Tracing the Metal Pollution History of the Tisza River Through the Analysis of a Sediment Depth Profile
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- Nguyen, H.L., Braun, M., Szaloki, I. et al. Water Air Soil Pollut (2009) 200: 119. doi:10.1007/s11270-008-9898-2
The vertical profiles of 20 major and trace metals were investigated along a 180-cm-long sediment core, which was sampled at Kiss-Janosne-Holt Tisza, an oxbow lake located in the upper part of the Tisza River in Hungary. The vertical profiles showed sharp peaks at different depths, reflecting historical pollution events and unusual changes of river water characteristics. Five different groups of metals, containing metals which were strongly correlated and showing a similar behaviour, could be distinguished by factor analysis. Six areas, with variable degrees and types of contamination, were classified in the sediment core with cluster analysis. The most polluted sections were found in the upper 50-cm part (significantly contaminated by Cu, Zn, Pb, Cd and Hg) and the deeper 100–120-cm part (characterised by high concentrations of metals associated with mining activities, such as Fe and Mn, as well as Cu, Zn and Pb). In recent years, important pollution events, such as the one which took place in March of 2000, were the reason for pollution of the upper sediment layers, whereas mining activities during the last century were responsible for the pollution of the deeper core sections.
KeywordsHeavy metalsSediment corePollutionTisza River
The origin of metals which accumulate in sediments is partly from natural sources through the weathering of rocks and partly arising from a variety of human activities, including mining, smelting, electroplating and chemical manufacturing plants, as well as domestic discharges, shipping, boating activities…(Forstner and Wittmann 1983). Due to their particle reactivity, trace metals tend to accumulate in sediments and may persist in the environment long after their primary source has been removed (Park and Presley 1997). Consequently, sediments are not only important carriers but also potential sources of contaminants in aquatic environments (Salomons and Forstner 1984). Being formed by the deposition of fine particles with their associated contaminants, each layer of buried sediment represents a record of the environmental conditions, reflecting the water quality and possible effects of anthropogenic contamination at a certain period (Von Gunten et al. 1997). As a result, sediments are one of the most important tools to assess the contamination level of aquatic ecosystems. It has been reported that both natural background levels and man-induced accumulation of metals over an extended period of time can be traced by studying dated sediment cores (Forstner 1990). Using this technique, a historical record of the various influences on the aquatic system can be reconstructed. A lot of research on sediment depth profiles in lakes or rivers has been carried out and has provided valuable information about temporal variation of metals, the relationship with the evolution of adjacent terrestrial ecosystems and emission sources or the reconstruction of the metal pollution history of the aquatic area, for example (Iskandar and Keeney 1974; Park and Presley 1997; Dauvalter and Rognerud 2001; Ying et al. 2002; Ciszewski 2003).
The Tisza river is the longest tributary (977 km) of the Danube river, having the largest drainage basin (157,200 km2), which is shared by five countries: Ukraine, Romania, Slovakia, Hungary and Servia (Laszlo et al. 2000). The upper Tisza river, flowing through Hungary, has been subject to severe metal pollution due to mining activities and major industrial complexes in Romania, near the border of Romania and Hungary. Heavy floods, which occur regularly due to snow-melting and intensive precipitation, are a great potential risk of enhancing these metal pollutions by leaching of the mine tailing, which are stocked nearby the river banks, as well as spreading of the contamination outside the river bed and further downstream (Hamar and Sarkany-Kiss 1999, Osan et al. 2002, 2007). The river has about 70 oxbow lakes, with surface areas larger than 5 ha, that are only connected to the main river during flooding events (Braun et al. 2000). During these events, a large amount of suspended particles and associated metals are transported by the river and deposited onto the floodplains. As a result, sequential sediment layers have built up, containing important geochemical information about changes in chemical and hydrological conditions of the river at the moments of deposition. This relation is most evident in areas of extensive ore exploitation and processing when the pollution with heavy metals is large (Ciszewski 2003). Therefore, sediment depth profiles, especially in oxbow lakes of Tisza River, are considered as historical records that provide valuable information on environmental changes and industrial or/and mining activities in the river catchment area.
In this study, the vertical distribution of major and trace metals in a sediment core of an oxbow lake was investigated. Different groups of metals were identified based on their common source, similar behaviour and distribution within the core. Dating the scale of the core was estimated by linking contaminated sections with flooding events. From these analyses, a reconstruction of the metal pollution history of the Tisza River could be made.
2 Materials and Methods
The analytical results were statistically processed with factor analysis to group metals with similar behaviour and with cluster analysis to identify sections with various levels of contamination in the sediment profile. Based on the obtained clusters and a vertical time scale using water levels and specific flooding and pollution events, the metal pollution history of the Tisza river sediment was reconstructed.
3 Results and Discussions
3.1 Vertical Variation of Heavy Metals in Sediment of Kiss Janosne Holt Tisza, Tisza
Hungarian authority standards for soil (mg·kg−1 dry weight)
A value (natural background)
B value (polluted)
C value (intervention to biological species)
Cu, Zn and Pb show sharp peaks at 15 and 120 cm from the top of the core, suggesting their common origin or the same effects for these metals happening at the same moment. In other words, the pollution events occurring at these moments showed elevated levels of Cu, Zn and Pb. The concentrations are twice, triple (Cu, Zn) or even six times (Pb) larger than their average or local background levels (Table 1) implying a severe pollution of the river sediments by those events. Actually, two recent mining accidents which took place in Romania are considered the most suspect for the metal pollution in the top 15 cm of the core. In March 2000, a solid waste pond of Baia Borsa mining field, broken due to heavy rainfalls and melting of large snow layers, released about 20,000 tons of sludge contaminated with Pb, Zn and Cu to the Tisza river. Together with the cyanide and heavy metal spill from Baia Mare in January 2000, it was recently the most serious environmental disaster in Tisza river (Laszlo et al. 2000; WWF 2002). In other studies (Laszlo et al. 2000; Fleit and Lakatos 2003; Osan et al. 2002, 2007), high concentrations of these elements were also noticed in 10-cm surface sediment layers of Tivadar village, as well as the upper Tisza, as indications of these newly arrived pollution events. For the peaks occurring at a depth of about 120 cm of the core, the most possible explanation is heavy runoff from historical metal exploitation. Mining has been going on for centuries in the Baia Mare region, surrounding the streambeds of the Szamos river basin, the most important tributary of the Tisza. Important pollution from mining origin occurred about 40 years ago, together with the flourishing of the non-ferro industry (Baciu 2002), and left an imprint on the sediment of the Tisza river, as we observed at the depth of 120 cm. Several small peaks which appeared in the middle of the core may be the result of heavy floods, which occur regularly in Tisza catchment area (Hamar and Sarkany-Kiss 1999).
Cd and Hg show similar profiles and seem to be related to the S content of the sediment. Significantly higher levels of these elements (four to five times the mean levels) are recognised at a depth of 30–50 cm of the core. This may be due to contamination arising from discharged wastewater from the gold mining activities in the upper river. The peaks observed at the top of the core (5 cm) in Cd and Hg may also have originated from the metal pollution catastrophes which occurred in early 2000. Fleit and Lakatos (2003) also found increases of Cd in surface sediment layers sampled at Tivadar and the surroundings. MeHg concentrations are correlated to the total Hg concentrations. MeHg in the sediments is formed by in situ methylation of inorganic Hg by sulphate-reducing bacteria and can be decomposed to inorganic Hg by demethylating bacteria. Higher levels of MeHg are observed in the top sediment layers (0–30 cm) and sharp peaks at 35 and 45 cm coincide with the total Hg peaks, indicating that, during pollution events, Hg accumulated in the sediments in a form which was readily available for methylation.
Similar to what was observed in the Cu, Zn and Pb patterns, sharp peaks appeared in the profiles of Fe and Mn (100–120 cm) and P (60–80 cm) which may be attributed to the chronically and accidentally polluted discharges from mining areas in the upper Tisza river during the last century. These peaks are as high as three to five times above the average background values of Fe (54 ± 6.5 mg g−1), Mn (630 ± 179 μg g−1) and P (842 ± 152 μg g−1). Consequences of these mining activities were also recognised, with high concentrations of Fe and Mn (at similar levels), in another sediment core collected at the oxbow lake Marot-zugi-Holt-Tisza situated in Upper Tisza river (Braun et al. 2000).
For the other major elements (Al, Ca, Mg, Na and Sr), apart from a decline at about 80-cm depth of the core, no clear pattern is determined along the sediment depth. Similar small peaks noticed at the same sediment layers reflect the impact of flooding frequency and intensity perturbing the normal sediment depth profile of an oxbow lake. Our observations are supported by the results of Braun et al. (2000), which showed similar consistent variations of these elements over an about 130-year time span.
3.2 Similar Behaving Metal Groups in Sediment of Kiss Janosne Holt Tisza Oxbow
Loadings (>0.6) of five extracted factors from principal component analysis
3.3 Cluster Analysis of the Metal Pollution along the Depth Profile
The first cluster includes sediment layers which are highly polluted by Cu, Zn and Pb in the top of the core (10–15 cm) as a result of the recent pollution events. A rapid decline of these metal concentrations before and after the pollution events is another characteristic of cluster 1. The second cluster shows maximum concentrations of Hg and Cd at 30–35-cm depth. The third one is related to the low concentrations of major metals Al, Ba, K, Mg, Na and Sr and trace metals at a depth of 80 cm, probably resulting from dry periods. Significantly higher concentrations of Fe and Mn at a depth of 115–120 cm are characteristic for cluster 4, which are the result of historical mining activities. Cluster 5 groups sediment sections, which are associated with minor variations in Cr, Co and Ni concentrations (90–110 and 125–130 cm). The last cluster, the largest one, reveals no information on metal pollution on important flood events and covers the section from 130 cm to the end of the core. This part reflects background levels of heavy metals in sediments of the Tisza catchment area (Table 1).
3.4 Reconstructing Pollution History of the Tisza River
To identify unusual increases of metal concentrations in different sections of the core as a result of pollution events or severe floods, it is necessary to determine the age profile of the core. Several methods exist to assemble an age profile of the sediment core. In areas polluted by mining or industrial discharges, production changes recorded in the economic history of those catchment areas make it possible to determine the age profile of deposited sediments (Knox 1987). Isotopic dating with 14C, 234U, 230Th (Schwarcz 2002), 210Pb (Appleby and Oldfield 1992; Godoy et al. 1998; Lee and Cundy 2001) and 137Cs (Delaune et al. 1978; Catallo et al. 1995; Gallagher et al. 1996; Gambrell et al. 2001) are also techniques widely applied for this purpose (Macklin et al. 1994; Kudo et al. 2000; Gambrell et al. 2001). Analyses of historical maps and/or artefacts are further possibilities.
Heavy metals which have accumulated in the sediment of the oxbow lake were correlated with each other based on either their common origin (Cu, Zn and Pb from recent severe mining accidents, Fe and Mn, as well as Cu, Zn and Pb, from mining runoff during the period of industrial development), their similar behaviour (Cd and Hg, associated to S) or their consistence distribution in the investigated area (Al, Ba, K, Mg, Na and Sr unaffected or Cr, Co and Ni moderately affected by the pollution events).
Based on the metal contamination levels and the metal groups involved, the depth profile of the Kiss Janosne Holt Tisza oxbow sediment can be separated into six parts. The most polluted sections were found in the upper 50-cm part (significantly contaminated by Cu, Zn and Pb with a peak at 15 cm and by Cd and Hg with a peak at 30 cm) and the deeper 100–120-cm part (characterised by high concentrations of metals associated with mining activities, such as Fe and Mn, as well as Cu, Zn and Pb). In recent years, important pollution events, such as the one which took place in March of 2000, were the reason for pollution of the upper sediment layers, whereas mining activities during the last century were responsible for the pollution of the deeper core sections. A decrease in metal concentrations, as well as major element concentrations, was observed at 80 cm, reflecting a change in sediment composition during an extended dry period. The bottom 45-cm section is more uniform, reflecting background levels, unaffected by severe pollution events.
The reconstruction of the metal pollution history of the Tisza river confirmed that the upper 15 cm of the core highly polluted by Cu, Zn and Pb was the result of metal spills from Baia Mare and Baia Borsa industrial complexes occurring in March 2000. Other mining or related metallurgical processing pollution events occurring during the 1950s and 1960s were the reason for high levels of these metals in the deeper part. Hg and Cd contaminations from the depth of 50 cm toward the surface originated from the Baia Mare gold mining area, where cyanide was used for gold extraction from ores. The time scale indicated that the pollution of these metals became severe in the period of 1975–1990. As a consequence of uncontrolled or improper mining discharges during the period of rapid industrialisation in the 1950s and 1960s, significantly large amounts of Fe, Mn and P, as well as Cu, Zn and Pb, accumulated in oxbow sediment. The extremely low water level of Tisza river observed during 1959–1961 was responsible for a sudden decline of Cr, Co, Ni and major element concentrations.
From all analyses, it can be concluded that the sediment core of the oxbow lake, containing valuable time-dependent information of the chemical and hydrological characteristics of the river, can be effectively used for reconstructing the metal pollution history of the river.
We are very grateful to Bela Csapo, teacher of Tarpa Primary School, for his help during the sampling expedition. This research was funded by the Flemish Government and Hungarian Education Ministry through the Flemish–Hungarian Bilateral Scientific and Technological Co-operation under contract project Nr. B-00/76 and by the Flemish government through a grant for H. L. Nguyen.