Effects of Waste Disposal on Groundwater ami Surface Water (Proceedings of the Exeter Symposium, July 1982). IAHS Publ. no. 139.

Help! Holland is plated by the Rhine (environmental problems associated with contaminated sediments)

W. SALOMONS1, W. VAN DRIEL2, H. KERDIJK1 & R. BOXMA2 1 Delft Hydraulics Laboratory, Haren Branch, c/o Institute for Soil Fertility, PO Box 30003, 9750 RA Haren (Gr), The Netherlands 2 Institute for Soil Fertility, PO Box 30003, 9750 RA Haren (Gr), The Netherlands

ABSTRACT Physical and chemical processes cause an accumulation of about 2/3 of the metal load of the rivers Rhine and Meuse in the Netherlands. Accumulation takes place in freshwater basins fed by rivers and by the disposal of dredged material on land. Agricultural use of the land-fill areas is restricted due to cadmium accumulation by crops. The accumulation of heavy metals by cows on river flood plains is low compared with uncon-taminated reference areas. The disposal of dredged material on land influences the composition of the groundwater, disposal of dredged material in the marine environment causes a mobilization of cadmium.

INTRODUCTION

Large amounts of trace metals are transported by various rivers in the direction of the Southern North Sea (figure 1).

The major part of the heavy metal load originates from antropo-genic sources, in fact the River Rhine has been polluted with heavy metals for more than 60 years (Salomons & de Groot, 1978). Cadmium concentrations in sediment samples from the River Rhine in 1920 (originating from archives and recently analysed for metals) are 4.4 ug g-1, which is already 15 times the base line level, whereas the zinc concentrations are more than 1000 ug g-1.

A history of the metal pollution of the River Rhine, as reflected in its sediments, is shown in figure 2. Between 1920 and 1958 all metal concentrations increased, while between 1958 and 1980 this rise continued for cadmium only. Striking are the decreases for arsenic (after 1958) and for mercury (after 1973).

Human activities not only change the metal load of the rivers, but also the pathways along which the dissolved and particulate metals are transported into the direction of the Southern North Sea.

Estuaries are normal pathways by which dissolved and particulate trace metals enter the marine environment, but nowadays two addi­tional pathways have to be considered:

Freshwater basins. The closing of river mouths (Haringvliet, 1970) and coastal lagoons (IJsselmeer, 1932) has created large freshwater basins (figure 1) in

255

256 W. Salomons et al.

FIG. 1 Particulate metal transport by rivers (tons/a) and major dredging activities (106 m3/a) in the Netherlands, Belgium and Germany.

which the river water, before it enters the marine envi­ronment, is subject to hydrodynamic and geochemical processes. Harbours and navigation channels. The construction of deep harbours and navigation channels has favoured the sedimentation of suspended matter and its associated heavy metals. The dredged material from these areas is partly dumped on land, while another part is dumped in the North Sea.

The processes affecting heavy metals during transport in the aquatic environment will be discussed in the first part of this paper, whereas the problems associated with the accumulation of contaminated sediments will be discussed in the second part.

PROCESSES AFFECTING HEAVY METALS IN THE AQUATIC ENVIRONMENT IN THE NETHERLANDS

The River Meuse and the lower course of the River Rhine, as well as the entire delta area, have been subject to extensive interference and alteration by man, so that the many works of civil engineering affect the regime of the Rivers Rhine and Meuse and the distribution over the several courses. In particular the enclosure of the Zuiderzee in 1932 (the present Ijsselmeer, receiving about 10% of the discharge of the River Rhine) and the Haringvliet in 1970 (see figure 1) had a great influence on the hydrology of the Rhine and Meuse and on the estuaries in the south-western part of the Netherlands. The distribution of the water of the Rhine is con­trolled by weirs and sluices in the enclosure dams. As a conse-

Holland plated by Rhine 257

2 0 0 0

1000

500

2 0 0

100

5 0

20

10

5

2

1 9 0 ° ^ 9 2 0 1930 1940 1950 1960 1970 1980

FIG. 2 History of trace metal pollution in the River Rhine, as reflected in its sediments.

quence, the Rotterdam Waterway is at present the main estuary of the Rhine and Meuse.

The Freshwater Basins

During transport in the River Rhine, the ratio of dissolved to particulate metals changes. Between the Dutch-German border and the outflow of the river IJssel in the Usselmeer (figure 1), the dis­solved cadmium concentration decreases, whereas those in the sus­pended matter increase (figure 4). During transport the pH also changes, and the absorption of cadmium is sensitive to pH-changes (figure 4C). The change in pH observed of about 0.3 units is suffi­cient to cause the change in ratio.

River water which enters the freshwater basins Haringvliet and Usselmeer loses, as may be expected, part of its suspended load. If the freshwater basins would act as a sink for particulate metals only, the dissolved concentrations and the concentrations in the suspended matter in the water leaving the basins through the sluices in the enclosure dams would be more or less equal to those in the river water. Figure 3A shows, however, that this is not the case. Metal concentrations become lower and furthermore, notable diffe­rences are observed between the two basins.

258 W. Salomons et al.

DISSOLVED METALS

Cu

n

CI] Haringvliet E3 Ysselmeer

Cd Zn

3

C

3

pH

>V'

x •

* * *

X observed • calculated

FIG. 3 A. Dissolved metal concentrations and the concentrations in the suspended matter in the water leaving the basins through the sluices in the enclosure dams (metal concentrations in the incoming riverwaters are 100%).

B. Seasonal cycle of pH in the LJSselmeer and observed and calculated dissolved zinc concen­trations in the lake water.

Three phenomen have to be explained in this respect: the decrease in dissolved metal concentrations the decrease in particulate metal concentrations the differences in behaviour between the two basins

Both basins are subject to a high nutrient load, which promotes the growth of algae. The residence time of the water in the IJsselmeer is about 6 months and massive algal blooms occur in summer. In the Haringvliet, the residence time is much shorter (weeks) and massive algal blooms are observed. Algal blooms are one of the major pro­cesses affecting the pH and are mainly responsible for the seasonal cycle in the pH observed for the IJsselmeer (figure 3B). In the Haringvliet these pH increases (compared with the river water pH of about 7.3) are not as pronounced as in the IJsselmeer; the water leaving the Haringvliet has a lower pH than in the IJsselmeer.

The extent of the adsorption of zinc, chromium and cadmium on the suspended matter depends to a large extent on the pH (figure 4C, Salomons and Mook, 1980) over the pH range observed in these two basins. The predominant processes responsible for the removal of dissolved Cd, Cr and Zn appears to be the pH-dependent adsorption on the suspended matter. In fact, a simple mathematical description, which includes only pH-dependent adsorption processes, was able to simulate quite satisfactorily the seasonal changes in the dissolved

Holland plated by Rhine 259

RHINE

C A D M I U M

drzissena

YSSEL

mm YSSELMEER

v •:;r"x^i

B par t icu la te

dissolved

cadmium

.

P b

*\\ • >

" i *" *

SO 60 70 80 9.0

- » PH

-

*•—— •

D • .—

• i (

Cd

• \

- - • - . \

\

, ,

p H , 6

PH : ?

pH - 8

\

^ ! ^ 20 50 200 500 ?O00 ^000

cod^IS"

E V

FIG. 4 A. Cadmium accumulation by Drissena Polymorphs based on data by Marquenie 1981.

B. Dissolved and particulate metal concentra­tions in the Rhine, IJssel and IJsselmeer. Adsorption of cadmium on the suspended matter, as influenced by pH (C), NTA (D) and chloride (E) concentrations.

zinc concentrations in the IJsselmeer (figure 3B). The fact that the absorption of copper is pH independent in the range 7-9 (01Conner & Rester, 1975; Grimme, 1968) explains the observation that copper is far less removed from solution. The predominant process affecting the concentrations of metals in the suspended matter appears to be the erosion of bottom sediments and their admixture with the suspended matter. The bottom sediments originate mainly from the period before the enclosure and, therefore, contain lower concentrations of metals. However, differences exist in the intensity of the erosion processes between the two basins. The IJsselmeer is shallow and has a large surface area. Winds with a strength of Beaufort 6 generate waves which are able to stir up the bottom sediments. The Haringvliet on the other hand is much smaller

260 W. Salomons et al.

and deeper, and conditions, therefore, are less favourable for a resuspension of bottom sediments. As a consequence metal concen­trations in the suspended matter in the Haringvliet remain high.

Jointly with the geochemical studies, biological studies have been carried out in the freshwater basins (Marquenie, 1981). Part of this study consisted of an active biological monitoring programme using Dreissena polymorphs. Dreissena Polymorphs is a bivalve which lives attached to solid surfaces in the water, consequently these organisms are subject to a trace metal burden from the dissolved and particulate phase. However, the mollusk is not subject, like, for instance, Anadonta or Unio, to metals present in the deposited sediments and in the pore waters.

Some results on the metal uptake by organisms sampled in unpol­luted areas and subsequently exposed in contaminated parts of the Rivers Rhine and IJssel and in the IJsselmeer are presented in figure 4A. Also average metal concentrations in solution and attached to the suspended matter are given over the exposure period (figure 4B).

Cadmium concentrations in Dreissena Polymorpha are high in the Rhine (about 60 ug g-1 at the Dutch-German border). This is close to the toxic level (100 ug g-1), as has been found in laboratory experiments (Marquenie, 1981, private communication). In the IJssel and in the IJsselmeer the cadmium concentrations are lower and reflect the decreasing dissolved cadmium concentrations, rather than the increasing concentrations in the suspended matter (figure 4B). Apparently, cadmium present in the suspended matter is less bio-available. Thus any change in the ratio of dissolved to particulate cadmium concentrations (total concentrations constant) will affect metal uptake by Dreissena Polymorpha and probably also by similar organisms.

Apart from the already discussed pH changes, complexing agents are able to change this ratio. Complexing agents such as chloride ion or NTA (a substitute for phosphate in detergents) can effec­tively compete with the suspended matter for complexation of dis­solved metals. Chloride ions are already present in the freshwater environment at values (200 mg l-1) well above base line levels. At these chloride levels the adsorption of cadmium on the suspended matter is affected (figure 4E). Therefore, a decrease of salt discharge into the River Rhine will promote the adsorption of cad­mium on the suspended matter and decrease its bio-availability.

Presently the NTA levels in, the River Rhine are low or absent ; however if in the drainage area of the river phosphates in deter­gents are replaced by NTA, the concentrations of this agent are expected to range between 40 and 560 ug l-1 at the Dutch-German border (Salomons & van Pagee, 1981). Low values are expected during periods of high discharge (more dilution) and high temperature (more decay) and high values during periods of low discharge (little dilution) and low temperature (decay). At concentrations, 200 ug l-1, the adsorption of cadmium will be influenced (figure 4D) and furthermore a desorption of cadmium (and also of zinc) from the suspended matter starts (Salomons & van Pagee, 1981). The resulting change in the ratio of dissolved to particulate metals may affect the bio-availability of cadmium.

Holland plated by Rhine 261

FIG. 5 Mean metal concentrations in dredged material from Rotterdam harbour.

The Estuaries and Harbour Areas

In harbours located along estuaries an extensive sedimentation takes place. The total amount of dredged material which is yearly removed from harbours in the Netherlands, Belgium and Germany is estimated at 75-80xi06m3. This is twice the total mud budget of the North Sea (Salomons & Eysink, 1981). Part of this spoil is permanently removed from the aquatic system (applied as land-fill), but most of it is dumped at selected sites in the North Sea.

The predominant process which determines metal concentrations in estuarine sediments (apart from local discharges) is the mixing of marine (low contents of metals) with highly polluted fluvial sedi­ments (Millier & Forstner, 1975; Salomons et al. , 1975; Salomons, 1975). The mixing ratio can be determined with the aid of natural tracers (e.g. natural differences in composition between marine and fluvial sediments) (Salomons, et al., 1975).

The total amount of dredged material from the Rotterdam harbour is about 20xi06m3, which respresents a 20-25 fold increase compared with 70 years ago. Particularly the spoil from the westerly har­bours (figure 5), which is less contaminated because of low admix­tures of fluvial sediment, is dumped at sea. The most severly

262 W. Salomons et al.

contaminated spoil from the easterly harbours, with high admixtures of fluvial sediment, is dumped on land. So the land-fill areas are an important sink for fluvial (and marine) particulate metals. A tentative balance for trace metals of fluvial origin in spoils from the Rotterdam harbour is presented in Table 1.

TABLE 1 Tentative balance of trace metals of fluvial origin (in tons) in dredged material from the Rotterdam harbours (1977)

Zn Cu Cr Pb Cd Ni Me

Transported to land-fill ^ ^ 4 ? 4 28Q ^ ^ 6Q 2 3 3 Q

areas

Transported to the ^ ? 8 ? 6 2Q ^ North Sea

Tentative Balance for the Trace Metals Transported by the Rivers Rhine and Meuse

Together with the data on the total amount of trace metals transpor­ted by the Rivers Rhine and Meuse, based on data from Rijkswater-staat (1977-1978), the studies carried out in the Haringvliet and LJsselmeer basins (Salomons & Eysink, 1981; Salmons, 1981; Salomons & Mook, 1980), dredging data obtained from the Municipality of Rotterdam (corrected for the amount of marine sediments) it is possible to present a tentative metal balance for the River Rhine and Meuse (Table 2).

TABLE 2 Tentative metal balance for the Rivers Rhine and Meuse

CU Ni Zn Pb Cd Cr Me

Input (tons/year) Rhine + Meuse 1215 886 10051 1707 195 2463 16517

Accumulation in the Netherlands (in % of the input by Rhine and Meuse) Freshwater basins 50 Land-fill areas 18

Output Through sluices in the enclosure dams Dumping in the North Sea 6

Not accounted for: 15

28 7

30

3

32

57 13

11

5

14

58 16

8

7

11

60 13

10

4

13

53 19

7

8

13

55 14

11

5

15

Of the total amount of metals transport by the Rivers Rhine and Meuse, about 2/3 accumulates in the Netherlands. Only about 1/3 is

Holland plated by Rhine 263

transported to the North Sea. The Netherlands acts in fact as an effective "treatment plant" for the River Rhine and Meuse, This situation may be regarded as beneficial for the North Sea, but poses problems for the management of inland waters and the land-fill areas in the Netherlands.

ENVIRONMENTAL PROBLEMS ASSOCIATED WITH CONTAMINATED SEDIMENTS

Part of the problems associated with heavy metals in surface waters has already been discussed in the section on freshwater basins. However, of major concern is the management of heavy metals in sediments deposited on land.

Contamination of Groundwater as a Consequence of the Disposal of Dredged Material on Land

About 6xi06m3 of dredged material from the easterly, most contamina­ted harbour basins are put on land. An important land-fill area has been the Broekpolder. The Broekpolder is a polder with a surface area of about 3.5 km2. It has been filled with 25xi06m3 of dredged material between 1959 and 1976. The subsoil of the Broekpolder and surrounding polders near Rotterdam consists of a 15 m thick semi­permeable and compressible peaty layer. It rests upon 1 20 m thick water-bearing layer of sand and gravel. Before the disposal star­ted, the surface of the Broekpolder and its surrounding areas laid about 2.5 m below mean sea level (figure 6). The water table was maintained some 0.5m lower. Therefore, water from the Rhine estuary seeped from the aquifer to the surface of the Broekpolder at a rate of 500 m2 per day, as calculated by a two-dimensional finite-element computer programme (Kerdijk, 1981).

FIG. 6 Groundwater movement in the Broekpolder and surrounding areas (after 2020).

264 W. Salomons et al.

The disposal of dredged material raised the surface about 7 m. The consequences have been a rise of the groundwater table in the polder and a consolidation of the peaty layer. The main effect of this consolidation is that the superstresed pore water will form a barrier against the downward flow until the year 2020, approximately (Kerdijk, 1981). Thereafter leakage of the water to the aquifer is expected at a rate of 2000 m2 per day and the contaminated ground­water will seep into the polders north and east of the Broekpolder up to a distance of 3.5 km (figure 6). Heavy metal concentrations in the interstitial waters of the spoil dump showed little variation with depth and are 2 to 10 times higher than in the Rhine estuary. Concentrations of the pesticides Mdrin, Isodrin and Telodrin, present in dredge spoil from a harbour at which a pesticide manufac­turing company was located, showed significant variations with depth, up to 500 times estuarine concentrations. Other pesticides like Dieldrin, Endrin, Lindane and DDT could not be detected. Because of adsorption and/or precipitation reactions, the concentra­tions in the ditches of the Broekpolder were somewhat lower. Only Dieldrin was higher, due to the transformation of Aldrin into Dieldrin under aerobic conditions. The ratio of soil bonded to water dissolved contaminants lies between 2500 for nickel and 98000 for chromium. Calculations indicated that if 0.5% of the bonded metals and pesticides is desorbed, a subsequent delivery to the interstitial water over a long period (300-3000 years) may be caused.

During transport through the subsoil the concentrations of con­taminants will be affected by diffusion and/or dispersion, adsorp­tion and chemical reactions. The transport of the different com­pounds was calculated with a one-dimensional dispersion programme (Kerdijk, 1981). Chloride, showing conservative behaviour, will become apparent in the adjacent poulders in the year 2100 approxi­mately, the heavy metals one to three centuries later, and pesti­cides several thousand years later.

Contamination of Seawater from the Disposal of Dredged Material

Dredged material from the westerly, less contaminated harbours (figure 5) is dumped at a site (loswal Noord) 5 km offshore north­westerly of the mouth of the Rotterdam Waterway. Since 1970 a mean annual discharge of about 14.5xi06m3 of spoil has been released there, which represents about 7xi0B tons of dry sediment per year. At least half to three-quarters of this spoil is mud (Terwindt, 1971, 1978; Van Oostrom, 1978). This muddy part of the spoil is for a great deal resuspended during the dumping process, as well as by wave and current action on the bottom (Terwindt, 1978). The (re) suspended mud is carried along the coast by the residual current and mixes with suspended matter already present in the North Sea. In this way anoxic sediments deposited at low salinity regions in the harbours are brought into an oxic, high salinity environment. Metal compounds, stable under anoxic conditions, may be transformed in compounds stable under oxic conditions. During this transformation metals may be released to the surrounding high salinity waters.

Holland plated bg Rhine 265

To simulate these processes and to study the effect of salinity on metal release, laboratory experiments were carried out simulating natural conditions as closely as possible. Anoxic fluvial mud and dredged material were held in suspension in large glass cylinders over periods of 6 weeks, during this period the suspension was slowly leached with seawater. In a control experiment to study the effect of salinity, the mud was leached with river water. Air was bubbled through the suspension to maintain oxic conditions during the experiment. Some results, representative for the different types of dredged material studied, are presented in Table 3.

TABLE 3 Release of metals from anonix Rhine sediment (in % of total concentrations) suspended in seawater and river water*. Negative numbers denote adsorption of metals from the leaching medium on the sediment

Zn Cu Ni Cd Pb

Seawater 2.2 2.0 2.5 49 0.1 River water -0.8* 0.9 -2.0* 1 0

The rate of release is slow and took about 4-5 weeks to level off, with a maximum in the release after 2-3 weeks.

Calculation on the dispersion of the mud at the dumping site, however, showed that the rate of dispersion is high as compared with the rate of release. Therefore, it is not expected that local high concentrations of dissolved metals are to be found as a consequence of the dumping of dredged material.

The Use of Contaminated Sediments for Agriculture in the Netherlands

Agriculture in the Netherlands is affected by contaminated sediments in some cases:

The river flood plains along the Rivers Rhine and Meuse, about 40,000 hectares, are flooded almost yearly. The upper soil layer consists entirely of contaminated river sediment. The flood plains are mainly in use as pastures for grazing dairy cattle. A limited part is used for the cultivation of mais. In the freshwater tidal area of the Rivers Rhine and Meuse, some polders have been enlarged by reclaiming adjacent marsh areas. The soils are moderately polluted with heavy metals. This area comprises about 100 hec­tares. The sediments dredged out of the harbours and waterways of Rotterdam are partly deposited on land. Depending on local conditions these polders contain layers of dredged material up to a thickness of 10 metres. About 300 hectares of these disposal sites potentially have agrono­mic destinations.

266 w. Salomons et al.

The degree of contamination of the river sediments is dependent on the year of deposition (figure 2), on the mixing ratio of marine to fluvial sediments (figure 5). In all cases the heavy metal contents are large as compared with that of the original, uncontami-nated river sediment. The total contents of As, Cd, Cr, Cu, Hg, Pb and Zn range from 2 to 50 times the base line levels. The levels of the chlorinated hydrocarbons and other organic pollutants in the river sediments are generally low. However, in some parts of the harbour discharges from factories cause high concentrations of pollutants in the sediments.

Reference Soil

Vday SHg 10 Cd 5D As 200 Pb

J l m n ; Flood plain

Cd As Pb Hg Cd As Pb

FIG. 7 Heavy metals ingested daily per cow grazing in river flood plains and on uncontaminated reference areas.

River flood plaints The cattle grazing in the river flood plains is exposed to three sources of heavy metals in the diet: the drinking (river)water, the herbage and the ingested soil particles. The contribution of the polluted drinking water is less than 1% of the total heavy metal intake. The contribution of the contaminated grass and soil particles is demonstrated in figure 7, in which the daily intake of As, Cd, Hg and Pb by a dairy cow on the river flood plains is compared with an animal in an uncontaminated situation. The heavy metal burden of the cattle in the flood plains was found to be considerably higher than that of the cattle at the reference pastures. Investigations of the heavy metal contents of the milk, kidney, liver and brains, however, revealed that this did not result into elevated concentrations. Thus, the considerable contamination of the soil and the grass by heavy metals is not reflected in the body of the cattle, and no risk has to be feared for the consumers of animal products of that origin.

Agronomic crops on contaminated dredged material Young sedi­ments (dredged material) generally have a very favourable structure and soil fertility. A minor problem is the occurrence of manganese deficiency, mainly in cereals and sugarbeets, because of the high pH of the soil. This deficiency may cause severe yield depressions, but it is easily cured by spraying with a solution of manganous sulphate during the vegetative growth. However, cultivation of consumable crops on sediments heavily polluted with heavy metals of

Holland plated by Rhine 267

12 -

to-

potato

FIG. 8 Cadmium accumulation in crops on dredged material.

ppm Cd cropl f resh weight)

0.16 r

012

010

008

/

/

proposed threshold value

/ /

Cd-upfoke by carro ts and potatoes

3 4 5 ppm Cd soil (dry weight}

FIG. 9 Relationship between cadmium content of soils and the accumulation by potato and carrot.

organic micro-pollutants, may involve risks of intolerably high levels of these contaminants in the products. The accumulation of metals and of chlorinated hydrocarbon in crops has been studied, both by performing pot experiments under controlled conditions and by surveying the contents of contaminants in the crops grown under field conditions.

The availability of most metals in these sediments is relatively low because of the high contents of CaCo , organic matter and clay. Nevertheless a significant accumulation of most metals in the crops has been observed as compared with non-contaminated substrates. In figure 8 the relative increase of the cadmium content of eight crops, cultivated on dredged sediments in a pot experiment, is demonstrated. The various crops differ widely in their capacity to accumulate heavy metals. The effect of an increasing soil cadmium content on the accumulation by two root crops (carrot and potato) is shown in figure 9. In this experiment the proposed threshold value for vegetables of 0.1 mg kg-1 fresh weight is exceeded at 2 mg kg-1

total cadmium in the soil. This level of cadmium was already pre-

268 w. Salomons et al.

sent in Rhine sediments deposited in 1920 on the river flood plains and in the freshwater tidal areas (figure 2).

For the judgement of the health implications of the enhanced metal contents, the actual concentrations and the proportion of the product in the total human diet must be considered. The toxicity of the investigated elements at the levels observed in the crops is relatively low for copper, chromium, nickel and zinc. Arsenic, lead and mercury are more toxic, but their concentrations of cadmium, however, approximates or exceeds the tolerable levels in several crops. In the green, leafy vegetables, what grain and other cereals and in most green fodders, the cadmium concentrations must be con­sidered too high for human and animal consumption.

REFERENCES

O'Conner, T.P. & Rester, D.R. (1975) Adsorption of copper and cobalt from fresh and marine systems. Geochim. Cosmochim. Acta, 39, 1531-1543.

Grimme, H. (1968) Die Adsorption von Mn, Co., Cu and Zn durch Geothit aux verdiinnter Losungen, Z. Pf lanzernernahr. Diign. Bodenkunde, 121, 58-65.

Kerdijk, H.N. (1981) Groundwater pollution by heavy metals and pesticides from a dredge spoil dump. In: Quality of Groundwater (ed. by W. van Fuyvenboden, P. Glasbergen, H. van Lelyveld) , 279-286, Elsevier Publishing Co.

Marquenie, J.M. (1981) The freshwater mollusc Dreissena Polymorphe. as a potential tool for assessing bio-availability of heavy-metals in aquatic systems. In: Proc. International Conference Heavy Metals in the Environment, 409-412, Amsterdam,September.

Millier, G. & Fôrstner, U. (1975) Heavy metals in sediments of the Rhine and Elbe estuaries. Environ. Geol. , 1, 33-39.

Oostrum, W.H.A. van (1978) MKO-project. De Ingénieur, 90, 776-779. Salomons, W. (1975) Chemical and isotropic composition of carbo­

nates in recent sediments and soils from Western Europe. J. Sediment. Petrol., 45, 440-449.

Salomons, W., Hofman, P., Boelens, R. & Mook, W.G. (1975) The oxygen isotopic composition of the fraction less than 2 microns (clay fraction) in recent sediments from Western Europea. Mar. Geol., 18, M23-M28.

Salomons, W. & de Groot, A.J. (1978) Pollution history of trace metals in sediments as affected by the River Rhine. In: Environmental Biogeochemistry rand Geomicrobilogy (ed by W.E. Krumbein), Volume I, 149-164, Ann Arbor.

Salomons, W. & Mook, W.G. (1980) Biogeochemical processes affecting trace metal concentrations in lake sediments (IJsselmeer, The Netherlands). Sci. Total Env., 16, 217-229.

Salomons, W. (1981) Impact of civil engineering on the pathways of heavy metals from rivers to the Southern North Sea. In: Proc. International Conference Heavy Metals in the Environment, 359-362, Amsterdam, September.

Salomons, W. & Pagee, J.A. (1981) Prediction of NTA levels in river systems and their effect on metal concentrations. In: Proc. International Conference Heavy Metals in the Environment, 694-697, Amsterdam, September.

Holland plated by Rhine 269

Salomons, W. & Eysink, W. (1981) Pathways of mud and particulate trace metals from rivers to the South North Sea. In Proc. Holecene Marine Sedimentation in the North Sea Basin, Blackwell.

Terwindt, J.H.J. (1971) Nader onderzoek naar de slibbewegingen in het kustwater voor het Deltagebied en de Schone kust in relatie tot het storten van slib op Loswal Noord. Rijkswaterstaat, Deltadienst W.A., Nota W-71.030.

Terwindt, J.H.J. (1977) Mud in the Dutch Delta area. Geol. Mijnb., 56, 203-210.