An Evaluation of Mercury Accumulation Dynamics in Tree Leaves Growing in a Contaminated Area as Part of the Ecosystem Services: A Case Study of Turda, Romania

Abstract

Mercury (Hg) poses a significant threat to human health and ecosystems, garnering increased attention in environmental studies. This paper evaluates the dynamics of Hg accumulation in various common tree leaves, specifically white poplar, linden, and cherry plum, throughout their growing season. The findings offer valuable insights into air quality and the ability of urban vegetation to mitigate mercury pollution in urban areas. A case study was conducted in Turda, a town in northwestern Romania, where a former chlor-alkali plant operated throughout the last century. Although the plant ceased its electrolysis activities over 25 years ago, the surrounding soil remains contaminated with mercury (Hg) due to the significant amounts released during its operation. The results indicated that the Hg concentration varied between 2.4 and 7.3 mg kg⁻¹ dry weight (dw), exceeding the intervention threshold for soil of 2.0 mg kg⁻¹. Additionally, the Hg content in the leaf samples consistently increased over time, influenced by leaf age and tree species. The Hg content increased in the following order: cherry plum < white poplar < linden. On average, white poplar leaves accumulated 72 ng Hg g⁻¹ dw, linden leaves 128 ng Hg g⁻¹ dw, and cherry plum leaves 47 ng Hg g⁻¹ dw during the six-month monitored period from April to September. The results obtained can be used to evaluate the potential of different tree species for mitigating atmospheric Hg contamination and to elaborate on the suitable management of fallen leaves in the autumn.

1. Introduction

Contamination of soil with toxic elements poses a significant risk to both the biosphere and the atmosphere. Consequently, the concentrations of these toxic elements in soil have been extensively studied over the past few decades [1,2,3,4]. Among the toxic elements, mercury (Hg) is garnering increasing attention in environmental studies [1,5]. When the Hg levels in the soil exceed specific thresholds, the risk of its transfer from the soil to the atmosphere and to living organisms increases [6]. Since mercury (Hg) is not an essential element, even trace amounts in the environment can have a significant impact on human health, resulting in neurological and kidney disorders [7].

Hg in soil can originate from both natural and anthropogenic sources. The natural concentration of Hg in soil is generally low, typically ranging from 0.01 to 0.2 mg kg⁻¹ [8,9]. Anthropogenic sources of mercury, including the chemical industry, chlor-alkali plants, fossil fuel combustion, and the mining industry, have led to a continuous increase in mercury levels in the environment over time [10,11,12,13]. It is estimated that soil is the largest reservoir of Hg in the environment, containing approximately 250 × 10⁶ to 1000 × 10⁶ kg of total mercury [14].

In the atmosphere, the majority of mercury exists as gaseous elemental mercury (GEM, Hg⁰), which accounts for over 95% of the total atmospheric Hg. Other forms of atmospheric Hg, although present in smaller quantities, include gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM) [15,16]. GEM is recognized for its long residence time in the atmosphere (6 to 24 months), which enables its global distribution as a widely dispersed atmospheric pollutant [17]. The exchange of elemental Hg between the atmosphere and soil or terrestrial surfaces remains incompletely understood. The deposition of Hg from the atmosphere involves a combination of the dry deposition of Hg⁰ and the deposition (either dry or wet) of particulate-bound mercury and GOM. The re-release of Hg from terrestrial surfaces primarily involves Hg⁰. In this exchange, vegetation represents a significant missing sink in the overall mass balance of mercury, indicating that vegetation sequesters more than 1000 tons of Hg annually from the atmosphere [18]. Thus, urban vegetation can significantly influence the Hg cycle, decreasing the Hg concentration in the urban atmosphere.

Tree leaves absorb Hg during the growing season, thereby contributing to air purification [19]. Due to the limited uptake of Hg from soil by plant roots and the low mobility of Hg in soil, the atmosphere serves as the primary source of Hg accumulation in vegetation [20,21,22]. Leaves serve as the primary pathway for the accumulation of elemental mercury (Hg⁰) in vegetation, a process facilitated by stomatal diffusion [15]. Therefore, the presence of Hg in soil, as well as in tree leaves and wood in contaminated areas, has been the subject of several recent research studies [1,15,17,18,19,20,23,24,25,26,27,28,29].

Turda is a town located in Cluj County in the central part of Romania, where a Chemical Plant was established in 1911 [30]. In 1964, the Chemical Plant started producing hexachlorocyclohexane, of which production stopped in 1983. Simultaneously, a chlor-alkali electrolysis plant has used liquid mercury as a cathode to produce NaOH and Cl₂ since 1958. Even though in 1998 the plant’s activity stopped, it caused significant Hg leaks, accounting for over 100 tons of Hg released into surroundings. After 2000, the Chemical Plant was demolished, but inadequate management during the closure process is believed to aggravate the issues of Hg pollution in the environment [31].

Previous studies [32] revealed elevated total Hg concentrations in soil, exceeding the legislative alert and intervention thresholds of 1 mg kg⁻¹ and 2 mg kg⁻¹, respectively, as established by Romanian legislation [33]. Additionally, air contamination with Hg in Turda was assessed through Hg analysis in the air and the examination of Salix alba L. tree samples [25]. However, an evaluation of Hg accumulation dynamics in the leaves of common tree species growing in Turda has not been conducted previously.

We hypothesized that Hg content would increase in leaf tissues with the duration of exposure, and this may contribute to the minimizing of Hg content in the air. Therefore, the objectives of this study are as follows: (1) to determine the total Hg concentrations in composite soil samples from four locations in the urban area of Turda, as well as in the leaves of common tree species found in these locations, including white poplar (Populus alba), linden (Tilia Europaea Pallida), and cherry plum (Prunus cerasifera); (2) to assess the dynamics of Hg accumulation in the tree leaves of three types of deciduous species collected at one-month intervals during the growing season from April to September 2024; and (3) to estimate the potential of tree leaves to mitigate air contamination by Hg as part of the ecosystem services provided by urban forests.

2. Materials and Methods

2.1. Study Area Description and Sampling

The Turda area is located in the Aries Valley hydrological corridor, traversed by the Aries River from west to east (Figure 1). The town of Turda, with approximately 56,000 inhabitants, consists of four neighborhoods: Turda Veche, Turda Nouă, Oprișani, and Poiana.

Four sampling locations (designated S1—Hărcana, S2—Poșta Rât, S3—Castrul Roman, and S4—industrial zone) were chosen due to containing specimens of the three common deciduous species: white poplar (Populus alba), linden (Tilia Europaea Pallida), and cherry plum (Prunus cerasifera). From each sampling location, a composite soil sample and leaves from each selected tree species were collected. Sampling locations in this study were distributed considering the four geographical directions around the former Chemical Plant, now in ruins (Figure 2).

Surface soil samples (1–10 cm) were collected from each sampling location in April 2024. Approximately 500 g of soil was obtained from three sampling points at each location after removing debris from the soil surface using stainless steel tools to create a composite soil sample for each site. At each sampling location, four trees from each tree species were selected, from which at least 20 leaves were collected at heights of approximately 1.5 to 2 m. A composite leaf sample was obtained from each tree (about five leaves in a composite sample). Photographs of the tree species used for leaf sampling are presented in Figure 3.

The collection of leaf samples was repeated at one-month intervals from April to September, resulting in a total of six collection campaigns. The samples were transported in plastic bags to the laboratory within 24 h, where they were washed with tap water followed by ultrapure water. Subsequently, the samples were dried in a laboratory oven at 40 °C for 36 h. Once dried, the samples were ground using a grinder and sieved to a particle size of less than 100 µm. The soil samples were air-dried at room temperature for 48 h, homogenized, and sieved to a particle size of less than 2 mm. Aliquots of the soil samples were ground in an agate mortar and sieved below 100 µm. All samples were stored in sealed bags before chemical analysis.

2.2. Sample Analysis

The total mercury (Hg) concentration in dried soil and leaf samples was determined using an automated Direct Hg Analyzer Hydra-C from Teledyne Instruments, Leeman Labs (Hudson, NH, USA), based on thermal desorption atomic absorption spectrometry (TD-AAS) [34,35]. The operating conditions included a drying temperature of 300 °C for 40 s, a decomposition temperature of 800 °C for 150 s, a catalyst temperature of 600 °C, a desorption time of 60 s, and a measurement time of 90 s, with a Hg absorbance wavelength of 253.7 nm. Aqueous Hg calibration standards were prepared from a stock solution of Hg (1000 mg L⁻¹) from Merck (Darmstadt, Germany), with metrological traceability to NIST SRM 3133.

Calibration standards of 0.1 mg L⁻¹ and 1.0 mg L⁻¹ were used at six different weights for each calibration curve (high-sensitivity and low-sensitivity curves). Two certified reference materials (CRMs) of soil (CRM048-50G trace metals sand 1) and a vegetable (GBW10020—citrus leaf standards) were used to validate the method’s trueness (recovery ± 15%) and precision (<10%). Triplicate analyses of each homogenized soil and leaf sample were carried out to determine Hg content. A check standard with a concentration of 1.0 mg L⁻¹ was analyzed for every 10 samples. The limits of detection (LOD) and quantification (LOQ) were 0.3 µg kg⁻¹ and 1.0 µg kg⁻¹, respectively, calculated based on the signal-to-noise ratio for a sample mass of 0.1 g [36].

2.3. Geo-Accumulation Index (Igeo)

The geo-accumulation index (Igeo) was used to assess the level of soil contamination with Hg by assessing the concentration of Hg above the normal value (0.05 mg kg⁻¹). We calculated the geo-accumulation index (Igeo) according to Equation (1) [18]:

$$I_{geo}={log}_2\left[{\displaystyle \frac{C_i}{K\times B_i}}\right]$$

(1)

where Igeo is the geo-accumulation index; C_i represents the determined content of Hg in soil (mg kg⁻¹); K signifies the modified index (1.5) for considering the variations in the background values; and B_i represents the soil background value (0.05 mg kg⁻¹). The Igeo consists of seven grades, ranging from unpolluted (Igeo < 1) to very highly polluted (Igeo ≥ 6).

2.4. Mercury Accumulation Percentage

For improved visualization of mercury accumulation dynamics in leaf samples, the data were also expressed as consecutive accumulation percentages (Ac, %). This was based on the Hg content accumulated, which was recorded every month during the growing period, and calculated according to Equation (2).

$$Ac\left(\%\right)={\displaystyle \frac{C_{HgTx}-C_{HgTx-1}}{C_{HgTf}}}\times 100$$

(2)

where C_HgTx is the Hg content of each tree species in a specific sampling location at sampling time T, C_HgTX−1 indicates the Hg content at the previous sampling time, while C_HgTf refers to the Hg content of each tree species at the last sampling campaign.

2.5. Statistics

Data analysis was conducted using OriginLab software (version 2023b; OriginLab Corporation, Northampton, MA, USA). To assess statistical differences among the three sample groups—white poplar (WP), linden (L), and cherry plum (CP)—the means were compared using Tukey’s HSD post hoc test at a significance level of p ≤ 0.05, implemented in OriginPro.

3. Results and Discussion

3.1. Concentration Levels of Mercury in the Soil and Geo-Accumulation Index

The concentrations of total Hg in the four composite soil samples from Turda ranged from 2.4 to 7.3 mg kg⁻¹, consistently exceeding the intervention threshold for soil in residential areas of 2 mg kg⁻¹ [33]. These elevated concentrations were detected not only near the Chemical Plant but also in more remote areas of the town. In a previous study, Frentiu et al. [32] also indicated that the soil in Turda is contaminated by Hg, and reported a larger concentration interval, with a range of 0.074 to 114 mg kg⁻¹. In a different region of France affected by a chlor-alkali plant, soil mercury concentrations were comparatively lower, ranging from 0.043 to 0.844 mg kg⁻¹ [10]. In contrast, significantly higher Hg levels have generally been observed in soils from cinnabar mining regions. For instance, Bauštein et al. [6] reported total mercury concentrations between 0.221 and 135 mg kg⁻¹ in soils from two former cinnabar mining sites in the Czech Republic. Even higher concentrations were documented in Spanish mining areas, with values ranging from 5 to 778 mg kg⁻¹ in Usagre [37] and from 122 to 550 mg kg⁻¹ in Almadén [38]. The geo-accumulation indices and their indication of the pollution grades for each sampling location are presented in

Table 1. Concentrations of Hg (average ± stdev) in composite soil samples from each sampling location and geo-accumulation indices.

Sampling Location	Hg Concentration (mg kg−1 dw)	Igeo
S1	7.3 ± 0.84	6.6
S2	4.6 ± 0.25	5.9
S3	2.4 ± 0.20	5.0
S4	5.5 ± 0.38	6.2

. According to Adriano [8], the normal content of Hg in uncontaminated soil is approximately 0.05 mg kg⁻¹.

The geo-accumulation index consists of seven grades of contamination, ranging from unpolluted to very highly polluted. A value of 0 indicates unpolluted soil, while a value of 6 signifies very highly contaminated. Based on the calculated I_geo values presented in Table 1, the soil at sampling locations S2 and S3 exhibited high to very high contamination levels of mercury (Hg), whereas locations S1 and S4 were classified as very highly contaminated. Among these sampling points, the location S4 is located close to the Chemical Plant in the industrial zone of Turda, which explains the high contamination of the soil. The samples S1 and S2 are located to the west direction in the Aries River corridor which influences the air current direction and thus transports the gaseous Hg in that direction. Interestingly, the S1 location, which is situated farther from the former Chemical Plant than the S2 location, is more contaminated with mercury. One possible explanation is that S1 is located on a hill at a higher altitude, making it more susceptible to atmospheric mercury deposition. Location S3 has the lowest mercury concentration in the soil, as it is situated in an area that is more sheltered from air currents. In addition, the variability of Hg dispersal and the high levels of pollution in areas outside the primary pollution source align with findings from other studies in this region, which report significant variances among sampling points that are near each other [32].

3.2. Mercury Concentrations in the Leaves of the Different Tree Species

Hg concentrations were measured in leaves collected from white poplar, linden, and cherry plum in each of the four sampling locations during the period from April to September 2024. The Hg content in cherry plum leaves ranged from 8 to 48 µg kg⁻¹ dw at location S1, 14 to 67 µg kg⁻¹ dw at location S2, 8 to 49 µg kg⁻¹ dw at location S3, and 22–75 µg kg⁻¹ dw at location S4. For white poplar, the Hg content in leaves ranged from 16 to 88 µg kg⁻¹ dw at S1, 23–98 µg kg⁻¹ dw at S2, 9 to 73 µg kg⁻¹ dw at S3, and 16 to 91 µg kg⁻¹ dw at S4. Linden leaves accumulated the highest Hg concentrations among the monitored species: 22 to 139 µg kg⁻¹ dw at location S1, 16 to 198 µg kg⁻¹ dw at S2, 12 to 89 µg kg⁻¹ dw at S3, and 18 to 154 µg kg⁻¹ dw at S4.

The variations in Hg concentrations in the leaves of the monitored tree species during the growing period in the four sampling locations are presented in Figure 4.

In a previous study conducted by Esbri et al. [25] in Turda, the leaves of Salix alba L. trees were utilized for air quality biomonitoring. The authors reported Hg contents in the leaves ranging from 23 to 140 µg kg⁻¹ in most of the leaf samples. However, one sample collected in the vicinity of a Chemical Plant exhibited an exceptionally high concentration of 4600 µg kg⁻¹. Excluding this outlier, the Hg levels reported in that study are consistent with the concentrations observed in our investigation. Esbri and collaborators [25] also measured total gaseous mercury and reported a maximum Hg level in the air of 140 ng m⁻³. This is below the U.S. Environmental Protection Agency’s (USEPA) Reference Concentration for Inhalation Exposure of 300 ng m⁻³ and the World Health Organization’s (WHO) reference level of 200 ng m⁻³ for long-term inhalation exposure.

In a study conducted in Spain, the Hg content accumulated in the leaves of olive trees ranged from 90 to 1270 µg kg⁻¹ in the vicinity of an active plant in Flix, and between 46 and 453 µg kg⁻¹ in the area of a former plant in Jódar [39]. In another study conducted near a mercury mine, significantly higher Hg concentrations were measured in the leaves of Salix atrocinerea [40].

As illustrated in Figure 4, the concentration of Hg in the leaves increased progressively from April to September, which corresponds to the aging of the leaves. Additionally, the accumulated Hg concentrations varied among tree species, following the following order: cherry plum < white poplar < linden.

Gustin and collaborators [26] also reported that Hg uptake varies depending on tree species and the age of the leaves. The authors stated that foliar uptake represents a significant sink for Hg from the atmosphere, and that the Hg present in the leaves is not substantially influenced by uptake from the soil through the plant roots. Millhollen et al. [41] demonstrated that there is an exchange of Hg between foliar surfaces and the atmosphere due to the processes of uptake and re-emission. This supports the hypothesis that the Hg found in leaves primarily originates from the atmosphere.

Assad et al. [42] combined laboratory and field experiments to assess the uptake pathways of Hg in the leaves of poplar species. The authors reported that Hg concentrations in the leaves increased with age at both polluted and unpolluted (control) sites, ranging from 6 to 120 µg kg⁻¹. Similar findings were reported by Laacouri et al. [15] in a study examining the leaves of six deciduous species. In that study, the authors demonstrated that leaf Hg content was positively correlated with the dry mass of the leaves. Additionally, a positive correlation was found between Hg content in the leaves and their surface area. The authors concluded that Hg accumulates on both the surface area (stomates) and through biomass pathways. In a study on atmospheric Hg in areas impacted by a chlor-alkali plant [10], it was found that the total Hg content in tree leaves was significantly higher than that accumulated by annual crops. The authors reported that different plant species have varying capacities for trapping atmospheric elements due to their distinct leaf structures and morphologies.

In a recent study, Pan et al. [18] evaluated Hg concentrations in the leaves of various tree species, including Populus tomentosa, willow, Canadian poplar, Mongolian oak, and elm, from forested areas in China. Generally, the reported Hg concentrations were lower than those measured in our study, ranging from 10 to 50 µg kg⁻¹, with an average value of 24 µg kg⁻¹.

The Hg concentrations in leaves collected from sampling location S3 were generally lower than those found in other locations. This observation correlates with the lower Hg content in the soil sampled from this area, which was 2.4 mg kg⁻¹, compared to the range of 4.6 to 7.3 mg kg⁻¹ in the other analyzed samples. Therefore, even if it was reported that the atmosphere represents the principal source of Hg accumulation in vegetation [20,21,22], our results indicate that the Hg levels in the soil of a specific region may influence the accumulation of Hg in leaves, most probably due to the uptake and even due to the higher Hg content in the atmosphere in higher-contamination areas.

3.3. Mercury Accumulation Percentage over the Growing Season

In order to facilitate the observation of mercury accumulation dynamics in leaf samples, the percentage of mercury (Hg) accumulated each month relative to the total Hg measured during the previous sampling campaign was calculated and expressed as the accumulation percentage (Ac, %). The data obtained are graphically presented in Figure 5.

Although a certain amount of mercury had already accumulated during the initial stages of leaf growth observed in April, which can be considered a background value, there is a noticeable and continuous increase in mercury concentration in the leaves. According to the accumulation percentage of Hg in the leaves of the studied tree species, it can be observed that leaf age and species influence the Hg accumulation. Generally, a more pronounced increase is observed during the months of May, June, and July, followed by a lower percentage of accumulation in the last monitored period. In most cases, the highest accumulation of mercury was recorded in June, accounting for approximately 15–40% of the total mercury accumulated throughout the entire monitoring period. This trend may be attributed to the fact that the leaves reached maturity during this time; however, it could also be explained by the higher ambient temperatures in the summer months, which may lead to a more significant release of mercury from the soil and an increased concentration of gaseous mercury in the atmosphere. This statement is based on the findings reported by Osterwalder et al. [16]. They recorded seasonal variations in gaseous mercury using a passive air sampler network and observed slightly higher levels of Hg⁰ in the atmosphere of urban areas in Switzerland during the spring and summer compared to the autumn and winter seasons. Moore and Castro [42] reported that higher temperatures in soil amplified the volatilization of mercury from the soil solid matrix into the soil pore spaces. From here, these could be released into the atmosphere. In a previous study, Pleijel et al. [17] investigated Hg accumulation in broadleaved tree species and the deciduous conifer Larix. They found a statistically significant difference in Hg content measured in June and September, as well as a variation in Hg content among different species.

Laacouri et al. [15] also reported that deciduous tree species monitored in the USA accumulated Hg throughout the entire growing season. They found that Hg concentrations in leaves showed no significant difference when the leaf Hg content was normalized by surface area. In their study, the monitoring period was divided into three intervals: May–June, July–August, and September–October. The researchers analyzed mercury (Hg) absorption rates over various time periods and determined the Hg uptake rate for each interval. They found that most of the Hg absorption took place during the peak growing season (July–August), when the leaves were fully mature and photosynthesis was most active. For most species, the rate of Hg uptake stabilized or declined near the end of their growing time. These results align with those seen in our research and validate them.

3.4. Assessment of Hg Accumulation by Tree Species over the Growing Season

The accumulation of Hg in tree leaves may contribute to the reduction of Hg content in the atmosphere. In this sense, we estimated the amount of Hg removed daily from the air per mass unit of the leaves, and the amount of Hg that could be removed by tree leaves during a growing season. For this estimation, we considered the initial content of Hg measured in April, and the content of Hg measured in the sampling campaign in October. The results are presented in

Table 2. Amount of Hg accumulated daily by each tree species in the four sampling locations.

Sampling Location	Tree Species	Hg/Day ng g−1	Hg/Season ng g−1
S1	white poplar	0.48	72
	linden	0.78	117
	cherry plum	0.27	40
S2	white poplar	0.50	75
	linden	1.21	182
	cherry plum	0.35	53
S3	white poplar	0.43	64
	linden	0.51	77
	cherry plum	0.27	41
S4	white poplar	0.50	75
	linden	0.91	136
	cherry plum	0.35	53
* Average ± SD	white poplar	0.48 b ± 0.03	75 b ± 5
	linden	0.78 a ± 0.29	136 a ± 44
	cherry plum	0.31 b ± 0.05	53 b ± 7

* Results are expressed as average ± SD (n = 4 composite samples from different locations). The different letters indicate significant differences (p < 0.05) based on Tukey’s test.

.

The results presented in Table 2 represent the averages for each point across the six sampling campaigns conducted from April to September. The average for each tree species was also calculated based on the results obtained from each location. The results showed the following mean ± standard deviation values: WP—0.45 ± 0.03 ng g⁻¹, L—0.85 ± 0.29 ng g⁻¹, and CP—0.31 ± 0.05 ng g⁻¹. Notably, the standard deviation for the linden group was considerably higher than that of WP and CP, indicating greater variability within this group. Moreover, the WP and CP groups exhibited lower standard deviations and fell into the same statistical category (“b”) based on the Tukey test results, suggesting no significant difference between them. According to Tukey’s test, significant differences (p < 0.05) were observed between the linden tree and the other two tree species: white poplar and cherry plum.

As shown in Table 2, white poplar leaves can accumulate, on average, approximately 72 ng Hg g⁻¹ dw, while linden leaves can accumulate about 128 ng g⁻¹ dw. Cherry plum leaves can uptake, on average, 47 ng g⁻¹ dw. Among the species investigated, linden leaves exhibit the highest potential to mitigate mercury contamination in the air due to their larger surface area.

Calculated as the rate of accumulation per day, white poplar leaves can uptake approximately 0.48 ng g⁻¹ dw, linden leaves can uptake about 0.85 ng g⁻¹ dw, and cherry plum leaves accumulate approximately 0.31 ng g⁻¹ dw during the growing season from April to September. In a study on Hg accumulation in poplar leaves, Assad et al. [43] compared daily Hg uptake at a polluted site and a control site. It was reported that at the polluted site, the rate of Hg accumulation was 5.2 ng g⁻¹ dw per day, a much faster rate than that observed at the control site.

In several studies examining mercury (Hg) accumulation in tree leaves within forest ecosystems, the authors calculated the uptake rate per square meter. For instance, Laacouri et al. [15] reported an uptake rate of 4.71 ng m⁻² day⁻¹. Bushey et al. [20] found that sugar maple had an average Hg uptake rate of 14.40 ng m⁻² day⁻¹, while Poissant et al. [44] reported an uptake rate of 13.20 ng m⁻² day⁻¹ for maple leaves.

Using USDA Forest Service information [45] to convert leaf area to biomass production, we estimated that a mature white populus can produce up to 110–150 kg total leaf biomass (dry mass). A mature linden can produce about 100–140 kg total leaf biomass (dry mass), while a cherry plum can produce annually about 10–20 kg total leaf biomass (dry mass). Considering the average amount of leaf biomass produced by each monitored tree species, it can be estimated that a white poplar can retain approximately 7.8–10.7 mg of Hg per mature tree, a linden can retain about 12.8–19.2 mg of Hg, and a cherry plum tree can retain around 0.5–0.9 mg of Hg during a growing season. Of course, these values may greatly vary with the tree height and crown spread. Therefore, trees growing on the surface in a city contribute to reducing mercury contamination in the air. The cultivation of broad-leaved trees can be an effective solution for enhancing the quality of life in urban areas [46,47]. Additionally, it is essential to manage the collection of fallen leaves during the autumn season properly and to identify suitable disposal methods for the collected leaves to prevent their reintroduction into the soil.

4. Conclusions

In this study, total Hg concentration was determined in composite soil samples from four sampling locations in the urban area of Turda, as well as in the leaves of three common deciduous species: white poplar (Populus alba), linden (Tilia Europaea Pallida), and cherry plum (Prunus cerasifera). In soils, the total mercury concentrations ranged between 2.4 and 7.3 mg kg⁻¹, exceeding the intervention threshold for residential areas (2 mg kg⁻¹) in all cases. According to the calculated I_geo values, the soil was highly to very highly contaminated. Our results indicate that the Hg levels in the soil of a specific sampling location may influence the accumulation of Hg in leaves. The Hg content in the leaf samples consistently increased over time, influenced by both leaf age and tree species, following this order of increasing Hg content: cherry plum < white poplar < linden. On average, during the six-month monitoring period from April to September, white poplar leaves accumulated 72 ng Hg g⁻¹ dw, linden leaves 128 ng Hg g⁻¹ dw, and cherry plum leaves 47 ng Hg g⁻¹ dw. Generally, a more pronounced increase can be observed during the months of May, June, and July, followed by a lower percentage of accumulation in the last monitored period.

We calculated that white poplar leaves can uptake approximately 0.48 ng g⁻¹ dw, linden leaves can uptake about 0.85 ng g⁻¹ dw, and cherry plum leaves can accumulate approximately 0.27 ng g⁻¹ dw. These findings suggest that trees growing within the urban environment can play a role in mitigating mercury contamination in polluted areas if the fallen leaves during the autumn season are collected and managed properly.