c a l c o m p o s i t i o n of s e d i m e n t s from various river s y s t e m s , and of marine s e d i m e n t s in the North Sea, are presented in Table 1.
1336 Table 1.
W. SALOMONSand W. G. MOOK
Concentrations o f trace elements in fluvial and marine sediments (in lag g 1 at 50% < 16 lam)
Fluvial Seine Thames Scheldt Rhine Meuse Marine Belgium coast Eastern Scheldt Dutch Wadden Sea
Ce
Cs
Eu
La
Sc
Ta
Th
Yb
Tb
95.0 25.6 93.8 96.2 104.0
6.58 3.01 6.50 10.70 8.90
1.02 0.63 0.89 0.39 1.83
43.6 14.6 36.7 37.0 47.9
7.45 3.85 10.20 9.00 11.20
1.22
3.73 1.47 2.47 2.04 2.70
0.67
1.81 1.32 1.09
12.90 3.56 9.11 8.44 11.50
61.7 57.3 69.5
5.29 6.04 6.30
0.83 0.65 0.75
26.3 24.4 29.8
6.62 6.90 7.74
0.90 0.75 1.03
7.48 6.30 8.16
1.72 1.45 1.82
0.82 0.94 1.26 0.68 0.81
An overview on differences between marine and freshwater muds has been given by SHIMP et al. (1969). The use of boron to distinguish between freshwater and marine deposits has been discussed by HARDER (1970). Once differences in composition between fluvial and marine sediments (the end-members) are observed, a number of conditions have to be met before these differences can be used as a tracer. In using the variations in chemical composition, one should be aware that a number of chemical elements are reactive during transport within estuaries. Removal and addition processes may occur, changing their concentrations. Also, changes may occur in the concentrations of certain elements after deposition, especially those which are prone to redox changes (e.g. Mn, Fe, P). These processes show up particularly in the oxidized surface layer of sediments, which are subject to fluxes from the reduced pore waters, or from the surface waters. This demonstrates one pitfall: the uppermost layer may be the most recently deposited, but may also be the most altered one. Such changes occur particularly in areas with low sedimentation rates, where there is sufficient time for a significant flux of elements from the reduced pore waters. An example is given in Table 2 for an area with a sedimentation rate of up to 0.5 m y-i (harbour area in the eastern Scheldt), and an area in which the sedimentation is 10-3-10-2 m y-I (the Dollard area in the Dutch Wadden Sea). Physical processes such as differential flocculation or differential sedimentation, as observed for some deltas and estuaries (GIBBS,1977; EDZWALDand O ' M E L I A , 1975), may also cause changes in the composition of sediments which are not related to differences in origin. However, despite the selective transport of clay minerals from the Amazon, for example, differences in composition between mud from the Amazon and from the Orinoco are sufficiently large to warrant the use of clay mineralogy as an indicator for sediments in the Orinoco Estuary (EISMAet al., 1978). Table 2. Concentrations (Zn and Mn in lag g 1 Fe in %) in the surface layers and in the reduced layers for two marine sedimentation areas. Eastern Scheldt is a high sedimentation area and the Dollard a low sedimentation area. All data are corrected for grain-size differences Eastern Scheldt
Zn Mn Fe (%)
Dollard
Surface
2-5 cm
Surface
2-5 cm
164 550 2.23
158 609 2.23
104 578 2.11
124 1226 2.59
1337
Natural tracers for sediment transport studies
Man-induced changes are possible due to discharges of elements in an estuary. In some cases they can be used to advantage if the chemicals added are unreactive in the estuarine environment. A useful tracer might be the more highly substituted cogeners of the polychlorinated biphenyls. They have a very limited solubility, a low or nearly nonexistent bio-degradability, and are very unreactive in the estuarine environment. Also, polyaromatic hydrocarbons may fall in this category. There is one problem with using these organic micro-pollutants: these types of compounds have a strong affinity for organic matter in the sediments, have a low specific gravity, and may have transport patterns different from that of the other components of the sediments. The conclusions derived from their use as a tracer may reflect only the origin of the organic matter. Separate studies are required to warrant an extrapolation of the results to the whole sediment. Apart from the requirement that tracers should be unreactive in the estuarine environment, the composition of the end-members (marine and fluvial) should be constant over the time period in which the sediment transport has taken place. Nevertheless, the composition of an end-member may change with time, and this is especially noticeable for common pollutants such as heavy metals. This criterion, however, depends on the characteristics of the estuary and the study zone. If sediment transport takes place over short time-periods (as in small tidal estuaries) this criterion is less stringent than for areas in which the sediments move relatively slowly. This means that elements which are temporally variable in supply might have some use for estuaries such as the Rhine. However, for studying the origins of sediments along the Dutch coast, the German Bight or the Wadden Sea they cannot be used. In Table 3, trace element concentrations in sediments from the river Rhine over the period 1922 to 1975 are given. They show that elements such as Cs, La and Ta can be used for long-distance transport studies, due to slow variations in their concentrations in the Rhine sediments. For each estuary the criteria have to be checked specifically. To summarize, one can conclude that a natural tracer has to fulfill the following conditions: conservative behaviour with time; conservative behaviour during transport; and conservative behaviour after deposition.
Tracers to determine the origin of individual sedimentary minerals Although determining the origin of the individual sedimentary particles is more complex than analysing the whole sediment, it can have a number of advantages: (a) Table 3. Relative concentrations in sediments from the Wadden Sea and the Rhine for various periods of sampling. The concentrations in these sediments in 1922 were 100 units W a d d e n Sea
Ce Cs Eu La Sc Ta Th Yb
Rhine
1958
1975
1958
1970
1975
227 93 109 107 97 104 109 111
122 88 100 111 96 103 117 125
116 140 93 103 107 88 93 112
150 135 109 150 107 102 99 114
118 148 92 108 104 152 93 100
1338
W. SALOMONSand W. G. MOOK
Information can be obtained on transport patterns of individual components; in this way a "high resolution" picture of sediment transport may be derived. (b) A smaller number of samples is needed compared with the "whole" sediment method, in which corrections for grain-size have to be applied. (c) If, for a certain area, information on the origin of one component can be extrapolated to the "whole" sediment, the effort required in sampling and analysis is greatly reduced. A large number of methods are available to determine the composition of individual sedimentary particles. Before the individual particles are analysed it is necessary to carry out a pre-treatment. Pre-treatments can be divided into two classes: (i) physical separation of individual sedimentary components and subsequent chemical, mineralogical or isotope geochemical analysis; and (ii) chemical separation, in which the isolated fractions can be analysed directly. Separation of grain-size fractions can be achieved with classical physical methods such as sieving and use of settling columns. The separated fractions can then be analysed for their mineralogical, chemical and isotopic composition. Each of these analyses may yield a suitable tracer. The clay fraction, which is monomineral, can be easily separated with physical methods. A combination of mineralogical and stable isotope analyses of the clay mineral fraction has been carried out for sediments from the southern North Sea and adjoining estuaries (SALOMONSet al., 1975). Isolation of the quartz fraction from grain-size fractions or from the whole sediment is possible with chemical methods (SvERS et al., 1968). After isolation, the chemical composition or the isotopic composition can be determined. The stable isotopic composition of quartz has been used to trace the origin of finely grained quartz particles in Pacific pelagic sediments (CLAYTONet al., 1972). DENNEN (1967) and HERRERA and HEURTEBISE (1974) used the chemical composition of quartz sands for provenance studies. Although no published data exist for estuaries, the combination of chemical and isotopic analyses of the quartz fraction might provide useful data - - especially as quartz is one of the major minerals which constitute estuarine sediments. The isotopic composition of the carbonate or organic matter fraction is determined after chemical isolation (e.g. acid treatment for carbonates and combustion for organic matter). The carbon and oxygen isotopic composition of the carbonates, and the carbon isotopic composition of the organic matter in sediments have often been used to distinguish between marine and fluvial-derived sources (SALOMONS, 1975; TAN and STRAIN, 1979; SCHULTZand CALDER,1976; FONTUGNEand JOUANNEAU,1981; SALOMONS and MooK, 1981, 1982). APPLICATIONS
OF THE
USE OF TRACERS SEDIMENTS
TO DETERMINE
THE
ORIGIN
OF
IN ESTUARIES
Changes in sediment origin over tidal cycles Trace metal concentrations in the suspended matter change during a tidal cycle. This effect was observed during several anchor stations in the Scheldt Estuary (Fig. 2). Samples from the suspended matter were taken each hour over the tidal cycle. During the ebb the concentrations were high, whereas during high water the concentrations were low. The suspended matter samples were analysed for heavy metals, and for the isotopic composition of the carbonates. The isotopic composition was used to calculate the
1339
Natural tracers for sediment transport studies ,ug/9
10 5
o! 5:
%
lOb
5f lllllm'
C~]
znoug Sahnlty (%o)
25
~
Marine mud (%)
20
3o L
~ 8
9
.
+
Cd/2 (ug/g)
j
10 11 12 13 14 15 16 17 18 19 20 21
Hour
Fig. 2. Changes in metal concentrations in the suspended matter, salinity of the water, and isotopic compositionof the carbonates of the suspended matter at an anchor station in the Scheldt Estuary.
percentage of marine suspended matter present. Over the tidal cycle, the salinity showed a sharp peak. The percentage of marine suspended matter started to increase more rapidly than the salinity - - small changes in salinity caused disproportionate changes in the proportion of marine suspended matter. This again reflected the non-conservative behaviour of the suspended matter. Changes in the particulate metal concentrations showed a similar trend to those in the suspended matter - - relatively large changes in concentration occurred for slight increases in salinity.
Changes in sediment origin in the freshwater tidal areas An earlier study in the Ems Estuary (SALOMONSand MOOK, 1977) gave strong evidence for a landward transport of marine sediments past the freshwater boundary. To study this process in more detail, a locality in the freshwater tidal area was selected (Diele), and sediment samples were taken every month over a period of one year. About 15 samples were taken for trace metal analysis. This number of samples was necessary to construct curves such as those presented in Fig. lb for grain-size correction. The concentrations at 50% < 16 ~tm were calculated from regression curves, and used for comparing data from the different sampling dates. The results (Fig. 3) showed a strong seasonal dependence of the metal concentrations, which correlated with the river discharge. It is known that river discharge can influence metal concentrations (SALOMONS and FORSTNER, 1984; SALOMONS and KERDIJK, 1986). However, the relationship was different from that previously observed: low discharge caused high concentrations, and vice versa. Inspection of the isotopic composition
1340
W. SALOMONSand W. G. MooK (a) ,ug/g 1CO
"x
80
60
40
20
Fe/10 Cd X 10 I
I
I
I
I
I
I
I
I
i
J
Zn/lO
I
Mar Apr May Jun Jul Aug Se~ Oct Nov Feb Mar Apr May
Month b)
200 mg/s 700
•
'
J'x \\X,,_~_?
,/
~
Detta ¢:-!3
--
D,sctTarge
-10 Carbon rsotopes c a r b o n a t e s I
I
i
I
I
I
I
I
I
I
I
%o I
I
Mar Apr May dun Jul Aug Sep Oct Nov Feb Mar Apr May
-2 0
Month Fig. 3. (a) Monthly changes in metal concentrations (corrected for grain-size differences) in sediments from the freshwater tidal area of the Ems Estuary. (b) Monthly changes in freshwater discharge and isotopic composition of the carbonates in the sediments.
showed that low metal concentrations were correlated with less-negative values of the carbon isotopic composition, expressed as ~lSC (marine influence), and high concentrations with more negative values (fluvial influence). The changes in metal concentrations can be explained simply by variations in the mixing ratio of marine to fluvial sediments. During periods of low discharge more marine sediments entered the freshwater tidal area, whereas during high discharge the proportion of marine sediments decreased. This is evident from an inspection of the isotopic composition of the carbonates. During periods of low discharge, the ~13 values were
1341
Natural tracers for sediment transport studies
lower compared with periods of high discharge. One important conclusion is that relatively large proportions of marine sediments are found in the freshwater tidal area of estuaries. These observations help to explain the variations in both metal and organic micropollutant concentrations. However, this physical phenomenon is not yet fully understood. A mechanism based on scour and settling lag proposed by POSTMA(1967) and by VANSTRAATENand KUENEN (1957) to explain accumulation of fine-grained sediments in the Dutch Wadden Sea might explain the transport of marine sediments past the freshwater boundary in estuaries. Irregular changes in metal concentrations in bottom sediments The pattern of metal concentrations in deposited sediments from the Scheldt are irregular. From data in Fig. 4 one might conclude that significant discharges of trace metals were taking place at Stas 3 and 4. However, the isotopic composition of carbonates in the deposited sediments show an irregular pattern which is similar to that for the metals. Low metal concentrations coincide with high 813C values (less negative), indicating a high proportion of marine-derived material. Because marine sediments have low metal concentrations (Sta. 11 in Fig. 4), this irregular variation can be explained by changes in the mixing ratio of marine to fluvial-derived sediments along the estuary. Origin of sediments in the Rhine and Scheldt estuaries Samples of the suspended matter were taken over a wide salinity range in the Rhine and Scheldt estuaries. The isotopic composition of the carbonates was determined. Earlier, unpublished studies showed that results from the isotopic composition analysis of the carbonates were similar to those of chemical methods (neutron activation analysis).
1400 I Carbon Isoto~,c ccmpus, t,on carbona}es}
~Jg/g 1200
J
J
l-2o
10,00 -15 8OO
'
6OO
-10
¥
4OO
-05
111
IHLIi
2OO 0
1
2
3
4
5
Zn
~
6
L ° 7
8
9
Cd X 20
m
Pb
10
11
*0.5
Station Fig. 4. Changes in metal concentrations in the Scheldt Estuary from the freshwater reach (Sta. 1) to the Western Scheldt (Sta. 11), and the isotopic composition of the carbonates in the sediments.
1342
w. SALOMONSand W. G. MOOK Percentage fluvial mud lOO
80
~ 1
l ~ i i eL
m - - - ~ - - - = - . . . ~ S c he i d t
% 40
20
5
10
15
20
25
30
35
Salinity %o Fig. 5.
Relationship between the percentage of fluvial mud in suspended matter from the Scheldt and the Rhine estuaries and salinity.
Figure 5 shows that the ratio of marine to fluvial mud in suspended matter is not a simple function of salinity. Furthermore, large differences are observed between the Scheldt and Rhine estuaries. The relationship for the Scheldt Estuary shows three distinct regions. At low salinities the percentage of fluvial mud decreases relatively quickly to about 70% and stays more or less constant for stations between 5 and 15%o. The amount of fluvial mud again decreases with salinities above 15%o. This second decrease coincides with a broadening of the estuary. The Rhine shows the same behaviour, except that the changes at low salinity are much more pronounced. In the Rhine Estuary, less than 20% of the suspended matter originates in the river at salinities in excess of 5%0. If the mixing ratio of marine to fluvial sediments is known, together with the metal concentrations in the fluvial and marine end-members, it is possible to calculate metal concentrations for various mixing ratios (assuming conservative behaviour) and compare these with measured values (SALOMONS and MooK, 1977). Negative deviations from the calculated values indicate release of metals from the sediments, and vice versa. REFERENCES CLAYTON R. N., R. W. REX, J. K. SYERS and M. L. JACKSON (1972) Oxygen isotope abundance in quartz from Pacific pelagic sediment. Journal of Geophysical Research, 77, 3907-3915. DENNEN W. H. (1967) Trace elements in quartz as indicators of provenance. Geological Society of America Bulletin, 78, 125-130. EDZWALD J. K. and C. R. O'MELIA (1975) Clay distribution in recent estuarine sediments. Clay Minerals, 23, 39-44. EISMA D., S. J. VAN DER GAAST, J. M. MARTIN and A. J. THOMAS (1978) Suspended matter and bottom deposits of the Orinoco delta: turbidity, mineralogy and elementary compositions. Netherlands Journal of Sea Research, 12, 224-251. FONTUGNE M. and J. M. JOUANNEAU(1981) S6dimentologie: la composition isotopique du carbone organique des mati6res en suspension de l'estuaire de la Gironde. Application ~ l'6tude de la distribution du plomb et du zinc particulaires. Compte rendu de l'Acad6mie des Sciences (Paris), 293, 389-392. FORSTNER U. and W. SALOMONS (1980) Trace metal analysis on polluted sediments. Part I. Assessment of sources and intensities. Environmental Technology Letters, 1,506-517.
Natural tracers for sediment transport studies
1343
GIBBS R. J. (1977) Clay mineral segregation in the marine environment. Journal of Sedimentary Petrology, 47, 236-243. HARDER H. (1970) Boron content of sediments as a tool in facies analysis. Sedimentary Geology, 4, 153-175. HERRERA R. and M. HEURTEBISE (1974) Neutron activation analysis of trace elements in quartz sands: its possibilities in the assessment of provenance. Chemical Geology, 14, 81-93. POSTMA H. (1967) Sediment transport and sedimentation in the estuarine environment. In: Estuaries, G. H. LAUFF, editor, A.A.A.S. Publication 83, Washington D.C., pp. 158-179. SALOMONS W. (1975) Chemical and isotopic composition of carbonates in recent sediments and soils from Western Europe. Journal of Sedimentary Petrology, 45,440-449. SALOMONS W. and W. G. MOOK (1977) Trace metal concentrations in estuarine sediments: mobilization, mixing or precipitation. Netherlands Journal of Sea Research, 11,199-209. SALOMONSW. and W. EYSlNK (1981) Pathways of mud and particulate trace metals from rivers to the southern North Sea. In: Holocene marine sedimentation in the North Sea Basin, S. D. NIO, R. T. E. SCHUTTENHELM and TJ. C. E. VAN WEERING, editors, Blackwell Science Publications, Oxford, pp. 429-450. SALOMONS W. and W. G. MOOK (1981) Field observations on the isotopic composition of particulate organic carbon in the southern North Sea and adjacent estuaries. Marine Geology, 41, M11-M20. SALOMONS W. and W. G. MOOK (1982) Natural tracers for sediment transport studies. Delft Hydraulics Laboratory Publication, No. 271, 49 pp. SALOMONS W. and U. FORSTNER (1984) Metals in the hydrocycle. Springer Publishing Co., Berlin, 349 pp. SALOMONS W. and H. N. KERDIJK (1986) Cadmium in fresh- and estuarine waters: A review. In: Cadmium in the environment, Vol. 50, H. MISL1N and O. RAVERAL, editors, Experienta Supllementum, pp. 24-28. SALOMONS W., P. HOFMAN, R. BOELENS and W. G. MOOK (1975) The oxygen isotopic composition of the fraction less than 2 microns (clay fraction) in recent sediments and soils from Western Europe. Marine Geology, 18, M23-M28. SCHULTZ D. J. and J. A. CALDER (1976) Organic carbon 13C/12Cvariations in estuarine sediments. Geochimica et Cosmochimica Acta, 40,381-385. SHIMP N. F., J. WI'/~ERS, e. E. POTI'ER and J. A. SCHI,EICHER (1969) Distinguishing marine and freshwater muds. Journal ~)f Geology, 77,566-580. VAN STRAATEN J. M. U. and P. H. KUENEN (1957) Accumulation of fine grained sediments in the Dutch Wadden Sea. Geologie en mijnbouw, 36, 329-354. SVERS J. K., S. L. CHAPMAN, M. L. JACKSON, R. W. REX and R. N. CLAYTON (1968) Quartz isolation from rocks, sediments and soils for determination of oxygen isotopic composition. Geochimica et Cosmochimica Acta, 32, 1022-1025. TAN P. M. and F. C. STRAIN (1979) Carbon and oxygen isotope ratios in Saguenay fjord and the St. Lawrence estuary and their implications for palaeoenvironmental studies. Estuarine and Coastal Marine Science, 8, 119-126.
0278~.343/87 $3.00 + 0.(~1 © 1987PergamonJournalsLtd.
Printedin Great Britain.
Natural tracers for sediment transport studies W. SALOMONS* and W. G. MOOK, (Received 5 September 1986; in revised form 29 May 1987; accepted 10 June 1987) Abstract--The use of natural differences in composition between marine and fluvial sediments makes it possible to determine their mixing ratio in estuarine deposits. Differences in chemical, mineralogical and isotope geochemical composition can be used as tracers, provided that three criteria are met: conservative behaviour during transport, with time, and after deposition. Examples of the utility of natural tracers are presented, including their use for the interpretation of pollutant patterns in estuaries. The application of natural tracers has shown that marine sediments may be transported past the freshwater boundary in estuaries, and thus contribute to the sedimentation in freshwater tidal areas.
INTRODUCTION
PARTICLES in estuaries have different origins. Rivers are supplying particles to the estuary; suspended matter may also be introduced into the estuary from the marine environment. Apart from these two primary sources, internal recycling through erosion and deposition takes place. Studies .of the fate and behaviour of trace metals and other pollutants in estuaries benefit from methods which distinguish between marine and fluvial sediment origins. Basically, two approaches are used to study processes affecting pollutants in the estuarine system, and to determine whether estuaries act as sinks or sources for contaminants. The first approach studies the changes in water chemistry, and the second the changes in composition of the sediments and suspended particles. In most estuarine studies, information on the behaviour of trace constituents has been derived from mixing curves, which depict the relationship between salinity and the dissolved constituents. In principle, these observations should be confirmed by analyses of the particulate components (i.e. losses from the dissolved phase should show up as elevated levels in the particulate fraction). However, due to lack of suitable tracers this approach has seldom been used (SALOMONS and MooK, 1977; SALOMONSand EYS1NK, 1981). If harbours are located within an estuary, accumulations of sediments in the harbour areas and navigation channels may have to be removed by dredging. Understanding the origin of the dredged material (fluvial or marine derived) is of great importance for dredging practices. Furthermore, if man makes changes in the morphology of estuaries * Delft Hydraulics Laboratory, c/o Institute for Soil Fertility, P.O. Box 30003, 9750 RA Haren (Gr), The Netherlands. t Centre for Isotope Research, University of Groningen, Westersingel 34, 9718 CM Groningen, The Netherlands. 1333
1334
w . SALOMONSand W. G. MOOK
(barrages, dams, harbours etc.), the changing sedimentation patterns may necessitate increased dredging operations. For predicting these changes, knowledge of the origins of sediments is indispensable. In this article, tracers are discussed which can be used to distinguish between different sediment sources in an estuary. The tracers are natural differences in composition between marine and fluvial sediments. The use of natural tracers has some advantages over the use of artificial tracers (radio-nuclide labelling or labelling with Ir or Ta). The method is far less expensive, the field work is relatively simple, and sampling can be repeated as often as is necessary (e.g. to detect changes during isolated events such as major storms). Artificial tracers have the advantage that transport pathways in an estuary can be traced. However, information can be obtained only over a relatively short time period. The choice between natural or artificial tracer methods is not an either/or case; the two approaches are complementary and provide their own solutions to sediment transport and sedimentation problems in estuaries. TRACERS FOR SEDIMENT TRANSPORT
If simple mixing of marine and fluvial suspended matter took place in an estuary, the relationship between suspended matter concentration and salinity would be a straight line; the mixing ratio of riverwater and seawater would determine the suspended matter concentration. If this were the case, salinity would be a good tracer for suspended matter. Unfortunately this is not so. A linear relationship is not observed; the relationship sometimes shows a turbidity maximum which may extend into the freshwater tidal area. This indicates that the movement of suspended matter is different from that of water. A conservative index of mixing should be based not on properties of the water, but on properties of the sediments (e.g. the composition of the fluvial and marine endmembers). It is convenient to distinguish between tracer methods in which the sediment sample as taken from the field is analysed ("whole" sediment tracer) and those in which, through a chemical or mechanical treatment, individual mineral components are isolated and subsequently analysed. Whole sediment tracers
The two "whole" sediment tracers are the chemical and the mineralogical composition. In using these two properties one should be aware that the composition of sediments depends on their grain-size distribution. Sandy samples from the same area have a different mineralogical composition from that of the clay-rich samples (Fig. 1). Also, the chemical composition depends on the grain-size distribution. A common method to correct for grain-size differences is to determine, in samples from the same locality, both the mineralogy and a variable which is representative of the grain-size distribution. Several methods are available (FORSTNER and SALOMONS, 1980; SALOMONS and FORSTNER, 1984). An example of the variation in mineralogical and chemical composition of sediment samples from one locality (Wadden Sea, The Netherlands) with the percentage of particles with diameter <16 gm (a variable which is representative of the grain-size distribution) is presented in Fig. 1. From these curves the particle composition at a certain grain-size composition (e.g. 50% < 16 gm) can be derived and used for comparison purposes.
Natural tracers for sediment transport studies
1335
(a) % 100
oi|i
iliii iiili
fill! ~i!iii!
;-;iii i~!!ii i
iilii
|
6O
Clays ~i~'J'~ Feldspars
40
Quartz
'
Org Matter m
i
Carbonates
2O
oiii | 17
21
24
29
32
34
37
43
51
57
62
68
% < 16 ~Jm
b) Jg/g 300
250
i
L f L
200
jxf
J
;z~ 4' ; J
I 150
f
100
0
i J
J 50
/
t J
0 0
10
20
30
40
50
60
70
80
Fig. 1. (a) The mineralogical composition of sediment samples from the Dutch Wadden Sea. (b) Zinc, copper and lead concentrations in sediments from the Dutch Wadden Sea as functions of the percentage of particles with diameters less than 16 lam (% < 16 I-tm).
B y c o m p a r i n g s e d i m e n t s a m p l e s corrected for different grain-size distributions, it is possible to d e t e r m i n e w h e t h e r significant differences exist which are potentially of use for tracer studies. E x a m p l e s of the differences in c h e m i c a l c o m p o s i t i o n of s e d i m e n t s from various river s y s t e m s , and of marine s e d i m e n t s in the North Sea, are presented in Table 1.
1336 Table 1.
W. SALOMONSand W. G. MOOK
Concentrations o f trace elements in fluvial and marine sediments (in lag g 1 at 50% < 16 lam)
Fluvial Seine Thames Scheldt Rhine Meuse Marine Belgium coast Eastern Scheldt Dutch Wadden Sea
Ce
Cs
Eu
La
Sc
Ta
Th
Yb
Tb
95.0 25.6 93.8 96.2 104.0
6.58 3.01 6.50 10.70 8.90
1.02 0.63 0.89 0.39 1.83
43.6 14.6 36.7 37.0 47.9
7.45 3.85 10.20 9.00 11.20
1.22
3.73 1.47 2.47 2.04 2.70
0.67
1.81 1.32 1.09
12.90 3.56 9.11 8.44 11.50
61.7 57.3 69.5
5.29 6.04 6.30
0.83 0.65 0.75
26.3 24.4 29.8
6.62 6.90 7.74
0.90 0.75 1.03
7.48 6.30 8.16
1.72 1.45 1.82
0.82 0.94 1.26 0.68 0.81
An overview on differences between marine and freshwater muds has been given by SHIMP et al. (1969). The use of boron to distinguish between freshwater and marine deposits has been discussed by HARDER (1970). Once differences in composition between fluvial and marine sediments (the end-members) are observed, a number of conditions have to be met before these differences can be used as a tracer. In using the variations in chemical composition, one should be aware that a number of chemical elements are reactive during transport within estuaries. Removal and addition processes may occur, changing their concentrations. Also, changes may occur in the concentrations of certain elements after deposition, especially those which are prone to redox changes (e.g. Mn, Fe, P). These processes show up particularly in the oxidized surface layer of sediments, which are subject to fluxes from the reduced pore waters, or from the surface waters. This demonstrates one pitfall: the uppermost layer may be the most recently deposited, but may also be the most altered one. Such changes occur particularly in areas with low sedimentation rates, where there is sufficient time for a significant flux of elements from the reduced pore waters. An example is given in Table 2 for an area with a sedimentation rate of up to 0.5 m y-i (harbour area in the eastern Scheldt), and an area in which the sedimentation is 10-3-10-2 m y-I (the Dollard area in the Dutch Wadden Sea). Physical processes such as differential flocculation or differential sedimentation, as observed for some deltas and estuaries (GIBBS,1977; EDZWALDand O ' M E L I A , 1975), may also cause changes in the composition of sediments which are not related to differences in origin. However, despite the selective transport of clay minerals from the Amazon, for example, differences in composition between mud from the Amazon and from the Orinoco are sufficiently large to warrant the use of clay mineralogy as an indicator for sediments in the Orinoco Estuary (EISMAet al., 1978). Table 2. Concentrations (Zn and Mn in lag g 1 Fe in %) in the surface layers and in the reduced layers for two marine sedimentation areas. Eastern Scheldt is a high sedimentation area and the Dollard a low sedimentation area. All data are corrected for grain-size differences Eastern Scheldt
Zn Mn Fe (%)
Dollard
Surface
2-5 cm
Surface
2-5 cm
164 550 2.23
158 609 2.23
104 578 2.11
124 1226 2.59
1337
Natural tracers for sediment transport studies
Man-induced changes are possible due to discharges of elements in an estuary. In some cases they can be used to advantage if the chemicals added are unreactive in the estuarine environment. A useful tracer might be the more highly substituted cogeners of the polychlorinated biphenyls. They have a very limited solubility, a low or nearly nonexistent bio-degradability, and are very unreactive in the estuarine environment. Also, polyaromatic hydrocarbons may fall in this category. There is one problem with using these organic micro-pollutants: these types of compounds have a strong affinity for organic matter in the sediments, have a low specific gravity, and may have transport patterns different from that of the other components of the sediments. The conclusions derived from their use as a tracer may reflect only the origin of the organic matter. Separate studies are required to warrant an extrapolation of the results to the whole sediment. Apart from the requirement that tracers should be unreactive in the estuarine environment, the composition of the end-members (marine and fluvial) should be constant over the time period in which the sediment transport has taken place. Nevertheless, the composition of an end-member may change with time, and this is especially noticeable for common pollutants such as heavy metals. This criterion, however, depends on the characteristics of the estuary and the study zone. If sediment transport takes place over short time-periods (as in small tidal estuaries) this criterion is less stringent than for areas in which the sediments move relatively slowly. This means that elements which are temporally variable in supply might have some use for estuaries such as the Rhine. However, for studying the origins of sediments along the Dutch coast, the German Bight or the Wadden Sea they cannot be used. In Table 3, trace element concentrations in sediments from the river Rhine over the period 1922 to 1975 are given. They show that elements such as Cs, La and Ta can be used for long-distance transport studies, due to slow variations in their concentrations in the Rhine sediments. For each estuary the criteria have to be checked specifically. To summarize, one can conclude that a natural tracer has to fulfill the following conditions: conservative behaviour with time; conservative behaviour during transport; and conservative behaviour after deposition.
Tracers to determine the origin of individual sedimentary minerals Although determining the origin of the individual sedimentary particles is more complex than analysing the whole sediment, it can have a number of advantages: (a) Table 3. Relative concentrations in sediments from the Wadden Sea and the Rhine for various periods of sampling. The concentrations in these sediments in 1922 were 100 units W a d d e n Sea
Ce Cs Eu La Sc Ta Th Yb
Rhine
1958
1975
1958
1970
1975
227 93 109 107 97 104 109 111
122 88 100 111 96 103 117 125
116 140 93 103 107 88 93 112
150 135 109 150 107 102 99 114
118 148 92 108 104 152 93 100
1338
W. SALOMONSand W. G. MOOK
Information can be obtained on transport patterns of individual components; in this way a "high resolution" picture of sediment transport may be derived. (b) A smaller number of samples is needed compared with the "whole" sediment method, in which corrections for grain-size have to be applied. (c) If, for a certain area, information on the origin of one component can be extrapolated to the "whole" sediment, the effort required in sampling and analysis is greatly reduced. A large number of methods are available to determine the composition of individual sedimentary particles. Before the individual particles are analysed it is necessary to carry out a pre-treatment. Pre-treatments can be divided into two classes: (i) physical separation of individual sedimentary components and subsequent chemical, mineralogical or isotope geochemical analysis; and (ii) chemical separation, in which the isolated fractions can be analysed directly. Separation of grain-size fractions can be achieved with classical physical methods such as sieving and use of settling columns. The separated fractions can then be analysed for their mineralogical, chemical and isotopic composition. Each of these analyses may yield a suitable tracer. The clay fraction, which is monomineral, can be easily separated with physical methods. A combination of mineralogical and stable isotope analyses of the clay mineral fraction has been carried out for sediments from the southern North Sea and adjoining estuaries (SALOMONSet al., 1975). Isolation of the quartz fraction from grain-size fractions or from the whole sediment is possible with chemical methods (SvERS et al., 1968). After isolation, the chemical composition or the isotopic composition can be determined. The stable isotopic composition of quartz has been used to trace the origin of finely grained quartz particles in Pacific pelagic sediments (CLAYTONet al., 1972). DENNEN (1967) and HERRERA and HEURTEBISE (1974) used the chemical composition of quartz sands for provenance studies. Although no published data exist for estuaries, the combination of chemical and isotopic analyses of the quartz fraction might provide useful data - - especially as quartz is one of the major minerals which constitute estuarine sediments. The isotopic composition of the carbonate or organic matter fraction is determined after chemical isolation (e.g. acid treatment for carbonates and combustion for organic matter). The carbon and oxygen isotopic composition of the carbonates, and the carbon isotopic composition of the organic matter in sediments have often been used to distinguish between marine and fluvial-derived sources (SALOMONS, 1975; TAN and STRAIN, 1979; SCHULTZand CALDER,1976; FONTUGNEand JOUANNEAU,1981; SALOMONS and MooK, 1981, 1982). APPLICATIONS
OF THE
USE OF TRACERS SEDIMENTS
TO DETERMINE
THE
ORIGIN
OF
IN ESTUARIES
Changes in sediment origin over tidal cycles Trace metal concentrations in the suspended matter change during a tidal cycle. This effect was observed during several anchor stations in the Scheldt Estuary (Fig. 2). Samples from the suspended matter were taken each hour over the tidal cycle. During the ebb the concentrations were high, whereas during high water the concentrations were low. The suspended matter samples were analysed for heavy metals, and for the isotopic composition of the carbonates. The isotopic composition was used to calculate the
1339
Natural tracers for sediment transport studies ,ug/9
10 5
o! 5:
%
lOb
5f lllllm'
C~]
znoug Sahnlty (%o)
25
~
Marine mud (%)
20
3o L
~ 8
9
.
+
Cd/2 (ug/g)
j
10 11 12 13 14 15 16 17 18 19 20 21
Hour
Fig. 2. Changes in metal concentrations in the suspended matter, salinity of the water, and isotopic compositionof the carbonates of the suspended matter at an anchor station in the Scheldt Estuary.
percentage of marine suspended matter present. Over the tidal cycle, the salinity showed a sharp peak. The percentage of marine suspended matter started to increase more rapidly than the salinity - - small changes in salinity caused disproportionate changes in the proportion of marine suspended matter. This again reflected the non-conservative behaviour of the suspended matter. Changes in the particulate metal concentrations showed a similar trend to those in the suspended matter - - relatively large changes in concentration occurred for slight increases in salinity.
Changes in sediment origin in the freshwater tidal areas An earlier study in the Ems Estuary (SALOMONSand MOOK, 1977) gave strong evidence for a landward transport of marine sediments past the freshwater boundary. To study this process in more detail, a locality in the freshwater tidal area was selected (Diele), and sediment samples were taken every month over a period of one year. About 15 samples were taken for trace metal analysis. This number of samples was necessary to construct curves such as those presented in Fig. lb for grain-size correction. The concentrations at 50% < 16 ~tm were calculated from regression curves, and used for comparing data from the different sampling dates. The results (Fig. 3) showed a strong seasonal dependence of the metal concentrations, which correlated with the river discharge. It is known that river discharge can influence metal concentrations (SALOMONS and FORSTNER, 1984; SALOMONS and KERDIJK, 1986). However, the relationship was different from that previously observed: low discharge caused high concentrations, and vice versa. Inspection of the isotopic composition
1340
W. SALOMONSand W. G. MooK (a) ,ug/g 1CO
"x
80
60
40
20
Fe/10 Cd X 10 I
I
I
I
I
I
I
I
I
i
J
Zn/lO
I
Mar Apr May Jun Jul Aug Se~ Oct Nov Feb Mar Apr May
Month b)
200 mg/s 700
•
'
J'x \\X,,_~_?
,/
~
Detta ¢:-!3
--
D,sctTarge
-10 Carbon rsotopes c a r b o n a t e s I
I
i
I
I
I
I
I
I
I
I
%o I
I
Mar Apr May dun Jul Aug Sep Oct Nov Feb Mar Apr May
-2 0
Month Fig. 3. (a) Monthly changes in metal concentrations (corrected for grain-size differences) in sediments from the freshwater tidal area of the Ems Estuary. (b) Monthly changes in freshwater discharge and isotopic composition of the carbonates in the sediments.
showed that low metal concentrations were correlated with less-negative values of the carbon isotopic composition, expressed as ~lSC (marine influence), and high concentrations with more negative values (fluvial influence). The changes in metal concentrations can be explained simply by variations in the mixing ratio of marine to fluvial sediments. During periods of low discharge more marine sediments entered the freshwater tidal area, whereas during high discharge the proportion of marine sediments decreased. This is evident from an inspection of the isotopic composition of the carbonates. During periods of low discharge, the ~13 values were
1341
Natural tracers for sediment transport studies
lower compared with periods of high discharge. One important conclusion is that relatively large proportions of marine sediments are found in the freshwater tidal area of estuaries. These observations help to explain the variations in both metal and organic micropollutant concentrations. However, this physical phenomenon is not yet fully understood. A mechanism based on scour and settling lag proposed by POSTMA(1967) and by VANSTRAATENand KUENEN (1957) to explain accumulation of fine-grained sediments in the Dutch Wadden Sea might explain the transport of marine sediments past the freshwater boundary in estuaries. Irregular changes in metal concentrations in bottom sediments The pattern of metal concentrations in deposited sediments from the Scheldt are irregular. From data in Fig. 4 one might conclude that significant discharges of trace metals were taking place at Stas 3 and 4. However, the isotopic composition of carbonates in the deposited sediments show an irregular pattern which is similar to that for the metals. Low metal concentrations coincide with high 813C values (less negative), indicating a high proportion of marine-derived material. Because marine sediments have low metal concentrations (Sta. 11 in Fig. 4), this irregular variation can be explained by changes in the mixing ratio of marine to fluvial-derived sediments along the estuary. Origin of sediments in the Rhine and Scheldt estuaries Samples of the suspended matter were taken over a wide salinity range in the Rhine and Scheldt estuaries. The isotopic composition of the carbonates was determined. Earlier, unpublished studies showed that results from the isotopic composition analysis of the carbonates were similar to those of chemical methods (neutron activation analysis).
1400 I Carbon Isoto~,c ccmpus, t,on carbona}es}
~Jg/g 1200
J
J
l-2o
10,00 -15 8OO
'
6OO
-10
¥
4OO
-05
111
IHLIi
2OO 0
1
2
3
4
5
Zn
~
6
L ° 7
8
9
Cd X 20
m
Pb
10
11
*0.5
Station Fig. 4. Changes in metal concentrations in the Scheldt Estuary from the freshwater reach (Sta. 1) to the Western Scheldt (Sta. 11), and the isotopic composition of the carbonates in the sediments.
1342
w. SALOMONSand W. G. MOOK Percentage fluvial mud lOO
80
~ 1
l ~ i i eL
m - - - ~ - - - = - . . . ~ S c he i d t
% 40
20
5
10
15
20
25
30
35
Salinity %o Fig. 5.
Relationship between the percentage of fluvial mud in suspended matter from the Scheldt and the Rhine estuaries and salinity.
Figure 5 shows that the ratio of marine to fluvial mud in suspended matter is not a simple function of salinity. Furthermore, large differences are observed between the Scheldt and Rhine estuaries. The relationship for the Scheldt Estuary shows three distinct regions. At low salinities the percentage of fluvial mud decreases relatively quickly to about 70% and stays more or less constant for stations between 5 and 15%o. The amount of fluvial mud again decreases with salinities above 15%o. This second decrease coincides with a broadening of the estuary. The Rhine shows the same behaviour, except that the changes at low salinity are much more pronounced. In the Rhine Estuary, less than 20% of the suspended matter originates in the river at salinities in excess of 5%0. If the mixing ratio of marine to fluvial sediments is known, together with the metal concentrations in the fluvial and marine end-members, it is possible to calculate metal concentrations for various mixing ratios (assuming conservative behaviour) and compare these with measured values (SALOMONS and MooK, 1977). Negative deviations from the calculated values indicate release of metals from the sediments, and vice versa. REFERENCES CLAYTON R. N., R. W. REX, J. K. SYERS and M. L. JACKSON (1972) Oxygen isotope abundance in quartz from Pacific pelagic sediment. Journal of Geophysical Research, 77, 3907-3915. DENNEN W. H. (1967) Trace elements in quartz as indicators of provenance. Geological Society of America Bulletin, 78, 125-130. EDZWALD J. K. and C. R. O'MELIA (1975) Clay distribution in recent estuarine sediments. Clay Minerals, 23, 39-44. EISMA D., S. J. VAN DER GAAST, J. M. MARTIN and A. J. THOMAS (1978) Suspended matter and bottom deposits of the Orinoco delta: turbidity, mineralogy and elementary compositions. Netherlands Journal of Sea Research, 12, 224-251. FONTUGNE M. and J. M. JOUANNEAU(1981) S6dimentologie: la composition isotopique du carbone organique des mati6res en suspension de l'estuaire de la Gironde. Application ~ l'6tude de la distribution du plomb et du zinc particulaires. Compte rendu de l'Acad6mie des Sciences (Paris), 293, 389-392. FORSTNER U. and W. SALOMONS (1980) Trace metal analysis on polluted sediments. Part I. Assessment of sources and intensities. Environmental Technology Letters, 1,506-517.
Natural tracers for sediment transport studies
1343
GIBBS R. J. (1977) Clay mineral segregation in the marine environment. Journal of Sedimentary Petrology, 47, 236-243. HARDER H. (1970) Boron content of sediments as a tool in facies analysis. Sedimentary Geology, 4, 153-175. HERRERA R. and M. HEURTEBISE (1974) Neutron activation analysis of trace elements in quartz sands: its possibilities in the assessment of provenance. Chemical Geology, 14, 81-93. POSTMA H. (1967) Sediment transport and sedimentation in the estuarine environment. In: Estuaries, G. H. LAUFF, editor, A.A.A.S. Publication 83, Washington D.C., pp. 158-179. SALOMONS W. (1975) Chemical and isotopic composition of carbonates in recent sediments and soils from Western Europe. Journal of Sedimentary Petrology, 45,440-449. SALOMONS W. and W. G. MOOK (1977) Trace metal concentrations in estuarine sediments: mobilization, mixing or precipitation. Netherlands Journal of Sea Research, 11,199-209. SALOMONSW. and W. EYSlNK (1981) Pathways of mud and particulate trace metals from rivers to the southern North Sea. In: Holocene marine sedimentation in the North Sea Basin, S. D. NIO, R. T. E. SCHUTTENHELM and TJ. C. E. VAN WEERING, editors, Blackwell Science Publications, Oxford, pp. 429-450. SALOMONS W. and W. G. MOOK (1981) Field observations on the isotopic composition of particulate organic carbon in the southern North Sea and adjacent estuaries. Marine Geology, 41, M11-M20. SALOMONS W. and W. G. MOOK (1982) Natural tracers for sediment transport studies. Delft Hydraulics Laboratory Publication, No. 271, 49 pp. SALOMONS W. and U. FORSTNER (1984) Metals in the hydrocycle. Springer Publishing Co., Berlin, 349 pp. SALOMONS W. and H. N. KERDIJK (1986) Cadmium in fresh- and estuarine waters: A review. In: Cadmium in the environment, Vol. 50, H. MISL1N and O. RAVERAL, editors, Experienta Supllementum, pp. 24-28. SALOMONS W., P. HOFMAN, R. BOELENS and W. G. MOOK (1975) The oxygen isotopic composition of the fraction less than 2 microns (clay fraction) in recent sediments and soils from Western Europe. Marine Geology, 18, M23-M28. SCHULTZ D. J. and J. A. CALDER (1976) Organic carbon 13C/12Cvariations in estuarine sediments. Geochimica et Cosmochimica Acta, 40,381-385. SHIMP N. F., J. WI'/~ERS, e. E. POTI'ER and J. A. SCHI,EICHER (1969) Distinguishing marine and freshwater muds. Journal ~)f Geology, 77,566-580. VAN STRAATEN J. M. U. and P. H. KUENEN (1957) Accumulation of fine grained sediments in the Dutch Wadden Sea. Geologie en mijnbouw, 36, 329-354. SVERS J. K., S. L. CHAPMAN, M. L. JACKSON, R. W. REX and R. N. CLAYTON (1968) Quartz isolation from rocks, sediments and soils for determination of oxygen isotopic composition. Geochimica et Cosmochimica Acta, 32, 1022-1025. TAN P. M. and F. C. STRAIN (1979) Carbon and oxygen isotope ratios in Saguenay fjord and the St. Lawrence estuary and their implications for palaeoenvironmental studies. Estuarine and Coastal Marine Science, 8, 119-126.