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Spatial and temporal variability of sediment deposition on artificial-lawn traps in a floodplain of the River Elbe
Environmental Pollution 148 (2007) 770e778 www.elsevier.com/locate/envpol
Spatial and temporal variability of sediment deposition on artificial-lawn traps in a floodplain of the River Elbe M. Baborowski a,*, O. Bu¨ttner b, P. Morgenstern c, F. Kru¨ger d, I. Lobe a, H. Rupp e, W. v. Tu¨mpling a a
Department of River Ecology, Helmholtz Centre for Environmental Research e UFZ, Bru¨ckstrasse 3a, 39114 Magdeburg, Germany Department of Lake Research, Helmholtz Centre for Environmental Research e UFZ, Bru¨ckstrasse 3a, 39114 Magdeburg, Germany c Department of Analytical Chemistry, Helmholtz Centre for Environmental Research e UFZ, Permoserstrasse 15, 04318 Leipzig, Germany d ELANA Boden Wasser Monitoring, Dorfstrasse 55, 39615 Falkenberg, Germany e Department of Soil Physics, Helmholtz Centre for Environmental Research e UFZ, Dorfstrasse 55, 39615 Falkenberg, Germany b
Received 29 January 2007; accepted 31 January 2007
The deposition of polluted sediments on floodplains is characterised by a high local variability. Abstract Artificial-lawn mats were used as sediment traps in floodplains to measure sediment input and composition during flood events. To estimate the natural variability, 10 traps were installed during two flood waves at three different morphological units in a meander loop of the River Elbe. The geochemical composition of deposited and suspended matter was compared. The sediment input showed weak correlations with concentration and composition of river water. It also correlated poorly with flood duration and level as well as distance of trap position from the main river. This is due to the high variability of the inundation, different morphological conditions and the variability of sources. The composition of the deposits and the suspended matter in the river water was comparable. Hence, for the investigated river reach, the expected pollution of the floodplain sediments can be derived from the pollution of the suspended matter in the river during the flood wave. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Trace elements; Floodplain; Sediment input; Spatial distribution; Measurement strategy
1. Introduction Sediments are the long-term memory of rivers and their catchments. Despite a significant improvement of the pollution situation in the River Elbe for more than 15 years (Guhr et al., 2006), sediment contamination with heavy metals and organic micro-pollutants still exists in the river basin (Heise et al., 2005). During floods as well as low-water periods, the sediments act as secondary sources of pollution for the river (Baborowski et al., 2004a,b; Stachel et al., 2004; Umlauf et al., 2005). To understand the transport and fate of pollutants
* Corresponding author. Tel.: þ49 391 8109630; fax: þ49 391 8109150. E-mail address: [email protected] (M. Baborowski). 0269-7491/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2007.01.032
in a river, as well as to assess the pollution situation in the whole river basin both the sources and the sinks of the pollutants must be considered. Groyne fields and extensive floodplains are the characteristic morphological features of the middle part of the River Elbe acting as source of and sink for sediments (Kozerski et al., 2006; Schwartz, 2006; Kru¨ger et al., 2005). Similar conditions were described also for other rivers (Zerling et al., 2006; Costa et al., 2006; Walling and Owens, 2003; Middelkoop, 2000; Engelhardt et al., 1999). Summary and event-related approaches are available to calculate sedimentation rates of floodplain soils (Kru¨ger et al., 2006). Such approaches include the comparison of surface elevations, the quantification of the sedimentation with the aid of anthropogenic and geogenic tracers, the application of sediment traps as well as the calculation of solid-matter load
M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
balances. Within these methods, artificial-lawn mats, which function as sediment traps, are an appropriate tool to measure short-term sedimentation processes that occur during flooding events. In this study, the variability of mass input was investigated by means of traps that were arranged in a meander loop of the Middle Elbe (Bu¨ttner et al., 2006). Investigations were performed during two waves of a spring flood in February and March 2005. The study should contribute to improve the understanding of sediment transport processes (including erosion and sedimentation) at the river basin-scale, which is consistent with the recommendations of the European Sediment Research Network (Report SedNet, 2005). The objective of the study was to investigate the variability of the flood-induced input of suspended matter into the floodplain in respect to its amount and composition. The following factors were under consideration: duration of the inundation, ground-level elevation, distance to the river, morphological structures, and pollution of the suspended matter in the river water. The results should improve the reliability of assessments of local pollution and clarify the uncertainty of input data for the modelling of pollutant and suspended-matter transport. 2. Material and methods 2.1. Study site Investigations were carried out in the floodplain Scho¨nberg Deich (Fig. 1). The floodplain is located in the middle reach of the River Elbe upstream of the city of Wittenberge on the left bank between river-kilometres 435 and 440 (German mileage). It covers an area of 2 km2. The site is representative for the lower part of the Middle Elbe.
2.2. Methodology To estimate the natural variability of the sediment input, artificial-lawn mats were placed at different morphological units (10 positions, 5 replicates), in a sub-area of the floodplain of about 0.2 km2 (Fig. 2). The sampling points covered the typical spectrum in riparian landscapes such as old arms, depressions without drainage, and plateaus. The traps were placed immediately before the time when the sample points became flooded, and they were recovered when the water level had receded. The traps (pieces of synthetic turf) simulated the natural soil cover such as pasture or mown grassland. Similar methods had been used by Asselmann and Middelkoop (1995) to determine sediment loads in the floodplains of the River Rhine, and by Kronvang et al. (2002) to measure sediment deposition during overbank flooding of the Gjern River. The investigations in the floodplain were completed by daily analyses of water samples during the flood and by analyses on a weekly basis between the two flood waves. The water samples were taken at the sampling site Wittenberge (river-km 454, left bank) under consideration of a local discharge threshold as the starting point for the measuring campaign. The local discharge threshold value has to be known to detect the beginning of the remobilisation of sediment in front of the flood crest (Baborowski et al., 2005). For Wittenberge, this threshold was determined at 1080 m3 s1. At this critical value, sediments that had deposited at times of low flow in the groyne field and had not yet consolidated become re-suspended and are transported into the main stream. At the same time, the river water began to flow into the floodplain (Fig. 1, right, below). Additionally, special SPM samples were collected upstream of the floodplain in perpendicular to the river flow and in the floodplain itself by boat during the flood crest (2005-02-22). Fig. 2 shows the position of the traps in the floodplain. The Traps No. 1, 2, 4, 7, 8, 9, 10 were collected from the floodplain when the water receded after
771
the passages of the two flood waves. Traps No. 3, 5 and 6 were situated in a flood channel and could therefore not be recovered before the end of the second event. Consequently, they were exposed to both flood waves (Fig. 4). During both waves, the suspended-particulate-matter (SPM) concentration and composition (inorganic and organic contents, trace metals) were measured in the water samples and in the deposits on the traps. The trap samples were collected and prepared as described in Friese et al. (2000). After freeze-drying, the collected sediments were analysed by means of energy dispersive X-ray fluorescence (EDXRF) for their contents of heavy metals, arsenic (As) and silica (Si). The total organic carbon of the sediments was analysed in triplicate with a C(H)NS analyser (Elementar vario EL) after drying of the samples at 105 C. The Si/Al ratio was used as a measure of the ratio of sand/silt and clay according to Hintze (1985). Consequently, the higher the sand content of a sample, the higher the Si/Al ratio, or the higher the fine grain content, the lower the Si/Al ratio. In the water samples, the dry weight of the SPM was measured according to the German Industrial Standard (DIN 38409 part H2, filtration onto Whatman GF/F glass fibre filter). Heavy metals and As were analysed in filtered (<0.45 mm PVDF Millipore syringe filter) and unfiltered samples. The filtered samples were acidified with HNO3, the unfiltered ones digested with HNO3/H2O2 in a microwave equipment. Aluminium (Al), iron (Fe), manganese (Mn) and zinc (Zn) were determined by optical emission spectrometry with inductively coupled plasma (ICP-OES), while arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), titanium (Ti), uranium (U), and zinc (Zn) were measured using mass spectrometry, also with inductively coupled plasma (ICPMS). Mercury (Hg) was analysed only as total content by atomic absorption spectrometry (AAS), cold-vapour technique. Si was analyzed according to Zwirnmann et al. (1999), a modified method of Krausse et al. (1983). The digestion method focuses on the biogenic part of silica in the water. Particulate element concentrations of the river water were calculated from the difference between the total and the dissolved fraction of the elements. The results were related to the dry substance of SPM. In the case of Hg and Cd, only data of the total fraction were available. The uncertainty of the trace-metal analyses and of the measurement of the particulate organic carbon (POC) in the water samples was below 10%. An overview of the analytical uncertainty for the sediment analysis is given in Table 2. Moreover, the relative maximal method error was estimated (Sachs, 1995) for the elements on the traps (35%).
3. Results The discharge curve and the variation of the suspendedmatter concentration during the flood wave are shown in Fig. 3. The maximum suspended-matter concentration was measured before the flood wave reached its crest: 4 days ahead in the first wave and 5 days in the second. The discharge threshold was exceeded for 16 days during the first wave and for 28 days during the second wave. The results of the SPM measurements upstream of the floodplain in cross direction of the river (24.4e27.3 mg L1) were comparable with the result obtained for the sampling site Wittenberge (31.1 mg L1) (Fig. 3). Consequently, the SPM concentrations measured in Wittenberge can be used as a good approach for SPM concentration of inflowing water from the river into the floodplain. As expected, the SPM concentrations decreased in direction from the main stream into the floodplain. The measured concentrations in the floodplain ranged between 12.4 and 17.2 mg L1. The variation of the trace-metal concentration of the river SPM is given in Table 1. Table 2 contains comparable values of floodplain sediments. Furthermore, values of previous studies
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M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
Fig. 1. Catchment of the River Elbe (left) with the Floodplain Scho¨nberg Deich at normal discharge (right, top) and at the beginning of the inundation (1080 m3 s1, right, bottom). The circles mark the locations, where the water begins to flow into the floodplain.
as well as information of background values, precaution values and action values are listed in Table 2. Findings from the Traps No. 3, 5 and 6 are not included in the presentations in Tables 1 and 2 and Fig. 5, because these traps were exposed to both waves. The suspended-matter input of the individual traps undergoes strong fluctuations, from trap to trap as well as from flood wave to flood wave. In general, the input of sediment was higher during the second wave (114.5e401.5 g m2, median value: 192.8 g m2) than during the first wave (9.2e 178.3 g m2, median value: 72.3 g m2). The concentrations of the suspended matter in the river water were comparable (Fig. 3). The input variation in dependence on the ground-level elevations of the trap positions is shown in Fig. 4 (above). In contrast to the high variability of the sediment inputs, the grain size of the deposited matter, characterised by the Si/Al ratio, was comparable (Fig. 4, lower section). The median values of the Si/Al ratio were 3.9 for the first and 3.7 for the second wave. The calculated ratios are characteristic for sediments with at least 60% fine grained particles, dominated by the silt fraction. The suspended-matter input, the concentrations of the trace elements and organic carbon in the sediments showed wide variations between the traps, the waves, and the elements. Here, the following differentiation is summarised: (i) less fluctuating concentrations between the traps and the waves for Cr and the geogenic elements Ti, Al, Fe, Si;
(ii) maximum concentration at Trap No. 8 for Mn, Zn, Pb and S; (iii) generally slightly increased Pb, Cu and C concentrations during the first wave; and (iv) generally slightly increased As concentrations during the second wave. An example for each of these groups is given in Fig. 5. 4. Discussion 4.1. Flood-dependent mass transport in the river The higher input of sediment into the floodplain with the second wave (Fig. 4, top) is probably due to the fact that the discharge threshold was exceeded 12 days longer than during the first wave. Furthermore, the assessment of the discharge hydrographs of the upper Elbe stretches and of the main tributaries Mulde and Saale (data not shown) suggests that a main part of the second wave originated in another part of the river basin than the first wave. More detailed discussion of the composition and transport behaviour of pollutants in the Middle Elbe between Magdeburg and Wittenberge during the flood 2005 underlines these findings (Baborowski et al., in press). As shown in Table 2, the organic-carbon content was slightly increased during the first wave compared to the second one. Results of previous studies of the SPM transport in the River Elbe 2002 (Baborowski et al., 2004b) showed that organic carbon can enhance the tendency of particles to remain suspended. From this perspective another possible explanation
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Fig. 2. Floodplain Scho¨nberg Deich (air view at low flow, top) and a detail of the Digital Elevation Model (DEM) overlaid with calculated currents (after Bu¨ttner et al., 2006) and the positions of the traps (bottom).
is that the settling rate of the SPM in the second wave was different (higher) due to their different material composition. DS
Q
Qs
2500
4.2. Variability of sediment deposition
80 60
1500 40 1000 20
500
DS (mg L-1)
Q (m3 s-1)
2000
0
20 0
05
-0 2
-1 5 502 -2 20 0 05 -0 225 20 05 -0 302 20 05 -0 307 20 05 -0 312 20 05 -0 317 20 05 -0 322 20 05 -0 327
0
20
With the high variability of flooding conditions, different morphological forms and the variability of sources, the sediment inputs were only weakly correlated to the investigated parameters. According to Fig. 4 (top), no correlation can be identified between the ground-level elevation and sediment input. Although Trap No. 1 and Trap No. 7 were situated at the same elevation, the input at Trap No. 1 is more than twofold higher than that of Trap No. 7. This higher input could be explained by the shorter distance to the main river channel. However, by comparing the distances of the traps from the main river (Fig. 2) with the inputs (Fig. 4, top), no general relation can be derived. For instance, Trap No. 5 was closer to the Elbe than Trap No. 3. Both traps were situated at the same height
Sampling day Fig. 3. Graph of the discharge (Q) with the value of the local discharge threshold (QS) and the dry substance (DS) of the suspended-particulate-matter at the sampling site Wittenberge, left bank.
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Table 1 Element concentrations of river SPM during both flood waves Element
Unit 1
Fe
g kg
Al
g kg1
Ti
g kg1
Si
g kg1
Mn
g kg1
As
mg kg1
Cda
mg kg1
Cr
mg kg1
Cu
mg kg1
Hg
mg kg1
Ni
mg kg1
Pb
mg kg1
Zn
mg kg1
Wave
Range
Median
1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd
30e49 29e41 28e52 30e48 1.8e3.2 1.4e2.4 5e26 12e27 1.7e3.0 1.4e2.4 32e46 23e49 5e8 4e8 79e145 67e96 91e208 78e147 1e2 1e3 58e87 41e67 148e202 114e205 402e814 438e774
37 38 37 37 2.4 2 17 19 2.1 1.8 40 43 e e 108 81 125 97 2 2 70 50 165 197 648 678
a Median values for Cd were not calculated, due to the fact that with increasing discharge the Cd concentrations in the river water decreased to the detection limit of the method (0.2 mg L1).
level and were flooded for the same period. However, the input in Trap No. 3 is almost twice of that of Trap No. 5. Otherwise, the input in Trap No. 6 (positioned in a depression, inputs from both waves) is comparable to the input e summarized over both waves e of Trap No. 9 (upland position on a plateau). Obviously, the complex flow regime in the considered floodplain is the dominating factor for the sediment inputs and their spatial distribution and thus causes the high local variability of sediment deposition. 4.3. Variability of sediment composition In contrast to the sediment input, the variability of the Al/Si ratio is more similar (Fig. 4, bottom). Due to the different analytical procedures involved, the pollution data for suspended matter (Table 1) and deposited matter (Table 2) are hardly comparable. On the one hand, the microwave assisted digestion of the water samples with HNO3/H2O2 leads to an underestimation of the soil reference compounds Al and Ti. On the other hand, the used analytical procedures for Si are not comparable for suspended matter and deposited matter. In the water samples, the Si concentrations comprise only the biogenic fraction, while in the deposited matter samples they cover the biogenic as well as the mineral fractions. However, the pollution data of the river SPM and floodplain sediments are comparable for the other investigated elements with the exception of Zn, Hg and Cd. Hence, the dominating
influence of the event-related composition of the river SPM on the composition of the floodplain sediments is evident. Therefore, the results given in Table 1 offer the possibility to estimate the pollution of the floodplain sediments to be expected from the pollution of the flood-induced river SPM. The difference between the Zn concentrations of the floodinduced river SPM and the floodplain sediments corresponds to the proportion of dissolved Zn, which amounts to 50% of the total Zn. Comprehensive investigations of the transport between Magdeburg and Wittenberge during the flood 2005 (Baborowski et al., in press) revealed a group behaviour of the elements under consideration. Thereby, the trapping of the clay fraction by sedimentation was evident and much higher than that observed with the other elements (Cr, Pb, Ni) considered. In contrast to Cr, Pb and Ni, the behaviour of Zn agreed with that of the clay reference compounds (Fe, Al, Mn), which were transported more or less in particulate form. Since Zn can be easily adsorbed by mineral and organic components (besides Fe/Mn co-precipitation) the accumulation in the floodplain sediments occurred at the later stage of the flood. This could explain the higher Zn content in the deposits compared with the river SPM. The reason for the differences between the Hg and Cd concentration of suspended and deposited matter is methodical. For Hg the poor measurement uncertainty of the analytical method applied to the deposited matter (35%, see Table 2) has to be considered. In the case of Cd with increasing discharge the concentrations in the river water decreased to the detection limit of the method (0.2 mg L1). During the first wave 40%, and during the second wave 60% of the measured concentrations were below the detection limit. The differentiation within the heavy-metal and As concentrations in the floodplain sediments, as presented in the results (Fig. 5), can be explained as follows: (i) The little-fluctuating concentrations of Ti, Al, Fe and Si result from the geogenic origin of these elements. Cr also showed little fluctuation because the recent concentrations of Cr in sediments and soils were in the range of background values (Table 2). (ii) Extremes of deposition and in the composition of the sediments at single sites, as shown for Trap No. 8, indicate local effects of other contamination sources, e.g. locally relocated old sediments of the nearby old river arms. (iii) As discussed before, the influences of the tributaries to the River Elbe were different during the two investigated flood waves. Imports from the tributary Saale were more important in the first wave, while entries from the tributary Mulde were more significant in the second wave (Baborowski et al., in press). In this context, the slightly higher concentrations of Cu and Pb in the deposits during the first wave can be explained by a higher flood-induced Pb and Cu pollution of the Saale River, compared with the Mulde River. Among other elements Cu and Pb reflect impacts of former mining industries, for example copper-shale mining (Schreck et al., 2004). Regarding Pb, the measurements of the
M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
775
Table 2 Composition of the trapped sediments in comparison with previous results, geochemical background values, precaution values and action values of German Federal Soil Protection and Contaminated Site Ordinance Parameter Unit Fe
g kg1
Al
g kg1
Ti
g kg1
Si
g kg1
Mn
g kg1
C
g kg1
N
g kg1
S
g kg1
As
mg kg1
Cd
mg kg1
Cr
mg kg1
Cu
mg kg1
Hg
mg kg1
Ni
mg kg1
Pb
mg kg1
Zn
mg kg1
a b c
Wave Range
Median Analytical Highest median Background Precaution value Action value uncertainty (%) 1997e2003a (mg kg1) valueb (mg kg1) loamy soilsc (mg kg1) grasslandc (mg kg1)
1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd
40 41 60 61 4.2 4.2 229 229 1.9 2.4 111 88.5 9.9 9.3 2.8 2.6 49 61 7 8 114 118 128 117 4 4 52 58 188 152 1015 1070
31e47 35e42 41e73 55e62 3.0e4.8 4.0e4.3 184e256 214e252 0.5e2.6 1.8e4.6 70.5e134 80e115 6.7e14.1 8.2e12 2.2e4.7 2.5e3.3 26e56 47e63 5e8 7e9 83e128 106e123 104e173 96e128 3e6 4e5 42e67 49e84 158e1138 141e163 700e3350 943e1182
0.5
e
e
e
e
0.5
e
e
e
e
0.7
e
e
e
e
0.6
e
e
-
e
1.0
e
e
e
e
10
e
e
e
e
10
e
e
e
e
10
e
e
e
e
2.5
68 (2002/2003)
19
e
50
15
8.4 (2002/2003)
0.2
1
20
6
210 (2002/2003)
94
60
e
2.0
144 (2002/2003)
24
40
1300/200a (for sheep)
35
4.9 (1997)
0.2
0.5
2
4.5
112 (2003)
41
50
1900
2.0
153 (2003)
22
70
1200
1.5
1149 (1997)
103
150
e
Kru¨ger et al. (2005). Kru¨ger et al. (1999). Federal Soil Protection and Contaminated Site Ordinance (Germany), BbodSchV.
SPM contamination contradict the sediment studies (Heise et al., 2005), which showed increased Pb concentrations in sediments of the Mulde River against those of the Saale River. However, the findings of the flood investigations 2005 are supported by studies made during a spring flood in the Elbe River 2006 (data not shown). Upstream of the mouth of the River Saale, the event-related maximum concentrations (total values) of Pb were up to two times higher, those of Cu up to three times higher than those measured upstream of the inflow of the River Mulde. (iv) The slightly increased As concentrations in the deposits of the second wave can be explained by the dominating influence of flood-induced imports from the Mulde River in the second flood wave, against those from the Saale River. In the catchment of the Elbe River, the As concentrations had the highest values in the Mulde River both in suspended matter and sediments (Heise et al., 2005). Although mining and other industrial activities had been brought to a close in some cases already decades ago, studies in the heavily polluted
Mulde basin showed that during flood events there is a permanent supply from point and non-point sources into the river system (Klemm et al., 2005). During the spring flood 2006 the maximum As concentrations (total values) upstream of the mouth of the Mulde River were two times higher than the concentrations measured upstream of the mouth of the Saale. The median value of the seven traps considered within a study area of 0.2 km2 gives a good overview of the distribution of elements. With 35%, the estimated total relative method error (including sampling) is comparable to other surface-sediment investigation (Truckenbrodt and Einax, 1995) with all the elements under consideration, except for lead and mercury during the first flood event. 4.4. Assessment of the heavy-metal and As pollution in the floodplains The contamination of the floodplain sediments with heavy metals and As in the year 2005 (Table 2) corresponds to
776
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Fig. 4. Sediment input (dry substance) into the floodplain in relation to the ground-level elevation (top) as well as the variation of the Si/Al ratio of the deposited matter (bottom).
findings of former investigations (1997e2003) presented in Kru¨ger et al. (2005). Altogether the data cover a period of 9 years and indicate largely constant pollution levels with heavy metals and As in the river basin of the Elbe, following a rapid decrease in the first years after the German reunification (Heininger and Pelzer, 1998; Lehmann and Rode, 2001). The findings reflect constant pollution levels and transport-proneness of the groyne field sediments of the middle part of the River Elbe during floods, as well as a regular supply of polluted sediments from the Elbe tributaries Mulde and Saale. In this river section the flood-dependent mass transport seems to be limited more by the transport capacity than by the supply (Baborowski et al., in press). Thereby, the transport capacity pertains to the maximum amount of suspended sediment that can be transported by the given physical properties of the water (e.g. viscosity) and the flow (e.g. current velocity) in the river. Supply refers to the amount of suspended sediment that is delivered
into the river by groyne fields and the Elbe tributaries. The amount of available suspended matter depends on the accumulated sediment prior the flood event and its erosion stability. When comparing results of heavy-metal and As of the floodplain deposits with the precaution values and the action values given in Table 2, the concentrations of Cd, Cu, Hg, Pb, and Zn exceed the precaution values for loamy soils. The concentrations of Cr and Cu exceeded the precaution values, but the natural background values of these elements are in the range of the precaution values as well. The measured As concentrations were near the action values for grassland, while the Hg concentrations were above these thresholds. However, the determined sedimentation rates and contamination loads (Table 2) indicate that single flood events in the area of the Lower Middle Elbe can hardly influence the quality of the soils in the actual floodplain where the pollutant loads are the result of pollutant inputs over decades and even
M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
1st wave
centuries. On the one hand, still today the contamination of the recent flood sediments corresponds to a large extent to the quality of the topsoils (Kru¨ger et al., 2005). On the other hand, the maximum sediment input of 580 g m2 (Fig. 4, above) with an assumed bulk density of 1 g cm3 (after Kru¨ger et al., 2005) amounts only to a maximum of 0.6% of the 0e 10 cm soil-depth layer that is used as standardized soil sample. This is not sufficient to alter the pollution state of this soildepth layer significantly.
2nd wave
50 40
Fe (g kg-1)
777
30 20 10
5. Conclusions
0 1
2
7
4
8
9
10
8
9
10
8
9
10
9
10
3600
Zn (mg kg-1)
3000 2400 1800 1200 600 0 1
2
4
7
200
Cu (mg kg-1)
160
120
80
40
0 2
1
4
7
80
As (mg kg-1)
60
40
20
Artificial-lawn mats were placed as sediment traps in floodplains and served as an appropriate tool to estimate the sediment supply during flood events. The method can easily be transferred to other rivers with comparable scale and morphological status. With such field techniques, high local variability has to be taken into account, especially since the number of available traps is limited. The variability also has to be considered in the assessment of the pollution with respect to the observance of precaution values and action values. Based on the findings of this study, the pollution of the flood-borne sediments to be expected in the considered floodplain can be derived from the pollution of the suspended matter in the river water. This assessment requires a measuring programme with a high resolution sampling based on a local discharge threshold that indicates the starting point of the sampling campaign. An estimation of the sedimentation rates in the floodplain on the basis of the suspended-matter content of the river water is not possible based on the above presented values. Nevertheless, for the study of the spatial distribution patterns of the deposited suspended matter in the floodplain, the installation of the traps proved to be indispensable. The wide variation range of the sediment inputs over two flood waves between the 10 different trap sites in the year 2005 shows that the sediment traps can help to estimate only local sediment inputs. The transferability of the findings to larger scales should be assured through a higher number of sites exposed to various flooding conditions. For the estimation of the large-scale sediment inputs, a more elegant approach should use numerical models that can be calibrated and validated by means of sediment-trap data. Ultimately, it would be possible to optimise measuring programmes through estimating the pollutant load of flood-borne sediments on the basis of the pollutant load of the suspended matter in the river water, when denser sediment-trap surveys of the inputs were available.
0 1
2
4
7
8
Acknowledgements
Trap number Fig. 5. Variations of iron (Fe), zinc (Zn), copper (Cu) and arsenic (As) concentrations of the trapped sediments.
This work was supported by the European Union FP6 Integrated Project AquaTerra (Project no. GOCE 505428) under the thematic priority ‘‘sustainable development, global change and ecosystems’’.
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References Asselmann, N.E.M., Middelkoop, H., 1995. Floodplain sedimentation: quantities, patterns and processes. Earth Surface Processes and Landforms 17, 687e697. Baborowski, M., Claus, E., Friese, K., Pelzer, J., von der Kammer, F., Kasimir, P., Heininger, P., 2005. Comparison of different monitoring programs of the 2002 summer flood in the River Elbe. Acta Hydrochimica et Hydrobiologica 22, 404e417. Baborowski, M., Guhr, H., Bu¨ttner, O., von Tu¨mpling, W., Friese, K., 2004a. The influence of extreme low water flow on the water quality of the Elbe River at Magdeburg monitoring station. In: Geller, W., Blachuta, J., Blazkova´, Sˇ., Claus, E., Dubicki, A., Feldmann, H., Guhr, H., HeliosRybicka, E., Holzmann, H., Szalinska, W., von Tu¨mpling, W., Zaray G. (Eds.), 11th Magdeburg Seminar on Waters in Central and Eastern Europe: Assessment, Protection, Management, 18e22 October 2004, Leipzig. UFZ Report 18/2004, ISSN: 0948-9452 18, pp. 23e24. Baborowski, M., von Tu¨mpling, W., Friese, K., 2004b. Behaviour of suspended particulate matter (SPM) and selected trace metals during the 2002 summer flood in the River Elbe (Germany) at Magdeburg monitoring station. Hydrology and Earth System Sciences 8 (2), 135e150. Baborowski, M., Kru¨ger, F., Bu¨ttner, O., Morgenstern, P., Lobe, I., von Tu¨mpling, W., Rupp, H., Guhr, H. Transport and fate of dissolved and suspended particulate matter in the Middle Elbe region during flood events. In: Westrich, B., Fo¨rstner, U. (Eds.), Sediment Dynamics and Pollutant Mobility in Rivers e Interdisciplinary Approach. Springer Verlag, in press. Bu¨ttner, O., Otte-Witte, K., Kru¨ger, F., Meon, G., Rode, M., 2006. Numerical modeling of floodplain hydraulics and suspended sediment transport and deposition at the event scale in the middle River Elbe, Germany. Acta Hydrochimica et Hydrobiologica 34 (3), 265e278. Costa, A.T., Arias Nalini Jr., H., de Tarso Amorim Castro, P., Carvalho de Lena, J., Morgenstern, P., Friese, K., 2006. Sediment contamination in floodplains and alluvial terraces as an historical record of gold exploitation in the Carmo River basin, Southeast Quadrilatero Ferrifero, Minas Gerais, Brazil. Acta Hydrochimica et Hydrobiologica 34 (3), 245e256. Engelhardt, C., Kru¨ger, A., Karrasch, B., Baborowski, M., 1999. Inputeoutput balances of nutrients and plankton in a flooded area of the lower Odra. Acta Hydrochimica et Hydrobiologica 27, 325e330. Friese, K., Witter, B., Brack, W., Bu¨ttner, O., Kru¨ger, F., Kunert, M., Rupp, H., Miehlich, G., Gro¨ngro¨ft, G., Schwartz, R., van der Veen, A., Zachmann, D.R., 2000. Distribution and fate of organic and inorganic contaminants in a river floodplain e results of a case study on the River Elbe, Germany. In: Wise, D.L., Trantolo, D., Cichon, E.J., Inyang, H.I., Stottmeister, U. (Eds.), Remediation Engineering of Contaminated Soils. Marcel Dekker, New York, Basel, pp. 375e428. Guhr, H., Spott, D., Dreyer, U., 2006. Die Wiedergenesung der Elbe nach dem gesellschaftlichen Umbruch in Deutschland und Tschechien. In: Handbuch Angewandte Limnologie-25. Erg. Lfg. 3/06, pp. 3e56. Hintze, B., 1985. Geochemie umweltrelevanter Schwermetalle in den vorindustriellen Schlickablagerungen des Elbe-Unterlaufs. Hamburger Bodenkundliche Arbeiten. ISSN: 0724-6382 2. Heise, S., Claus, E., Heininger, P., Kra¨mer, Th., Kru¨ger, F., Schwartz, R., Fo¨rstner, U., 2005. Studie zur Schadstoffbelastung der Sedimente im Elbeeinzugsgebiet, Ursachen und Trends. Hamburg Port Authority, Hamburg. Heininger, P., Pelzer, J., 1998. Trends und Verteilungsmuster in der Schadstoffbelastung von Sedimenten aus o¨stlichen Bundeswasserstraßen. Acta Hydrochimica et Hydrobiologica 24, 218e225. Klemm, W., Greif, A., Broekaert, J.A.C., Siemens, V., Junge, F.W., van der Veen, A., Schultze, M., Duffek, A., 2005. A study on arsenic and the heavy
metals in the Mulde River system. Acta Hydrochimica et Hydrobiologica 33 (5), 475e491. Kozerski, H.-P., Schwartz, R., Hintze, T., 2006. Tracer measurements in groyne fields for the quantification of mean hydraulic residence times and the exchange with the stream. Acta Hydrochimica et Hydrobiologica 34 (3), 188e200. Krausse, G.L., Schelske, C.L., Davis, C.O., 1983. Comparison of three wet alkaline methods of digestion of biogenic silica in water. Freshwater Biology 13, 73e81. Kronvang, B., Falkum, O., Svendsen, L.M., Laubel, A., 2002. Deposition of sediment and phosphorus during overbank flooding. Verhandlungen Internationale Vereinigung fu¨r theoretische und angewandte Limnologie 28, 1289e1293. Kru¨ger, F., Schwartz, R., Kuhnert, M., Friese, K., 2006. Methods to calculate sedimentation rates of flood plain soils in the middle region of the Elbe River. Acta Hydrochimica et Hydrobiologica 34 (3), 175e187. Kru¨ger, F., Meissner, R., Gro¨ngro¨ft, A., Grunewald, K., 2005. Flood-induced heavy metal and arsenic contamination of Elbe River floodplain soils. Acta Hydrochimica et Hydrobiologica 33 (5), 455e465. Kru¨ger, F., Prange, A., Jantzen, E., 1999. Ermittlung geogener Hintergrundwerte an der Mittelelbe und deren Anwendung in der Beurteilung von Unterwassersedimenten. Hamburger Bodenkundliche Arbeiten 44, 39e51. Lehmann, A., Rode, M., 2001. Long-term behaviour and cross correlation water analysis of the River Elbe, Germany. Water Research 9, 2153e2160. Middelkoop, H., 2000. Heavy-metal pollution of the River Rhine and Meuse floodplains in the Netherlands. Geologie en Mijnbouw/Netherlands Journal of Geosciences 79 (4), 411e428. Report SedNet, 2005. Contaminated sediments in European river basins. Available from:, Brussels, RTD/1.2/AT/JHB D. Sachs, L., 1995. Angewandte Statistik, 5 Auflage. Springer Verlag, Berlin, Heidelberg, New York, p. 79. Schreck, P., Wennrich, R., Sta¨rk, H.J., Schubert, M., Weiß, H., 2004. Mansfeld e The Contribution of a Mining-affected Catchment Area to Regional Riverine Pollution. UFZ Report 18/2004. UFZ Centre for Environmental Research, Leipzig, pp. 169e170. Stachel, R., Go¨tz, R., Herrmann, T., Kru¨ger, F., Knoth, W., Pa¨pke, O., Rauhut, U., Reincke, H., Schwartz, R., Steeg, E., Uhlig, S., 2004. The Elbe flood in August 2002 e occurrence of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F) and dioxin-like PCB in suspended particulate matter (SPM), sediment and fish. Water Science and Technology 50 (5), 309e316. Schwartz, R., 2006. Geochemical characterisation and erosion stability of finegrained groyne field sediments of the Middle Elbe River. Acta Hydrochimica et Hydrobiologica 34 (3), 223e233. Truckenbrodt, D., Einax, J., 1995. Sampling representativity and homogeneity of river sediments. Fresenius Journal of Analytical Chemistry 5, 437e443. Umlauf, G., Bidoglio, G., Christoph, E.H., Kampheus, J., Kru¨ger, F., Landmann, D., Schulz, A.J., Schwartz, R., Severin, K., Stachel, B., Stehr, D., 2005. The situation of PCDD/Fs and dioxin-like PCBs after the flooding of River Elbe and Mulde in 2002. Acta Hydrochimica et Hydrobiologica 33 (5), 543e554. Walling, D.E., Owens, P.N., 2003. The role of overbank floodplain sedimentation in catchment contaminant budgets. Hydrobiologica 494 (1e3), 83e91. Zerling, L., Hanisch, C., Junge, F.W., 2006. Heavy metal inflow into the floodplains at the mouth of the river Weiße Elster (Central Germany). Acta Hydrochimica et Hydrobiologica 34 (3), 234e244. Zwirnmann, E., Kru¨ger, A., Gelbrecht, J., 1999. Analytik im Zentralen Chemielabor des IGB: Gesamtsilicium (TSi) ohne mineralische Bestandteile. Reports IGB, 9, pp. 3e24.
Spatial and temporal variability of sediment deposition on artificial-lawn traps in a floodplain of the River Elbe M. Baborowski a,*, O. Bu¨ttner b, P. Morgenstern c, F. Kru¨ger d, I. Lobe a, H. Rupp e, W. v. Tu¨mpling a a
Department of River Ecology, Helmholtz Centre for Environmental Research e UFZ, Bru¨ckstrasse 3a, 39114 Magdeburg, Germany Department of Lake Research, Helmholtz Centre for Environmental Research e UFZ, Bru¨ckstrasse 3a, 39114 Magdeburg, Germany c Department of Analytical Chemistry, Helmholtz Centre for Environmental Research e UFZ, Permoserstrasse 15, 04318 Leipzig, Germany d ELANA Boden Wasser Monitoring, Dorfstrasse 55, 39615 Falkenberg, Germany e Department of Soil Physics, Helmholtz Centre for Environmental Research e UFZ, Dorfstrasse 55, 39615 Falkenberg, Germany b
Received 29 January 2007; accepted 31 January 2007
The deposition of polluted sediments on floodplains is characterised by a high local variability. Abstract Artificial-lawn mats were used as sediment traps in floodplains to measure sediment input and composition during flood events. To estimate the natural variability, 10 traps were installed during two flood waves at three different morphological units in a meander loop of the River Elbe. The geochemical composition of deposited and suspended matter was compared. The sediment input showed weak correlations with concentration and composition of river water. It also correlated poorly with flood duration and level as well as distance of trap position from the main river. This is due to the high variability of the inundation, different morphological conditions and the variability of sources. The composition of the deposits and the suspended matter in the river water was comparable. Hence, for the investigated river reach, the expected pollution of the floodplain sediments can be derived from the pollution of the suspended matter in the river during the flood wave. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Trace elements; Floodplain; Sediment input; Spatial distribution; Measurement strategy
1. Introduction Sediments are the long-term memory of rivers and their catchments. Despite a significant improvement of the pollution situation in the River Elbe for more than 15 years (Guhr et al., 2006), sediment contamination with heavy metals and organic micro-pollutants still exists in the river basin (Heise et al., 2005). During floods as well as low-water periods, the sediments act as secondary sources of pollution for the river (Baborowski et al., 2004a,b; Stachel et al., 2004; Umlauf et al., 2005). To understand the transport and fate of pollutants
* Corresponding author. Tel.: þ49 391 8109630; fax: þ49 391 8109150. E-mail address: [email protected] (M. Baborowski). 0269-7491/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2007.01.032
in a river, as well as to assess the pollution situation in the whole river basin both the sources and the sinks of the pollutants must be considered. Groyne fields and extensive floodplains are the characteristic morphological features of the middle part of the River Elbe acting as source of and sink for sediments (Kozerski et al., 2006; Schwartz, 2006; Kru¨ger et al., 2005). Similar conditions were described also for other rivers (Zerling et al., 2006; Costa et al., 2006; Walling and Owens, 2003; Middelkoop, 2000; Engelhardt et al., 1999). Summary and event-related approaches are available to calculate sedimentation rates of floodplain soils (Kru¨ger et al., 2006). Such approaches include the comparison of surface elevations, the quantification of the sedimentation with the aid of anthropogenic and geogenic tracers, the application of sediment traps as well as the calculation of solid-matter load
M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
balances. Within these methods, artificial-lawn mats, which function as sediment traps, are an appropriate tool to measure short-term sedimentation processes that occur during flooding events. In this study, the variability of mass input was investigated by means of traps that were arranged in a meander loop of the Middle Elbe (Bu¨ttner et al., 2006). Investigations were performed during two waves of a spring flood in February and March 2005. The study should contribute to improve the understanding of sediment transport processes (including erosion and sedimentation) at the river basin-scale, which is consistent with the recommendations of the European Sediment Research Network (Report SedNet, 2005). The objective of the study was to investigate the variability of the flood-induced input of suspended matter into the floodplain in respect to its amount and composition. The following factors were under consideration: duration of the inundation, ground-level elevation, distance to the river, morphological structures, and pollution of the suspended matter in the river water. The results should improve the reliability of assessments of local pollution and clarify the uncertainty of input data for the modelling of pollutant and suspended-matter transport. 2. Material and methods 2.1. Study site Investigations were carried out in the floodplain Scho¨nberg Deich (Fig. 1). The floodplain is located in the middle reach of the River Elbe upstream of the city of Wittenberge on the left bank between river-kilometres 435 and 440 (German mileage). It covers an area of 2 km2. The site is representative for the lower part of the Middle Elbe.
2.2. Methodology To estimate the natural variability of the sediment input, artificial-lawn mats were placed at different morphological units (10 positions, 5 replicates), in a sub-area of the floodplain of about 0.2 km2 (Fig. 2). The sampling points covered the typical spectrum in riparian landscapes such as old arms, depressions without drainage, and plateaus. The traps were placed immediately before the time when the sample points became flooded, and they were recovered when the water level had receded. The traps (pieces of synthetic turf) simulated the natural soil cover such as pasture or mown grassland. Similar methods had been used by Asselmann and Middelkoop (1995) to determine sediment loads in the floodplains of the River Rhine, and by Kronvang et al. (2002) to measure sediment deposition during overbank flooding of the Gjern River. The investigations in the floodplain were completed by daily analyses of water samples during the flood and by analyses on a weekly basis between the two flood waves. The water samples were taken at the sampling site Wittenberge (river-km 454, left bank) under consideration of a local discharge threshold as the starting point for the measuring campaign. The local discharge threshold value has to be known to detect the beginning of the remobilisation of sediment in front of the flood crest (Baborowski et al., 2005). For Wittenberge, this threshold was determined at 1080 m3 s1. At this critical value, sediments that had deposited at times of low flow in the groyne field and had not yet consolidated become re-suspended and are transported into the main stream. At the same time, the river water began to flow into the floodplain (Fig. 1, right, below). Additionally, special SPM samples were collected upstream of the floodplain in perpendicular to the river flow and in the floodplain itself by boat during the flood crest (2005-02-22). Fig. 2 shows the position of the traps in the floodplain. The Traps No. 1, 2, 4, 7, 8, 9, 10 were collected from the floodplain when the water receded after
771
the passages of the two flood waves. Traps No. 3, 5 and 6 were situated in a flood channel and could therefore not be recovered before the end of the second event. Consequently, they were exposed to both flood waves (Fig. 4). During both waves, the suspended-particulate-matter (SPM) concentration and composition (inorganic and organic contents, trace metals) were measured in the water samples and in the deposits on the traps. The trap samples were collected and prepared as described in Friese et al. (2000). After freeze-drying, the collected sediments were analysed by means of energy dispersive X-ray fluorescence (EDXRF) for their contents of heavy metals, arsenic (As) and silica (Si). The total organic carbon of the sediments was analysed in triplicate with a C(H)NS analyser (Elementar vario EL) after drying of the samples at 105 C. The Si/Al ratio was used as a measure of the ratio of sand/silt and clay according to Hintze (1985). Consequently, the higher the sand content of a sample, the higher the Si/Al ratio, or the higher the fine grain content, the lower the Si/Al ratio. In the water samples, the dry weight of the SPM was measured according to the German Industrial Standard (DIN 38409 part H2, filtration onto Whatman GF/F glass fibre filter). Heavy metals and As were analysed in filtered (<0.45 mm PVDF Millipore syringe filter) and unfiltered samples. The filtered samples were acidified with HNO3, the unfiltered ones digested with HNO3/H2O2 in a microwave equipment. Aluminium (Al), iron (Fe), manganese (Mn) and zinc (Zn) were determined by optical emission spectrometry with inductively coupled plasma (ICP-OES), while arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), titanium (Ti), uranium (U), and zinc (Zn) were measured using mass spectrometry, also with inductively coupled plasma (ICPMS). Mercury (Hg) was analysed only as total content by atomic absorption spectrometry (AAS), cold-vapour technique. Si was analyzed according to Zwirnmann et al. (1999), a modified method of Krausse et al. (1983). The digestion method focuses on the biogenic part of silica in the water. Particulate element concentrations of the river water were calculated from the difference between the total and the dissolved fraction of the elements. The results were related to the dry substance of SPM. In the case of Hg and Cd, only data of the total fraction were available. The uncertainty of the trace-metal analyses and of the measurement of the particulate organic carbon (POC) in the water samples was below 10%. An overview of the analytical uncertainty for the sediment analysis is given in Table 2. Moreover, the relative maximal method error was estimated (Sachs, 1995) for the elements on the traps (35%).
3. Results The discharge curve and the variation of the suspendedmatter concentration during the flood wave are shown in Fig. 3. The maximum suspended-matter concentration was measured before the flood wave reached its crest: 4 days ahead in the first wave and 5 days in the second. The discharge threshold was exceeded for 16 days during the first wave and for 28 days during the second wave. The results of the SPM measurements upstream of the floodplain in cross direction of the river (24.4e27.3 mg L1) were comparable with the result obtained for the sampling site Wittenberge (31.1 mg L1) (Fig. 3). Consequently, the SPM concentrations measured in Wittenberge can be used as a good approach for SPM concentration of inflowing water from the river into the floodplain. As expected, the SPM concentrations decreased in direction from the main stream into the floodplain. The measured concentrations in the floodplain ranged between 12.4 and 17.2 mg L1. The variation of the trace-metal concentration of the river SPM is given in Table 1. Table 2 contains comparable values of floodplain sediments. Furthermore, values of previous studies
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Fig. 1. Catchment of the River Elbe (left) with the Floodplain Scho¨nberg Deich at normal discharge (right, top) and at the beginning of the inundation (1080 m3 s1, right, bottom). The circles mark the locations, where the water begins to flow into the floodplain.
as well as information of background values, precaution values and action values are listed in Table 2. Findings from the Traps No. 3, 5 and 6 are not included in the presentations in Tables 1 and 2 and Fig. 5, because these traps were exposed to both waves. The suspended-matter input of the individual traps undergoes strong fluctuations, from trap to trap as well as from flood wave to flood wave. In general, the input of sediment was higher during the second wave (114.5e401.5 g m2, median value: 192.8 g m2) than during the first wave (9.2e 178.3 g m2, median value: 72.3 g m2). The concentrations of the suspended matter in the river water were comparable (Fig. 3). The input variation in dependence on the ground-level elevations of the trap positions is shown in Fig. 4 (above). In contrast to the high variability of the sediment inputs, the grain size of the deposited matter, characterised by the Si/Al ratio, was comparable (Fig. 4, lower section). The median values of the Si/Al ratio were 3.9 for the first and 3.7 for the second wave. The calculated ratios are characteristic for sediments with at least 60% fine grained particles, dominated by the silt fraction. The suspended-matter input, the concentrations of the trace elements and organic carbon in the sediments showed wide variations between the traps, the waves, and the elements. Here, the following differentiation is summarised: (i) less fluctuating concentrations between the traps and the waves for Cr and the geogenic elements Ti, Al, Fe, Si;
(ii) maximum concentration at Trap No. 8 for Mn, Zn, Pb and S; (iii) generally slightly increased Pb, Cu and C concentrations during the first wave; and (iv) generally slightly increased As concentrations during the second wave. An example for each of these groups is given in Fig. 5. 4. Discussion 4.1. Flood-dependent mass transport in the river The higher input of sediment into the floodplain with the second wave (Fig. 4, top) is probably due to the fact that the discharge threshold was exceeded 12 days longer than during the first wave. Furthermore, the assessment of the discharge hydrographs of the upper Elbe stretches and of the main tributaries Mulde and Saale (data not shown) suggests that a main part of the second wave originated in another part of the river basin than the first wave. More detailed discussion of the composition and transport behaviour of pollutants in the Middle Elbe between Magdeburg and Wittenberge during the flood 2005 underlines these findings (Baborowski et al., in press). As shown in Table 2, the organic-carbon content was slightly increased during the first wave compared to the second one. Results of previous studies of the SPM transport in the River Elbe 2002 (Baborowski et al., 2004b) showed that organic carbon can enhance the tendency of particles to remain suspended. From this perspective another possible explanation
M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
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Fig. 2. Floodplain Scho¨nberg Deich (air view at low flow, top) and a detail of the Digital Elevation Model (DEM) overlaid with calculated currents (after Bu¨ttner et al., 2006) and the positions of the traps (bottom).
is that the settling rate of the SPM in the second wave was different (higher) due to their different material composition. DS
Q
Qs
2500
4.2. Variability of sediment deposition
80 60
1500 40 1000 20
500
DS (mg L-1)
Q (m3 s-1)
2000
0
20 0
05
-0 2
-1 5 502 -2 20 0 05 -0 225 20 05 -0 302 20 05 -0 307 20 05 -0 312 20 05 -0 317 20 05 -0 322 20 05 -0 327
0
20
With the high variability of flooding conditions, different morphological forms and the variability of sources, the sediment inputs were only weakly correlated to the investigated parameters. According to Fig. 4 (top), no correlation can be identified between the ground-level elevation and sediment input. Although Trap No. 1 and Trap No. 7 were situated at the same elevation, the input at Trap No. 1 is more than twofold higher than that of Trap No. 7. This higher input could be explained by the shorter distance to the main river channel. However, by comparing the distances of the traps from the main river (Fig. 2) with the inputs (Fig. 4, top), no general relation can be derived. For instance, Trap No. 5 was closer to the Elbe than Trap No. 3. Both traps were situated at the same height
Sampling day Fig. 3. Graph of the discharge (Q) with the value of the local discharge threshold (QS) and the dry substance (DS) of the suspended-particulate-matter at the sampling site Wittenberge, left bank.
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774
Table 1 Element concentrations of river SPM during both flood waves Element
Unit 1
Fe
g kg
Al
g kg1
Ti
g kg1
Si
g kg1
Mn
g kg1
As
mg kg1
Cda
mg kg1
Cr
mg kg1
Cu
mg kg1
Hg
mg kg1
Ni
mg kg1
Pb
mg kg1
Zn
mg kg1
Wave
Range
Median
1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd
30e49 29e41 28e52 30e48 1.8e3.2 1.4e2.4 5e26 12e27 1.7e3.0 1.4e2.4 32e46 23e49 5e8 4e8 79e145 67e96 91e208 78e147 1e2 1e3 58e87 41e67 148e202 114e205 402e814 438e774
37 38 37 37 2.4 2 17 19 2.1 1.8 40 43 e e 108 81 125 97 2 2 70 50 165 197 648 678
a Median values for Cd were not calculated, due to the fact that with increasing discharge the Cd concentrations in the river water decreased to the detection limit of the method (0.2 mg L1).
level and were flooded for the same period. However, the input in Trap No. 3 is almost twice of that of Trap No. 5. Otherwise, the input in Trap No. 6 (positioned in a depression, inputs from both waves) is comparable to the input e summarized over both waves e of Trap No. 9 (upland position on a plateau). Obviously, the complex flow regime in the considered floodplain is the dominating factor for the sediment inputs and their spatial distribution and thus causes the high local variability of sediment deposition. 4.3. Variability of sediment composition In contrast to the sediment input, the variability of the Al/Si ratio is more similar (Fig. 4, bottom). Due to the different analytical procedures involved, the pollution data for suspended matter (Table 1) and deposited matter (Table 2) are hardly comparable. On the one hand, the microwave assisted digestion of the water samples with HNO3/H2O2 leads to an underestimation of the soil reference compounds Al and Ti. On the other hand, the used analytical procedures for Si are not comparable for suspended matter and deposited matter. In the water samples, the Si concentrations comprise only the biogenic fraction, while in the deposited matter samples they cover the biogenic as well as the mineral fractions. However, the pollution data of the river SPM and floodplain sediments are comparable for the other investigated elements with the exception of Zn, Hg and Cd. Hence, the dominating
influence of the event-related composition of the river SPM on the composition of the floodplain sediments is evident. Therefore, the results given in Table 1 offer the possibility to estimate the pollution of the floodplain sediments to be expected from the pollution of the flood-induced river SPM. The difference between the Zn concentrations of the floodinduced river SPM and the floodplain sediments corresponds to the proportion of dissolved Zn, which amounts to 50% of the total Zn. Comprehensive investigations of the transport between Magdeburg and Wittenberge during the flood 2005 (Baborowski et al., in press) revealed a group behaviour of the elements under consideration. Thereby, the trapping of the clay fraction by sedimentation was evident and much higher than that observed with the other elements (Cr, Pb, Ni) considered. In contrast to Cr, Pb and Ni, the behaviour of Zn agreed with that of the clay reference compounds (Fe, Al, Mn), which were transported more or less in particulate form. Since Zn can be easily adsorbed by mineral and organic components (besides Fe/Mn co-precipitation) the accumulation in the floodplain sediments occurred at the later stage of the flood. This could explain the higher Zn content in the deposits compared with the river SPM. The reason for the differences between the Hg and Cd concentration of suspended and deposited matter is methodical. For Hg the poor measurement uncertainty of the analytical method applied to the deposited matter (35%, see Table 2) has to be considered. In the case of Cd with increasing discharge the concentrations in the river water decreased to the detection limit of the method (0.2 mg L1). During the first wave 40%, and during the second wave 60% of the measured concentrations were below the detection limit. The differentiation within the heavy-metal and As concentrations in the floodplain sediments, as presented in the results (Fig. 5), can be explained as follows: (i) The little-fluctuating concentrations of Ti, Al, Fe and Si result from the geogenic origin of these elements. Cr also showed little fluctuation because the recent concentrations of Cr in sediments and soils were in the range of background values (Table 2). (ii) Extremes of deposition and in the composition of the sediments at single sites, as shown for Trap No. 8, indicate local effects of other contamination sources, e.g. locally relocated old sediments of the nearby old river arms. (iii) As discussed before, the influences of the tributaries to the River Elbe were different during the two investigated flood waves. Imports from the tributary Saale were more important in the first wave, while entries from the tributary Mulde were more significant in the second wave (Baborowski et al., in press). In this context, the slightly higher concentrations of Cu and Pb in the deposits during the first wave can be explained by a higher flood-induced Pb and Cu pollution of the Saale River, compared with the Mulde River. Among other elements Cu and Pb reflect impacts of former mining industries, for example copper-shale mining (Schreck et al., 2004). Regarding Pb, the measurements of the
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775
Table 2 Composition of the trapped sediments in comparison with previous results, geochemical background values, precaution values and action values of German Federal Soil Protection and Contaminated Site Ordinance Parameter Unit Fe
g kg1
Al
g kg1
Ti
g kg1
Si
g kg1
Mn
g kg1
C
g kg1
N
g kg1
S
g kg1
As
mg kg1
Cd
mg kg1
Cr
mg kg1
Cu
mg kg1
Hg
mg kg1
Ni
mg kg1
Pb
mg kg1
Zn
mg kg1
a b c
Wave Range
Median Analytical Highest median Background Precaution value Action value uncertainty (%) 1997e2003a (mg kg1) valueb (mg kg1) loamy soilsc (mg kg1) grasslandc (mg kg1)
1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd
40 41 60 61 4.2 4.2 229 229 1.9 2.4 111 88.5 9.9 9.3 2.8 2.6 49 61 7 8 114 118 128 117 4 4 52 58 188 152 1015 1070
31e47 35e42 41e73 55e62 3.0e4.8 4.0e4.3 184e256 214e252 0.5e2.6 1.8e4.6 70.5e134 80e115 6.7e14.1 8.2e12 2.2e4.7 2.5e3.3 26e56 47e63 5e8 7e9 83e128 106e123 104e173 96e128 3e6 4e5 42e67 49e84 158e1138 141e163 700e3350 943e1182
0.5
e
e
e
e
0.5
e
e
e
e
0.7
e
e
e
e
0.6
e
e
-
e
1.0
e
e
e
e
10
e
e
e
e
10
e
e
e
e
10
e
e
e
e
2.5
68 (2002/2003)
19
e
50
15
8.4 (2002/2003)
0.2
1
20
6
210 (2002/2003)
94
60
e
2.0
144 (2002/2003)
24
40
1300/200a (for sheep)
35
4.9 (1997)
0.2
0.5
2
4.5
112 (2003)
41
50
1900
2.0
153 (2003)
22
70
1200
1.5
1149 (1997)
103
150
e
Kru¨ger et al. (2005). Kru¨ger et al. (1999). Federal Soil Protection and Contaminated Site Ordinance (Germany), BbodSchV.
SPM contamination contradict the sediment studies (Heise et al., 2005), which showed increased Pb concentrations in sediments of the Mulde River against those of the Saale River. However, the findings of the flood investigations 2005 are supported by studies made during a spring flood in the Elbe River 2006 (data not shown). Upstream of the mouth of the River Saale, the event-related maximum concentrations (total values) of Pb were up to two times higher, those of Cu up to three times higher than those measured upstream of the inflow of the River Mulde. (iv) The slightly increased As concentrations in the deposits of the second wave can be explained by the dominating influence of flood-induced imports from the Mulde River in the second flood wave, against those from the Saale River. In the catchment of the Elbe River, the As concentrations had the highest values in the Mulde River both in suspended matter and sediments (Heise et al., 2005). Although mining and other industrial activities had been brought to a close in some cases already decades ago, studies in the heavily polluted
Mulde basin showed that during flood events there is a permanent supply from point and non-point sources into the river system (Klemm et al., 2005). During the spring flood 2006 the maximum As concentrations (total values) upstream of the mouth of the Mulde River were two times higher than the concentrations measured upstream of the mouth of the Saale. The median value of the seven traps considered within a study area of 0.2 km2 gives a good overview of the distribution of elements. With 35%, the estimated total relative method error (including sampling) is comparable to other surface-sediment investigation (Truckenbrodt and Einax, 1995) with all the elements under consideration, except for lead and mercury during the first flood event. 4.4. Assessment of the heavy-metal and As pollution in the floodplains The contamination of the floodplain sediments with heavy metals and As in the year 2005 (Table 2) corresponds to
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M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
Fig. 4. Sediment input (dry substance) into the floodplain in relation to the ground-level elevation (top) as well as the variation of the Si/Al ratio of the deposited matter (bottom).
findings of former investigations (1997e2003) presented in Kru¨ger et al. (2005). Altogether the data cover a period of 9 years and indicate largely constant pollution levels with heavy metals and As in the river basin of the Elbe, following a rapid decrease in the first years after the German reunification (Heininger and Pelzer, 1998; Lehmann and Rode, 2001). The findings reflect constant pollution levels and transport-proneness of the groyne field sediments of the middle part of the River Elbe during floods, as well as a regular supply of polluted sediments from the Elbe tributaries Mulde and Saale. In this river section the flood-dependent mass transport seems to be limited more by the transport capacity than by the supply (Baborowski et al., in press). Thereby, the transport capacity pertains to the maximum amount of suspended sediment that can be transported by the given physical properties of the water (e.g. viscosity) and the flow (e.g. current velocity) in the river. Supply refers to the amount of suspended sediment that is delivered
into the river by groyne fields and the Elbe tributaries. The amount of available suspended matter depends on the accumulated sediment prior the flood event and its erosion stability. When comparing results of heavy-metal and As of the floodplain deposits with the precaution values and the action values given in Table 2, the concentrations of Cd, Cu, Hg, Pb, and Zn exceed the precaution values for loamy soils. The concentrations of Cr and Cu exceeded the precaution values, but the natural background values of these elements are in the range of the precaution values as well. The measured As concentrations were near the action values for grassland, while the Hg concentrations were above these thresholds. However, the determined sedimentation rates and contamination loads (Table 2) indicate that single flood events in the area of the Lower Middle Elbe can hardly influence the quality of the soils in the actual floodplain where the pollutant loads are the result of pollutant inputs over decades and even
M. Baborowski et al. / Environmental Pollution 148 (2007) 770e778
1st wave
centuries. On the one hand, still today the contamination of the recent flood sediments corresponds to a large extent to the quality of the topsoils (Kru¨ger et al., 2005). On the other hand, the maximum sediment input of 580 g m2 (Fig. 4, above) with an assumed bulk density of 1 g cm3 (after Kru¨ger et al., 2005) amounts only to a maximum of 0.6% of the 0e 10 cm soil-depth layer that is used as standardized soil sample. This is not sufficient to alter the pollution state of this soildepth layer significantly.
2nd wave
50 40
Fe (g kg-1)
777
30 20 10
5. Conclusions
0 1
2
7
4
8
9
10
8
9
10
8
9
10
9
10
3600
Zn (mg kg-1)
3000 2400 1800 1200 600 0 1
2
4
7
200
Cu (mg kg-1)
160
120
80
40
0 2
1
4
7
80
As (mg kg-1)
60
40
20
Artificial-lawn mats were placed as sediment traps in floodplains and served as an appropriate tool to estimate the sediment supply during flood events. The method can easily be transferred to other rivers with comparable scale and morphological status. With such field techniques, high local variability has to be taken into account, especially since the number of available traps is limited. The variability also has to be considered in the assessment of the pollution with respect to the observance of precaution values and action values. Based on the findings of this study, the pollution of the flood-borne sediments to be expected in the considered floodplain can be derived from the pollution of the suspended matter in the river water. This assessment requires a measuring programme with a high resolution sampling based on a local discharge threshold that indicates the starting point of the sampling campaign. An estimation of the sedimentation rates in the floodplain on the basis of the suspended-matter content of the river water is not possible based on the above presented values. Nevertheless, for the study of the spatial distribution patterns of the deposited suspended matter in the floodplain, the installation of the traps proved to be indispensable. The wide variation range of the sediment inputs over two flood waves between the 10 different trap sites in the year 2005 shows that the sediment traps can help to estimate only local sediment inputs. The transferability of the findings to larger scales should be assured through a higher number of sites exposed to various flooding conditions. For the estimation of the large-scale sediment inputs, a more elegant approach should use numerical models that can be calibrated and validated by means of sediment-trap data. Ultimately, it would be possible to optimise measuring programmes through estimating the pollutant load of flood-borne sediments on the basis of the pollutant load of the suspended matter in the river water, when denser sediment-trap surveys of the inputs were available.
0 1
2
4
7
8
Acknowledgements
Trap number Fig. 5. Variations of iron (Fe), zinc (Zn), copper (Cu) and arsenic (As) concentrations of the trapped sediments.
This work was supported by the European Union FP6 Integrated Project AquaTerra (Project no. GOCE 505428) under the thematic priority ‘‘sustainable development, global change and ecosystems’’.
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