Distribution of Pb, Sr, U isotopic signature and multielement composition of sediment in Lake Balaton (Hungary) at a sediment trap deep dredged 40 years ago near Balatongyörök – Search for routes of recent pollution

doi:10.21203/rs.3.rs-2001357/v1

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Distribution of Pb, Sr, U isotopic signature and multielement composition of sediment in Lake Balaton (Hungary) at a sediment trap deep dredged 40 years ago near Balatongyörök – Search for routes of recent pollution

https://doi.org/10.21203/rs.3.rs-2001357/v1

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published 17 Jan, 2023

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We studied the Pb, Sr, U isotopic composition and the concentration of toxic elements in sediment core samples collected in Lake Balaton at a sediment trap that was deep dredged in 1979, to analyze their changes in the last 40 years. Pb isotopic composition profiles of cores taken from the sediment trap showed a shift in ²⁰⁶Pb/²⁰⁷Pb from 1.206 ± 0.002 to 1.185 ± 0.002 going from bottom towards the top of the sediment, at 80–100 cm depth. Concomitantly Pb concentration together with Zn, Sb, Cu, Cd and Fe showed elevated, 2–4 times higher values in the top phase of the sediment. Calculated Pb isotopic composition of pollutant Pb fraction was 1.177 ± 0.005 in the case of the ²⁰⁶Pb/²⁰⁷Pb and 2.456 ± 0.004 for ²⁰⁸Pb/²⁰⁷Pb, which shows good agreement with literature data for lead ores in Poland and Germany, but it is distinct from literature data for leaded fuel concerning Middle and Eastern Europe. The marked change of the Pb signatures enabled the construction of a sediment deposition rate map. U and Mo showed a characteristic concentration peak in positioned exactly at the depth of Pb signature transition. Isotopic signature of U based on ²³⁴U and ²³⁵U also showed similar pattern. We suggest that the deposition of U and Mo can be related to cyanobacterial blooms in Lake Balaton in late1970s and early1980s.

Lake Balaton

Pb isotopic composition

pollutant elements

sediment deposition

lake bed dredging

tracing pollutant source

Sediment geochemistry

Deposition of sediments in lakes, marshes, peats (Kamenov et al. 2014, .Kylader et al. 2005, Brannvall et al. 2001), or even Arctic ice (Alfonzo et al. 2001) preserved a historic record of inputs from erosion or atmospheric fallout. The source of these deposits is mostly natural, crystalline or authigenic, volcanic ash etc. but in the last centuries a significant proportion can be attributable to anthropogenic pollution. This fraction usually contains potentially hazardous metals. Studying chemical composition of sediment cores can lead to large amounts of retrospective data regarding trends in hydrogeology, climate, geology or pollution. The careful analysis of Krachler et al. (2003) on V, Ni, Cr and Pb in Switzerland ombrotrophic peats shows the timeline of climatic and anthropogenic deposition history.

The early reports on historic pollution events based on sediment core elemental analysis pointed out major shortcomings of such measurements: they need labor intensive sequential elution methods and extreme caution regarding to potential contaminants (White and Gubala 1990). Statements on pollution history based solely on element concentration record is prone to high levels of uncertainty (White and Gubala 1990, Boyle 2001). Isotopic composition of Pb, Sr and U on the other hand is highly indicative regarding the source of pollution, this holds the possibility to distinguish e.g. Pb of anthropogenic origin from geological background (Ettler et al. 2006, Escobar et al. 2013, Hopper et al. 1991, Graney et al. 1995). Pb, Sr and U isotopic composition remains unaltered in biological and chemical processes during deposition in lake sediments (Ault et al. 1970, Graney et al. 1995). Besides Pb, Sr and U isotope ratios show great diversity due to their radiogenic and radioactive nature, therefore they are putative tracers of geological sources or pollutants.

Pb poses a severe health hazard, associated with its neurotoxic effects. A recent paper by McFarland (2022) made a gross conclusion that at least half of the USA population may have a significant drop in intelligence quotient that is attributable to the lead pollution mainly from the use of lead bearing antiknock agents.

In many cases lead pollution is due to, for the most part, leaded fuels, and to a lesser extent mining, smelting or combustion of fossil energy carriers in the 20th century (Kamenov et al. 2020, 2009, Bölhoffer and Rosman, 2001). Earlier deposits, from 100 to 3000 years old, may also contain significant amounts of anthropogenic lead as shown by Hopper et al. (1991), Kylander et al. (2005), Brannvall et al. (2001) Alfonzo et al. (2001), Hong et al. (1994), although this is only seen clearly in ombrotrophic lakes or peats.

Slightly elevated lead concentration does not necessarily mean risk to health, many rocks and geological formations do contain considerable amount of lead.

Isotopic composition analysis method comparison.

For many years, Thermal Ionization Mass Spectrometer (TIMS) was the dominant analytical method for lead isotopic analysis, however recent advances in ICP-MS methods made them applicable for isotopic analysis. Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) is the benchmark method for isotopic analysis, but modern single collector High Resolution Inductively Coupled Plasma Mass Spectrometer (HR-ICP-MS) instruments have the precision for the analysis of radioactive and radiogenic elements, although, they cannot measure the potentially extremely small differences due to chemical isotope fractionation of heavy elements. Pb isotopic composition analysis is usually less prone to errors from contamination. A 1 percent contaminating Pb can, in worst case, cause a 0.2‰ shift in isotopic composition, therefore single collector HR-ICP-MS methods will not be significantly affected.

Determination of sediment deposition rate

Dating methods in sedimentology, such as radiometric ²¹⁰Pb and ¹³⁷Cs based systems that can be used for the latest 100 years, provide the most accepted ways to connect the geometric position of a marker to the timeline, thus providing a measure for sediment deposition rate (Sanchez-Cabeza and Ruiz-Fernandez 2012, Barsanti et al. 2020). Dating methods are also able to expose disturbances, including discontinuation of layers or layer inversion. Radiometric methods are labor-intensive, require special equipment and it takes in some cases weeks to perform, therefore it is not in the scope of a commercial environmental inorganic chemistry laboratory. Classic radiometric methods however are not always needed, since historical facts studied together with geochemical data can be used to resolve historic depositions and their date accurately. Kemp et al. (2012) showed clear correspondence between Pb records of a salt marsh and historic records of pollution, they managed to fill in a gap between 100 and 200 years that cannot be covered by ²¹⁰Pb or ¹³⁷Cs based geo-chronologies, and ¹⁴C methods are not accurate.

Lake Balaton

Lake Balaton is the largest freshwater lake in Central Europe, it is located at the Transdanubian region of Hungary. It is a shallow lake with an average depth of 3 m and maximum depth of 10 m. Its surface area is 600 km². Its primary inflow is Zala River, the primary outflow is Sió Channel. It is a tectonic depression originating from the mezozoic era. The final geometry of the lake was generated through several events of sinking. Catchment area of Lake Balaton is 5,174 km², it includes Balaton Uplands, Keszthelyi Mountains, Zala Valley, Zala Hills, Baranya Hills (Fig. 1, Jambor 2012).

Balaton Uplands and Keszthelyi Mountains is part of the Transdanubian Mountains. It consists mainly of Triassic carbonates, limestones and dolomites, and Pliocene basalts from volcanic activities at the end of this era. To the south and southwest of Lake Balaton, Quarternary loess is the predominant sediment, deposited by the wind. The origin of the material deposited as loess is most likely the eroded fraction of Transdanubian Mountains, therefore it is expected to have similar geochemical characteristics (isotopic compositition) to their piedmonts (Jambor 2012).

The high carbonate content of Lake Balaton can be explained by the karst activity of Keszthelyi Mountains or Balaton Uplands (Csepregi 2007). These karst waters also find their way to Zala, through torrents Köszvényes, Kis-Zala, Gyöngyös (Raucsik et al. 2006, Budai and. Koloszar 1987, Sztano et al. 2013, Jambor 2012).

Zala River and its drainage basin is one of the major water sources for Lake Balaton. It is originating from Őrség region in Vas County. It receives inputs from many torrents like Harkály Torrent, Sárvíz Torrent etc. also has karst water inflow. Until the beginning of the restauration of Kis-Balaton it served as the major source of agriculture related pollutant input to the lake. Torrents from northern coast of Balaton are the other major water inputs. Dolomites, lime stones, marl, basalts make this source an important contributor to the carbonaceous materials found in lake sediments (Jámbor 2012).

Prior to the governmental restriction of fertilizer usage in Hungary, Lake Balaton suffered from water blooming and had on many occasions eu- or hypertophic water. The era of reconstruction in the 1950s in Hungary was marked by a gross increase in the output of agriculture. This was made possible by mechanization of crop cultivation and the uncontrolled use of fertilizers (Pálffy 2013). Agriculture in itself can generate higher amounts of dust in the air greatly contributing to lake sediment matter (Sharrat et Auvermann 2014). Pollution from anthropogenic sources culminated in massive fish kills in 1965, and in the late 70s water quality issues of Lake Balaton were calling for intervention. High input of organic matter was generating anaerobic conditions. Some experts were conscious about the tourism, other felt that the lake’s very existence is at stake (Takacs et al. 2013). In 1979, deep dredging operation that effected the lake bed 100 m from the northern shore near Balatongyörök took place. This involved deep dredging to generate sludge traps, a 200m wide and 2m deep trench. Since then at Balatongyörök, no deep dredging was performed, therefore the deposited material that contains the pollution record of the past 40 years is still affecting the area (Fig. 2, Kutics 2021).

Besides agriculture, massive urbanisation and industrialisation was taking place in the Eastern Central European region that included mining and smelting. In Hungary, bauxite and brown coal were the most prominent minerals, while in the Czech Republic and Poland, among others Fe, Pb, Zn, Ni, Cu ores were mined and processed. A massive growth of motor vehicle usage was also taking place. Taking into account the prevailing northwestern wind directions (Mezősi et al. 2015) also support the view that southern Poland, Czech Republic and, to some extent, Vienna is a putative source of pollutant aerosol. Aerosol particles, including those that contain lead and other metals, are able to travel long or even extreme distances (Hammelin et al. 1989, Boy and Wilcke 2008).

Effects of dredging

Dredging is a controversial intervention method for preserving the quality and touristic value of surface water bodies, a dredging project may permanently damage the lake bed ecosystem whose role in water quality is undisputed. Dredging without precise planning and detailed study of possible effects is highly unrecommended. In shallow lakes, superficial streams and wind does influence sedimentation, at some places at least temporarily erosion occurs and in the case of depressions, deposition prevails. Performing deep dredging and superficial dredging, the right places are beneficial to the water quality as it removes organic material and phosphorous and slows down lake succession. (Zhang et al. 2010, Kutics 2021). Finding the right place for dredging may need the establishment of the sediment accumulation rate distribution in the area of interest.

Certified reference materials

Throughout the study, we used NIST 981 CRM (Sigma) for Pb that is elemental lead wire. A piece of 0,025 g Pb wire was left in concentrated nitric acid solution (67% m/m). Complete dilution took roughly 3 weeks. At laboratory temperature. The concentration of the Pb CRM stock was 1000 ppm. For the evaluation of the signal stability and Dead time of the Electron Multiplier, series of Pb solutions were prepared with concentrations of 0.1, 0.2, 0.4, 0.8, 1 ppb, in 0.4M HNO₃. For Sr isotopic composition, NIST NBS 987, ⁸⁶Sr/⁸⁷Sr = 0.71025 was used as reference standard for external bracketing. In the case of U, since there is no internationally accepted ²³⁴U/²³⁵U standard reference material, we performed a comparative analysis using a non-isotopic standard reference material of significant U content, NIST SRM 2710, a polluted soil from Montana. We performed the separation of U from this reference sample using UTEVA resin (Eichrom) according to methods provided by the manufacturer.

Sampling area

Sample cores from B1-B15 were collected from the sediment trap dredged in 1979 nearby Balatongyörök, the sampling sites and their WGS88 coordinates are as follows, B1: 46.745414, 17.341175; B2: 46.744839, 17.340903; B3: 46.744208, 17.340606; B4: 46.743628, 17.340294; B5: 46.743036, 17.340011; B6: 46.745664, 17.340178; B7: 46.745058, 17.339886; B8: 46.744433, 17.339564; B9: 46.743881, 17.339289; B10: 46.743281, 17.338983; B11: 46.745861, 17.339142; B12: 46.745283, 17.338894; B13: 46.744647, 17.338572; B14: 46.744122, 17.338333; B15: 46.743517, 17.338056 (Fig. 5a-b)

Sample collection and preparation

Samples were collected using a piston core borer with internal diameter of 2.5 cm. Core lengths ranged from 80 to 140 cm. Cores were sliced to 1 cm discs. Every tenth disk was sampled (1 cm diameter cylinder from the center of the core slices), in some regions, every second disk was analyzed (transition zone).

The reagents used in leaching experiments was HCl, as a simplified method of Graney et al. 1995, 10 ml of 1M HCl was added to 1 g of sediment sample in a plastic tube (Sarstedt), digestion took 90 min at 45 ° C and another 24 hours at ambient temperature.

Complete digestion of the sediment samples was implemented using microwave acid digestion system (Milestone UltraWAVE, Sorisole, Italy). The digestion was performed according to factory recommendation (method #OF03). In brief, 0.25 g of samples were placed to glass digestion tubes, 1.5 ml HCl 30% m/m and 4.5 ml HNO₃ 67% m/m was added. Samples were digested for 2 times 10 minutes (factory program). After digestion, samples were transferred to 25 ml volumetric flasks, through paper filter. The resulting solution was diluted by filling the vials to 25 ml using de-ionized Milli Q type 2 water.

Many authors state that in order to measure isotopic composition, one has to bring the matrix of the standard (CRM) and unknown sample as close as possible in terms of composition (Irrgeher 2015), Matrix matching in our practice meant the purification of analyte, matching the concentration with the standard prior to its introduction to the instrument in 0.4 N HNO₃. For efficient and fast purification of Pb, we used a method originally described for Eichrom Pb-resin by the manufacturer. It proved to be adaptable to Sr-resin that has the same functional group as Pb-resin but in lower concentration. The method is optimal for samples containing low or trace amounts of Pb, and also provides the ability to purify Sr, and separate from Rb in one single chromatographic run. Columns of 7mm internal diameter filled with 0.5 ml Pb-resin were washed with 0.2M sulfuric acid followed by multiple washes (totaling 20 ml) of H₂O. Before each run, columns were equilibrated by 5ml solution of 4M HNO₃. Digested samples were 5 times diluted (the resulting solution contained approximately 3M HNO₃ and 0,7M HCl) prior to the introduction to the column. For leachates, the HNO₃ concentration was brought to 3M prior to sample introduction. Our experience is that both HCl and HNO₃ can be used to introduce sample, but the acid concentration has to exceed 2M. After washing with 5ml 4M HNO₃, 2 ml 0.1M HNO₃ was added to strip Sr from the column. This results a Sr bearing solution that contains minor amount of Rb, the NIST 987 CRM SrCO₃ has similarly extremely low concentration of Rb. It is important to note that for a count rate of 2 million at ⁸⁸Sr, a rate of 1000 cps ⁸⁵Rb was detected, while the standard deviation of the ⁸⁸Sr signal was about 30000. The calculation of ⁸⁷Rb correction factor can eliminate the potential error of measurement at the precision level of single collector ICP-SF-MS in terms of isotope ratio. After stripping off Sr, 3ml 0.1M HNO₃ was added, followed by washing with 0.5 ml 0.1M ammonium oxalate (Sigma, trace metals grade). Pb was released from the column using 0.5 ml 0.1 M ammonium oxalate. The resulting samples were diluted to 1.5 ppb Pb concentration and 3 ppb Sr concentration for ICP-MS measurement.

Separation of U was performed according to a method described by the manufacturer of UTEVA resin (Eichrom). The method was slightly modified by omitting steps related to the separation of trace radioactive elements Po, Np etc. that did not represent a concern for ICP-MS analysis. Elution was performed using 0.1M HCl. Samples containing > 5 µg L^− 1 U were used for measuring ²³⁴U/²³⁵U ratios.

Analysis of concentration and isotopic composition

Analysis of concentrations was carried out on an Agilent 8800 triple quadrupole ICP-MS according to EPA 6020B:2014 method for analysis of toxic metals. For isotopic composition measurements, Attom ES (Nu Instruments) Sector Field type HR-ICP-MS was used. For Pb analysis, magnet was parked to 204.29285 m/z, peak jump method was used, that means that the Fast-scan electrostatic deflector system augmenting the magnet was jumping between single voltage values corresponding to the analyzed nucleon, while the magnet is fixed. For Sr, park mass was 99 m/z, peak jump method. Resolution was set to 300 m/dm. In the case of U, magnet was set to 235.66 m/z.

Data handling

Statistical analyses of concentration of samples and evaluation of Pb isotopic composition. Standard Deviation (SD) calculation, assessment of standard errors and confidence intervals, linear fitting of data was carried out in Microsoft Excel. Assessment of significance of data was carried out using Student’s T test. For Pb, Sr and U isotopes, external bracket calibration was performed. Expected isotope ratio of contaminants was calculated by rearranging the equation presented by Kylander et al. (2005).

Equation 1.

$${{}^{206}Pb}\!\left/ \!{{}^{207}{Pb}_{pollution}}\right.= \frac{{\left[\text{P}\text{b}\right]}_{sample}}{{\left[\text{P}\text{b}\right]}_{pollution}} {{}^{206}Pb}\!\left/ \!{{}^{207}{Pb}_{sample}$}\right.-\left(1-\frac{{\left[\text{P}\text{b}\right]}_{sample}}{{\left[\text{P}\text{b}\right]}_{pollution}}\right) {{}^{206}Pb}\!\left/ \!{{}^{207}{Pb}_{background}$}\right.$$

Where ${{}^{206}Pb}\!\left/ \!{{}^{207}{Pb}_{pollution}$}\right.$ represents the expected isotopic composition of the contaminating or polluting Pb. ${{}^{206}Pb}\!\left/ \!{{}^{207}{Pb}_{background}$}\right.$ is the isotope ratio of the bottom of core. ${{}^{206}Pb}\!\left/ \!{{}^{207}{Pb}_{sample}$}\right.$ is the isotope ratio of the polluted sample (top of core). ${\left[\text{P}\text{b}\right]}_{sample}$ is the concentration of lead in the upper part of core, ${\left[\text{P}\text{b}\right]}_{pollution}$ is the difference between the Pb concentration of top sediment and bottom sediment.

For U isotopic signature, δ (‰) value was calculated (Eq. 2.) with respect to an arbitrary standard: NIST CRM 2710 polluted soil sample that underwent the same extraction and purification process than sediment samples.

Equation 2.

$$\delta =\left(\frac{{IR}_{SMP}}{{IR}_{CRM}}-1\right)\times 1000$$

Where IR_SMP is the isotope ratio of sample and IR_CRM is the isotope ratio of reference material.

Calculation of relative sediment thickness in order to enable comparative evaluation of isotope ratio and concentration trends, was carried out by first appointing a reference core, then the depth of Pb isotope ratio transition of all cores was taken. All the transition depth values were then divided by the reference value (B11, 80 cm), resulting a quotient specific for all individual cores. All the depth values of a core were multiplied by the quotient specific to that given core. This way, all the profiles were corrected, so that Pb isotope ratio transitions of different cores overlap. The depth values generated this way are called depth in phase space.

SD (standard deviation) values in the results section are not the measure of analytical error, but represent the standard deviation of data when considering all samples from all cores for a given phase, e. g. all data for the phase-2 samples.

Pb isotopic profile showed a characteristic transition at depths from around 80 to 100 cm in all cores, except for the B5 which resides on the periphery of the sludge trap. The transition from higher 1.206 (SD = 0.002) ²⁰⁶Pb/²⁰⁷Pb ratio (in the deepest parts of the sediment) towards a lower value 1.185 (SD = 0.001) was detected in most of the cores. Transition also seen in case of ²⁰⁸Pb/²⁰⁷Pb drop from 2.491 to 2.460 (SD = 0.003), and ²⁰⁸Pb/²⁰⁶Pb increase from 2.061 to 2.093 (SD = 0.005), In the case of ²⁰⁶Pb/²⁰⁴Pb a drop from 18.93 to 18.51 (± 0.04). Based on these results, we have assigned phase-1 label to the bottom part of the core, while phase-2 to the upper part of the sediment (Fig. 3).

In the case of B5, there was no significant change in Pb isotopic composition. This sample was drawn from the margin of the sediment trap, where erosion is predominant, ²⁰⁶Pb/²⁰⁷Pb values 1.198–1.207; ²⁰⁸Pb/²⁰⁷Pb values 2.479–2.487; ²⁰⁶Pb/²⁰⁴Pb values ranging from 18.77 to 18.92 stayed similar to values measured on the bottom of the core (Fig. 4a.). The onset of isotope ratio change seen in the rest of the cores is known to be the bottom of the original sediment trap deep dredged in 1979. Depth of this transition therefore is a measure of how much deposit has accumulated over the area in the last 40 years. We calculated the amount of deposited sediment (cm) by finding the point where Pb isotopic ratio shift started, this is clearly visible from the Pb isotope ratio profiles (Fig. 3). Depth of this point was used for the sediment deposition rate map. Estimated amount of deposited sediment in cm is shown on Fig. 5c. Maximum level of deposit was seen on the Western margin of the sediment trap (maximum 130 cm), and to lesser extent (max 110 cm) at the Northern margin.

Analysis of segments belonging to the transition shows areas where occasional sediment overturn or mixing may have occurred. We have identified 3 types of cores: type-1 showing abrupt transition (B2, B3, B4, B14, B15) type-2 showing smoother transition and presence of peaks of Pb isotope ratio reversal to the values of old sediment (B1, B6, B7, B8, B12, Fig. 4a). Map of different types of sediments is shown on Fig. 5d. Type-3 (B5) did not show transition of Pb Isotope ratios.

In type-2, reversal peaks were seen in a 10–20 cm range atop of transition at the bottom of the original sediment trap in 1979.

In cores B6, B2 and B13, a drop of ²⁰⁶Pb/²⁰⁷Pb to 1.175 ± 0.003 (Fig. 4b) at the transition zone was detected. These represent the minimum values of ²⁰⁶Pb/²⁰⁷Pb of the whole study. Similar differences were seen in case of ²⁰⁸Pb/²⁰⁷Pb where the minimum ratio is 2.457 ± 0.005 and in case of ²⁰⁶Pb/²⁰⁴Pb 18.46 ± 0.02. This extremity was also seen in the Pb concentration that peaked at 23.8 ± 0.5 mg kg^− 1 d. m. in B13 (Fig. 4c).

In the 1M HCl leaching experiment, phase-1 sample showed ²⁰⁶Pb/²⁰⁷Pb ratio 1.209 ± 0.001 corresponding to the MW digested phase 1 sample, phase-2 leachate Pb isotopic composition 1.182 ± 0.001 was also highly similar to the MW digested samples. It is worth mentioning that leachate of phase-1 contained roughly 30 times more Pb than phase-2, based on raw counts per second data.

Pb concentration was 5.04 mg kg^− 1 d. m. (SD = 1.17 mg kg^− 1 d. m.) on the bottom of the core, that jumped to 19 mg kg^− 1 d. m at the onset of new sediment accumulation for most of the cores. A gradual drop to 16–19 mg kg^− 1 was present as seen in Fig. 6a but it proved not to be significant by linear fitting of phase-2 data points after converting depth values to depth in phase space. Pb isotope ratios stayed constant in phase-2 (Fig. 6b).

Estimation of the isotope ratio of contaminating lead was performed according to Eq. 1. The resulting ²⁰⁶Pb/²⁰⁷Pb value is 1.1775 ± 0.003 and for ²⁰⁸Pb/²⁰⁷Pb 2.4560 ± 0.004 (Fig. 7).

Uranium isotopic composition showed a characteristic peak at the zone of transition between phase-1 and phase-2. The relative shift in ²³⁴U/²³⁵U isotope ratio (δ) was 27.5–36.5‰ compared to U isotopic signature of background (bottom of the core). This change was also seen in U concentration as well. It was a 2.7–2.9 times increase compared to background value of 1.3–1.7 mg kg^− 1 d. m. Mo showed similar behavior to U staring at 0.5–0.8 mg kg^− 1 d. m. and peaking at 2.5–5.1 mg kg^− 1 d. m. (Fig. 8).

Sb concentration was 0.21 mg kg^− 1 d. m. (SD = 0.12 mg kg^− 1 d. m.) in phase-1, while an increase to 0.49 mg kg^− 1 d. m. (S.D.=0.061) was shown in phase-2. Correlation coefficient calculated for Pb and Sb data was 0.96. From phase-1 mean concentrations of Zn: 41.43 mg kg^− 1 d. m. (SD = 4.98 mg kg^− 1 d. m.), Cu: 6.6 mg kg^− 1 d. m. (SD = 2.9 mg kg^− 1 d. m.), Cd: 0.082 mg kg^− 1 d. m. (SD = 0.001 mg kg^− 1 d. m.), Fe: 13.04 g kg^− 1 d. m. (SD = 1.03 g kg^− 1 d. m.), significant increase to 63.23 mg kg^− 1 d. m. (Zn, SD = 8.09 mg kg^− 1 d. m.), 16.57 mg kg^− 1 d. m. (Cu, SD = 1.58 mg kg^− 1 d. m.), 0.30 mg kg^− 1 d. m. (Cd, SD = 0.001 mg kg^− 1 d. m.), 17.15 g kg^− 1 d. m. (Fe, SD = 1.44 g kg^− 1 d. m.) was seen in phase-2 samples (Fig. 9).

Sr isotopic composition was determined for 3 complete cores, B1, B5 and B8. Bottom of core B1 had a value of 0.70835 ± 0.0005 for ⁸⁷Sr/⁸⁶Sr, then showed a slight increase and stayed constant from 80 cm to 10cm at value 0.7101 ± 0.0005, in the layers close to the surface it turned back towards smaller values (0.7093 ± 0.0005, Fig. 10). Core B8 had ⁸⁷Sr/⁸⁶Sr values 0.7097 ± 0.0005 at the bottom, 0.7104 ± 0.0005 at the middle and 0.7099 ± 0.0005 at the top. B5 showed a different behavior in terms of Sr isotopic composition, it had a constant value 0.7094 ± 0.0005 at the majority of the core, and increasing to 0.7102 ± 0.0005 at the uppermost 20 cm (Fig. 10). Although the differences presented do not reach a considerable significance level in t-test, the trends and the fact that these trends are visible in both biphasic cores analyzed for Sr, suggest that the difference may be present (Fig. 10). Concentration of Sr did not change considerably (400–500 mg kg^− 1 d. m.)

A marked transition of Pb concentration (3.8 times increase) and isotopic composition (²⁰⁶Pb/²⁰⁷Pb from 1.21 to 1.18) was detected in all but the B5 core upon reviewing the core profiles (Fig. 3–4). Data collected shall be handled with care since only the upper part of the sediment (called phase-2) represents material deposited on a known time scale. This is the fraction that was deposited after the deep dredging of the sludge trap in 1979. Phase-1 sediment is much older than 40 years since it is known (Kutics 2021) that deep dredging affected 200 cm of sediment, leaving a bed that is expected to be older than 150–200 years. Many experts showed significantly altered Pb composition due to anthropogenic effects as early as 2000 years ago (Kamenov et al. 2020, Brannvall et al. 2001, Kylander 2005), but in the case of Balaton, it is expected that in the era preceding the industrial revolution, the contribution of lead from distant mining fields to the lake deposit was low, natural clastic or authigenic Pb deposition prevailed. Dolomites and other carbonate minerals dominant in the catchment area of Balaton, thus determining the natural sediment composition can contain amounts of Pb that are sufficient to explain Pb concentration of lower sediment phase. Lake Balaton has a large water capture area and a significant proportion of its deposit is derived from erosion through e.g. Zala River and Tapolca torrent. It is not ombrotrophic like many waters analyzed by Brannvall et al. (2005). Fluctuations in the proportion of inputs could also potentially explain minor shifts in isotopic composition of lead, but unlikely to cause the large differences presented in this paper. Increase of concentrations of Pb and other pollutants also point towards an anthropogenic source for the Pb profiles measured.

B5 sediment core, from the margin of the sludge trap, was distinct from the other cores as it did not show transition of Pb isotopic signatures (Fig. 4a), in contrast to that, it had a signature closer to deep core phases showing only a week trend from 1.206 towards lower values (1.198, R² of fitting is 0.559). We concluded that B5 site did not accumulate significant amount of sediment, instead it was slightly eroded after 1979 deep dredging and affected by the 1999 thin layer (10-20cm layer) dredging, the surface of this site is believed to be between 50 and 100 years old. This suggests that the transition of Pb isotope ratios has happened in the early-mid 20th century, corresponding to exponential growth of industrialization at that era.

We compared Pb isotopic compositions with data gathered by Bölhoffer and Rosman (2000, 2001) and Hopper et al. (1991), and other papers (Fig. 7). on Pb isotopic composition of aerosols, fuel samples and ore samples in the scope of the prevailing wind direction that is from northwest. The results of the comparison are summarized in Fig. 7. The distribution of values of ²⁰⁶Pb/²⁰⁷Pb for many Central European countries is in the range 1.13–1.17. By supposing that Pb in the deep core is the natural background and the average concentration of Pb in phase-2 sample is the sum of the background and pollutant Pb, we calculated the expected isotope ratio of pollution to be 1.1775 ± 0.003 based on Eq. 1. This equation is the rearranged form of equation used by Kylander et al. (2005) for estimating the contribution of pollutant lead. This value is distinct from the data presented by Bölhoffer and Rosman (2001) for central Europe, but Hopper et al showed a value of 1.174 ± for ²⁰⁶Pb/²⁰⁷Pb in the 1988 leaded fuel. They have found leaded fuel ²⁰⁶Pb/²⁰⁷Pb ratios of 1.072 for Hungary that is closer to the Vienna leaded fuel 1.111 but also to the Broken Hill signature 1.039 (Richards, 1986, Reynolds, 1971) than to the expected pollutant lead isotopic composition. It must be mentioned that although the Hungarian leaded fuel has ²⁰⁶Pb/²⁰⁷Pb value distinct from the expected value for pollutant, and also ²⁰⁸Pb/²⁰⁷Pb values in the leaded fuel from Poland was 2.418 that is dissimilar compared to phase-2 samples 2.460 ± 0,003 and the expected value of pollutant lead 2.456 ± 0,004, two fact have to be pointed.

1: The values presented by Hopper (1990) for Hungary are measured in 1990 when Hungary was out of the scope of Warsaw Convention, becoming an open market for western companies, the samples from Poland on the other hand were collected in 1988 when it was part of Eastern Block.

2: Validity of Hopper’s data may be questionable based on the fact that they measured isotope ratio and concentration simultaneously, meaning no concentration matching before isotope ratio measurement was performed, also, they do not present the detector used, and whether or not they have performed dead time correction. ²⁰⁸Pb/²⁰⁷Pb values in a sample whose Pb concentration is multiple times higher than the external bracket standard can be underestimated if proper dead time calculation is omitted, but this effect will be totally absent for samples with concentration matching the standard solution, and this effect is also missing when ²⁰⁶Pb and ²⁰⁷Pb are measured since the abundance of these two is highly similar (our unpresented laboratory experience).

The differences between the literature data for leaded fuel samples and data presented here for phase-2 samples do not support the idea that leaded fuels are the main anthropogenic contributors to the Balaton sediment lead isotope signature. Data retrieved from the literature for different lead bearing ores show ²⁰⁶Pb/²⁰⁷Pb 1.179 (Russell and Farquahar, 1960) and 1.171 (Binczycki et al. 2014) with respective ²⁰⁸Pb/²⁰⁷Pb values 2.469 ± 0.003 and 2.456 ± 0.003 show great similarity to our data, suggesting that this signature is linked lead mining and subsequent utilization of Pb from these sources. Aerosol formed during mining and smelting of Pb does have the ability to travel long many thousand miles distances as seen in more than 2000 years old deposits in ombrotrophic peats and lakes in Spain and Sweden (Brannval et al. 2001 and Kylander et al. 2005). Lead was used for producing dyes and acid-lead batteries. Both of these industrial processes and the inadequate storage of waste containing these products may give rise to Pb emission. Lead is also an alloying material in steel manufacturing. Wear of machines produced from such alloy may also give rise to Pb emission. Leaded fuel Pb isotopic signatures in the literature suggest that ores from Harz Mountains and Southern Poland were not widely used for production of leaded fuel. Considering the geography and the prevailing wind directions in the Central European area encompassing Southern Poland, the Czech Republic, Austria and Hungary does support a model where lead from mines northwest to Krakow could reach Lake Balaton. In Second World War, there were heavy fights in Western Hungary. Considering lead containing projectiles, they can also account for lead burden significantly (Broomandi et al. 2020).

We recorded the change in Pb isotopic composition seeking for changes in the last decades. Pb isotopic composition is shifted between the upper new deposit and the bottom of the trench. In the newly formed deposit, we expected at least a gradual shift towards the uncontaminated bottom. To our surprise, such change was absent (Fig. 6). This is in contrast with the findings in USA regarding Great Lakes and lakes in Florida (Graney et al. 1995, Escobar et al. 2013). Where researchers suggested a shift in Pb isotopic composition towards the geologic Pb, this was accompanied by a decrease in Pb concentration due to slow resolution of pollution. They also showed changing Pb isotope ratio fingerprint for the pollutant tetraethyl lead due to a change in the lead ore used for the production of the fuel additive. Although levels of lead did not reach dangerous levels in Lake Balaton, the unchanging Pb isotopic composition in the phase-2 of cores suggests that Pb contamination in the catchment area can persist for extremely long times.

The most likely cause of such persistence is dust from urban area and agricultural soils (supported by our unpresented results showing that Hungarian soils of Pb pollution). Findings of Resongles et al. (2022) showed that Pb signatures in London urban area did not change significantly after drastic decrease of Pb emission, they predict that lead concentrations cannot be expected to drop any further in the near future. Persistence of Pb in agricultural soils are also shown by many other papers (Wang et al. 2021). Dust produced during ploughing may release Pb containing dust to atmosphere (Sharrat and Auvermann 2014) that can reach and contaminate Lake Balaton due to prevailing winds from northwest (Mezősi et al. 2015).

Our leaching experiment showed that extractability of Pb shows correlation with the extraction of Fe suggesting its association to Fe/Mn hydroxide particles, and not to carbonaceous fraction. 1M HCl leaching extract of phase-1 samples was characteristic of high concentration of Fe and Pb, while phase-2 showed low extraction yield for both Fe and Pb. Concentration of Pb in latter is 50 times lower than in the leachate of phase-1. This result is strikingly different from microwave digested (complete digestion) samples where phase-2 contained 4 times more Pb than phase-1.

The area having the highest amount of deposit 110-130cm (sediment depth of ²⁰⁶Pb/²⁰⁷Pb phase transition) was the western margin of the sediment trap, cores B11-B15. This area corresponds to the direction from which the predominant stream arrived to the sediment trap, there was a slightly elevated sediment thickness in the B1 and B6 (108-110cm, northern margin). The rest of the area showed somewhat smaller deposition rates, 80–96 cm. This shows that careful analysis of stream directions in the water body is of high importance in achieving the highest level of efficiency, while keeping the extent of the dredged area at a minimum level (Fig. 5b). If instead of a 100 ± 10 m trench perpendicular to the direction of dominant stream, two trenches of 40–50 m were created, the efficiency would grow by 10–15%, while the same area would be affected.

Transition of Pb isotopic composition in the phase limit is not always abrupt. There are peak like changes and/or smoother ramps in some of the cores (Fig. 4a). Peaks belong to occasional turbulences and sediment overturn, while smooher ramps to thorough mixing. We labelled the three main types of cores with types 1–3, where 1 means abrupt change in Pb isotopic composition, 2 means peak like changes or smooth transition or occasionally both, while 3 is B5 that was taken outside of the sediment trap. Type 1 is characteristic of the western margin of the trench and shows less disturbance. We conclude that the area covering the first few meters from the entry point of dominant water stream will have higher accumulation rate and can preserve pollution records better. The middle of the sludge trap shows characteristics of sediment inversion suggesting higher level of mixing and remobilisation (Fig. 5c).

Concentration of many elements that are traditionally associated with agricultural and heavy industrial activities showed correlation with the Pb isotopic signature, being elevated in the upper (phase-2) part (Fig. 7). Many authors (Nriagu and Pacyna 1988, Kemp et al. 2012) attribute Sb to the extent of emission of road vehicles, thus expecting higher concentration values for the areas where fuel additive derived Pb contamination is present. We found slightly elevated Sb in phase-2 compared to phase-1. Fe concentration was slightly elevated concomitantly with Pb. The major source of iron is probably dust associated with agricultural activities, but other industrial activities can also be significant contributors. Deposition of iron is sensitive to different chemical environments. Iron and manganese hydroxides that form during precipitation provide high adsorbent surface that can aid the deposition of other metal elements to a large extent (Boyle 1995).

The increase of Zn, Cu and Cd concentration in phase-2 compared to phase-1 was significant. Since a discontinuity exists in the cores due to dredging, the correlation of these elements with the Pb concentration needs to be regarded carefully, they do not represent a real causative connection, but it is worth noting that expansion of agricultural, urban and industrial areas is in most cases associated with elevated concentrations or, in some cases, toxic levels of these elements (Wang et al. 2021). It is important to note at this point that none of the examined elements reached hazardous level in any of the samples, but the changes were significant.

Prominent changes in ²³⁴U/²³⁵U composition was seen in the four sediment records examined for U isotopic composition, a peak like transient change is seen in the profilae roughly overlapping with the inflexion of Pb isotope ratio profile (Fig. 9). U in these peaks is not enriched or depleted, only the proportion of isotope ²³⁴U is varying considerably. Ratio of ²³⁵U and ²³⁸U is unaltered. The U and Mo peaks emerged at the time of deep dredging and slightly afterwards. This is supported by the fact that dredging is an invasive intervention leading to large amounts of suspended matter that can fall back to the lake bed (Zhang et al. 2010, Van der Berg et al. 2001).

U can be immobilized by bacteria by bio-sorption or reduction under anaerobic circumstances (Kalin et al. 2005, Anderson and Fleischer 1991, Mann and Fyfe 1985). Such circumstances can arise in the aftermath of algal and cyanobacterial blooms as a result of increased organic matter load provided by the mass of dead cells (Rastogi et al. 2015). From the 1970s until the early 1990s, overproduction of N fixing cyanobacteria Cylindrospermopsis raciboskii was significant, leading to water blooms (Pálffy 2013). There is evidence that molybdenum is accumulated in algal cells and can get trapped in the sediment during water blooming, therefore an increased biomass production in the lake may increase the level of Mo in the sediment. (Glass et al. 2008). U isotopic composition, as well as its concentration together with Mo might be a retrospective marker of water trophity and algal blooms. Dredged material is likely to have composed of such sludge containing high amounts of organic matter and having reductive nature. The fall out after dredging may be the source of the characteristic U and Mo peaks. The source of background U is likely to be lithogenic as carbonaceous materials may contain U (Nyachoti et al. 2017), karst water in the Transdanubian Mountains does indeed contain U, as shown in many karst waters in Hungary (Jobbágy et al. 2010).

Trends of Sr isotopic compositions in cores that have 2 separate phases, in terms of Pb, shows a gradual increase in the bottom core and a reversal at the top of the core (Fig. 10). Although data presented are not significant per se but considering the presence of the same trend in two cores suggests that the changes are real. Besides motor vehicle usage and industrial emission, there was another important anthropogenic factor at the presented time scale, namely the massive extraction of karst water from the reservoirs at Transdanubian Mountains, associated with mining activity that was gaining momentum in the 1950s which lead to a massive drop in karst water level, and ended in the early 1990s, followed by a slow but more or less constant replenishment (Csepregi 2007). It is highly probable that slight changes in the Sr isotopic composition are at least partially attributable to the fluctuations of karst water level and thus the carbonate input to the Lake Balaton from that particular source. The small change in Sr isotopic composition is strikingly dissimilar to the massive changes in Pb isotopic composition, this further strengthens the view that elevation of Pb cannot be merely the result of changing geological environment.

Pb isotopic signature was significantly shifted in the upper fraction of the sediment cores of Lake Balaton concomitant with a 4 times increase in concentration. Isotopic signature shows close similarity to ores from Germany and Poland. Depth of Pb signature transition was used to map the sediment deposition rate in the dredged area. U and Mo concentrations and U isotopic signature is transient, peak like. We conclude that these markers are related to algal or cyanobacterial blooms and consequent nutrient burden near the shores.

Acknowledgments

We would like to express our gratitude to Zoltán Világosi and the sample collection team of Balint Analitika LLC. We wish to thank for precise work of Anna J. Horváth, Csongor Szalóki and Mária B. Nándori, assistants of inorganic chemistry laboratory. Financial support was granted by the National Research, Development and Innovation Office (Hungary, NKFIH, Nemzeti Kutatási Fejlesztési és Innovációs Hivatal), grant number: KFI_16-1-2017-0240.

Ethical Approval

Authors of this paper declare that due to the nature of research published (inorganic analysis of sediment samples), it does not require ethical approval.

Consent to participate

Human individuals, or samples derived from human sources were not subjects of this study

Consent for publication

All authors have given their full consent to publish their research.

Conflict of interest

Authors declare that there was no conflict of interest among participants of the project.

Author contributions

Daniel. E. Beyer and János K. P.ánczél designed the study and carried out data evaluation of isotopic analysis, Mária Varga and Páter Fazekas carried out measurements and data evaluation of concentrations.

Funding

This work was supported by the National Research, Development and Innovation Office (Hungary, NKFIH, Nemzeti Kutatási Fejlesztési és Innovációs Hivatal) grant number: KFI_16-1-2017-0240.

Author contributions: D. E. B. and J. K. P. designed the study and carried out data evaluation of isotopic analysis, M. V. and P. F. carried out measurements and data evaluation of concentrations.

Availability of data

Raw data generated during this study is available in .csv format.upon request to the contributing author.

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Distribution of Pb, Sr, U isotopic signature and multielement composition of sediment in Lake Balaton (Hungary) at a sediment trap deep dredged 40 years ago near Balatongyörök – Search for routes of recent pollution

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Abstract

Figures

Introduction

Sediment geochemistry

Isotopic composition analysis method comparison.

Determination of sediment deposition rate

Lake Balaton

Effects of dredging

Methods

Certified reference materials

Sampling area

Sample collection and preparation

Analysis of concentration and isotopic composition

Data handling

Results

Discussion

Conclusion

Declarations

References

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Journal Publication

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