Propagation of a cadmium spill through an impounded river system
dr. G.T. Klaver dr. J. Joziasse I. Bakker, MSc. (RWS Waterdienst)
© Deltares, 2009
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Inhoud
Glossary 1
Abstract 2
1 Introduction 3
2 Materials and methods 5
3 Result 8 3.1 Variation of discharge and SPM content during the Cd spill 8 3.2 Background values for Cd, Cu, Ni, Pb and Zn in total water and SPM 10 3.3 The 2005-2006 Cd spill in the Dutch part of the River Meuse 13 3.3.1 The Cd spill monitored in Eijsden, upstream of the impundments and stream components 13 3.3.2 The Cd spill in the monitoring stations downstream of Eijsden 16 3.3.3 The Cd load of the Cd spill 19 3.4 Extent of SPM contamination before and during the Cd spill, a comparison with environmental standards 20
4 Discussion 22 4.1 Signaficance of the Cd spill in understand river system processes and river basin management effects 22 4.2 Fate of the Cd-enriched SPM in the Meuse 23 4.3 Remobilization of sediment-bound Cd towards the dissolved phase 23
5 Conclusions and recommendations 24
6 References 25
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Glossary
BKB-B Upper limit of class B concentration values in the Dutch ‘Besluit Bodemkwaliteit’ (VROM, 2007) EC European Commission EF Enrichment Factor IMC International Meuse Commission MLI Metal Load Index NBM Natural Background concentration Meuse RIZA Institute for Inland Water Management and Waste Water Treatment rkm River kilometer number SPM Suspended Particulate Matter TM Target value for the Meuse river VROM Dutch Ministry of Housing, Spatial Planning and the Environment V&W Dutch Ministry of Transport, Public Works and Water Management
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Abstract
In this paper, the influence of impoundments (sluices, weirs, etc.) and stream components (tributaries, river branches, associated canals) on the metal content in water and suspended particulate matter (SPM) in the Dutch part of the River Meuse is assessed using the decrease in the cadmium content of the particulate and dissolved phase during the transport of a distinct cadmium spill through the river. This anthropogenic spill lasted from July 2005 to June 2006 and is documented by the weekly monitoring results of the Meuse in Eijsden at the Belgian-Dutch border. The monitoring data indicate that cadmium was discharged as a dissolved phase. Redistribution of water towards canals is the cause that during low flow conditions only a limited amount of water with an Eijsden geochemical signature arrives in Keizersveer (near the mouth of the river). During such periods various tributaries and groundwater have significant contributions to the discharge measured in Keizersveer.
The monthly variations of cadmium concentrations in total water and SPM, upstream and downstream of the series of impoundments are calculated for the period 1993-2004. Next, the transfer of the cadmium through the impounded part of the river is determined based on the monitoring results obtained in the stations in between and downstream from the impoundments. Finally, possible lag effects of the cadmium spill are quantified using monitoring data obtained in 2006 and 2007. It is concluded that it takes about six months before the cadmium spill is detected in the SPM at the monitoring station of Keizersveer. Leaching of cadmium from the sediment to the surface water may result in high dissolved cadmium concentrations.
For a better understanding of the processes in the river system it is essential that the SPM monitoring frequency in river sections downstream of Eijsden is increased, starting with the Keizersveer station. A better system understanding is important in order to define appropriate measures to meet the goals of the Water Framework Directive concerning the chemical status of the Meuse River.
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1 Introduction
For long times in the past, the natural functioning of river basins has been distorted by high input of contaminants in the environment (Rang et al., 1986; Négrel et al., 2000; Grosbois et al., 2006). Contaminants are distributed through the river either dissolved in the water phase, or adsorbed to the suspended particulate matter (SPM). A strong affinity for the particulate phase, especially the < 63 µm fraction, leads to accumulation of contaminants on the SPM (Van der Weijden and Middelburg, 1989; Horowitz, 1995; Roy et al., 1999; Négrel and Grosbois, 1999). River systems, with their associated impoundments (weirs, sluices) and stream components (tributaries, river branches, associated canals), provide ideal settling conditions for the particulate phases that are transported through the watershed. However, some fractions remain in suspension and are transported to downstream river sections. Additional material can also be introduced from tributaries (Klaver et al, 2007). The amount of SPM in the Meuse River can be directly related to river discharge. During summer, low flow conditions enhance deposition of SPM and only a small part (mainly the finest fraction) will be in suspension (wash load). With high rainfall, both riverbed material and sediment from the surrounding area erode and contribute to large SPM concentrations. The Dutch part of the Meuse receives a large quantity of SPM from the Ardennes area, because of the direct water influx (Middelkoop, 1998; Doomen, 2003; Wijma, 2005). SPM quality is controlled by the input of different sources. Large industries exist near the river channel, especially in the Ardennes area (Liege, Maastricht) for the exploration of the calcareous areas and for metal mining. Most of these point sources are closed down already, but they still can contribute to higher contamination levels by leaching of waste dumps and memory effects (Swennen, van Keer and de Vos, 1994). Diffuse source inputs mainly come from agriculture (fertilizers, pesticides), households and industries.
Meuse basin characteristics The river Meuse originates at the plateau of Langres in France, at a height of 409 meter above sea level. Its drainage basin covers 36.000 km2, including parts of France, Luxembourg, Belgium, Germany and the Netherlands (Figure 1). The main source of water in the river Meuse is rainfall. In France, the river discharge is low, with little relief and only a small contribution of tributaries. River discharge increases in Belgium, with contributions from the tributaries Lesse, Ourthe and Sambre (IMC, 2004). The high relief in the Belgian Ardennes leads to high river water flow. As the impermeable, calcareous soil of the Ardennes area can not accommodate high rainfall, the water will immediately flow to the flattened, lower lying areas of the Netherlands (Wijma, 2005; de Wit, 2009). The river discharge at the Dutch-Belgian border amounts to 230 m3/s, on average, but during flooding conditions the discharge can peak to values of more than 3000 m3/s (Middelkoop, 1998; Doomen, 2003). The whole basin of the river Meuse is artificially controlled. Canals, dikes, weirs and sluices and reservoirs (river works) have been constructed in order to maintain navigation, ecological functions and water supply during low flow periods and to improve flood protection during high flows.
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Figure 1 The drainage basin of the River Meuse with the locations of the IMC monitoring stations (www.meuse-maas.be). Also shown is the location of monitoring station Stevensweert (www.waterbase.nl ).
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2 Materials and methods
Data from the four different monitoring stations (Figure 1, Table 1) in the Dutch part of the river Meuse were obtained from the former RIZA (Institute for Inland Water Management and Waste Water Treatment), now known as the Centre for Water Management of the Ministry of Transport, Public Works and Water Management (V&W). This institute monitors the larger rivers and lakes in the Netherlands for possible contaminants.
Monitoring stations Location Eijsden (rkm 615), at the border, monitors the state of transgressing water, from Belgium into the Netherlands. General measurements and sampling are performed at a pontoon, positioned in the river channel. Stevensweert (rkm 677) is situated in the so called “Grensmaas” (Border Meuse). This part of the Meuse (from Borgharen to Linne) is not navigable, as the water level is low and the river course has lots of meanders. Monitoring location Belfeld (rkm 711) is positioned close to the weir of Belfeld at the upstream side (Figure 1). After the last weir (near Lith), the Meuse splits into the “Bergsche Maas” and the “Afgedamde Maas” (Dammed Meuse). Keizersveer (rkm 855) is the last monitoring location before the river Meuse joins the river Rhine and finally enters the sea. Tidal movements at Keizersveer sometimes causes some material from the river Rhine to be transported into the Meuse.
Suspended matter is sampled with an overflow centrifuge. River water is fed into the centrifuge, equipped with a teflon sheet onto which suspended matter is collected by centrifugal forces. The dissolved phases (nutrients and metals) are analyzed after filtration of the water over a 0.45 µm filter. The dataset comprises the concentration of cadmium, copper, nickel, lead and zinc in total water, dissolved water and suspended matter of these four monitoring stations. Additionally general parameters discharge, pH, suspended matter content, organic carbon content of SPM and grain size fractions <2 ȝm (clay) and <63 ȝm (silt) of SPM are also considered. Table 1 gives a survey of the sampling frequency and period considered in this research for the different parameters. Sampling and storage of water and suspended matter samples are carried out according to standard protocols. Chemical analyses are performed according to standard methods by the Centre for Water Management. Organic carbon is determined by flash combustion (CN-analyzer), grain size distribution by sedimentation (Sedigraph), total and dissolved metals in water are analysed by AAS until 1998 and afterwards by ICP-MS, metals in suspended matter are analysed by AAS until 1998 and afterwards by ICP-AES. All data have been made accessible on the internet by the Centre for Water Management (www.waterbase.nl).
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Keizersveer Maas Bergsche Afgedamde Maas Dommel/Dieze
Hertogswetering Grote Wetering
Lith Nieuwe Wetering
Grave
Maas-Waal canal
Niers
Sambeek
Eckelsche beek Afleidingskanaal
Gelderns-Niers canal
Belfeld
Afwateringskanaal
Neerbeek Swalm
Roermond
Roer
Linne
Thorner beek
Maas Geleen beek Stevensweert
Geul Juliana canal Grens- Zuid-Willemsvaart
Borgharen
Jeker Voer
Eijsden
Figure 1 Schematic overview of weirs, tributaries and canals in the Dutch part of the Meuse Yellow blocks: monitoring locations; orange blocks: weirs
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Table 1 Frequency and period of monitoring of various parameters in Eijsden, Stevensweert, Belfeld and Keizersveer (www.waterbase.nl), for locations see Figure 1
Parameter Discharge SPM Org. C in pH Metals Metals Metals in SPM total dissolved SPM Monitoring m3/s mg/l wt % - mg/l µg/l mg/kg station Eijsden Daily Daily Weekly Weekly Weekly Weekly Weekly 1993-2007 1993- 1993-2007 1993- 1993-2007 2002-2007 1993-2007 2007 2007 Stevensweert 6 x year-1 6 x year-1 Monthly 6 x year-1 6 x year-1 1993- 1993-2007 1993- 1993-2007 1993-2007 2007 2007 Belfeld 6 x year-1 6 x year-1 Monthly Monthly Monthly 6 x year-1 1993- 1993-2007 1993- 1993-2007 2006-2007 1993-2007 2007 2007 Keizersveer Daily Monthly Monthly Monthly Monthly Monthly Monthly 1997-2007 1993- 1993-2007 1993- 1993-2007 2005-2007 1993-2007 2007 2007
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3 Result
3.1 Variation of discharge and SPM content during the Cd spill From Eijsden to Keizersveer various impoundments and stream components are present that influence the transfer of water and SPM throughout the watershed (Figure 1). The impoundments consist of weirs and sluices; the stream components consist of outflowing and parallel canals and incoming tributaries. As will be shown below, the Cd spill lasted from July 2005 to June 2006. Based on the transfer of Cd through the Meuse, the spill can be divided into three phases (Table 2). These phases not surprisingly coincide with variations in the discharge regime and SPM contents present during the Cd spill. After these three phases, a fourth phase represents the period in which lag effects of the Cd spill can be recognized.
Table 2 The four phases distinguished in the 2005-2006 Cd spill on basis of the Cddis and CdSPM transfer through the Meuse river. Also the summary statistics of the discharge in Eijsden, the Border Meuse (Borgharen) and Keizersveer are presented during the phases distinguished Cd Dissolved Particulate matter Period Discharge Station Average P90 P50 P10 spill matter phase 1 No transfer of No transfer of SPM 21/6/’05 – Low Eijsden 45 76 38 22 Cd Sedimentation of SPM 30/11/’05 Border Meuse 29 61 20 11
SPMKeiz not enriched in 163 days Keizersveer 86 146 79 35 Cd 2 Transfer of Cd Limited transfer of 1/12/’05 – Low Eijsden 164 240 152 96 SPM 14/2/’06 Border Meuse 153 233 141 87 Sedimentation of SPM 76 days Keizersveer 228 330 223 143
SPMKeiz enriched in Cd 3 No transfer of Intensive transfer of 15/2/’06 – High Eijsden 371 721 277 162 Cd SPM 12/6/’06 Border Meuse 360 729 268 142
Levelling of CdSPM 117 days Keizersveer 454 817 381 216
SPMKeiz enriched in Cd 4 Remobilisation No transfer of SPM 12/6/’06 – Low Eijsden 77 133 64 30 of Cd from Sedimentation of SPM 20/11/’06 Border Meuse 60 118 47 13
river bed SPMKeiz enriched in Cd 162 days Keizersveer 118 215 97 48 sediments
In Figure 3, the daily discharge in Eijsden, the Border Meuse (Borgharen) and Keizersveer, and the daily SPM content in Eijsden, Stevensweert, Belfeld and Keizersveer are plotted against time. This figure shows that for discharges below 100 m3/s, the values in Eijsden, Border Meuse and Keizersveer differ significantly. Especially in the Border Meuse and in Keizersveer, large discharge fluctuations can be observed. These discharge differences can be as high as one order of magnitude. The low discharges in the Border Meuse compared to those in Eijsden are caused by the distribution of water between the Border Meuse and the outflowing and parallel canals downstream of Eijsden (see Figure 1). This water distribution is regulated in the Meuse Discharge Treaty of 1995 and has a strong impact on the discharge regime of the Border Meuse (Jaskula-Joustra, 2001). Furthermore, Figure 2 shows that for discharges above 100 m3/s, the values in Eijsden and the Border Meuse (Borgharen) are very similar. Various tributaries and contributions from groundwater cause the discharge in Keizersveer to be distinctly higher than in Eijsden and the Border Meuse. It takes about two days for a discharge peak or flood to travel from Eijsden to Keizersveer. The discharge peaks
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in phase 3 of the Cd spill were not high enough to cause flooding of the floodplains and the weirs were still functioning (these weirs are completely opened at discharges over 1200 m3/s).
Eijsden Border Meuse Keizersveer
Eijsden SPM Belfeld SPM Keizersveer SPM
1000
100
10 Discharge (m3/s)SPM (mg/l) Discharge and
1 2 3 4 1 May-05 Sep-05 Jan-06 May-06 Sep-06
Figure 2 Discharge in Eijsden, the Border Meuse and Keizersveer and SPM in Eijsden, Belfeld and Keizersveer during the period of the Cd spill. The black dashed lines indicate the Cd spill duration as observed in Eijsden; blue arrows indicate the four phases distinguished (see Table 2).
During the first phase of the Cd spill, the average discharge in Eijsden was low (45 m3/s) compared to the long-term average (104 m3/s, 1993-2007). The effects of the distribution between the Border Meuse and the parallel and outflowing canals downstream of Eijsden can be seen in the low average discharge in the Border Meuse (29 m3/s, Table 2). In Stevensweert, the water originates from the Border Meuse, the tributary Geul and inflowing groundwater. Even during extremely low discharge conditions, the geochemical signature of the water will resemble that of Eijsden. In Belfeld, the water originates from the Border Meuse, the Juliana canal, inflowing groundwater and tributaries. The Roer is the most important tributary in terms of water quantity. During the summer, the discharge of the Roer is fixed at 15 m3/s and during low flow periods, the discharge of the Roer is larger than that of the Border Meuse at its gauging point Borgharen (de Wit, 2008). To make navigation possible in the Juliana canal during such periods, the water has to be pumped back from the Meuse downstream of the Juliana canal sluices into the Juliana canal (Jaskula-Joustra, 2001). Compared to Belfeld, the water discharge in Keizersveer is further increased by inflowing groundwater and tributaries. The Niers and the Dommel/Dieze are the main tributaries. Consequently, in Belfeld and Keizersveer only the water originating from the Border Meuse has an Eijsden geochemical signature. Discharge data are available only for Keizersveer, not for Belfeld. During phase 1 of the Cd spill, the average discharge in Keizersveer was 86 m3/s.
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From this total discharge a maximum of 29 m3/s (average discharge of the Border Meuse) originated from the undivided Meuse in Eijsden, which amounts to about 30 %.
3.2 Background values for Cd, Cu, Ni, Pb and Zn in total water and SPM In order to interpret the Cd spill, the background values of the total metal content in water and in SPM have to be considered. The Meuse basin has a temperate climate, with tributaries that are dominated by a rainfall-evaporation regime. This causes low flows during summer and high flows during winter. Because of this highly variable discharge regime, background values were calculated on a monthly basis for the period 1993-2004. In order to facilitate the interpretation, the monthly variation is plotted in the same order as the hydrological year in the Meuse (April to March). For this comparison, the monitoring results (1993-2004) were extracted from the database of RIZA (www.waterbase.nl). This time period was chosen, because it immediately precedes the Cd spill of 2005-2006 and because detection limits were not low enough in the years before 1993. The summary statistics for the total metal concentrations (Cdtot, Cutot, Nitot, Pbtot and Zntot) in water and the contents of these metals in SPM are given in Table 3. In this period, no metal concentration data for the dissolved phase are available. The dissolved concentration of an element is defined as the concentration in water after filtration over 0.45 µm.
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Table 3 Summary statistics of discharge1,2, SPM1 content, <2 µm and < 63 µm and organic C in SPM1 and total metal content in water3 and SPM3 in Eijsden and Keizersveer. All data obtained from www.waterbase.nl. 1Data 1993-2007; 2 Data discharge in Keizersveer 1997-2006; 3 Data 1993-2004 Parameter Discharge (m3/s) SPM (mg/kg) < 2um fraction in SPM < 63um fraction in SPM Org.C in SPM (wt%) Aver P90 P50 P10 Aver P90 P50 P10 Aver P90 P50 P10 Aver P90 P50 P10 Aver P90 P50 P10 Month Eijsden 1 564 1030 474 134 29 58 13 3 38 43 39 28 63 71 67 45 7 11 6 5 2 518 1019 435 141 30 73 16 3 34 43 34 27 65 74 67 53 7 12 6 4 3 419 773 333 138 18 47 8 3 35 40 34 29 62 72 64 47 9 15 8 5 4 290 618 210 88 13 27 7 3 34 43 34 27 59 77 59 43 13 19 13 6 5 174 289 149 74 8 15 6 3 34 41 33 26 57 70 58 42 13 19 13 7 6 98 170 85 35 9 14 7 3 29 37 31 19 53 67 54 37 16 23 14 8 7 79 141 58 25 11 17 8 4 30 40 31 24 54 70 55 42 16 23 16 7 8 64 126 45 19 8 13 7 3 25 35 26 14 53 69 54 37 16 24 16 9 9 75 149 51 20 9 12 6 3 28 34 29 24 57 65 58 48 13 19 13 8 10 70 223 70 20 10 15 5 2 30 35 30 23 58 66 60 49 12 16 11 8 11 238 551 134 40 25 63 7 3 31 41 32 22 60 71 61 49 10 15 10 6 12 420 852 291 93 30 78 9 3 32 39 32 22 61 71 63 50 8 13 7 5 Month Keizersveer 1 582 1067 506 190 18 43 16 4 39 45 39 33 71 76 71 65 5 7 5 4 2 611 1148 555 179 29 78 13 4 39 43 41 33 70 74 71 65 6 7 6 4 3 549 1049 435 234 18 29 10 6 37 39 36 35 67 71 67 63 7 9 6 5 4 360 730 291 147 11 17 9 4 38 40 39 34 65 69 67 60 9 13 8 6 5 252 396 221 133 9 14 8 5 40 40 39 33 64 71 63 60 9 12 9 7 6 147 231 131 71 11 16 7 3 39 43 40 35 65 71 69 53 8 10 7 6 7 129 217 110 52 7 9 7 4 43 49 42 38 68 73 68 63 7 9 7 6 8 110 207 93 41 7 11 7 5 43 45 43 41 68 72 69 60 7 8 7 6 9 120 233 90 37 9 9 5 3 43 47 42 39 70 74 70 66 7 9 6 5 10 156 291 126 47 6 8 4 4 41 48 39 36 71 78 71 63 6 8 6 5 11 329 724 192 74 15 30 7 3 43 50 43 35 74 76 71 70 6 7 6 5 12 432 811 365 153 14 42 8 3 37 42 36 31 70 73 69 64 6 7 6 5 Element Cdtot (ug/l) Cutot (ug/l) Nitot (ug/l) Pbtot (ug/l) Zntot (ug/l) Month Eijsden 1 0,61 1,56 0,20 0,10 5,5 8,2 3,8 2,2 4,9 8,5 3,4 1,8 7,6 10,9 4,2 2,2 66 91 36 18 2 0,38 0,86 0,30 0,10 4,6 7,4 3,8 2,3 3,3 5,7 2,9 1,7 6,2 13,0 3,7 1,6 62 81 43 23 3 0,26 0,50 0,21 0,10 4,5 7,0 3,7 2,5 3,3 6,1 2,8 1,5 4,9 9,3 3,0 1,5 42 79 31 19 4 0,34 0,48 0,20 0,10 4,3 6,2 3,8 2,5 2,7 4,5 2,3 0,8 3,3 4,9 2,7 1,7 41 69 28 18 5 0,21 0,30 0,16 0,09 4,5 8,4 3,4 2,3 3,1 5,4 2,4 1,2 3,0 5,4 2,2 1,4 29 48 24 15 6 0,29 0,30 0,13 0,08 3,9 5,9 3,1 2,1 3,6 5,9 3,0 1,4 2,9 4,9 2,4 1,0 32 47 24 13 7 0,15 0,29 0,10 0,07 3,7 6,1 3,3 1,8 3,9 5,9 3,5 1,8 3,4 6,1 2,8 1,5 29 46 26 13 8 0,25 0,30 0,10 0,04 3,5 5,0 3,1 2,2 3,6 5,3 3,1 2,1 2,6 4,5 2,4 1,2 39 44 21 13 9 0,24 0,37 0,15 0,06 4,5 6,3 3,7 2,5 5,2 7,8 4,4 2,7 3,7 5,5 2,5 1,3 31 47 24 15 10 0,22 0,43 0,20 0,08 6,0 9,1 5,6 2,7 5,2 7,6 4,5 2,5 4,8 7,8 3,1 1,2 38 59 32 16 11 0,50 1,30 0,30 0,10 5,9 10,4 4,6 2,4 4,6 7,5 3,6 1,9 6,3 14,5 3,5 1,5 47 99 34 18 12 0,59 1,30 0,31 0,10 8,2 17,6 4,9 2,8 4,6 9,1 3,6 1,9 9,6 16,0 4,8 2,1 69 129 42 20 Month Keizersveer 1 0,34 0,49 0,24 0,15 4,5 7,8 3,8 2,1 5,2 7,2 5,0 3,4 9,4 26,5 5,0 1,8 54 83 44 32 2 0,29 0,48 0,26 0,13 4,5 5,3 4,4 3,0 4,5 5,0 4,0 4,0 6,0 9,0 6,2 2,5 42 62 45 23 3 0,20 0,30 0,19 0,09 4,0 5,0 3,6 3,0 4,5 5,1 4,3 3,7 3,4 5,7 3,4 1,8 36 45 35 25 4 0,19 0,31 0,17 0,10 3,4 4,1 3,1 2,9 4,4 4,9 4,0 4,0 3,3 4,6 3,7 2,0 26 38 24 18 5 0,15 0,24 0,15 0,06 3,2 4,0 3,1 2,9 4,6 5,7 4,0 3,9 2,9 4,6 2,6 1,7 25 33 26 17 6 0,14 0,24 0,11 0,08 4,1 5,1 3,8 3,0 5,2 6,8 4,9 3,9 2,3 3,9 2,3 1,0 24 36 23 16 7 0,17 0,26 0,16 0,12 4,0 5,0 4,0 3,0 5,5 6,8 5,0 4,1 2,3 3,1 2,1 1,2 25 34 26 17 8 0,14 0,23 0,12 0,09 4,4 5,2 4,0 3,6 5,2 6,9 5,0 3,5 2,6 2,9 2,0 1,3 25 30 25 17 9 0,20 0,20 0,13 0,10 4,1 5,0 4,0 3,0 5,1 6,1 5,0 4,0 1,7 2,0 1,7 1,0 23 26 23 19 10 0,15 0,21 0,14 0,09 3,9 5,0 4,0 3,0 5,1 7,3 5,0 3,8 2,6 3,4 2,0 1,0 26 28 25 19 11 0,18 0,31 0,12 0,08 4,9 6,8 4,0 3,1 5,2 6,2 5,0 4,0 4,1 8,5 2,5 1,1 35 54 27 20 12 0,22 0,42 0,15 0,08 4,2 6,8 3,0 2,4 4,9 6,0 5,0 3,6 4,6 12,8 2,8 1,5 41 66 33 21 Element Cdspm (mg/kg) Cuspm (mg/kg) Nispm (mg/kg) Pbspm (mg/kg) Znspm (mg/kg) Month Eisden 1 7 20 7 3 112 192 92 52 63 81 54 46 158 287 145 96 968 1728 927 560 2 9 20 6 3 133 280 100 52 66 90 59 44 195 310 160 100 1089 1722 897 526 3 14 23 9 3 133 225 128 59 59 77 56 45 184 267 184 112 1244 1900 1194 614 4 15 28 11 4 148 191 146 90 58 76 54 43 200 271 201 109 1417 1878 1257 775 5 13 21 11 6 167 262 157 96 60 75 58 42 219 309 213 114 1495 2186 1330 1058 6 13 23 10 6 149 230 140 92 56 70 57 39 223 351 203 122 1376 1954 1275 927 7 10 16 9 5 161 235 146 90 52 68 50 34 206 332 185 114 1253 1766 1140 837 8 14 21 8 5 147 220 134 92 51 69 49 39 202 295 185 125 1406 1760 1220 840 9 15 22 15 7 204 281 201 125 65 87 62 50 234 312 226 166 1559 2113 1570 1007 10 16 22 15 11 281 412 279 173 74 90 73 54 277 338 280 196 1837 2303 1835 1392 11 21 32 16 5 252 395 279 74 74 95 73 52 276 423 278 116 1805 2871 1842 678 12 22 33 10 3 183 359 131 60 70 93 66 48 230 418 190 99 1415 2674 1107 564 Month Keizersveer 1 9 13 8 6 111 163 92 70 58 75 52 47 177 200 174 148 915 1088 865 766 2 8 9 8 6 113 125 100 70 57 71 53 48 165 198 167 137 905 1070 839 752 3 9 12 9 6 98 113 87 77 56 71 55 41 168 200 165 150 1006 1209 1003 810 4 7 10 8 4 84 103 77 61 46 54 48 34 140 170 143 102 851 1039 860 625 5 9 10 9 5 94 103 81 64 58 66 49 45 182 196 166 127 962 1065 985 787 6 9 11 9 8 87 99 90 70 57 70 62 41 178 205 178 151 998 1117 991 865 7 10 14 10 8 94 110 94 77 57 65 57 50 198 224 201 174 1070 1226 1058 959 8 10 14 10 7 97 119 89 79 58 66 60 46 188 224 187 158 974 1168 993 864 9 8 9 8 6 90 110 91 67 61 69 62 45 191 233 195 142 978 1120 1004 800 10 8 11 8 6 87 106 87 68 57 66 58 47 203 232 201 184 979 1073 1018 843 11 7 9 8 5 104 136 92 80 61 76 61 52 195 236 197 157 906 1000 941 798 12 8 12 8 5 106 136 105 74 61 71 61 51 189 227 189 145 972 1214 908 772
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1 000 1000 Eijsden Cdspm (mg/kg) Stevensweert Cdspm (mg/kg)
100 100
10 10
1 1 A J A O D F A J A O D F
10 00 1000 Belfeld Cdspm (mg/kg) Keizersveer Cdspm (mg/kg)
1 00 100
10 10
1 1 A J A O D F A J A O D F Figure 3 Monthly variation of Cdspm in Eijsden, Stevensweert, Belfeld and Keizersveer in the period 1993-2004 Note that on the X-axis the months are plotted according to the hydrological year in the Meuse starting in April and ending in March of the next year. The blue lines in the plots are the monthly 10 and 90 percentile lines calculated for the period 1993-2004. For comparison purposes, in the Stevensweert plots, the monthly 10 and 90 percentile lines of Eijsden are given and in the Belfeld plots those of Keizersveer (blue dashed lines).
10 10 Eijsden Cdtotal (ug/l) Stevensweert Cdtotal (ug/l)
1 1
0.1 0.1
0.01 0. 01 A J A O D F A J A O D F
10 10 Belfeld Cdtotal Keizersveer Cdtotal (ug/l) (ug/l)
1 1
0.1 0.1
0. 01 0.01 A J A O D F A J A O D F Figure 4 Monthly variation of Cdtot in Eijsden, Stevensweert, Belfeld and Keizersveer in the period 1993-2004 Note that on the X-axis the months are plotted according to the hydrological year in the Meuse starting in April and ending in March of the next year. The blue lines in the plots are the monthly 10 and 90 percentile lines calculated for the period 1993-2004.
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In Figure 3 and Figure 4, the monthly variations of Cdspm and Cdtot in the period 1993-2004 are plotted in hydrological order (April to March) for Eijsden, Stevensweert, Belfeld and Keizersveer. The variation of Cdtot and Cdspm as shown in these figures is also representative for Nitot and Nispm, Cutot and Cuspm, Pbtot and Pbspm and Zntot and Znspm.. Therefore, these elements are presented in the graphs. Because of the low monitoring frequency (see Table 1), no monthly summary statistics for the metal content of the SPM in Stevensweert and Belfeld could be calculated. The major differences of the metal content in SPM (Figure 3) before and after the impoundments in the Dutch part of the River Meuse (locations Eijsden and Keizersveer, respectively) are: x The systematic decrease of the metal content in SPM from Eijsden towards Keizersveer. x The narrow ranges for the metal contents in SPM from Belfeld and Keizersveer, compared to the large ranges in SPM from Eijsden. x The 10 percentile values for metal content in SPM from Eijsden are lower during the high discharge period (December to March, also see Table 3), compared to the 10 percentile values in the low flow period (April to November). In Keizersveer, such a trend is not visible. x The monthly variations of the metals in SPM from Stevensweert resemble those in SPM from Eijsden; the monthly variations and metal contents in SPM from Belfeld are very similar to those in SPM from Keizersveer. In Figure 4, the monthly and the spatial variation of Cdtot for the Dutch part of the Meuse are plotted. These graphs show the systematic decrease in the total Cd concentration in water from Eijsden towards Keizersveer and the higher values of Cdtot during the high flow periods. When 90 percentile values for Cdtot, are exceeded, this is regarded as a first indicator that the metal concentration is enriched by the Cd spill (see next section). The elevated concentration values can also be observed for Cutot, Pbtot and Zntot (not shown here) and are caused by the higher SPM contents during the high flow period (see Figure 2) and the affinity of Cd, Cu, Pb and Zn for the particulate phase. The contribution of the 1 dissolved phase to the Nitot concentration in the Meuse is larger than 50% (van Vliet and Zwolsman, 2008, figure 9). The contribution of the SPM fraction to the Nitot concentration during the high flow period will therefore be limited.
3.3 The 2005-2006 Cd spill in the Dutch part of the River Meuse
3.3.1 The Cd spill monitored in Eijsden, upstream of the impundments and stream components The 2005-2006 Cd spill is best documented in the data available from monitoring station Eijsden, because this station is close to Liege, from where the Cd spill probably originated and because this station has the highest monitoring frequency (weekly). In Figure 5, the variation of the total and dissolved fraction of the Cd concentration in Eijsden for the period 2002-2007 is compared with the monthly total metal 10 and 90 percentiles, calculated for the period 1993-2004 (Table 3). For the other elements (Zn, Cu, Ni, Pb), no influence of the Cd spill can be seen. Almost all total concentration values for these elements are between the 10 and 90 percentile envelopes. Cdtot is the only element that repeatedly plots above the 90 percentile line. The duration of the Cd spill is defined as the period in which the Cdtot values in Eijsden are continuously higher than the Cdtot 90 percentile values (Table 2). Figure 5 shows that - using this definition - the Cd spill lasted from the end of June 2005 to June 2006 (phase 1, 2 and 3 of the Cd spill).
1 -3 Calculated as Nidis [µg/l] = Nitot [µg/l] - Nispm [mg/kg] x 10 x SPM [mg/l]
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100 Cdtotal (ug/l) Cddis (ug/l)
10 CdtotalP90
1
0.1
CdtotalP10 0.01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06 Jan-07
Figure 5 The variation of Cdtot and Cddis in Eijsden for the period 2002-2007
The blue lines in the plots are the monthly 10 and 90 percentile lines for Cdtot calculated for the period 1993-2004.
In Figure 6, the Cdspm, Cdtot and Cddis values in Eijsden are compared with one another for the period 2004-2007. The Cddis concentrations and the Cdtot concentrations were distinctly higher during phase 1 and 2. The peak values occurred during phase 2 of the Cd spill. In the period before the Cd spill, frequently large Cddis peaks were present. These peaks th disappeared after the last Cddis peak on the 13 of June 2006 (Figure 5 and Figure 6). During these Cddis peaks also the Cdspm contents are high (Figure 6). A fast sorption process of Cd from the dissolved phase to the particulate phase is plausible, given the high organic matter concentrations in Eijsden during the summer period. The data suggest that the Cd was (mainly) discharged in the Liege surroundings, as a dissolved phase. However, we do not have more explicit data to prove this. The Zndis values were distinctly higher as well, especially during phase 2 of the Cd spill, but this increase did not cause Zntot concentrations to exceed the 90 percentile values. Similarly as observed for Cdtot, from July 2005 to June 2006, the Cdspm values are above the CdP90spm-line and reached maximum values between November 2005 and February 2006 (Figure 6). In this period the Cdspm contents exceeded the 100 mg/kg value and were between 5 to 10 times higher than the monthly CdP90spm values. In the succeeding high flow period (spring 2007) the Cdspm content in Eijsden decreased.
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Cdspm Cdspm P90 Cdtot Cdtot P90 Cddis 10000 Eijsden Discharge (m3/s)
1000 250 m3/s 100
10
1
0.1 Cd in water (ug/l) and SPM (mg/kg) SPM water (ug/l) and in Cd 1 23 4 0.01 Jan/04 Jul/04 Jan/05 Jul/05 Jan/06 Jul/06 Jan/07 Jul/07
Figure 6 Variation of Cdtot, Cddis and Cdspm concentration in Eijsden
Legend: Cdtot: black line, Cddis: red lines, Cdspm: black squares and CdP90spm: blue line with small dots. The black dashed lines indicate the period of elevated Cd concentration values in Eijsden; blue arrows indicate phases in the Cd spill (see Table 2).
In Figure 7 the discharge (m3/s) and SPM concentration (mg/l) in Eijsden are plotted for the period May 2005 to January 2007, together with the Cdspm contents in the other monitoring stations. As described above, from February to June 2006 four discharge peaks occurred. These peaks are associated with large SPM concentrations (Figure 2 and Figure 7). After these four discharge peaks the Cdspm content in Eijsden decreased, from 120-400 mg/kg before the first discharge peak towards an average of 30 mg/kg after the last one (Figure 7). During the discharge peaks, the Cdspm contents are distinctly lower than before or after the discharge peaks. For the first discharge peak the Cdspm concentration during the peak is 60 mg/kg; after the peak it is 80 to 90 mg/kg. For the other three discharge peaks the Cdspm contents during the peaks are 15 to 10 mg/kg; after the peaks the Cdspm contents are 30 mg/kg. After the fourth discharge peak the Cdspm concentration in Eijsden was high again (60 mg/l). Probably, this was caused by the final contribution of the Cd spill in the Meuse (Figure 6). Contrary to the Cdtot concentrations, the Cdspm contents remained high after the end of the Cd spill, plotting around the CdP90spm-line, until the beginning of the high flow period in December 2006.
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Eijsden (m3/s) Eijsden SPM (mg/l) Cdspm Eijsden Cdspm Bel Cdspm Keizersveer 1000
100
10
1 2 3 4 Discharge (m3/s), SPM (m g/l)and Cdspm (m g/kg) 1 May-05 Jul-05 Sep-05 Nov-05 Jan-06 Mar-06 May-06 Jul-06 Sep-06 Nov-06
Figure 7 Variation of Cdspm in Eijsden, Belfeld and Keizersveer from May 2005 to January 2007, showing the
rapid decrease in Cdspm in Eijsden and Belfeld in February and March 2006, while Cdspm in Keizersveer still shows some increase in this period. The black dashed lines indicate the period of elevated Cd concentration values in Eijsden; blue arrows indicate phases in the Cd spill (see Table 2).
3.3.2 The Cd spill in the monitoring stations downstream of Eijsden In Figure 7, the Cdspm contents in Belfeld are plotted together with the data from Eijsden and Keizersveer. Figure 8 shows the Cdspm, Cdtot and Cddis values in Keizersveer for the period 2004-2007. The Cddis concentration in Belfeld is presented in Figure 9 (data available for 2006 and 2007 only), together again with data from Eijsden and Keizersveer. Figure 7 and Figure 8 show that during phase 1 of the Cd-spill the Cd spill was not detected in Keizersveer, in both the dissolved and the particulate phase. From Figure 8 it is clear that the Cd spill arrived in Keizersveer with a delay of six months. The aftermath effects in Keizersveer, documented by the higher Cddis and Cdspm, lasted until January 2007. For the Belfeld monitoring station, only Cdtot and Cdspm data were available during that period. In this station as well, the Cd-spill was not detected during phase 1.
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Cdspm Cdspm P90 Cdtot Cdtot P90 Cddis 10000 10000 Eijsden Discharge (m3/s) Keizersveer Discharge (m3/s) 1000 1000 250 m3/s 250 m3/s 100 100
10 10 1 1 0.1
Cd in water(ug/l) and SPM (m0.1 g/kg) 1 2 3 4
Cd in water(ug/l)and SPM (mg/kg)0.01 2 Jan/04 Jul/04 Jan/05 Jul/051 Jan/063 Jul/064 Jan/07 Jul/07 0.01 Jan/04 Jul/04 Jan/05 Jul/05 Jan/06 Jul/06 Jan/07 Jul/07
Figure 8 Variation of Cdtot , Cddis and Cdspm concentration in Keizersveer
Legend: Cdtot: black line, Cddis: red lines, Cdspm: black squares and CdP90spm: blue line with small dots. The black dashed lines indicate the Cd spill duration in Keizersveer; arrows indicate phases in the Cd spill.
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100 Eijsden Cdtotal (ug/l) Cddis (ug/l)
10
1
0.1
2 3 4
0.01 Jan/06 Apr/06 Jul/06 Oct/06 Jan/07 Apr/07 Jul/07 100 Belfeld Cdtotal (ug/l) Cddis (ug/l)
10
1
0.1
2 3 4 0.01 Jan/06 Apr/06 Jul/06 Oct/06 Jan/07 Apr/07 Jul/07 100 Keizersveer Cdtotal (ug/l) Cddis (ug/l)
10
1
0.1
2 3 4 0.01 Jan/06 Apr/06 Jul/06 Oct/06 Jan/07 Apr/07 Jul/07
Figure 9 Variation of Cdtot and Cddis in Eijsden, Belfeld and Keizersveer from January 2006 to December 2007, showing that the dissolved Cd concentrations are distinctly higher in Belfeld and Keizersveer during the summer of 2006, compared to the summer of 2007. This indicates that upstream of Belfeld, Cd was transferred from the settled suspended matter (enriched in Cd as consequence of the Cd spill) towards the dissolved phase
In phase 2, from December 2005 to February 2006, in the monitoring stations Belfeld and Keizersveer Cd values were high in both the dissolved and the particulate phase, but the enrichment was distinctly higher in Belfeld than in Keizersveer (Figure 7 and Figure 9). In the succeeding high flow period in phase 3 (March to June 2006), the Cdspm content in Keizersveer increased a little, while the values in Belfeld decreased (Figure 7). From March to December 2006, the Cdspm contents in Eijsden, Stevensweert (not shown in the graphs),
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Belfeld and Keizersveer were very similar. Only during the last three discharge peaks in phase 3, the Cdspm contents in Eijsden were distinctly lower (Figure 7). During the low flow period after the Cd spill (phase 4, July-November 2006), an increase in the Cddis concentration is observed in Belfeld and Keizersveer, while in Eijsden the Cddis concentration varies around its baseline (0.05-0.1 µg/l, see Figure 9). In Belfeld and Keizersveer the Cddis concentrations also dropped to the baseline values (0.05-0.1 µg/l) after the last Cddis peak in February 2006. But contrary to Eijsden, both in Belfeld and Keizersveer the Cddis values increased to 0.4-0.5 µg/l from July to November 2006 (see Figure 9). During this period the organic matter content of the SPM was low in Belfeld en Keizersveer, enabling the Cd to be leached from the particulate phase into the dissolved phase. Keizersveer differed from Belfeld in that the Cddis increase was more slowly. In the following high flow period (starting in December 2006), the Cddis concentrations in Belfeld and Keizersveer dropped to the baseline values.
3.3.3 The Cd load of the Cd spill The average daily Cd load in the Meuse for each month in the period 1993-2004 was calculated from the Cdtot data in the stations Eijsden and Keizersveer. The average annual Cd load in the period 1993-2004 in Eijsden was 4260 kg.yr-1 and in Keizersveer 2490 kg.yr-1. For Eijsden this estimate is reasonably accurate (weekly analysis frequency) but for Keizersveer this is only a rough estimate (monthly analyses). These data suggest that about 40% of the Cdtot load passing Eijsden is retained in the river between Eijsden and Keizersveer (assuming that no additional Cd is emitted into this river stretch by tributaries or other Cd sources). In Figure 10, the daily Cd loads in Eijsden and Keizersveer during the spill are compared with the monthly averages of the daily Cd loads in the period 1993-2004. Figure 10 shows that the main part of the Cd load during the spill passed Eijsden in phase 2 and 3 (note the logarithmic scale). During the Cd spill, from July 2005 to June 2006, the extra Cd load in Eijsden amounted to 10480 kg, about 2.5 times the annual average. In phase 2 the extra load was 4000 kg, mainly in the dissolved phase; in phase 3 it was 6400 kg, mainly in the particulate phase.
1000 1000 Eijsden Cd load (kg/day) Keizersveer load (kg/day)
100 100 Cd LoadP90 Cd Load P90
10 10
1 1
Cd Load P10 Cd LoadP10 1 2 3 4 1 2 3 4 0.1 0.1 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-07 Jul-07 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-07 Jul-07 Figure 10 The daily total Cd load in Eijsden and Keizersveer, during the period 2004-2007, compared to the monthly average of 90 and 10 percentile daily loads (black lines), calculated for the period 1993-2004
For Keizersveer, it is not possible to calculate the extra Cd load during the spill, because of the low analysis frequency (Table 1, Figure 10). It is clear that most of the Cd is transferred in the particulate phase and that the Cdspm contents are more or less constant in Keizersveer (Figure 3 and Figure 8). However, the SPM concentrations during the discharge peaks in phase 3 are not known, rendering it impossible to estimate the Cd load transferred in the particulate phase.
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3.4 Extent of SPM contamination before and during the Cd spill, a comparison with environmental standards In Table 4, the monthly median metal concentrations found in SPM for the period 1993-2004 and during the Cd spill are compared with natural background values measured in a pond in the floodplain of the Meuse (NBM, Bakker, 2006), with the target values for SPM in the Meuse (TM, Actief Bodem Beheer Maas, 2003) and with the Dutch BBK-B (upper limit of class B) values (BBK, VROM, 2009). For this comparison the monthly median Mespm values given in Table 3 are used, because these are less affected by extreme values. The evaluation, based on metal enrichment factors (EF) and metal load indices (MLI) is adapted from Pease et al. (2007) and is described below.
Table 4 Metal Load Indices calculated for SPM in the period1993-2004, for SPM during phase 1 and 2 of the Cd spill and for SPM during phase 3 and 4 of the Cd spill, using the Natural Background values measured in the Meuse (NBM, Bakker, 2006), Target Values Meuse (TM, Actief Bodem Beheer Maas, 2003) and the Dutch BBK-B values (BBK,VROM, 2001)
MLIm (mg/kg) MLIt (mg/kg) Monitoring Standard Cd Cu Ni Pb Zn Total metals Station (mg/kg) Median values SPM 1993-2004, related to NBM Eijsden 17.0 5.1 1.2 3.6 6.3 4.7 Keizersveer 14.3 3.0 1.1 3.3 4.8 3.8 Median values SPM 1993-2004, related to TM Eijsden 3.0 2.9 2.3 1.4 2.3 2.3 Keizersveer 3.0 2.5 1.7 2.2 1.2 1.8 Median values SPM 1993-2004, related to BBK-B Eijsden 0.7 0.8 0.3 0.3 0.6 0.5 Keizersveer 0.6 0.5 0.3 0.3 0.5 0.4 Values SPM related to BKB-B during Cd spill phase 1 and 2 Eijsden 7.6 1.0 0.3 0.5 0.7 1.0 Keizersveer 0.9 0.7 0.3 0.3 0.5 0.5 Values SPM related to BKB-B during Cd spill phase 3 and 4 Eijsden 2.0 0.7 0.3 0.3 0.5 0.6 Keizersveer 1.9 0.5 0.3 0.3 0.5 0.5
Standard Cd Cu Ni Pb Zn (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) NBM 0.6 30 50 55 200 TVM 3.4 53 26 145 543 BBK-B 14 190 210 580 2000
First, for each data point (sample), the EF values (cf. Lawson and Winchester, 1979; Cabrera et al., 1999) are calculated for the three sets of reference values given in Table 4. An EF is the ratio between sample concentration (Cs) and a reference or background value (Cr), which takes the form EF= Cs/Cr Next, metal MLI values (MLIm) are calculated for each metal (m) as the geometrical mean of all samples measured in a specific period of time (see Table 4). The MLIm values for individual metal m represent a mean EF value for this metal and characterize the overall
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impact of pollution in the Meuse during the period 1993-2004 or during the Cd spill for each individual metal: 1/n MLIm = (EFm1 * EFm2 * EFm3 * …… EFmn ) where n equals the number of samples in the period of time considered. The total MLI (MLIt) represents an overall indication of the contamination level of SPM during the period 1993-2004 and the Cd spill. It is the geometrical mean for the five metals given in Table 4 and is calculated as follows (Tomlinson et al., 1980): 1/5 MLIt = (MLICd*MLICu*MLINi*MLIPb*MLIZn) MLI values greater than 1 indicate an overall elevation, when they are compared with the NBM and TM values, or severe contamination, when they are compared to the BKB-B values. The MLIt values for SPM in the period 1993-2004 in Eijsden and Keizersveer relative to the NBM values are 4.7 and 3.8 and relative to the TM values they are 2.3 and 1.8. This indicates that the SPM in the period 1993 and 2004 had an elevated heavy metal content. The elevated values compared to the TM value indicate that still a large effort is needed to lower the heavy metal content of the SPM in the Meuse. This is needed to minimize contamination of the newly formed floodplains, which are under construction to mitigate the effects of future major flood events that are expected to occur more frequently due to climate change. Table 4 show that the high MLIt values relative to the TM values are the consequence of the elevated values of all five metals. The MLIt values for SPM in the period 1993-2004 in Eijsden and Keizersveer relative to the BKB-B values are 0.5 and 0.4, indicating that the overall metal content of SPM is below the BKB-B values and does not call for remediation (Table 4). The MLIt values of SPM relative to the BKB-B values during the phases 1 and 2 of the Cd spill are about 1 in Eijsden and 0.5 in Keizersveer. The higher MLIt values in Eijsden compared to the 1993-2004 value is largely caused by the much higher contribution of Cd (MLICd = 7.6,), which was also concluded based on the excess values of Cdtot during the Cd spill (Figure 4). During the phases 3 and 4 of the Cd spill both the overall MLIt and the individual MLIm values of Eijsden and Keizersveer are similar, showing the efficiency of the SPM mixing process in the Meuse. The MLIt values of the SPM in the phases 3 and 4 are below 1 and now only Cd is above the BKB-B value. The MLICd value during the active Cd spill exceeds the BKB-B value in Eijsden by a factor 7.6. This severe contamination in Cd occurs not only in the particulate phase, but also in the dissolved phase. During the Cd spill, the Cddis concentrations in Eijsden and Belfeld regularly exceeded the EC directive concerning the quality required for surface water used for the abstraction of drinking water (5 µg/l, see Figure 6and Figure 9)
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4 Discussion
4.1 Signaficance of the Cd spill in understand river system processes and river basin management effects In the first phase of the Cd spill, from June to December 2005 (see Table 2), no transfer of Cd from Eijsden towards Belfeld and Keizersveer was observed, neither in the dissolved phase, nor in the SPM (see Figure 5, Figure 6 and Figure 8). Table 3 and Figure 3 show that especially during the low flow period, there is significantly more variation in metal content of the SPM in Eijsden compared to Belfeld and Keizersveer. This indicates that during low flow conditions the transport of SPM is limited and the SPM settles down between Eijsden and Belfeld. The reason for the absence of the Cddis signal in the Keizersveer station is that the amount of water with an Eijsden signature is too low in Keizersveer to enrich the dissolved phase in Cd (as discussed in section 3.1). During phase 2, the peak of the Cd spill occurred and the average discharge in Eijsden was more than three times higher than in stage 1. As discussed in section 3.1, under these conditions the water in Keizersveer is predominantly water with an Eijsden geochemical signature. Because of this predominance of Eijsden water in phase 2, high Cddis values were found in Belfeld and Keizersveer (Figure 8, Figure 9). The absence of extremely high Cddis concentrations in Keizersveer is most likely due to the low monitoring frequency (see Table 1). The only Cdspm data available for Belfeld in phase 2 of the Cd spill (Figure 7), are elevated (78 mg/kg), indicating that Cd enriched SPM is transferred through the Meuse. The discharge in phase 2 is not constant, but shows five relatively small discharge peaks (Figure 9). During these discharge peaks, transfer of SPM from Eijsden towards the downstream river sections (ending up in Keizersveer) occurred. As a result of these discharge events, the SPM will settle in front of the weirs and sluices. In Keizersveer the Cdspm contents are elevated in phase 2, but the contents are three to four times lower than in Belfeld. These distinctly lower Cd contents suggest that between Belfeld and Keizersveer mixing with less contaminated SPM occurred. The consequence of the impoundments and stream components downstream of Eijsden is that the river continuity is disrupted in periods with low river discharges. In these periods, the water quality in Belfeld and Keizersveer is determined by the water quality of the inflowing tributaries and by the groundwater quality downstream of Eijsden and not by the undivided Meuse entering the Netherlands in Eijsden. During periods with small discharge peaks, e.g. in phase 2 of the Cd spill, the disruption of the river continuity is switched on and off. In a study of the effects of extreme droughts in the Meuse in 1976 and 2003 (van Vliet and Zwolsman, 2008), it was shown that during the extreme drought of 2003 the water quality was much better in Keizersveer than in Eijsden. This better water quality was explained by the higher discharge in Keizersveer compared to Eijsden and similarly as in the first phase of the Cd spill, this was caused by inflowing tributaries and groundwater downstream of Eijsden. This situation results in a much better water quality, not only in terms of heavy metals concentrations, but also in terms of eutrophication (Van Vliet and Zwolsman, 2008). The monitoring data available from www.waterbase.nl confirm that the water quality is always better in Keizersveer compared to Eijsden during periods with low flows, especially with respect to eutrophication. Therefore, impoundments and stream components can have positive effects, not only for the Meuse case, but more in general, as stated by Bosch et al. (2007): “Impoundments placed near river mouths or in N and P source areas were most effective at reducing (contaminant) export”.
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4.2 Fate of the Cd-enriched SPM in the Meuse Impoundments are often associated with the retention of sediments (e.g. Vorosmarty et al., 2003; Meybeck, 2002, Klaver et al., 2007). The impact of the impoundments (weirs and sluices) in the Dutch Meuse, depends on the season: retention of SPM in the summer and autumn (low flow period, discharges < 250 m3/ s) and remobilization and transfer of SPM in the winter and spring (high flow period, discharges > 250 m3/s). The compositional differences and similarities of SPM between Eijsden and Keizersveer in the summer and autumn and the compositional similarities in winter and spring are caused by this temporal impact (Table 3, Figure 3). This temporal variation is even better illustrated by the variation in the Cdspm content in Eijsden, Belfeld and Keizersveer during the Cd spill. During the first phase, Cd-enriched SPM was observed in Eijsden, but not in Belfeld and Keizersveer (Figure 7), indicating that in this phase the SPM settled on the river bed between Eijsden and Belfeld. In phase 3, the Cdspm content in Eijsden decreased from over 100 mg/kg in phase 2 to 30 mg/kg in phase 4. This decrease was stepwise: After each discharge peak the Cdspm in Eijsden was lower than after the previous one. The Cdspm content decreased from a minimum value of 60 mg/kg during the first discharge peak to a minimum value of 10 mg/kg during the fourth peak. At the falling limb of each discharge peak the Cdspm content was higher than at the maximum of the discharge peak. The high Cdspm content in the first discharge peak can only be explained by erosion of Cd-enriched sediments from the river bed. The limited set of data available for the monitoring stations downstream of Eijsden shows that in phases 3 and 4, the Cdspm contents are very similar in all four monitoring stations (Figure 7). This indicates that the amount of SPM originating between Eijsden and Keizersveer was only minor and that most of the SPM originated upstream of Eijsden. Figure 7 finally shows that with the high discharge peaks occurring after phase 4 (one and a half year after the start of the Cd spill) the SPM in the Meuse is not enriched in Cd anymore. The SPM enriched in Cd must be deposited on the riverbed downstream of Keizersveer or transported further to the sea, as the discharge peaks were not high enough to cause flooding of the floodplains.
4.3 Remobilization of sediment-bound Cd towards the dissolved phase There is general consensus that sediment-bound substances are of major importance for water quality, as well as for the fate and effects of trace contaminants in aquatic systems (Wöltz et al., 2009). Sediments can act as sinks for various pollutants, but can also become a contaminant source, as a consequence of remobilization of particulates, or leaching of contaminants to surface water (Heise and Foerstner, 2006, Hollert et al., 2007, Foerstner and Salomons, 2008, Schipper et al., 2009). Up to this moment, the focus is on the effect of sediment remobilization during strong floods. However, the variation of the Cddis and Cdtot concentrations during the period 2006- 2007 in Belfeld and Keizersveer (Figure 9) indicates that Cd was released from the river bank sediments during the low flow periods and remained in the dissolved phase. Figure 9 shows that in the period from July to November 2006, Cddis became a significant carrier of the cadmium. In the high discharge period starting in December 2006), cadmium was carried mainly in the SPM again. The larger Cddis contents in 2006 compared to 2007 can only be caused by river bank sediments enriched in Cd as a consequence of the Cd spill. The increase in the Cddis, Cudis and Nidis concentrations during the low flow periods in 2006 and 2007 indicates that the release of metals from the river bank sediments in the Dutch Meuse is a common process during low flow periods.
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5 Conclusions and recommendations
In this paper, the influence of impoundments and stream components on the metal content in water and SPM in the Dutch part of the River Meuse was assessed using the variation in the cadmium content of the particulate and dissolved phase during the transport of a distinct cadmium spill through the river. The spill must have originated from the Liege region, but the exact source has not been made public. The monitoring data indicate that cadmium was discharged as a dissolved phase. Only cadmium was significantly enriched by the spill. It takes about six months before the cadmium spill is detected in the SPM at the monitoring station of Keizersveer, near the mouth of the river. The amount of water with an Eijsden signature in Keizersveer is limited during low flow conditions, because of a redistribution of water towards canals (viz. the Zuid-Willemsvaart). Leaching of cadmium from the sediment to the surface water may result in high dissolved cadmium concentrations. During low flow conditions the dissolved phase is a significant transport path of cadmium.
The transport paths of the cadmium through the impounded part of the river were determined based on the monitoring results obtained in the monitoring stations between and downstream of the impoundments. Interpretation of these monitoring results was hampered as a consequence of the low analysis frequencies at these stations. For a better understanding of the processes in the river system it is essential that the SPM monitoring frequency in river sections downstream of Eijsden is increased, starting with the Keizersveer station. A better system understanding is important in order to define appropriate measures to meet the goals of the Water Framework Directive concerning the chemical status of the Meuse river.
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6 References
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Wöltz, J, Cofalla, C., Hudjetz, S., Roger, S., Brinkmann, M., Schmidt, B., Schäffer, A, Kaufmann, U., Lennartz, G., Hecker, M, Schüttrumpf, H., Hollert, H. (2009) In search for the ecological and toxicological relevance of sediment remobilization and transport during flood events. J. Soils Sediments, 3, pp. 37-42
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