Lead Fixation in Sediments of Protected Wetlands in Lithuania

Abstract

Lead (Pb) is a persistent and toxic heavy metal that threatens aquatic ecosystems. Wetlands act as natural filters, while beaver dams influence sediment deposition and metal retention. This study investigates Pb fixation in wetland sediments by analyzing its spatial and temporal variations, considering organic matter content and sediment composition. Pb concentrations were determined using flame atomic absorption spectrometry (FAAS), and fixation processes were assessed using concentration coefficients relative to background values (15 µg g⁻¹, Lithuanian Hygiene Standard HN 60:2004). A total of 165 sediment samples were collected during the spring and the autumn of 2022 and 2023 across three study sites. The results indicate that Pb fixation strongly correlates with organic carbon content, while sediment texture influences its mobility. A key finding is that beaver dams contribute to Pb retention by altering hydrodynamic conditions and sedimentation patterns. Despite sediment stability, new Pb inputs continue to enter water bodies, depending on pollution sources. However, Pb concentrations remain within background levels and do not exceed the Maximum Allowable Concentration (MAC). These findings are essential for wetland conservation and contribute to sustainable strategies for mitigating heavy metal contamination in aquatic ecosystems.

1. Introduction

1.1. Importance of Wetlands and Their Functions

Wetlands are transitional ecosystems between terrestrial and aquatic environments, characterized by the presence of water, hydric soils, and vegetation adapted to saturated conditions. According to the Ramsar Convention, wetlands include marshes, swamps, peatlands, and bodies of water—whether natural or artificial, permanent or temporary, with static or flowing water, fresh, brackish, or saline [1]. Wetlands are recognized to be a vital natural resource of great economic, cultural, scientific and recreational values to human society. They provide habitats for numerous species and are keystones in global carbon storage. Despite their ecological significance, wetlands have historically been perceived as low-value land in economic terms [2]. Historically, wetlands were formed due to prolonged waterlogging of the land surface, triggered by landslides, floods, fires, and deforestation [3,4,5]. The most important wetland ecosystems and their functions, biodiversity and human living conditions are protected by intergovernmental treaty, the Ramsar Convention [1], EU directives and national legal acts, realizing sustainable socio-economic development through local, regional and national action and international cooperation [6].

Also, wetlands provide countless opportunities for recreation, including hunting, fishing, hiking, and observing wildlife, closely linked with forestry and agriculture ensuring water supply, particularly in times of droughts. Wetlands purify water by removing sediments and other pollutants, including chemicals [7,8,9]. In Lithuania, wetlands do not achieve good status due to concentrated and spread contamination [10]. Acidic and anaerobic conditions in wetlands contribute to the slow decomposition of plant residues, leading to the accumulation of organic matter and the formation of peat. One of the essential functions of peat is to accumulate atmospheric carbon by storing photosynthesis products [11,12,13]. The content of organic matter in the peatlands can reach 18–95% of global terrestrial C biomass [14,15,16]. Wetlands improve the quality of water by acting as “living filters” and ecotones between terrestrial and aquatic systems [17]. Thus, wetlands accumulate heavy metals, and lead (Pb) is one among them. In a short time, Pb ions lose mobility in the soil because of the formation of poorly soluble compounds, absorption by colloids, and strong binding to humus [18,19,20].

1.2. Lead Pollution and Global Trends

Lead is attributed to cumulative toxicants [18,19,20,21,22,23,24]. The biological role of Pb in plants and animals remains unclear; however, its toxic effects have been well documented [25,26]. Its harmful effects have been proven due to its ability to replace essential magnesium and potassium in biologically active substances due to its ability to mimic and displace biologically essential metal ions, such as magnesium and potassium [27]. In accordance with EU’s obligations under the Geneva Convention on Long-Range Transboundary Air Pollution [17,28], emissions of heavy metals declined in comparison with levels in 1990, e.g., between 2005 and 2022, emissions have continued to decline, with lead emissions decreasing by 44%, mercury emissions by 53% and cadmium emissions by 39% across the EU-27 Member States [17] while in Lithuania, Pb emission shown relatively small increases in absolute emissions (4%) from low baseline levels [17]. The 2030 Agenda for Sustainable Development [29] provides for the improvement of water quality by reducing pollution at international and national levels. Mineralization of the dried organic soils and unreasonable use of fertilizers cause contamination of nearby water bodies and groundwater. Wetlands, namely swamps, marshes and bogs, are the most vulnerable to natural disturbances and anthropogenic activities [30]. Buffer zones and wetland systems serve as natural filters, mitigating nutrient and pollutant inputs from surrounding landscapes [21,22,23,24,25,26,27,28,29,30,31,32,33]. The importance of determination of buffer zones is underlined worldwide and as far back as 1933 [31]. The concept of buffer zones was used in the UNESCO program “Man and Biosphere” (MAB) [34], and the main attention was addressed to protected areas as biosphere reserves and national parks. In the last period, the question of buffer zones for wetlands had emerged due to possible Pb contamination.

Despite global efforts to reduce Pb emissions, this metal remains persistent in ecosystems due to its low mobility in the soil and strong affinity for organic matter. Wetlands act as natural filters for heavy metals, yet the effectiveness of Pb retention is highly dependent on local environmental conditions. The role of organic carbon, hydrodynamic processes, and ecosystem engineering by semi-aquatic species, such as beavers, in Pb stabilization has not been extensively studied. This study aims to address these gaps by investigating Pb accumulation and fixation mechanisms in sediments influenced by beaver activity, providing new insights into the biogeochemical regulation of Pb in protected wetlands.

1.3. Lead Distribution in Lithuanian Wetland Soils

The average lead (Pb) content in soils worldwide is 27 µg g⁻¹, while its background concentration ranges between 0.1 and 20 µg g⁻¹, depending on soil type and geochemical conditions [13,19]. However, in recent years, an increasing trend in the background level of Pb in soil has been observed due to anthropogenic activities [19].

According to the Lithuanian Hygiene Standard HN 60:2004 [35], the background Pb concentration in sandy and loamy soils in Lithuania is 15 µg g⁻¹. In comparison, in the neighbouring Grodno region of Belarus, the background Pb concentration is lower, at 6 µg g⁻¹ [36]. This difference is primarily attributed to geological characteristics and the composition of parent rock formations.

Lead is a major component of more than 200 known minerals [13], and its concentration in rock-forming minerals can reach 280 µg g⁻¹ [37]. In granite, Pb concentrations can reach 30 µg g⁻¹, while in basalt, it is approximately 5 µg g⁻¹ [37] (Taylor, 1964). The average natural Pb content in the Earth’s crust is estimated to be 12.5–16 µg g⁻¹ [38,39].

The main source of lead (Pb) contamination in soils remains anthropogenic activities, including heavy metal mining, industrial emissions, transportation, and wastewater discharge. Domestic wastewater, with varying levels of treatment, often introduces Pb into natural water bodies, contributing to its accumulation in sediments and surrounding soils [21].

The extent of Pb contamination varies by region and is strongly influenced by economic activities. In China, long-term studies from 1979 to 2016 demonstrate a clear relationship between Pb concentrations in soils and socio-economic factors, such as gross domestic product (GDP) [40].

However, not all Pb contamination is purely anthropogenic. On the Qinghai–Tibet Plateau, Pb levels in soils are primarily influenced by the composition of parent rock materials, highlighting the role of natural geological processes in certain environments [41].

In Lithuania, Pb concentrations in soils follow a similar pattern, where background levels are typically exceeded only in the upper soil layers (0–10 cm). However, in areas near industrial zones and contaminated sites, Pb levels are 1.5 to 2 times higher than background values, reflecting the impact of localized pollution sources [42]. Additionally, an analysis of bottom sediments in the Šventoji River confirmed significant heavy metal pollution in Lithuanian wetlands, particularly in areas subjected to high industrial and anthropogenic impact [43].

The regulation of lead (Pb) content in soil varies significantly worldwide, depending on soil type, composition, land use, and pH levels. In urban soils, Pb standards differ: in England, the permissible level reaches 300 µg g⁻¹, while the background concentration in Birmingham is only 19.02 µg g⁻¹ [21]. In Canada, Pb limits range from 500 to 1000 µg g⁻¹ [44], whereas in the United States, they can be as high as 2000 µg g⁻¹ [45].

For agricultural soils, Pb regulations are generally stricter. The maximum allowable concentration (MAC) varies by country: in Serbia [46] and South Africa [47], it is set at 100 µg g⁻¹, as in Lithuania [35]. Despite these limits, no significant Pb contamination has been detected in the agricultural soils of the Baltic States [42].

In Lithuania, natural Pb concentrations in soils range from 4.97 to 9.85 µg g⁻¹, increasing along the seacoast up to 15.4 µg g⁻¹, reflecting regional variations in geological composition and environmental factors [48].

1.4. Impact of Lead Contamination on Vegetation and Its Role in Wetland Buffer Zones

Lead (Pb) contamination in soils directly affects plant health, metal mobility, and overall ecosystem stability, particularly in wetland and riparian environments. The extent of Pb accumulation depends on soil properties, including pH, organic matter content [18], and mineral composition, which influence Pb bioavailability and plant uptake [13]. In wetlands and floodplain buffer zones, these processes are further modified by hydrodynamic conditions and biological interactions, making Pb distribution highly variable.

A key indicator of Pb impact on vegetation is phytotoxicity, defined as the metal concentration in soil that reduces plant productivity by 10% compared to a clean control sample [13]. However, Pb toxicity varies significantly depending on species-specific tolerance mechanisms and environmental factors, such as water saturation and redox conditions in wetland ecosystems [33].

Plants respond to Pb contamination by restricting metal translocation beyond the root zone, thereby limiting its accumulation in aboveground tissues. For instance, early studies demonstrated that Pb levels in wheat grains remained largely unchanged even when soil Pb concentrations exceeded 200 µg g⁻¹, suggesting low mobility beyond the root system [49]. In most species, Pb is predominantly retained in below-ground tissues, with normal concentrations ranging from 15 to 140 µg g⁻¹ [50]. However, variations exist: background Pb levels in plants typically range from 0.5 to 10 µg g⁻¹, with toxicity thresholds between 30 and 300 µg g⁻¹ [51,52,53].

Our previous research in wetland buffer zones found that Pb concentrations in aboveground plant tissues ranged from 0.95 to 6.84 µg g⁻¹, indicating that vegetation in these zones effectively limits Pb bioavailability [53]. Since these buffer zones serve as transition areas between terrestrial and aquatic ecosystems, their ability to retain contaminants before they reach open water bodies is crucial.

1.5. Lead Mobility, Bioavailability, and Remediation Strategies in Beaver-Modified Wetlands

Assessing Pb accumulation in plants and sediments within beaver-modified wetlands provides valuable insight into the role of ecosystem engineering in contaminant distribution. Understanding these interactions is essential for evaluating the long-term stability of Pb in wetland ecosystems and the effectiveness of natural filtration mechanisms in beaver-impounded areas. Lead phytotoxicity decreases with time spent in the soil. Initially, heavy metals accumulate in the top layer (0–10 cm) of the soil profile but eventually migrate deeper [54], typically in the form of carbonate or organic complexes [19]. However, it may become bioavailable over time if the soil becomes waterlogged and oxygen deficient [55].

With a low lead content, the soil itself can convert it into low-toxic immobile forms due to the organic component, primarily humic acids [52], which have a free pair of electrons—forming a coordination bond with the lead ion [19]. In addition, the mobility of lead ions in the soil quickly decreases due to the formation of poorly soluble phosphates, sulphates, carbonates, chromates, molybdates and hydroxides [18].

The highest concentrations of heavy metals are typically found in soil horizons with finer granulometric composition [42]. In uncontaminated sandy soils, Pb levels average around 6.8 µg g⁻¹ [53]. Pb is also present in fine particulate and sandy fractions due to its occurrence in rock-forming minerals and external pollution sources [13].

The mineral composition of soil determines its buffering capacity for heavy metals. Pb concentrations are often highest in silty and pre-silty layers of the upper soil horizon, where clay minerals, such as montmorillonite and illite, enhance Pb retention [18]. These minerals primarily adsorb Pb in exchangeable forms, making it more bioavailable. Montmorillonite, for instance, has an absorption capacity of 80–150 mg-eq per 100 g, with only 10% of Pb forming strong bonds, while the rest remains in a mobile, exchangeable state. Due to their high cation exchange capacity, silty soils are particularly prone to Pb accumulation, increasing both retention and mobility of heavy metals [56].

Although Pb is generally less mobile than other heavy metals, its interactions with soil organic matter significantly influence its migration. Humic acids bind up to 40% of the total Pb content in the upper soil horizon, with a retention capacity of 400 mg-eq per 100 g [51]. However, soluble humic acids and fulvic acids can transport Pb down the soil profile, especially under low pH conditions or in leaching environments, where they facilitate its movement into illuvial horizons [18,52,57,58]. Pb can also migrate in carbonate-bound forms (PbCO₃(aq) and Pb(CO₃)₂²⁻), though overall, it is less susceptible to leaching than cadmium, zinc, and copper [51].

Wetland soils form a dynamic interface between aquatic and terrestrial ecosystems, influencing sediment composition and contaminant retention [51,59]. Sediments originate from weathered rock particles that are transported and redeposited through erosion processes. In wetland environments, soil erosion contributes to sediment accumulation in streams and rivers, where heavy metals, including Pb, become trapped in relatively immobile forms.

1.6. Sediments as Indicators of Lead Contamination and the Role of Beaver Dams in Their Accumulation

This study focuses on Pb content in the sediments of Lithuanian water bodies, where pond–dam systems created by beavers (Castor fiber L.) serve as natural sediment traps. By reducing water velocity, beaver dams facilitate the settling of suspended particles, effectively storing large amounts of sediment and associated contaminants [31].

Sediments are widely recognized as indicators of water quality, as they accumulate insoluble heavy metals from suspended matter, aquatic biota, and wastewater. Global studies illustrate the extent of Pb contamination in sedimentary environments (

Table 1. Pb Concentrations in bottom sediments of various water bodies.

Location	Pb Concentration (µg g−1)	Possible Source of Contamination	Reference
Nile River	3.1–79.6	Urban runoff	[60,61]
Northern Black Sea	Increase by 1.2 times in 20 years	Industrial and port activities	[62]
Barents Sea (coastal sediments)	30–91.6	Wastewater discharges from mining	[63]
Oualidia Lagoon, Morocco	Moderate levels	Below toxic thresholds for biota	[64]

).

Beaver dams further enhance Pb retention by creating zones of reduced flow and increased sedimentation, functioning as natural filtration systems [31]. Beyond heavy metal sequestration, these structures play a crucial role in nutrient cycling and wetland ecosystem stability, making them important regulators of sediment-bound contaminants.

Beaver-modified wetlands create hydrologically distinct environments where Pb retention and mobility may significantly differ from naturally flowing waterbodies. Investigating these processes will contribute to a better understanding of the long-term stability of Pb in sedimentary systems, particularly in Natura 2000-protected areas [65], where conservation strategies must consider both biological and geochemical factors.

Therefore, this study aims to evaluate Pb fixation processes in the sediments of protected Lithuanian wetlands, assess the influence of organic carbon and anthropogenic factors on Pb accumulation, and investigate the role of beaver dams in mitigating Pb pollution and preserving ecosystem stability. Understanding Pb dynamics in these ecosystems is essential for developing sustainable wetland conservation strategies, enhancing biogeochemical models of heavy metal transport, and informing policy decisions on pollution mitigation and land management.

2. Materials and Methods

2.1. Study Area

The study was conducted in three territories (Figure 1) of Lithuania with various wetland types, such as wet forests, dried peatlands, drainage ditches, streams, bogs, other waterbodies. The zones were marked as follows (

Table 2. Characteristics and labelling of study territories.

Territories	1M MMMPV (Plateliai)	2K Kretinga (Lenkimai)	3G Dubrava (Girionys)
Part of Lithuania	Northwestern	Northwestern	Central
Locality	Žemaitija National Park	The Northern part of the Žemaitija Upland on the border with Latvia	Kauno Marios, Nemunas River
Sampling locality and coordinates
A	Ditch 56.019060868038; 21.865494507859	Riverside 56.149076; 21.228591	Ditch 54.793712; 24.078733
B	Stream 56.023279513354; 21.921513405938	Ditch (former railway) 56.140218; 21.261297	Pond 54.828811; 24.120541
C	Old “mother” beaver site 56.042144080106; 21.913789218674	Ditch (meadows) 56.113681; 21.341182	Lagoon stream 54.856685; 24.035010

): 1M—MMMPV (Plateliai) in the northwestern part of Lithuania, within the Žemaitija National Park; 2K—Kretinga (Lenkimai) in the northern part of the Žemaitija Upland, bordering Latvia; and 3G—Dubrava forests (Girionys) in the central Lithuania, close to the largest artificial waterbody, Kauno Marios, and the Nemunas River (Kauno Marių Regional Park). Each zone was divided into three sampling areas (Figure 2 1M, 2K, 3G) along a transect. Sampling points within each study site were labeled A, B, and C to ensure consistency with previous research. These labels correspond to distinct landscape features (e.g., ditches, streams, ponds) and are critical for tracking Pb accumulation patterns across different environmental matrices (sediments, vegetation, and beaver fur). The names, labels, and coordinates of the study sites are presented in Table 2.

The first study territory (1M) is characterized by mixed spruce-broadleaf forests (Oxalidosa). The predominant soils in this territory are Arenosols and Base-saturated Albeluvisols, with low to medium humus content (https://www.vle.lt/straipsnis/lietuvos-dirvozemiai/ accessed on 24 March 2025). The average annual temperature is +6.5 °C, and the total annual precipitation is 1012 mm.

The climate in the second study area (2K) is cooler, with less precipitation. The average annual temperature here is +5.4 °C, with 897 mm of annual precipitation. The soils include base-saturated Fluvisols, deeper loamy Podzols, and ordinary shallow clayey Luvisols.

The third study area (3G) is characterised by typical deeper loamy soils (Podzols). The average annual precipitation is 721 mm, and the average annual temperature is +7 to 7.5 °C (https://www.vle.lt/straipsnis/lietuvos-klimatas/ accessed on 24 March 2025).

Wetland vegetation plays a key role in Pb retention and redistribution within sediments. The dominant plant species in the study areas vary by ecosystem type and influence Pb dynamics through root adsorption, organic matter accumulation, and sediment stabilization.

In 1M (Žemaitija National Park), the landscape is characterized by mixed spruce-broadleaf forests and wetland areas, with tree species such as Picea abies (Norway spruce), Betula pendula (silver birch), Pinus sylvestris (Scots pine), and Salix spp. (willows), known for their ability to retain Pb in organic-bound forms. Wetland plants, including Carex spp. (sedges), Molinia caerulea (purple moor grass), and Typha latifolia (broadleaf cattail), create anoxic conditions, facilitating Pb precipitation in less mobile forms.

In 2K (Kretinga District), a former railway site, the vegetation consists mainly of marshy meadows and floodplain wetlands, with hyperaccumulator species such as Phalaris arundinacea (reed canary grass), Juncus effusus (soft rush), and Deschampsia cespitosa (tufted hairgrass), which actively retain Pb in roots and sediments. Aquatic macrophytes, including Potamogeton natans (floating pondweed) and Nymphaea alba (white water lily), serve as bioindicators of Pb contamination.

In 3G (Dubrava Forests, Kaunas Reservoir), riparian forests dominate, with Pinus sylvestris, Quercus robur (pedunculate oak), and Alnus glutinosa (black alder) influencing Pb retention in organic matter. Wetland species such as Phragmites australis (common reed) and Carex rostrata (beaked sedge) promote Pb stabilization in sediments, while semi-aquatic plants like Myriophyllum spicatum (Eurasian watermilfoil) help monitor Pb contamination in aquatic environments.

The studied wetlands include protected areas, with Žemaitija National Park, part of the Kaunas Reservoir region, and one site in Kretinga District belonging to the Natura 2000 network [61]. These areas are designated for conservation due to their ecological importance and biodiversity. Understanding the processes of lead accumulation and fixation in these regions is crucial for ensuring sustainable environmental management.

2.2. Sampling

In each study area, composite samples of bottom sediment were collected using a vacuum sediment borer (inner diameter—63 mm, height—485 mm). At each site, five composite samples were taken from the beaver dam (n = 2), upstream (n = 2), and mid-pond (n = 1) (Figure 3). All collected sediment samples were immediately stored in sterile polyethylene containers and delivered to the laboratory. Until the analyses, the samples were stored in the freezer at −18.

A total of 165 sediment samples were collected over a two-year period (2022–2023), covering two seasonal sampling periods per year. However, in 2023, sampling at the 3G site was not conducted during the autumn period, reducing the total number of samples. Sampling was conducted at nine study sites (as detailed in Table 2), with five sediment samples taken at each site during two distinct periods: late May–June (spring) and late October–November (autumn) of 2022 and 2023.

The sediment samples collected from the study areas reflected distinct textural and compositional properties, influenced by underlying soil types and hydrodynamic conditions.

In 1M (Žemaitija National Park), where Arenosols and Base-saturated Albeluvisols dominate, sediments were primarily sandy or mixed clay–sandy, with moderate organic content. These soils promote high permeability and moderate organic matter accumulation, which affects Pb sorption potential. The presence of mollusk shells and coniferous litter indicates active biological input and sediment reworking. Sampling sites included a ditch (1M-A) and a stream (1M-B), with an old beaver-modified water body (1M-C).

In 2K (Kretinga District), sediments varied significantly across sampling sites due to the presence of Fluvisols, Podzols, and Luvisols. Territory 2K-A contained darker, organic-rich sediments, corresponding to Fluvisols, while 2K-B exhibited compact, fine-textured sediments with high organic matter content, characteristic of Luvisols. The lighter-colored, sandy sediments at 2K-C, likely associated with Podzols, suggest higher Pb mobility due to lower sorption capacity. Sampling sites included a riverside (2K-A), a ditch along a former railway (2K-B), and meadow-ditch sediments (2K-C).

In 3G (Dubrava Forests, Kaunas Reservoir), where Podzols are prevalent, sediments exhibited a mix of sandy and clay-rich textures, with coniferous litter and mollusc shells widely present. This reflects the influence of riparian forests and hydrological processes affecting sediment deposition. Sampling sites included a ditch (3G-A), a pond (3G-B), and a lagoon stream (3G-C).

These variations in sediment texture, organic matter content, and mineral composition play a crucial role in Pb retention and mobility, providing a geochemical framework for interpreting Pb accumulation patterns in the studied wetlands.

2.3. Sample Preparation and Analysis

The sediment samples were dried to a constant weight at 45 °C, ground in a mill Mortar Grinder Pulverisette 2 (Fritsch GmbH, Idar-Oberstein, Germany), and sieved through a 2.0 mm mesh. Wet mineralization of samples was conducted with 50% nitric acid (HNO₃) and hydrogen peroxide (H₂O₂) at 80–140 °C in a FOSS KelROS Digestor. The lead content in the sediments was determined using the atomic absorption method on an Atomic Absorption Spectrometer AAnalyst 200 (Perkin Elmer Inc., Shelton, CT, USA) at a wavelength of 283.0 nm.

The organic carbon content in sediment samples was determined using a modified method for soil organic carbon determination in agricultural soils [66]. This method relies on the oxidation of organic matter in an acidic medium, with quantification of OC performed spectrophotometrically at a wavelength of 590 nm using glucose as a standard.

2.4. Statistical Analysis

The statistical analysis was conducted using IBM SPSS Statistics version 25 (IBM Corp., Armonk, NY, USA). A linear regression analysis was performed to evaluate the relationship between Pb concentration in sediments and organic carbon content. The coefficient of determination (R²), sample size (n), and p-value were calculated to assess statistical significance. Prior to regression analysis, the normality of data distribution was tested using the Shapiro–Wilk test. Differences in Pb concentrations between study sites and seasons were analyzed using one-way ANOVA followed by Tukey’s post hoc test. Pearson correlation coefficients (r) were used to examine the strength of associations between variables. Statistical significance was set at p < 0.05, and all results are presented as mean ± standard deviation (SD) unless otherwise specified.

The relationship between Pb concentration in sediments and organic carbon content was analyzed using linear regression. The regression equation was determined as follows:

$${\mathrm{C}}_{\mathrm{P}\mathrm{b}}=\mathrm{16,297}+6466·\mathrm{O}\mathrm{C}$$

(1)

3. Results

No significant lead pollution was detected in sediments in wetland areas, with the maximum lead concentration in the sediments being 45.1 µg g⁻¹, while the Lithuanian Hygiene Standard HN 60:2004 [35] sets the maximum permissible lead concentration in soil at 100 µg g⁻¹.

Relatively high lead content was found in the territory 2K, where the average range was 18.05–34.54 µg g⁻¹, with a median of 29.99 µg g⁻¹ (Figure 4). The medians for the other two territories were 16.13 and 20.7 µg g⁻¹, respectively.

The highest lead levels were recorded close to the former railway in 2K-B (Railway), with a maximum of 45.1 µg g⁻¹ and a median of 33.8 µg g⁻¹ (Figure 5). High levels of Pb were also recorded in the meadows of 2K-C: maximum 40.38 µg g⁻¹, with a relatively low median of 15.45 µg g⁻¹. In Žemaitija National Park 1M-C, the maximum was 40.09 µg g⁻¹, with a low median of 18.36 µg g⁻¹. The sampling area 2K-A had a high median of 25.92 µg g⁻¹ and a maximum of 37.48 µg g⁻¹. The lowest median of 13.27 µg g⁻¹ was recorded at 1M-A in Žemaitija National Park.

The pollution level of site 2K-B is assessed as moderate, with the median concentration coefficient relative to the background value (15 µg g⁻¹ established by the Lithuanian Hygiene Standard HN 60:2004 [35]) being 2.25 (Figure 6), while the maximum value of this coefficient does not exceed 3.01. At site 1M-A, the concentration coefficient is below the background value (0.89), possibly due to the presence of mobile forms of lead that are easily leached by water. This issue requires further investigation. Territories 1M-B and 2K-C can be considered unpolluted, as their concentration coefficients are close to the background lead content (medians of 1.14 and 1.06, respectively). The remaining sites in the study can be considered moderately polluted.

The analysis of Pb concentrations in bottom sediments across different years (2022 and 2023) showed no statistically significant differences, with median values of 18.28 and 20.79 µg g⁻¹, respectively (Figure 7). Seasonal variations were also minimal, contributing to only 0.2% of the overall variability (Figure 8). These findings suggest that temporal factors play a minor role in Pb accumulation dynamics in the studied wetlands.

Regarding spatial distribution, higher Pb concentrations were consistently observed at sampling points located in flowing water areas (points 3, 4, 5; Figure 9), with median values of 21.14, 22.42, and 22.51 µg g⁻¹, respectively. In contrast, Pb levels were lower within beaver-dammed zones (points 1 and 2), where median concentrations were 15.88 and 16.91 µg g⁻¹. These results suggest that beaver dams function as natural sediment filters, trapping fine particles and reducing Pb mobility within the aquatic system.

A correlation was also determined between lead content in sediments and the presence of organic carbon (Figure 10). Higher lead accumulation was observed in sediments with elevated organic carbon concentrations (15–30%), supporting the role of organic matter in Pb fixation (Figure 10). The most contaminated sites, 2K-A and 2K-B, also had the highest organic carbon content (11.8–27.0%), suggesting that lead is primarily immobilized as metal–organic complexes.

The statistical analysis revealed a moderate but significant correlation between Pb concentration and organic carbon content in sediments. The Pearson correlation coefficient (r = 0.485, p < 0.0001) and Spearman correlation coefficient (r = 0.417, p < 0.0001) indicate that organic carbon plays a role in Pb retention. However, the coefficient of determination (R² = 0.235) suggests that only 23.5% of the variation in Pb concentration can be explained by organic carbon content, implying that additional factors, such as sediment texture, hydrological conditions, and local pollution sources, contribute to Pb accumulation patterns.

Moreover, the analysis of variance (ANOVA, p = 0.21) and the Kruskal–Wallis test (p = 0.66) showed no significant differences in Pb concentrations between years or seasons, indicating that Pb retention in sediments is relatively stable over time. Given these findings, future research should incorporate additional variables, such as sediment granulometry and pH, to better understand Pb fixation mechanisms in wetland ecosystems.

The spatial and temporal analysis of Pb concentrations in bottom sediments showed differences in accumulation patterns across study sites over the two-year observation period. Figure 11 classifies sampling sites based on Pb concentration levels relative to the background value (15 µg g⁻¹) using a color-coded scheme:

• Red marks areas where Pb concentrations exceeded the background level by more than 20%, indicating active Pb input from external sources.
• Yellow represents sites where Pb concentrations remained within ±20% of the background level, suggesting stabilization in relatively immobile forms.
• Green indicates areas where Pb content was lower than the background level, which may be associated with leaching, plant absorption, or sediment transport.

The results showed that 2K-B exhibited the highest Pb concentrations, exceeding background levels by 2.5–3 times, while 2K-A and 2K-C demonstrated decreasing trends over time. Pb accumulation in 1M and 3G was more stable, with variations depending on site-specific hydrological conditions.

These findings provide a quantitative assessment of Pb distribution patterns in wetland sediments. The interpretation of these trends and their environmental implications are discussed in Section 4.4.

4. Discussion

This study demonstrated that despite the relatively low lead concentrations (<100 µg g⁻¹) in the bottom sediments of water bodies in northwestern and central Lithuania, Pb continues to enter these ecosystems annually from external sources, including atmospheric deposition, soil leaching, and plant uptake. However, the magnitude and dynamics of Pb accumulation vary across the study areas, primarily depending on sediment composition, proximity to pollution sources, and local hydrological conditions. The results indicate that Pb retention is strongly linked to organic matter content, sediment texture, and the presence of beaver-modified hydrological systems (Figure 11).

4.1. Role of Organic Carbon in Lead Fixation

One of the key findings of this study is that sediments rich in organic carbon (11.8–27.0%) exhibit a strong Pb retention capacity, leading to its immobilization in less bioavailable forms. Organic matter facilitates Pb sequestration through chelation, complexation with humic acids, and precipitation as organo–metallic compounds [18]. Additionally, the presence of anaerobic conditions in organic-rich sediments enhances Pb stabilization, promoting its incorporation into sulphide and carbonate phases, thereby reducing its mobility [13,67,68,69].

The influence of soil-derived organic matter on Pb retention is particularly evident in 2K-B, where sediments contain high levels of decomposed plant material and clay fractions, which are characteristic of Fluvisols and Luvisols. These conditions enhance Pb complexation with humic substances, reducing its solubility. Conversely, sandy sediments in 2K-C, associated with Podzols, exhibit lower Pb retention capacity, likely due to reduced surface area and weaker sorption properties. The trends observed in Pb dynamics (Figure 11) confirm that organic-rich sediments maintain higher Pb concentrations, whereas lighter sandy sediments facilitate Pb leaching and transport over time.

4.2. Impact of Beaver Dams on Lead Distribution

The results indicate that bottom sediments in beaver-dammed water bodies contain significantly lower Pb concentrations (15.88–16.91 µg g⁻¹) compared to open-flowing sites, suggesting that Pb is primarily transported in dissolved or colloidal forms, with hydrological flow playing a major role in its redistribution (Figure 11).

Previous studies have suggested that beaver dams act as natural sediment traps, limiting the downstream transport of Pb and contributing to its accumulation in bottom sediments [30,69,70,71,72,73]. However, our results indicate that Pb concentrations are lower within the beaver dam itself compared to downstream flowing water (Figure 9), suggesting that Pb accumulation may occur further along the sedimentation pathway rather than directly within the dam structure. This discrepancy may be explained by differences in sediment texture, water flow dynamics, and organic matter composition, which influence Pb retention and mobility in beaver-modified wetlands. Further research is needed to assess the long-term stability of Pb within different compartments of beaver-impounded waterbodies.

Additionally, the organic-rich environment within beaver-dammed water bodies may further facilitate Pb stabilization, as organic matter enhances metal complexation, reducing Pb bioavailability and preventing its remobilization [30,74]. Similar patterns have been observed for nutrient and carbon retention in beaver-impounded wetlands, highlighting the importance of these structures in shaping biogeochemical cycles.

Since some of the studied wetlands are part of the Natura 2000 network, including Žemaitija National Park, one location in Kretinga District, and part of the Kaunas Reservoir region, understanding lead fixation in these areas is crucial for conservation and pollution management strategies. The role of beaver dams in sediment retention within these protected areas may contribute to the maintenance of water quality and overall ecosystem stability.

4.3. Spatio-Temporal Variability of Lead Concentrations

The spatial and temporal analysis of Pb distribution in bottom sediments (Figure 11) demonstrated that Pb accumulation patterns vary across study sites, influenced by local pollution sources, hydrodynamic conditions, and sediment characteristics. Over the two-year observation period, Pb concentrations showed significant spatial differences. Site 2K-B, located near a former railway, exhibited notably higher Pb accumulation, suggesting continued anthropogenic input. In contrast, sites 2K-A and 2K-C displayed self-cleaning tendencies, likely due to the leaching of soluble Pb forms or sediment transport. Sites in 1M and 3G showed annual Pb accumulation trends, indicating that these wetland ecosystems serve as long-term Pb retention zones.

Seasonal fluctuations in Pb concentrations were minimal, with differences accounting for only 0.2% overall (Figure 8). This suggests that Pb remains relatively stable in sediment matrices with limited seasonal mobility. However, studies in other regions, such as the Uzh River (Ukraine) [75], report higher Pb migration in spring and summer, likely due to increased hydrological activity. The absence of pronounced seasonal variation in our study may be attributed to local sediment composition and hydrodynamic conditions.

These findings confirm that Pb retention in wetland sediments depends on a combination of organic matter content, sediment texture, and external pollution sources. The role of beaver dams in modifying sedimentation patterns and Pb fixation further highlights the need for considering natural ecosystem engineering in pollution management strategies.

4.4. Long-Term Monitoring and Environmental Management

The observed Pb concentrations in bottom sediments suggest the need for long-term monitoring to track accumulation trends and potential environmental risks. While Pb levels remain below critical thresholds, continuous inputs from anthropogenic sources indicate the possibility of future increases, particularly in areas influenced by industrial activities and transportation.

Given the variability in Pb concentrations across sites (Figure 4 and Figure 5), further research should focus on identifying the key environmental factors controlling Pb retention and mobility in wetland sediments. Future studies could incorporate detailed geochemical analyses to assess Pb speciation and potential bioavailability under changing environmental conditions.

Regular sediment monitoring in protected wetland areas [76], including Natura 2000 sites, will provide valuable insights for conservation planning and pollution mitigation efforts.

4.5. Future Research Directions: Bioaccumulation and Remediation Strategies

Beyond sediment analysis, understanding Pb bioaccumulation in wetland vegetation and wildlife is a crucial research direction. Identifying key plant and animal species that accumulate Pb could provide valuable indicators for ecological monitoring. In this context, phytoremediation using native wetland plants may offer a sustainable approach to mitigate Pb contamination [77], leveraging plant uptake and stabilization mechanisms.

Furthermore, the application of natural sorbents [78,79,80], such as biochar, clay minerals, and organic amendments, to reduce Pb mobility in soils and sediments warrants further study. Assessing the effectiveness of these materials under field conditions would provide valuable insights for developing cost-effective remediation strategies.

4.6. Implications for Wetland Conservation and Policy Recommendations

The preservation and restoration of wetlands require a multifaceted environmental protection strategy. One key approach involves establishing buffer zones of forest and meadow ecosystems around wetlands, which can act as natural barriers against Pb input from surrounding areas. Additionally, developing a network of monitoring stations to conduct regular Pb assessments in water, sediments, and biological indicators is necessary for long-term ecological risk assessment.

Public awareness and community engagement also play a crucial role in wetland conservation. Educational programs targeting local communities and environmental management authorities can improve the understanding of Pb contamination risks and promote the adoption of sustainable technologies for wetland protection.

5. Conclusions

This study examined lead (Pb) fixation in the sediments of protected wetlands in Lithuania, highlighting the role of organic carbon in Pb immobilization. The findings confirm that wetlands, even in conservation areas, continue to receive Pb inputs from external sources, with spatial variability influenced by local pollution sources and sediment composition.

Pb concentrations in bottom sediments showed a significant correlation with organic carbon content, indicating that organic matter plays a key role in Pb retention. However, the strength of this correlation suggests that other factors, such as sediment texture and hydrological conditions, also contribute to Pb mobility.

Beaver dams were found to influence Pb distribution in sediments, with Pb concentrations inside the dams being lower than in the surrounding waterbodies. This suggests that beaver-modified landscapes create conditions for the dilution or stabilization of Pb rather than its accumulation. These findings emphasize the ecological role of beavers in maintaining sediment quality and reducing contamination levels in wetland ecosystems.

The study confirms that wetlands function as natural filters for heavy metals, but continued monitoring is necessary to assess long-term Pb accumulation trends and potential environmental risks. Future research should focus on detailed Pb speciation analysis, long-term monitoring of Pb fluxes in wetland ecosystems, and the role of bioaccumulation in wetland vegetation and fauna.