The composition of suspended particulate matter during floodings of the river Meuse

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The composition of suspended

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Master report of Ingrid Bakker

Supervised by Prof. Dr. H.A.J. Meijer (RUG/CIO)

Dr. G. Klaver (TNO)

University of Groningen

CIO, Center for Isotope Research

IVEM, Center for Energy and Environmental Studies

Nijenborgh 4

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Preface and acknowledgements

This report summarizes the results of my master thesis at TNO in Utrecht, on the subject of the variability in contamination degree of suspended matter and floodplain sediment in the Dutch part of the river Meuse. Despite the numerous questions still not answered on this subject, it became clear that main variation in contamination degree can be correlated to the variable input of different grain size fractions, originating from different sediment sources.

This research would not have been completed without the help of several people, guiding me in multiple ways. At first I would like to thank Gert-Jan Zwolsman, from Kiwa Nieuwegein, for providing me inside information and literature about research already performed on this subject, specific for the case of the river Meuse.

Special gratitude goes to the inorganic laboratory of RIZA in Lelystad, especially to Rembert Breidenbach, Onno Epema and Rene Geerdink, as they provided me with the fundamental results and samples for this research. Without their contribution, this research would still be in its starting phase and would include results of only a small set of parameters.

Additional measurements were performed at the laboratory of TNO. Therefore, I would like to thank Jorica Baars, Rob van Galen and Erik van Vilsteren for their contribution to a quick and accurate analysis and Bertil van Os and Harry Veld for their contribution to the data interpretation.

Last but not least, I would like to thank my two supervisors, Harro Meijer from the University of Groningen and Gerard Klaver from TNO in Utrecht. Harro Meijer gave me the opportunity to combine my fields of interest, environment and chemistry, at an external company and he did not mind to travel between Groningen and Utrecht for a discussion about the results. Gerard Klaver assisted me well to reduce my shortcoming knowledge about the field of geochemistry. His contribution by new perspectives, ideas and interesting discussions all guided me to the right direction and helped me out in the world of rivers and sediment.

For now, I hope that this research can contribute to a better understanding of the distribution of contaminants in the river sediment environment and will lead to new perspectives for future research.

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Abstract

Since the Industrial Revolution, mankind has disturbed the natural functioning of river floodplains by a high input of contaminants in the environment. Riverine suspended matter is the most important source for elevated levels of contaminants in floodplain areas. Previous research mainly focused on processes affecting the distribution of contaminants on the floodplain areas themselves, while the influence of the ultimate polluting source, river suspended matter, is poorly understood. Only minor knowledge is available on the temporal and spatial variability of contaminants in suspended matter.

This research focuses on the variability in contamination degree of both suspended matter and floodplain sediment for the Dutch part of the river Meuse.

Large seasonal variability of suspended matter composition was observed close to the Dutch-Belgian border in the river Meuse. Summer suspended matter is affected by a high input of organic matter, related to primary production (algae and nutrients), and a variable influx of polluting point sources. The high input of primary production and polluting point sources decreases, going downstream, until a steady state is reached, with only minor fluctuations in the composition of suspended matter. During winter time, the composition is very similar for all locations in the Dutch part of the river Meuse because of dilution of polluted base flow by coarse, not polluted, sediment.

The input of not contaminated, coarse material is the main factor for variation in the suspended matter composition during a flood, diluting the contaminated, fine material. Variability in dilution effect is attributed to the attainability and availability of coarse sediment sources. Resuspension of contaminated river bed sediment causes a variable input of elevated metal contents during the rising part of the flooding. Reaching the floodplain areas, initial spatial variability of contaminant content is caused by variation in grain size. Fine material will travel much further over the floodplain, leading to an increased contribution of fine material with increasing distance to the river channel. As most contaminants are adsorbed to the fine clay fraction, the highest degree of contamination will be found further away on the floodplain. In the aftermath of a flooding event, the contribution of river suspended matter will decrease, caused by resuspension of locally available, clean material.

The temporal variability of the floodplain sediments was investigated by sediment cores, originating from two dike breach ponds. The cores showed clearly the pollution history of the river Meuse, indicating that sediments in the studied dike breach ponds are valuable archives for floodplain research.

The composition of river suspended matter and that of the floodplain sediment showed a clear correlation. The main new finding of this work is that the pollution level of suspended matter during a flooding gives a good indication of the maximum level found back on floodplains.

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Samenvatting

Sinds de Industriële Revolutie verstoort de mensheid het natuurlijk functioneren van

overstromingsgebieden (Uiterwaarden) door een hoge toevoer van verontreinigingen in het milieu. De belangrijkste bron van een verhoogde verontreiniginggraad op overstromingsgebieden is het door de rivier aangeleverde zwevend slib. Voorgaand onderzoek concentreert zich hoofdzakelijk op de processen die de verspreiding van verontreinigingen op de overstromingsgebieden zelf beïnvloeden. De invloed van de grootste vervuilingsbron, het zwevend slib, wordt niet meegenomen. Er is daarbij nog maar weinig bekend over de temporele and spatiele variatie van verontreinigingen in het zwevend slib.

Dit onderzoek richt zich op de variatie van de vervuilingsgraad van zowel het zwevend slib als het overstromingssediment voor het Nederlandse deel van de rivier de Maas.

Een grote seizoensvariatie in de samenstelling van het zwevend slib treedt op bij de Nederlands/Belgische grens van de rivier de Maas. In de zomer wordt het zwevend slib beïnvloed door een hoge toevoer van organisch materiaal, wat gerelateerd is aan primaire productie (algen en nutriënten), en een variabele toevoer van vervuilende puntbronnen. Er treedt daarnaast een stroomafwaartse afname van de hoge en variabele toevoer van primaire productie en vervuilende puntbronnen op. Dit leidt tot een stabiele situatie, benedenstrooms, met slechts kleine variaties in de samenstelling van het zwevend slib. In de winter is de samenstelling van het zwevend slib vergelijkbaar voor alle locaties in het Nederlandse deel van de Maas, doordat de vervuilde zomertoevoer verdund wordt door een grotere bijdrage van grof, niet vervuild sediment.

Variatie in de samenstelling van het zwevend slib tijdens een overstroming wordt hoofdzakelijk bepaald door de grotere toestroom van niet vervuild, grof materiaal, wat zorgt voor een verdunning van het vervuilde, fijne materiaal. De variatie in verdunningseffect kan toegeschreven worden aan de bereikbaarheid en beschikbaarheid van de grovere sedimentbronnen. Heropname van vervuild beddingsediment zorgt voor een variabele invloed van verhoogde metaal gehaltes in het zwevend slib tijdens stijgend water.

De spatiele spreiding in vervuilingsgraad van het overstromingssediment wordt initieel bepaald door variatie in korrelgrootte. Fijn materiaal wordt verder over de overstromingsgebieden getransporteerd, wat leidt tot een toenemende bijdrage van fijn materiaal met toenemende afstand tot de rivier. Verontreinigingen zijn grotendeels geadsorbeerd aan het fijne, kleiachtige materiaal en zullen daardoor verder op de overstromingsgebieden afgezet worden. In de nasleep van een overstroming neemt de bijdrage van het door de rivier aangevoerde zwevend slib af door een toenemende heropname van het lokaal aanwezige, schone materiaal.

De temporele variatie van het overstromingssediment is onderzocht aan de hand van wielen, ontstaan na een dijkdoorbraak van de Maas. De metaalgehaltes, gerelateerd aan diepte, laten daarbij duidelijk de vervuilinggeschiedenis van de Maas zien. Dit geeft aan dat deze wielen nuttig zijn voor onderzoek van verontreinigingen in overstromingsgebieden.

Er is een duidelijke correlatie tussen de samenstelling van het zwevend slib en het overstromingssediment terug te vinden. De belangrijkste bevinding van dit onderzoek is dat de vervuilingsgraad van zwevend slib tijdens stijgend water een goede indicatie geeft van het maximale gehalte wat terug gevonden kan worden op de overstromingsgebieden.

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1 Introduction

1.1 Background

For centuries, floodplains fulfill multiple purposes for both human and nature. Not only as a protection zone between river channel and human habitats, but also as a fertile place for unique flora and fauna and agricultural use. Human disturbance of the natural floodplain functioning started in the Industrial Revolution. The construction of industries and cities near the river channel and the exploration of Earth’s material for human development led to unnatural high amounts of contaminants in the environment, mostly distributed by river water [Dennis, 2005]. As river water deposits its carrying material on floodplains, during high water flow, also floodplains became contaminated.

Contaminants travel through rivers mostly by binding to suspended particulate matter (SPM) [White, 2003]. During floodings, both river water and SPM are transported to the floodplain areas, where the particulate matter deposits. Contaminants, like nutrients, organic micro pollutants and heavy metals, accumulate at the river floodplain sections by long time storage. Especially storage of heavy metals represents a problem, as they are not degraded in nature (persistent) and accumulate in organic tissue, thereby threatening plants and organisms. Also human health can be affected by the consumption of contaminated agricultural products and leaching of contaminants into the groundwater [Thonon, 2006]. As river suspended matter is the ultimate source of contaminated floodplain areas, additional research is needed to understand the prevailing processes and transport modes affecting the contamination degree in suspended matter and floodplain sediments. In addition, information on the sources of transported suspended sediments has to be obtained since sediment source may have a key control on both the physical and chemical properties of the important fine sediment fraction [Walling, 2005].

1.2 Previous research

This research focuses on the situation of the Dutch part of the river Meuse (see paragraph 1.4). Therefore, the following literature references will all originate from research on the river Meuse or related Dutch rivers like the Rhine (separated in the Waal and Ijssel after entering the Netherlands) and the Geul.

Research on suspended matter is mainly focused on processes affecting the quantity of suspended matter. In the river Meuse, a positive correlation between suspended matter quantity and river discharge (amount

of water flowing through the river channel, expressed as m3/s) is observed [Doomen, 2003; Wijma, 2005].

During a flooding, a relative high amount of suspended matter will be transported to the floodplain sections. Higher amount of suspended matter can transport more contaminants to the floodplain areas, on total, but research found out that the contamination degree decreases with increasing discharge [Van der Heijdt and Zwolsman, 1997; Zwolsman et al., 2000; Doomen, 2003; Wijma, 2005]. According to previous research, the contamination degree decreases due to an increasing input of clean material of the surrounding area leading to dilution of contaminated material.

Floodplains are more extensively investigated looking at both sediment quantity and quality (suspended matter is called sediment when it deposits on floodplains). Sedimentation amounts and patterns vary widely within and between floodplains [Thonon, 2006]. Three primary factors determine where and how much sediment is deposited on floodplains: topography, grain size and flood magnitude [Asselman and Middelkoop, 1995; Middelkoop and Asselman, 1998]. Contaminants are associated with the finest particles and are deposited further away from the river channel [Middelkoop, 2000]. Models try to predict the distribution of sediment and contaminants on floodplains, but still include a large spread [Boogaerdt, 1996; Thonon, 2006]

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Still high contamination levels are maintained partly caused by memory effects: the storage of contaminated sediments in the river system at upstream floodplain areas and river banks with the possibility of redistribution during a next flooding [Swennen et al., 1994]. The Geul river catchment is a nice example of the interaction of memory effect and floodplain contamination, as the contamination level of suspended matter during a flooding is still influenced by mining activities of the past [Leenaers, 1989, 1991a, 1991b; Swennen et al., 1994].

The largest pollution levels of the past originated from industry and households with a direct outlet of pollution to river water. Since the direct outlet of contaminating material to river water is restricted, by laws and quality control, the main polluting sources shift from point to diffuse sources. This shift makes it hard to predict the quality of river water and particulate matter, nowadays, as much more processes are involved [Middelkoop, 2000; Driesprong and de Rijk, 2003].

Investigating the processes affecting the contamination degree of both suspended particulate matter and floodplain sediment is relatively new, as most research is focused on only one part of the story. Especially the temporal and spatial variability of SPM composition is a subject which is still poorly understood. Temporal and spatial variability in the composition of both suspended matter and floodplain sediment indicates the largest variation possible in contamination degree and might provide the required information about the main processes influencing the variation.

1.3 Research aims

The general aim of this research:

“Receive a better understanding of the processes affecting the contamination degree of (past, present and future) floodings in the Dutch part of the river Meuse by combining the composition of suspended particulate matter and floodplain sediment.”

The general aim will be answered with the following sub aims:

1) Temporal and spatial variability of suspended particulate matter composition in the hydrographical year (April) 2001- 2002.

2) Determination of the suspended particulate matter composition during the extreme flooding of the river Meuse in December 1993.

3) Determination of the sediment composition of an end member floodplain in two dike breach ponds

near the mouth of the river Meuse.

For a better understanding of the natural processes involved, a broad set of parameters already exist and additional parameters will be measured and implemented in the final results.

1.4 Research area

The river Meuse originates from France at 409 meter above sea level at the plateau of Langres. Traveling through Belgium and the Netherlands, the Meuse reaches the sea after 935 km (figure 1.1). Its drainage

basin (total land area that supplies the river with surface water [Bridge, 2003]) covers 36.000 km2 and also

includes part of Luxembourg and Germany [Middelkoop, 1998; Doomen, 2003].

The main source of water in the river Meuse is rain fall. In France the river discharge is low because of small tributary contributions and little relief. River discharge increases in Belgium with significant contribution from the tributaries Lesse, Ourthe and Sambre. The Meuse crosses the Belgian Ardennes and the present high relief leads to high river flow. As the impermeable, calcareous soil of the Ardennes area can not store high rain fall, river water will immediately flow to the flattened, lower lying areas of the

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The amount of suspended matter in the river Meuse can directly be related to river discharge. During summer, low flow enhances deposition of sediment and only a few milligrams of particulate matter will be in suspension. With high rain fall both bed material and sediment from the surrounding area erode and contribute to high suspended matter contents. The Dutch part of the Meuse receives a high amount of suspended matter from the Ardennes area because of the direct water influx.

The contamination degree of suspended matter is mainly controlled by the input of different anthropogenic sources. Large industrial areas exist near the river channel, especially in the Ardennes area (Liege, Maastricht), for the exploration of calcareous areas and metal mining [IMC, 2004]. Most of the industrial activities are tempered, nowadays, but still can contribute to high contamination levels by leaching of waste dumps and memory effects [Swennen et al., 1994]. Input of diffuse sources mainly originates from agriculture (fertilizers), households and industries.

This research will be restricted to the Dutch part of the river Meuse. Its position at the lower end of the river basin makes it sensitive to differences in rain fall (discharge), especially with the large influx from the Ardennes area. Almost the whole Dutch river course is artificially controlled by weirs, canals and dikes and is the only part of the river where room is

available for large floodplain

inundation.

The composition of suspended matter will be influenced by input from the Belgian Ardennes, erosion

of the surrounding area,

resuspension of river sediment and tributary inputs, though the exact contribution of each source still has to be determined.

Appendix A gives an overview of the weirs and most important tributaries of the Dutch part of the Meuse, going from the Dutch-Belgian border (Eijsden) to its confluence with the river Rhine

(after Keizersveer). The four

locations mentioned in Appendix A are adapted from the regular monitoring of the river by the State institute RIZA (Institute for Inland Water Management and Waste

Water Treatment) and these

locations are used for the

measurements in this research. In chapter 4 a broader view will be given of the Dutch part of the

Meuse and the measurement

locations.

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1.5 Report outline

In the following chapters the aforementioned research aims will be treated broadly, according to the following scheme:

Chapter 2 and 3 summarize the available information on suspended matter and floodplains. Every chapter includes a part, which specifies the situation of the river Meuse.

Chapter 4 presents the datasets used for this research and will expand the information about the Dutch part of the Meuse. Also the used measurement techniques for data analysis are briefly presented.

Chapters 5 to 8 give the results of this research and are divided in the sub aims mentioned at paragraph 1.3. Every chapter treats a sub aim. The results are finally combined in chapter 8.

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2 Suspended particulate matter

2.1 Composition of suspended particulate matter

2.1.1 Natural composition

Suspended particulate matter is defined as the material that is retained on a 0.45 µm filter, otherwise the material is dissolved or a colloid [Eisma, 1993]. Suspended particulate matter is naturally composed of two major groups. The contribution of each group depends on the prevailing chemical conditions in the river water.

1. Minerals originating from rock weathering

Different kind of minerals and rocks are present all over the world, which contribute to a major variety of minerals as particulate matter in rivers. Climate mostly determines the dominant group of minerals present in river water and the conditions for weathering [Berner and Berner, 1996]. Weathering is the process by which rock or sediment material is physically and chemically broken down into relatively fine solids and dissolved components [White, 2003]. Reactions in and at the Earth’s surface will continue until all elements are dissolved in ground or surface water or an unreactive mineral is formed. In Temperate Climate Zones with sufficient relief transport is very fast and weathering reactions cannot proceed to its end member.

Most minerals originate from the silicate group and will mainly weather to the end member quartz (SiO2).

Regions with dominance of calcareous rocks have carbonate (CaCO3) as another abundant mineral. Both

minerals exist in a crystal structure and appear as fine or coarse mineral, depending on the prevailing conditions for chemical and physical break down (abrasion) [Bridge, 2003].

The smallest fraction (< 2 µm) of minerals is called clays and has a layered structure. Figure 2.1 represents the general structure of clays with two silicate layers surrounding an aluminum hydroxide layer. Clay material can also consist of only one silicate layer or two layers of both [White, 2003].

During the formation of clays, both silicon and aluminum can be exchanged by other elements

from the surrounding area.

Basically all cations (positively

charged elements) can be

exchangeable, but the available amount in nature determines the relevance of each element. So

formation mainly involves

exchange with the major Earth elements (elements like iron,

magnesium and potassium).

Charge deficiencies caused by

exchange of, for example, Al3+

by Mg2+ are balanced by the

presence of exchangeable cations at the interlayer positions (figure 2.1) [White, 2003].

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The most important characteristic of clay material is its electrically charged surface layer. River water is generally pH neutral or a little alkaline, which makes the clay surface layer negatively charged. Positive elements (cations) are attracted by the negative charge and will be electrically adsorbed to the clay surface layer [Eisma, 1993]. The capacity to adsorb cations to the surface layer and exchange internal cations varies between minerals and is expressed as the ion exchange capacity [White, 2003].

When rock minerals reach the Earth’s surface and travel along by river water, weathering reactions proceed further, but also interactions with river water start. Most important is the affinity of an element to dissolve in water or remain in or at the particulate matter. Each element has its own affinity for a certain phase and, together with water chemistry, this will determine the exact distribution between the two phases. Of the major Earth elements, calcium has the highest affinity for the dissolved phase and is transported for only 40% as particulate matter in river water. On the other hand, aluminum rather remains in the particulate form (table 2.1) [Berner and Berner, 1996]. For this reason aluminum can be used as a proxy of the particulate clay content (fine fraction) in sediment, as the element mainly originates from clay minerals and remains in the particulate phase.

Table 2.1: Average concentration of major Earth elements at the Earth’s surface and in rivers [adapted from Berner and Berner, 1996]

Continents

Rivers

Element Surficial rock concentration (g/kg) Soil concentration (g/kg) Particulate concentration (g/kg) Dissolved concentration (mg/l) Al 69.3 71.0 94.0 0.05 Ca 45.0 35.0 21.5 13.4 Fe 35.9 40.0 48.0 0.04 K 24.4 14.0 20.0 1.3 Mg 16.4 5.0 11.8 3.35 Na 14.2 5.0 7.1 5.15 Si 275 330 285 4.85 P 0.61 0.8 1.15 0.025

River water in the Meuse originates from rainwater or groundwater at low river water discharge. Both rainwater and groundwater are also mainly composed of the major Earth elements. Therefore, input of these sources will not lead to major differences in the natural composition of the suspended particulate matter [Berner and Berner, 1996].

2. Organic matter

Organic matter mainly consists of plant remains, wood and detritus (non-living remains of an organism). It is supplied to rivers by wind or erosion from land material. During summer, low water flow can cause an increase of primary production [Eisma, 1993]. In the upper part of the River Meuse, development of aquatic plants and filamentous algae is the visible result of this increased primary production. After deposition, organic matter can be degraded by micro-organisms, which leads to a decreased level of organic matter. Resuspended bed material, therefore, contains lower levels of organic material.

Organic matter can be transported in river water as separate particles or attached to clay minerals in suspension. Part of the organic matter is charged by reactions in river water (both positively and negatively), which can lead to adsorption of cations to the organic surface layer or adsorption of organic matter onto a clay mineral. Organic matter competes with the positive elements in solution, which makes the relation between cations and clay material a bit more complicated.

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For river water, different sources of anthropogenic pollution contribute to recent contamination levels and can be divided in point and diffuse sources [Thonon, 2006]:

Point sources:

• Direct industrial effluents

• Wastewater treatment plants

Diffuse sources:

• Agriculture (application of manure, fertilizers and pesticides)

• Urban areas with atmospheric deposition of traffic and incinerations

• Surface runoff (including load from atmospheric deposition)

• Combined sewer overflows

Especially heavy metals and nutrients pose a problem for river water, as they are easily distributed in river water and affect river water quality. Anthropogenic sources overrule the natural background values by many times. The main contributions to a high contamination degree come from urban areas, wastewater treatment plants and, despite the restrictions, the direct emission of some industries to surface water [Thonon, 2006].

The natural background concentration of heavy metals in suspended matter is hard to determine, as recent pollution levels disturb the determination. Different methods are used to find a representative concentration of heavy metals in suspended matter. In van Tilborg (2002) several methods are discussed for the determination of natural background concentration of both the particulate and dissolved phase. Van den Berg and Zwolsman (2000) made an approach by using old river sediment to derive the natural background content of the river Rhine. Table 2.2 presents the results of this method for suspended matter. The heavy metals, mentioned in table 2.2, are considered as the most polluting elements for current river water.

Table 2.2: Natural background contents of heavy metals in the river Rhine [adapted from Van den Berg and Zwolsman, 2000]

Element Background content in suspended matter (mg/kg) Cd 0.3 Cr 80 Cu 20 Hg 0.2 Ni 30 Pb 25 Zn 100

Heavy metals enter the river system in three phases: dissolved, as aerosols (from the atmosphere) or attached to suspended matter [Thonon, 2006]. Most heavy metals have a higher affinity for the particulate phase and are adsorbed to clay particles (fine material) or organic matter (see also 2.1.1) [White, 2003]. Distribution and final deposition of heavy metals, therefore, depend on the characteristics and transport capacity of the fine particulate matter.

2.2 Transport of suspended particulate matter

Transport of suspended matter depends largely on the water flow, as it exerts different hydrostatic forces to a particle. Figure 2.2 represents the forces acting on a particle:

• Gravity: downward force, depending on the mass or

density of the particle

• Buoyancy: upward force, caused by larger hydrostatic

forces acting on the lower surface of a particle than on its upper surface

• Bernoulli: increasing velocity at the boundaries of a

particle, perpendicular to the river water flow, leading to upward forces

• Drag: tractive forces acting on a particle by flowing water

Figure 2.2: Hydrostatic forces acting on a suspended particle

Gravity Drag

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A particle remains in suspension as long as the conditions for transport are positive and the upward forces exceed the downward forces. Water flow determines the contribution of the upward forces. Forward transport will continue until both upward and downward forces become equal. At this point deposition of the particle starts at a critical velocity, also called the settling velocity. Particles with a high density or large particle size have higher downward forces and will deposit faster [Eisma, 1993; Bridge, 2003]. Suspended matter will, therefore, dominantly be composed of fine material (< 63 µm). Contaminants, adsorbed to the fine fraction, will travel a longer distance through the river channel, along with the river water flow.

Figure 2.3: Modes of sediment transport in river water

The deposited coarse material (generally >100 µm) at the bottom of the river channel is called the bed load. The transport of this material is much slower, as particles are heavier and cohesive forces at the bottom stick particles close together. Suspended matter travels along with river water by laminar, straight flows at the same velocity. At the bottom, laminar flows are disturbed and lead to turbulent eddies. Figure 2.3 illustrates the three different turbulent movements of bed load close to the bottom: saltation, scouring and rolling [Bridge, 2003]. Coarse material will travel a much shorter distance than fine material.

The exact grain size boundary between suspended material and bed load is not very clear, as water flow determines which particle will be in suspension and which particle will stay close to the river bed. During summer, water flow will be low and much more particles are deposited, leading to low suspended matter concentrations (only a few milligrams per liter of water). Summer is a deposition dominated time. When river discharge increases, more and more material will be resuspended. Coarse material will be transported both as bed load and suspended matter. During high rainfall, erosion of hill slope material and river beds also contribute to the total amount of suspended material. Deforestation and intensive agriculture led to increased erosion of river surrounding areas in the last decennia, contributing to higher amounts of suspended matter in the river channel [Berner and Berner, 1996; Thonon, 2006]. After a long period with high discharges, the input of some sediment sources can become exhausted, leading to lower suspended matter amounts than expected [Asselman, 1999; Doomen, 2003; Wijma, 2005].

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2.3 Predictability of suspended particulate matter fluxes and

composition of the river Meuse

2.3.1 Predicting suspended particulate matter fluxes

According to paragraph 2.2, the amount of suspended matter, transported in river water, seems to be directly related to water flow or water discharge. Most rivers, however, are sediment supply-limited and discharge only explains part of the variation [Wijma, 2005; Thonon, 2006].

Doomen (2003) and Wijma (2005) determined the influence of different processes to the variation in SPM quantity of the river Meuse. These processes can also be applied to other rivers, but will include possible different condition settings.

The direct relation of discharge with SPM quantity results in the following fitting curve for the river Meuse:

SPM = 2.18 + 0.012*Q

1.28

4594

.

0

2

=

R

Figure 2.4: Sediment fitting curve (predicting SPM quantity with discharge) of the river Meuse at location Eijsden (Dutch-Belgian border) [adapted from Doomen, 2003, including years 1988-2002]

The direct relation of SPM quantity and discharge leads to a correlation of about 0.46, which means that about 46% of the variation in SPM quantity can be explained by discharge.

Two main processes are involved which influence the SPM quantity in time:

1. Stock piling

The term stock piling was introduced by Wijma (2005) and contains the temporary storage of sediment waiting for transport. In the summer time, discharge is low, deposition of suspended matter will dominate. Only the finest fraction will remain in suspension, which is called the wash load (a few milligrams per liter). Sediment and organic matter is deposited in dry valleys, at river beds or behind weirs and create a stock of future suspended matter.

Sediment will remain deposited until the critical discharge has been reached. The discharge for river bed erosion will be higher than the discharge for deposition because, once a particle is deposited, it encounters

forces of cohesion. Doomen (2003) determined the critical discharge for the River Meuse at 250 m3/s while

Wijma (2005) fine-tuned it to 280 m3/s.

The stock of sediment will increase, during the summer, until the discharge finally exceeds its critical resuspension value. The first high water of the hydrological year (starting in April) carries a relative high content of suspended matter, as the stock is still large. With higher discharge more sediment will be released. As more high waters follow, the stock will decrease and lead to relative lower suspended matter concentrations compared to the start of the winter period. In a year with continuous high discharge, the stock will finally be empty and lead to exhaustion of sediment material. The supply of other sediment sources will dominate during the bed exhaustion time.

0 100 200 300 400 500 600 700 0 500 1000 1500 2000 2500 3000 3500 Q (m^3/s) SPM (mg/l)

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2. Hysteresis

During floodings (peak discharge > ± 1000 m3/s for the river Meuse) hysteresis can occur. Hysteresis is here defined as the difference in suspended matter quantity between the rising and falling limb of the flood. Figure 2.5 visualizes the four possible types of hysteresis for the river Rhine, as comparable figures are not available for the river Meuse.

Clockwise hysteresis is also the most dominant type in the river Meuse. The suspended matter quantity is higher during the rising limb than during the falling limb of a particular flooding: SPM quantity peaks before the water discharge peaks. River discharge is high and removes a lot of deposited sediment. This leads to temporal exhaustion of river bed material before the end of the flood. During a flooding, the supply of hill slope erosion and material from the upstream Ardennes area also increases and, therefore, will have a relative higher contribution when bed material gets exhausted [Wijma, 2005].

Figure 2.5: The four different types of hysteresis (difference in SPM quantity between rising and falling limb of a flooding) in the river Rhine [adapted from Asselman, 1999]

Implementing both stock piling and hysteresis in the sediment fitting curve of the river Meuse improves the correlation between discharge and the quantity of SPM to 0.62. For further improvement of the fitting curve, other sediment sources, outside hill slope material and river bed material, should be implemented [Wijma, 2005].

2.3.2 Predicting the composition of suspended particulate matter

Metal rating curves show a very low correlation between discharge and metal contents of suspended matter (see appendix B) [Doomen, 2003]. Not only sediment supply determines the SPM composition, but also metal characteristics, grain size distribution (metals are adsorbed to finer particles) and anthropogenic additions. To improve the predictability of heavy metal contents, all the complex processes should be implemented. So far no one succeeded to involve all these processes in the prediction.

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The following sources contribute to the SPM composition in the Dutch part of the Meuse (taking into account that summer time includes low discharges and winter time includes high discharges).

Sources of SPM composition always present at a certain point:

• Anthropogenic pollution

• Supply of upstream area

Sources of SPM composition in the Dutch part of the Meuse during summer:

• Tributaries (relative contribution higher than in the winter time)

• Possible higher organic content by input of primary production

Sources of SPM composition in the Dutch part of the Meuse during winter:

• Surface runoff (erosion of surrounding area)

• Memory effect

• Resuspension of bed sediment, sediment behind weirs and at floodplains

• Higher erosion of sediment in Ardennes area (upstream supply)

The memory effect is a combination of anthropogenic pollution and resuspension of bed sediment. High metal contents of a certain point source (e.g. mining industry) still can be found back far away from the source, but in a diluted form. Part of the polluted suspended matter will be deposited on river beds, while the other part remains in suspension and mixes with clean sediment from the surroundings. When floodings occur, the polluted river bed sediment will be picked up again with the suspended matter stream and deposits at floodplains. The next flooding can resuspend the polluted material from the floodplain areas and distribute it to downstream areas. This process can continue for a long time. The memory effect is clearly observed in areas with influence of past mining activities [Leenaers, 1991a, 1991b; Swennen et al., 1994; Dennis, 2005].

Despite the minimal knowledge on variation in suspended matter composition, appendix B makes clear that, on average, heavy metal contents decrease with increasing discharge. During floodings, the supply of clean, coarse material from the surrounding areas will lead to a dilution of contaminants in fine suspended matter [van der Heijdt and Zwolsman, 1997; Zwolsman et al., 2000; Wijma, 2005]. The contribution of coarse material will finally determine the composition of suspended matter deposited on floodplains.

2.3.3 Suspended particular matter during a flooding

For a better understanding of the influence of floodings to the contamination degree of suspended matter, the State institute RIZA made a special, intensive, sampling campaign during specific floodings of the river Meuse. The floods of 1991, 1993 and 1995 were reported by Zwolsman et al. (2000) and their results are summarized by the plots of figure 2.6 for the December 1993 flood.

The dilution effect, by input of clean material, becomes very clear for all plotted heavy metal and particulate organic matter (POC) contents in figure 2.6. During the rising limb of the flood, the metal content reaches a maximum value, caused by an increased contribution of contaminated bed material. As the stock of bed material runs out, the contribution of another sediment source starts to dominate.

The other, dominant sediment source consists of low organic and metal contents and both Van der Heijdt and Zwolsman (1997) and Zwolsman et al. (2000) attributed this to the influx of terrestrial silt or loess, originating from the river area in the Southern of the Netherlands (grain size: 30-60 µm). Loess is supplied by wind and is largely eroded to fine, clean, sand material. The larger influx of loess material should also be traced back with a lower contribution of fine material, but this can not be found back properly in the < 16 µm fraction (fine material) plot of figure 2.6.

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Figure 2.6: Discharge, suspended matter quantity, metal and organic matter content of the December 1993 flooding of the river Meuse at location Keizersveer [Adapted from Zwolsman et al., 2000]

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3 Floodplains

3.1 Sedimentation on floodplains of the river Meuse

Once river discharge is high enough to cross the river banks, water and suspended matter will travel along the floodplains. Suspended matter will be deposited on the floodplain area and is distributed along to where river water reaches. Transport of material is highest close to the river channel and decreases with increasing distance. Coarse material will be mainly transported as bed load (figure 2.3) and is deposited close to the river channel. Fine material travels in suspension and is deposited all over the floodplain [Middelkoop and Asselman, 1998].

Floodplain topography, suspended matter supply and flood magnitude determine the distribution and sediment quantity on the floodplain. Coarse material will be deposited directly behind elevated parts, like dikes and (natural) levees, and sediment deposition decreases rapidly going inland, as water flow is low. Fine material travels in suspension and will be deposited as a fine blanket on the inundated floodplain area, leading to a general trend of increasing contribution of fine material with increasing distance to the river channel. Intensive floodings can release more material from river bed and surroundings, which lead to transport of higher sediment amounts to the floodplain sections. Possible exhaustion of sediment sources leads to a lower suspended matter supply than expected [Asselman and Middelkoop, 1995, 1998; Middelkoop and Asselman, 1998].

Clay minerals, which are in a close proximity to each other, tend to flocculate (stick together), producing larger compound grains that settle faster than individual clay particles [Bridge, 2003]. Therefore, the in-situ distribution pattern can differ from the expected pattern of decreasing grain size with increasing distance to the river channel [Thonon, 2006]. This deficiency is still not investigated intensively and will not be taken into account for the total results, though it should be kept in mind.

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Figure 3.1 visualizes the influence of floodplain topography and flood magnitude during the January and December 1993 flood events in the river Meuse. Sediment was collected on the floodplain by a so called sediment trap, which consists of a plate of known size (mostly 1 m2), with artificial grass on top to trap sediment [Middelkoop and Asselman, 1998].

Figure 3.1a represents the elevation map of the research area and shows that elevation exists close to the river channel and in the middle of the research area. Accumulation of coarse material only takes place at the elevated sites, close to the river channel (figure 3.1c and 3.1f). The silt and clay fraction is deposited all over the floodplain. High deposition can be observed at the upstream site of the elevated bank in the middle of the floodplain.

The largest differences between the flooding of January 1993 and December 1993 were the flood

magnitude and duration. The January 1993 flood had a peak value of about 2300 m3/s and lasted only for a

few days. The December 1993 flood had a peak value of more than 3000 m3/s and remained at a high

discharge for more than ten days. During the December 1993 flood, almost ten times more sediment was deposited on total. Especially more coarse material was deposited at the elevated sites close to the floodplain compared to the January 1993 flood. Coarse material also traveled further into the floodplain. During a longer time frame, also the inundation frequency determines the sedimentation pattern and the amount of sediment on the floodplain.

Areas with a low inundation frequency will only be inundated during high floods and receive a relative high amount of fine sediment. High discharge floods can not only transport more material to the floodplain area, but can also release a part of the earlier deposited floodplain material, closer to the river channel. Areas with low inundation frequencies will be dominated by sediment deposition [Asselman and Middelkoop, 1995; Thonon, 2006].

Areas with a high inundation frequency receive the highest amount of sediment during both low and high floodings (see also figure 3.1), as the areas are mostly close to the river channel. Both fine and coarse material will be deposited, depending on the flood magnitude. Part of the deposited material can be transported further into the floodplain area, when the subsequent flooding has a water flow which is high enough for resuspension. This area is very dynamic and can be characterized by both deposition and erosion. On total, sedimentation rates will be highest close to the river channel.

3.2 Spatial and temporal variability of floodplain composition of

the river Meuse

The composition of floodplain sediment mainly depends on the supplied suspended matter composition. As mentioned in paragraph 2.3.2, different sediment sources influence the suspended matter composition during a flooding. The suspended matter composition depends on the conditions for erosion of possible sediment sources, the anthropogenic pollution degree and the influence of the dilution effect (see paragraph 2.3.3). The dilution effect provides clean, natural sediment to the river channel. The lowest metal contents, however, appear just a few days after the discharge peak. Therefore, floodplain sediment will always receive some elevated metal contents.

The following sections discuss the variability of the floodplain composition on both the spatial and temporal scale.

3.2.1 Spatial variability of floodplain composition

Spatial variability of floodplain composition can be investigated in twofold:

• Variability of floodplain composition on one location going inland, perpendicular to the river

channel

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Major variability in floodplain sediment composition can be observed looking at the heavy metal content. Metals are distributed by adsorption to the fine, suspended fraction (clay and organic matter) [White, 2003]. Fine material is deposited all along the floodplain section, but, relatively, the fine fraction increases with distance to the river channel, as the contribution of coarse material decreases [Middelkoop and Asselman, 1998]. Therefore, the heavy metal content also increases with distance to the river channel [Middelkoop, 2000; CSO, 2005].

Figure 3.2 confirms this distribution of the heavy metal content for the river Meuse during the December 1993 flood. Samples were taken near Bern, downstream of Keent (figure 3.1) by sediment traps. A natural levee exists near the river channel and, afterwards, elevation decreases with increasing distance to the river channel. Heavy metal content increases together with clay and organic matter content and reaches a maximum value at a distance of 200 meter from the river channel. Afterwards the metal content decreases rapidly.

Figure 3.2: Spatial distribution of clay, organic matter and heavy metal content with increasing distance to the river channel, during the December 1993 flood of the river Meuse near Bern [Adapted from Middelkoop, 2000]

The absolute amount of metals deposited during a flooding is highest near the river channel, because the highest amounts of suspended matter are deposited in these areas. Metal deposition coincides with the sedimentation pattern of figure 3.1 and is represented for the December 1993 flood, near Bern, in figure 3.3. The peak values of sediment and metal deposition do not completely correspond to the same distance, because the sediment deposition peak includes a high amount of coarse material. Low sediment deposition, more inland on the floodplain, provides a thinner layer of sediment, but contains higher metal contents according to figure 3.2.

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Variability of floodplain sediment composition, going downstream along the river channel, was investigated by CSO (2005) for the Dutch part of the Meuse. A comparable increase of heavy metal content, with increasing distance to the river channel, was also found back in this research. Going downstream, the heavy metal content on the floodplains decreases. This is attributed to a decreasing pollution degree of suspended matter in the river Meuse [CSO, 2005].

3.2.2 Temporal variability of floodplain composition

The temporal variability of floodplain composition represents the river pollution degree of the past. Rising industry in the nineteenth century increased the exposure of metals and nutrients to the environment. According to Middelkoop (2002), two pollution peak periods can be distinguished in the early 1930’s and early 1960’s for the river Rhine. After the second pollution peak, the Surface water Pollution Act restricted the direct outlet of polluted material to the surface water and steadily the pollution degree decreases [Middelkoop, 1998].

The river Rhine, likewise the river Meuse, receive industrial pollution from different anthropogenic sources. This situation is different for the river Geul, where two past mining industries determine the contamination level of both water and suspended matter. The increase and decrease of heavy metal content (especially lead and zinc) in floodplain sediment cores represents the rise and fall of the mining industry. Looking at the major shifts in metal content, known mining history dates can be connected to sediment core depths. Past changes in metal content are useful tracers for time [Swennen et al., 1994; Stam, 2002]. Another method, for dating of the floodplain sediment cores, is the application of radioactive tracers [Leenaers, 1991a, 1991b; Stam, 1998; Van den Berg and van Wijngaarden, 2000; Passier and Frapporti,

2003]. Especially the radioactive isotope 137Cs is useful, as it is appears in the environment only by human

influence. It is one of the nuclear waste products of major nuclear events, released into the atmosphere. Once 137Cs deposits on the soil, it rapidly and strongly adsorbs to the upper surface layer and remains immobile at the surface, until the following layer of sediment deposits above [Leenaers, 1991a, 1991b].

Two nuclear events led to high 137Cs contents in the atmosphere. The first event is the nuclear bombing test

period with a maximum peak at 1960 (according to KVI, Groningen). The second event is the large nuclear power plant accident at Chernobyl of 1986.

The relation between metal content and time is not always straightforward, as most floodplain sections are dynamic. The position on the floodplain section determines the amount of sediment received and the metal content adsorbed. A subsequent flooding can erode part of the previously deposited top layer and deposits the eroded sediment more inland. Floodplain sections, which receive material at different flood magnitudes, will also receive sediment of different composition. This leads to a variety of metal contents in the floodplain section. Once deposited, metals can also be remobilized downward by influence of chemical reactions (redox reactions) or burrowing of plant roots and organisms (bioturbation) [van den Berg et al., 1999]. Most of the remobilization reactions will come from bioturbation.

3.2.3 Combining spatial and temporal variability of floodplain sediment

composition

Floodplain sediment dynamics of the river Meuse were investigated for the area of Borgharen-Itteren [Van den Berg and van Wijngaarden, 2000; Passier and Frapporti, 2003]. This research combines both spatial and temporal variability of floodplain composition. The area is located close to the Dutch-Belgian border, directly behind the weir of Borgharen (appendix A), where the river Meuse is separated into the Juliana canal and the so called “Grensmaas” (literally “Border Meuse”, section with lots of meanders and low water flow).

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Sedimentation rates

The two peak values of the 137Cs-activity profile represent the years 1960 and 1986, as mentioned before,

and can be ascribed to a certain depth. The distance between the two peaks and between the 1986 peak and the top layer (time of core logging) can be used to determine the sedimentation rates (expressed in cm

sediment/year). The deeper the 137Cs-activity peaks are situated, the more sediment is deposited above and

the higher the sedimentation rates are. Core 1160 has higher sedimentation rates than core 1161 (137

Cs-activity peak values very close to each other) and confirms a higher sedimentation rate at positions close to the river channel.

0.0 10.0 20.0 Cd (mg kg-1) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 D ie p te ( c m -m v ) 0 200 400 Cu (mg kg-1) 0 400 800 Pb (mg kg-1) 0 2000 4000 Zn (mg kg-1) 20000 40000 60000 Al (mg kg-1) 0 12 24 Activiteit 137Cs (Bq kg-1) kern 1160

Figure 3.4: Variability of 137Cs-activity and metal content with core depth of core 1160 at the Borgharen-Itteren area, river Meuse [Adapted from Passier and Frapporti, 2003]

Metal profile

The metal profiles of core 1160 and 1161 differ very much. Core 1161 show a clear distinction between background, peak and recent values. The core location is only inundated during reasonable high floodings and can be considered as an end member of the flood. All plotted metals have a comparable profile. Core 1160, however, does not show a clear metal profile. Its close position to the river channel makes it accessible for both high and low floodings. The metal content of the core will be a mixture of different suspended matter compositions of different flood magnitudes.

0.0 10.0 20.0 Cd (mg kg-1) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 D ie p te ( c m -m v ) 0 100 200 Cu (mg kg-1) 0 200 400 Pb (mg kg-1) 0 1000 2000 Zn (mg kg-1) 20000 40000 60000 Al (mg kg-1) 0 12 24 Activiteit 137Cs (Bq kg-1) kern 1161

Figure 3.5: Variability of 137Cs-activity and metal content with core depth of core 1161 at the Borgharen-Itteren area, river Meuse [Adapted from Passier and Frapporti, 2003]

Metal content

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4 Materials and methods

4.1 Introduction

Previous research showed a clear relation between metal content and the finer grain size fractions during floodplain inundation. Furthermore, the input of coarse, clean material determines the contamination degree of suspended matter during a flooding. The contamination degree is clearly related to the grain size distribution of the river sediments and indicates the importance of knowledge of this distribution.

Until now, minor knowledge is gained about the parameters, which influence the contamination degree of river related sediments. Variation in metal content of suspended matter was only investigated by relating the metal content to discharge and showed a minor correlation, as many processes and parameters influence the variability of the contaminants. These processes result in a high variation of the contamination degree in time and space, which set the boundaries of this research. Investigating the temporal and spatial variability of contaminant content and related parameters of both suspended matter and floodplain sediment provide knowledge of the main processes involved. This kind of approach to the contamination problematic is only minimal applied in previous research.

In this research, the start will be given for an integrated analysis of the composition of suspended matter and floodplain sediment in time and space, with the application of a broader set of possible relevant parameters. Focus will be on the Dutch part of the river Meuse. The following, general parameters are implemented in this research, applied in different datasets:

• Discharge and suspended matter content (transport of sediment/sediment supply)

• Grain size distribution (contribution of fine fraction and relation coarse/fine material)

• Organic analysis (input of organic material)

• Element analysis with variation of contaminants and major Earth elements (natural occurring

elements)

The following sections describe the composition of the different datasets used for this research and the measurement techniques to determine the different parameters, mentioned above.

4.2 Datasets

Three different datasets were used to investigate the sub aims of paragraph 1.3. Sub aim 1 and 2 are investigated by suspended matter samples of the State institute RIZA (Institute for Inland Water Management and Waste Water Treatment), which monitors all important rivers and lakes of the Netherlands for possible high contaminant levels.

4.2.1 RIZA monitoring

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Location Eijsden (rkm 615), at the border, monitors the state of transgressing water, originating from Belgium. General measurements and sampling are performed at a pontoon, positioned a few meters from the river side. Stevensweert (rkm 677) is situated in the so called “Grensmaas”, or literally translated “Border Meuse”. This part of the Meuse is impassable for navigation, as the water level is low and the river course has lots of meanders. Low flow conditions promote the deposition of suspended matter and coarse material dominates the river bed [Doomen, 2003]. Navigation is diverted to the Juliana Canal (see appendix A) and re-enters the Meuse upstream of Roermond. The subsequent river course is dominated by anthropogenic adjustments, like weirs, canals and dikes, and is called the “Zandmaas” (“Sand Meuse”). Monitoring location Belfeld (rkm 711) is positioned close to the weir of Belfeld, at the upstream side. After the last weir (Lith), the Meuse is separated into the “Bergsche Maas” and “Afgedamde Maas”. Keizersveer (rkm 855) is the last monitoring location before the river Meuse joins the river Rhine and finally enters the sea. Tidal influence at Keizersveer can bring some suspended material from the river Rhine into the Meuse, occasionally.

Suspended matter sampling of the

monitoring locations is established at a fixed time interval. As the priority of some locations is higher, the sampling period is not equal for all locations:

Eijsden: weekly

Stevensweert: 8-weekly

Belfeld: 8-weekly

Keizersveer: 4-weekly

Figure 4.1: Monitoring locations of RIZA in the Dutch part of the Meuse [Adapted from www.waterplan.nl]

Suspended matter is collected by a centrifuge. Water is pumped up from the middle of the river course and rotated in the sampling centrifuge, where suspended matter sticks to the wall by the centrifugal forces. Before analysis can take place, the samples are freeze dried and grinded to a powder.

4.2.2 Regular monitoring of suspended matter

Regular monitoring data of RIZA was used to determine the temporal and spatial variability of suspended matter composition in the Dutch part of the river Meuse. Suspended matter data of the four RIZA monitoring locations in the river Meuse can be found back on the internet (www.waterbase.nl). Discharge is determined daily, only for Eijsden and Keizersveer. The other parameters are measured in the aforementioned time interval of each monitoring location. The availability of results on the internet differs for each station and parameter, but from 1997 onward the data set is complete.

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Previous research already showed [Doomen, 2003] that relating metal contents of suspended matter directly to discharge will give a blur of points and no clear correlation. The variability within this dataset is not only caused by differences within a year, but also by differences between the years. For a better understanding of the differences within a year, a representative year for the river Meuse was chosen to investigate the seasonal influences and spatial differences between the monitoring locations.

The results, obtained from RIZA, do not represent the total natural element contents. The destruction method used at RIZA, to break all the chemical bonds of suspended matter and release elements into the destruction solution (acid), is not complete. This means that some elements are only partly released into solution, caused by strong molecular bonding, and will lead to lower contents than expected. The measured metal content provides enough information for the biological availability and toxicity of the heavy metals in suspension, but does not represent the natural variation. Therefore, SPM samples of the representative year are re-measured by TNO with the measuring techniques of 4.2. A complete destruction can be performed and more than 60 elements can be measured.

The hydrological year (April) 2001 - 2002 was chosen for re-measurement, as both periods of low and high discharge appear in this time frame. The dataset consists of the following samples:

Table 4.1: Dataset of the re-measured RIZA monitoring samples of the river Meuse for the hydrological year 2001-2002

Station Total samples First sample date Last sample date Eijsden 26 3-27-2001 4-16-2002

Stevensweert 8 4-19-2001 5-16-2002

Belfeld 8 4-17-2001 5-13-2002

Keizersveer 15 4-17-2001 5-14-2002

4.2.3 Suspended matter sampling during the December 1993 flood

At the start of the nineties, the Meuse was affected by two large floodings. Especially the December 1993

flood was severe, with a discharge peak of more than 3000 m3/s. Such a high discharge occurred only once

in the whole history of the river Meuse. RIZA made a special sampling campaign for the December 1993 flood, to investigate the variability in water and suspended matter composition. The report of Kos (1995) includes the most important results of this campaign and focused mostly on the heavy metal content. Van der Heijdt and Zwolsman (1997) and Zwolsman et al. (2000) also used results from this research.

A part of the suspended matter samples, taken during the campaign, are also re-measured by TNO to provide a better understanding of the total natural variation. This dataset provides the information about the suspended matter composition during a high flooding (sub aim 2).

The dataset consists of the following samples:

Table 4.2: Dataset of the re-measured RIZA suspended matter samples of the December 1993 flood of the river Meuse

Station Total samples

Start sampling

Sampling interval End sampling Eijsden 15 12-20-1993 10:00 AM 2 hours until 12-21-1993* 12-27-1993 until 1-3-1994 daily sampling Stevensweert 23 12-20-1993 2:00 PM 2 hours 12-22-1993 4:00 PM Keizersveer 43 21-12-1993 8:00 AM 2 hours until 12-24-1993 12-24-1993 until 12-27-1993 few samples a day 12-27-1993 until 1-3-1994

daily sampling

* = Centrifuge wire was blocked at 12-21-1993, sampling could not proceed until 12-27-1993

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4.2.4 Sediment cores of the dike breach ponds at Ammerzoden and Empel

To determine the sediment

composition of an end member floodplain, sediment cores were taken at two dike breach ponds near Den Bosch. These ponds were created, several centuries ago, after a dike burst of the river Meuse. Water and suspended matter are supplied to the ponds only during

high floods, where sediment

material deposits as an end member of the flooding transport. Core sampling took place in the summer of 2005 and was executed on the deepest part of the pond to guarantee a minor contribution of bed erosion inside the pond and anaerobic conditions for minor

intervention of plants and

organisms (bioturbation).

Figure 4.2: Sampling of the sediment core at the pond near Ammerzoden; in the right corner a part of the Meuse is visible

The characteristics of the two ponds and sediment cores are summarized in table 4.3.

Table 4.3: Dataset of sediment cores Ammerzoden and Empel in two dike breach ponds of the river Meuse

Pond Distance to the river channel (m) Position Core length (cm)

Sampling interval Total samples Empel 450 Left hand site of the

Meuse; upstream of Den Bosch

70 3-4 cm 17

Ammerzoden 150 Right hand site of the Meuse; downstream of Den Bosch 660 First meter: 3-4 cm Following meters: 10 cm 96

Pollen and 137Cs-activity analyses are performed to date back the sediment cores. Measurement of 137

Cs-activity is significant for the first meter of the core, as only recent nuclear events (bomb testing and Chernobyl) introduced the radioactive isotope in the environment. Samples were taken at the same sampling interval of table 4.3. Pollen analysis is useful when the history of agriculture and flora is known of the researched area. A detailed pollen diagram was made, only for the Ammerzoden core, with a sampling interval of 5 centimeter for the whole core.

The composition of suspended particulate matter during floodings of the river Meuse