Spatio-Temporal Patterns of Polycyclic Aromatic Hydrocarbons and Phthalates Deposition in Sediments of Reservoirs: Impact of Some Environmental Factors

Abstract

The aim of this study is to assess the accumulation rates of polycyclic aromatic hydrocarbons (PAHs) and phthalic acid esters (PAEs) in the Nielisz Reservoir. Sediment traps were used at three sites: the upper (N1 station), central (N2 station), and near-dam (N3 station) zones, where they were installed at the bottom in the littoral zone of the reservoir at depths ranging from approximately 1.6 m to 2.5 m. Sampling took place from June to August 2019, and entrapped sediments were analyzed for 16 PAHs and 11 PAEs. Four PAHs, naphthalene (NAP), phenanthrene (PHE), benzo(b)fluoranthene (B(b)F), and benzo(a)pyrene (B(a)P), and two PAEs—di-n-butyl phthalate (DBP) and bis(2-ethylhexyl) phthalate (DEHP)—were identified. Among PAHs, 5-ring compounds dominated, while DBP was the most common PAE. PAH and PAE concentrations in entrapped sediments showed both temporal and spatial variability, but no clear trends were established. The accumulation rate of Σ₄PAH ranged from 47.8 to 458 μg/m²d, with a decreasing trend from the inflow to the dam. The light-to-heavy PAH ratio suggests a pyrogenic origin. Σ₂PAE fluxes were lower, between 1.81 and 17.6 μg/m²d, with no clear spatial variation along the reservoir. Ecotoxicological analysis indicated that PAHs and PAEs could occasionally harm benthic organisms, mainly due to B(b)F. The deposition rates of both PAHs and PAEs are strongly linked to the accumulation of suspended particulate matter (SPM) and organic carbon, particularly of autochthonous origin. Additionally, the pH and salinity of the water significantly influence the accumulation rate of the studied contaminants, especially PAHs.

1. Introduction

Reservoirs, as anthropogenic freshwater ecosystems, perform diverse and significant functions such as electricity generation, provision of drinking water, and the creation of recreational spaces. A considerable amount of organic and inorganic particles of varying composition, size, and shape are transported into the reservoir waters from the catchment area [1,2,3]. Additionally, within the reservoirs themselves, the production of organic matter occurs. This heterogeneous mixture of solid particles, known as suspended particulate matter (SPM), is maintained in suspension by the water’s drag force. Under the influence of gravity or as a result of physico-chemical processes, these particles can undergo sedimentation and settle at the bottom of the reservoirs, forming bottom sediments. SPM particles create a type of micro-ecosystem in which numerous processes occur, including the colonization of living organisms, interactions between particles, and, most importantly, the sorption of chemical pollutants [4]. Therefore, they act as carriers of anthropogenic micropollutants and play a crucial role in their transport to bottom sediments, which in turn serve as long-term “repositories” [5]. Pollutants deposited in the surface layer of sediments pose a direct threat to benthic organisms and an indirect threat to organisms at higher trophic levels [6]. Moreover, they do not remain permanently trapped there and, under favorable conditions, can be re-released, resulting in secondary contamination of the water column.

Some of the common, potentially toxic micropollutants that can be adsorbed and transported to bottom sediments on SPM include polycyclic aromatic hydrocarbons (PAHs) [7] and phthalic acid esters (PAEs, phthalates) [8].

Polycyclic aromatic hydrocarbons (PAHs) are classified as persistent organic pollutants (POPs) and can originate from both natural sources (forest fires, volcanic eruptions, and biosynthesis) [9] as well as anthropogenic activities (wood, coal, and grass burning, incomplete combustion of fossil fuels, vehicle emissions, and oil spills) [10]. They can enter surface waters through surface runoff, wastewater discharge, atmospheric deposition, and other pathways such as oil spills [11,12,13]. PAHs are characterized by low solubility and hydrophobicity, which makes them readily adsorb onto solid particles when introduced into water, leading to their sedimentation and accumulation in bottom sediments [14]. The presence of PAHs in aquatic environments has become the subject of intense research due to their toxicity, persistence, and tendency to bioaccumulate. Many PAHs exhibit mutagenic, carcinogenic, teratogenic, and immunotoxic effects on living organisms, including animals and humans [15]. Acute health effects include eye irritation, vomiting, diarrhea, disorientation, and skin irritation. Chronic health effects include eye cataracts, kidney and liver damage, respiratory issues, reduced immune function, abnormal lung function, and asthma-like symptoms [16].

The presence of phthalates in the aquatic environment is primarily associated with the ubiquitous contamination of the environment by microplastics (MPs) [6,17]. During the degradation of MP particles, the polymer chain breaks down, releasing toxic chemical substances into the aquatic environment. These substances, which are components of plastics, include oligomers, monomers, oxidized intermediates, and additives that enhance the properties of polymers, such as stabilizers, dyes, flame retardants, plasticizers, antioxidants, and others [8,18]. Among these substances, plasticizers constitute the largest proportion, with phthalates being among the most commonly used. Phthalates can have harmful effects on aquatic organisms, including fish, crustaceans, mollusks, and phytoplankton. Long-term exposure to these substances can lead to hormonal imbalances, internal organ damage, reduced fertility, developmental disorders, and other adverse health effects in aquatic organisms [19,20]. Additionally, some phthalates, such as DEHP (di(2-ethylhexyl) phthalate) and DBP (di-n-butyl phthalate), have been classified as carcinogenic or potentially carcinogenic to humans [21,22]. Furthermore, PAEs adversely affect reproductive health by blocking testosterone production in men and causing premature puberty in women. They are also capable of crossing the placental barrier, consequently affecting brain development and causing changes in the lungs of children, as well as disrupting the development of reproductive organs in male fetuses [23,24]. PAEs have also been shown to influence the development of mental disorders in children, such as autism and attention deficit hyperactivity disorder (ADHD) [25], exhibit toxic effects on the liver and kidneys, and potentially increase the risk of breast cancer, asthma, and allergies [26,27].

The aim of this study is to quantitatively and qualitatively determine the concentrations of PAHs and PAEs in the suspended particulate matter (SPM) deposited on the bottom of the Nielisz Reservoir, as well as to assess the accumulation rates of these contaminants across different zones of the reservoir. Additionally, the study examined the influence of selected environmental parameters (such as pH, conductivity, organic carbon content in water and sedimenting particulate matter, and the type of organic matter) on the deposition rates of the analyzed pollutants, along with an evaluation of the potential toxicity of the sedimented particulate matter to benthic organisms. This research fills a significant gap in the existing literature, as, unlike most studies that focus on analyzing the concentrations of PAHs and PAEs in water, suspended matter, or sediments, our study concentrates on the accumulation process of these contaminants in the bottom sediments of retention reservoirs due to the sedimentation of suspended solids. Furthermore, it provides valuable and rare data (particularly for PAEs) regarding the deposition rates and spatiotemporal trends of these compounds in the bottom sediments of retention reservoirs. The results of this study contribute to a better understanding of the mechanisms governing the transport and distribution of PAHs and PAEs in aquatic environments and offer an important reference for future research on the transport and accumulation of micropollutants in reservoirs.

2. Materials and Methods

2.1. Study Area

The subject of the study was the Nielisz Reservoir, located in the upper reaches of the Wieprz River (km 235 + 200) in southeastern Poland (Figure 1). The main tributaries of the reservoir are the Wieprz River and the Por River, on which a preliminary reservoir with an area of 179 ha and a capacity of 1.18 million m³ is situated. The purpose of this reservoir is to improve water quality (sedimentation of suspended solids) flowing into the main reservoir [28]. A hydroelectric power plant with a capacity of 362 kW is located on the dam.

The primary functions of the Nielisz Reservoir include flood control, regulation of water level fluctuations during bird breeding periods, energy production, and recreational activities, such as amateur fishing. The western catchment area (Por catchment) primarily consists of agricultural land, while the eastern catchment (Wieprz catchment) is characterized by a significant presence of coniferous and mixed forests interspersed with small agricultural areas. Agricultural land within the catchment area ranges from 27% to 86%, with the largest areas occupied by arable land, orchards, meadows, and pastures.

Within the immediate catchment area of the reservoir, there are two villages: Nielisz, located in the northeastern part, and Sułów, situated in the southwestern part. The entire catchment encompasses several small villages located near the main watercourses and two small towns. A special bird protection area has been designated within the reservoir area, which includes both the reservoir itself and a portion of the Por valley. The main point sources of pollution are wastewater treatment plants. Significant contamination of the water and sediments in the reservoir primarily results from agricultural activities that dominate this region. A linear source of pollution is the national road no. 74, which crosses the Wieprz River above the dam. Within the catchment area of the reservoir, there are no significant point sources of air pollution emissions. Surface emissions have the greatest impact on air quality.

2.2. Sampling

Suspended particulate matter (SPM) was collected using sediment traps designed by one of the authors (Figure 1). The traps were made of “plexiglass” tubes, inside which there was a sedimentation funnel with a deflector placed at the bottom. The detailed construction and operating principle of the trap have been described in a previous article [29].

The traps were installed on the bottom at three research stations: in the river zone (N1), in the central part (N2), and near the dam (N3) (distance: 3–10 m from the shoreline, depth: 1.6–2.5 m). This distribution was chosen in order to capture the potential influence of the variable dynamics of water flow, which is characteristic of reservoir systems, on the process of sedimentation of suspended particulate matter. This allowed for the determination of spatial variation in the accumulation of the studied pollutants along the reservoir. Samples were collected during the spring/summer period of 2019, with the research period divided into four series (SI: 08.05.2019–10.06.2019; SII: 10.06.2019–09.07.2019; SIII: 09.07.2019–07.08.2019; SIV: 07.08.2019–27.08.2019). The sediment collection period was divided in such a way as to allow the accumulation of the largest possible amount of sediment while avoiding the overflow of the sediment traps. At the end of each series, sediments accumulated in the traps (entrapped sediments) were collected to determine particulate organic carbon (POC) content (including δ¹³C-POC isotopic composition) and PAHs and PAEs concentration. Additionally, in situ measurements of water pH and electrical conductivity (EC) were conducted, and water samples were taken to analyze total organic carbon (TOC) and chlorophyll-a (Chla) concentrations.

2.3. Analytical Procedures

2.3.1. Analysis of POC and δ¹³C-POC

The contents of particulate organic carbon (POC) were determined using a CN elemental analyzer (Flash EA 1112, ThermoQuest, Wilmington, DE, USA). To ensure the accuracy and reliability of the measurements, quality control procedures included the analysis of blank samples to assess potential contamination and instrumental drift. Standard reference samples with a known elemental composition (acetanilide–C₈H₉NO) were analyzed regularly to verify instrument calibration and analytical performance. The calibration curve was established using certified standard materials, and sample measurements were corrected accordingly. The precision of the method was about ±3%.

Stable isotopic composition of particulate organic carbon (δ¹³C-POC) was determined using an IRMS isotope mass spectrometer (DELTAPlus, Finnigan Mat, San Jose, CA, USA) coupled with a CN elemental analyzer (Flash EA 1112, ThermoQuest). The isotopic ratios were reported in standard δ-notation (δ¹³C): δR(‰) = (Ra/Rb(sample)/Ra/Rb(standard)−1) × 10³, where Ra/Rb are the ¹³C/¹²C ratios relative to the PDB standard. The method was calibrated using the certified reference material NBS 22 for δ¹³C. Quality control procedures included the periodic analysis of NBS 22 to monitor instrumental stability and ensure measurement accuracy. The analytical precision of the method was ±0.1‰ for δ¹³C.

Prior to the determination of POC and δ¹³C-POC content, carbonates were removed from the sediment samples. For this purpose, the dried and ground sediment was placed in a desiccator with concentrated HCl vapor for 72 h. Prior to analysis, the sediment sample was again dried to constant weight at 60 °C.

2.3.2. Analysis of EC, pH, TOC, and Chla

Electrical conductivity (EC) and pH were measured in situ using a MultiLine P4 (WTW, Reutlingen, Germany). Total organic carbon (TOC) was determined using a TOC-VCPN analyzer (Shimadzu, Kyoto, Japan). Chlorophyll-a (Chla) was determined spectrophotometrically (Aquamate, Woodbridge, NJ, USA) after hot extraction with ethanol.

2.3.3. Analysis of PAHs and PAEs

Sediment samples were processed using a MARS 6 mineralizer/extractor from SELWALab (Warsaw, Poland). The extraction process was carried out at a maximum temperature of 120 °C with a maximum power of 1400 W. To extract analytes from the sediment, a mixture of 10 mL hexane and 10 mL acetone (1:1, v/v), along with internal standards, was employed. Following extraction, the samples underwent filtration, leading to the separation of organic and aqueous layers. The obtained extract was then treated with anhydrous sodium sulfate for drying, concentrated to a final volume of 1 mL, and subsequently subjected to chromatographic analysis. Quantitative assessments of PAHs and PAEs were conducted using capillary gas chromatography. This involved utilizing a gas chromatograph coupled with a GC-MS mass detector (Thermo Scientific, Waltham, MA, USA), enabling accurate and precise determinations. Chromatographic separation was achieved utilizing a DB-5MS chromatographic column coated with a 5% phenyl-95% dimethylpolysiloxane composition (30 m × 0.25 mm, with a 0.25 μm film thickness). The operational parameters of the GC–MS system are detailed in

Table 1. GC-MS system operating conditions.

	PAHs	PAEs
Temperature program	40 °C hold 3 min, 40 to 300 °C rate 8 °C/min, 300 °C hold 4 min	40 °C hold 1 min, 40 to 300 °C rate 9 °C/min, 300 °C hold 5 min
Dosing system	Split–Splitless injector with automatic sample feeder
Operating mode	Splitless
Sample volume (µL)	1.0
Temperature injector (°C)	300	300
Carrier gas	Helium
Carrier gas flow rate (mL/min)	1.1	1.0

. All samples underwent triplicate analysis, and the outcomes are reported as the arithmetic mean.

The standard solution contained 16 substances of polycyclic aromatic hydrocarbons, each present at a concentration of 2000 μg/mL, dissolved in toluene. The substances included acenaphthene, anthracene, benz(a)anthracene, chrysene, fluoroanthene, fluorene, naphthalene, phenanthrene, pyrene, benzo(a)pyrene, benzo(b)fluoroanthene, benzo(g,h,i)perylene, dibenz(a,h)anthracene, benzo(k)fluoroanthene, indeno(1,2,3-cd)pyrene, and acenaphthylene. Additionally, a standard solution of phthalic acid esters (PAEs) with a concentration of 2000 μg/mL for each substance was procured from Sigma-Aldrich (Darmstadt, Germany). This solution consisted of 11 substances, including 4-chlorophenylphenyl ether, 4-bromophenylphenyl ether, bis(2-chloroethoxy)methane, bis(2-chloroethyl)ether, bis(2-chloroisopropyl)ether, bis(2-ethylhexyl) phthalate (DEHP), butylbenzyl phthalate (BBP), diethyl phthalate (DEP), dimethyl phthalate (DMP), di-n-butyl phthalate (DBP), and di-n-octyl phthalate (DOP).

The validation process relied on data collected from six replicates of calibration solutions prepared at varying concentrations (

Table 2. Quality assurance/quality control parameters for the extraction and analysis of PAEs.

PAEs	Retention Time RT (min)	Linearity R2	CV (%)	Recovery (%) n = 3	LOQ (μg/kg)	LOD (μg/kg)
Bis(2-chloroethyl)ether	6.92	0.9482	2.55	91	25	8.33
Bis(2-chloroisopropyl)ether	8.13	0.995	1.86	96	10	3.33
Bis(2-chloroethoxy)methane	9.94	0.9919	2.25	105	15	5
Dimethyl phthalate	14.41	0.9939	1.58	98	12	4
Diethyl phthalate	16.31	0.995	0.94	105	15	5
4-Chlorodiphenyl ether	16.50	0.9775	2.33	89	18	6
4-Bromodiphenyl ether	17.79	0.9872	1.57	69	22	7.33
Di-n-butyl phthalate	20.71	0.9885	0.87	109	8	2.67
Butylbenzyl phthalate	24.68	0.9776	0.92	111	13	4.33
Bis(2-ethylhexyl) phthalate	26.31	0.9747	1.11	95	6	2
Di-n-octyl phthalate	27.96	0.9446	1.23	95	10	3.33

and

Table 3. Quality assurance/quality control parameters for the extraction and analysis of PAHs.

PAHs	Retention Time RT (min)	Linearity R2	CV (%)	Recovery (%) n = 3	LOQ (μg/kg)	LOD (μg/kg)
Naphthalene	12.93	0.9708	3.25	95	10	3.33
Acenaphthylene	17.57	0.9659	3.10	104	35	11.67
Acenephthene	18.09	0.9952	2.42	97	21	7
Fluorene	19.63	0.974	3.34	94	25	8.33
Phenenthrene	22.45	0.9633	4.12	89	30	10
Anthracene	22.60	0.9905	2.50	94	18	6
Fluoranthene	25.98	0.9567	3.88	110	45	15
Pyrene	26.62	0.9624	3.98	89	51	17
Benz(a)anthracene	30.23	0.9588	3.51	91	40	13.33
Chrysene	30.32	0.9685	3.22	90	45	15
Benzo(b)fluoranthene	33.21	0.9619	3.60	92	55	18.33
Benzo(k)fluoranthene	33.28	0.9516	3.79	89	35	11.67
Benzo(a)pyrene	34.02	0.9774	3.15	92	40	13.33
Indeno(1,2,3-c,d)pyrene	36.74	0.9891	2.86	96	24	8
Dibenz(a,h)anthracene	36.84	0.9625	3.55	91	42	14
Benzo(g,h,i)perylene	37.42	0.9674	3.74	90	50	16.67

). Key parameters such as linearity, repeatability, and accuracy were evaluated. Linearity was determined by analyzing the calibration curve, while repeatability of the method was assessed through the coefficient of variation (CV), presented as a percentage. Accuracy was determined by calculating the recovery rates. The limit of detection (LOD) was estimated using chromatograms, applying a criterion where the signal-to-noise ratio (S/N) of the analytical signal to background noise was at least three. For the limit of quantification (LOQ), it was assumed to be three times the calculated LOD.

Internal standards such as benzyl benzoate (1g/mL), naphthalene-d8 (2000 μg/mL in dichloromethane), phenanthrene-d10 (2000 μg/mL in dichloromethane), and perylene-d12 (2000 μg/mL in dichloromethane) were obtained from Sigma-Aldrich. Methanol, n-hexane, acetone, Na₂SO₄, NaCl, and NaOH solutions were purchased from POCH (Gliwice, Poland). Ultra-pure water was sourced from a Purix CNX-100 system by Polwater (Krakow, Poland).

To ensure cleanliness, all glassware underwent a 24-h soaking in NaOH solution, followed by rinsing with ultra-pure water and drying for 5 h. An additional rinse with acetone was performed before heating at 280 °C for 5 h.

2.4. Methodology of the Results Elaboration

2.4.1. Determination of the Accumulation Rates

The rate of SPM accumulation was calculated as Flux_SPM (g/m²d) from Equation (1) [30]:

$${\mathrm{F}\mathrm{l}\mathrm{u}\mathrm{x}}_{\mathrm{S}\mathrm{P}\mathrm{M}}={\displaystyle \frac{\mathrm{m}}{\mathrm{A}·\mathrm{t}}}$$

(1)

where m is the dry mass of entrapped sediment (g), A is the sediment-trap cross-sectional area (m²), and t is the exposure time of the trap in the water column (days).

The rates of accumulation of POC, PAHs, and PAEs were calculated by multiplying the Flux_SPM by the content of the analyzed substance in the entrapped sediment.

2.4.2. Origin of Organic Matter in Entrapped Sediments

The origin of organic matter in the entrapped sediments was calculated using the two-source model [31]. The value of δ¹³C-POC = −27.73‰ for autochthonous organic matter was determined for particles of suspended matter collected in summer in the central part of the reservoir. The value of δ¹³C-POC = −14.0‰ for organic matter of terrestrial origin was assumed based on the literature data [32].

2.5. Ecotoxicological Assessment

The assessment of the impact of sedimenting particulate matter on aquatic organisms was conducted based on the sediment quality indicators TEC and PEC. TEC (Threshold Effect Concentration) represents a threshold value used to identify contaminant concentrations below which no harmful effects on benthic organisms are expected. PEC (Probable Effect Concentration) is a probable value indicating the concentration at which adverse effects on benthic organisms are expected to occur. To assess the potential ecological risk, the concentration levels of the studied micropollutants in the entrapped sediments were compared with the TEC and PEC values reported in the literature [33,34].

2.6. Statistical Analysis

For the obtained results, basic descriptive statistics, such as the minimum, maximum, mean, and standard deviation values, were calculated using the MS Excel 2013 program. For linear relationships, the coefficient of determination with the corresponding level of significance p < 0.05 was calculated using Statistica 13 PL.

3. Results and Discussion

3.1. Concentrations and Distributions of PAHs and PAEs in Entrapped Sediments

In the entrapped sediments, only 4 out of 16 analyzed PAHs and 2 out of 11 analyzed PAEs were detected. They were the following: naphthalene NAP (2-rings), phenanthrene PHE (3-rings), benzo(b)fluoranthene B(b)F (5-rings), benzo(a)pyrene B(a)P (5-rings), di-n-butyl phthalate DBP, and bis(2-ethylhexyl) phthalate DEHP. Basic statistical parameters for all stations and research series are presented in

Table 4. The basic statistical parameters for PAHs and PAEs in the studies (μg/kg). LOD—limit of detection; LMW-PAHs—low molecular weight PAHs; HMW-PAHs—high molecular weight PAHs.

	Min	Max	Mean	Median	SD
B(b)F	213	679	464	488	150
B(a)P	74.3	242	143	133	43.0
NAP	<LOD	15.8	15.3	15.5	0.6
PHE	31.5	208	129	128	49.2
LMW-PAHs	31.5	208	134	129	51.6
HMW-PAHs	361	876	606	661	169
Σ4PAHs	393	1084	740	783	212
DBP	10.9	210	41.8	21.0	54.4
DEHP	<LOD	18.7	13.0	14.5	3.9
Σ2PAEs	13.7	225	49.4	29.8	57.3

, while the results are shown in Figure 2. The presence of a small number of PAHs and PAEs congeners in the studied sediments may be due to the specific characteristics of the Nielisz Reservoir catchment, such as the dominance of agricultural activity, the lack of significant industrial sources, and the presence of forests acting as natural pollution filters (see Section 2.1).

The range of total PAH concentrations in entrapped sediments was from 393 to 1084 µg/kg, with a mean value of 740 µg/kg and a median of 783 µg/kg. The mean concentrations of all detected PAHs were very close to the median value, suggesting the absence of significant outliers. The analyzed sediments were characterized by the highest mean content of B(b)F (464 µg/kg), followed by B(a)P (143 µg/kg) and PHE (129 µg/kg), with the lowest mean concentration observed for NAP (15.3 µg/kg), (5 rings > 3 rings > 2 rings). Furthermore, the mean concentration of high molecular weight PAHs (HMW-PAHs) (PAHs with 5 rings) was higher than the mean concentration of low molecular weight PAHs (LMW-PAHs) (PAHs with 2–3 rings). HMW-PAHs are characterized by higher lipophilicity and lower solubility in water compared to LMW-PAHs, making them more likely to accumulate in sediments [35,36]. Analyzing the partial results, it was found that for all locations and research series, high molecular weight PAHs significantly dominated over low molecular weight PAHs, constituting 77 to 94% of ∑₄PAH. Low molecular weight PAHs are indicators of petrogenic sources and low-temperature pyrolytic sources, whereas high molecular weight PAHs are evidence of pyrolytic origin [37]. Therefore, when the ratio of LMW-PAHs to HMW-PAHs is greater than 1, it is assumed that the PAHs are of petrogenic origin, whereas when it is less than 1, a pyrogenic source is assumed [38,39]. The analysis of the obtained research results showed that in all cases, the LMW-PAHs/HMW-PAHs ratio was significantly lower than 1, ranging from 0.08 to 0.30 (mean 0.22), strongly suggesting a pyrogenic origin of PAHs in the entrapped sediments. The ratios of individual PAH concentrations to each other (e.g., Phenanthrene/Anthracene PHEN/ANTH, Chrysene/Benz(a)anthracene CHRY/B(a)A, Fluoranthene/Pyrene FLU/PY; Fluoranthene/Pyrene+Pyrene FLU/(FLU + PY)) can provide more accurate insights into their source [10,38,40], however, due to the obtained results, such an analysis was not possible.

Analyzing the spatiotemporal distribution of PAH concentrations in entrapped sediments revealed that NAP was detected exclusively at station N1, located in the upper part of the reservoir, and maintained a nearly constant level over the study period, averaging approximately 15 µg/kg (Figure 2A–C). The concentration of PHE showed an increasing trend over time and a decreasing trend along the reservoir from station N1 to N3. The average concentration of this substance during the entire study period was approximately 148, 134, and 104 µg/kg, respectively, for N1, N2, and N3 (Figure 3). For B(a)P, an increasing concentration trend was noted along the reservoir, from top to bottom, averaging from 122 to 154 µg/kg (Figure 3), but no significant temporal change was observed (Figure 2A–C). In contrast, the concentrations of B(b)F did not show any clear temporal or spatial trends (Figure 2A–C and Figure 3).

Total concentrations of PAEs were not high, ranging from 13.7 to 225 µg/kg, with a mean value of 49.4 µg/kg and a significantly lower median value of 29.8 µg/kg. Among them, DBP exhibited a considerably higher mean concentration (41.8 µg/kg) compared to DEHP (13 µg/kg). DEHP and DBP are typically the most dominant congeners in environmental matrices, as they are the most abundantly produced and used phthalate esters [41,42,43]. Additionally, DBP, with its lower molecular weight, tends to be the dominant congener in water, while DEHP, with its higher molecular weight, predominates in sediments [8,44]. Due to its low water solubility, high organic carbon–water partition coefficient (K_oc), and hydrophobicity, DEHP tends to adsorb onto suspended particles and ultimately accumulates in sediments [45].

However, the results of the conducted studies showed that in entrapped sediments, the predominant congener was DBP, which exhibits higher solubility and lower lipophilicity compared to DEHP, theoretically indicating a lesser propensity to adsorb onto sedimentary organic matter particles. Such results strongly suggest that the studied reservoir area was more contaminated with DBP. Findings from some studies also indicate higher concentrations of DBP compared to DEHP in bottom sediments [46,47].

Spatial-temporal analysis of phthalate concentrations in entrapped sediments revealed that DBP was present in all examined samples, while DEHP was not always detected (Figure 2D–F). A clear increase in DBP concentration over time was observed only at station N3 near the dam, ranging from approximately 19 µg/kg to 210 µg/kg; no temporal trend was observed at other stations. In the case of DEHP, no temporal variability was found at any research station.

Spatial distribution analysis showed a nearly 4-fold increase in DBP concentration at station N3 compared to stations in the upper parts of the reservoir (Figure 3), whereas DEHP did not exhibit clear spatial variability. For comparison, He et al. [48] found in the system of water-suspended particulate matter of Chaohu Lake (China) that, while PAEs with varying hydrophobicity showed seasonal variability in the water phase, all exhibited the same temporal distribution in the particulate phase (SPM).

3.2. Findings of the Ecotoxicological Assessment

Due to the lack of specific standards for permissible concentrations of PAHs and PAEs in aquatic sediments, the analysis of sedimenting SPM particles for their ecotoxicological impact is a crucial element in monitoring the quality of the aquatic environment. The ecotoxicological assessment was conducted by comparing the detected contaminant concentrations with threshold values established in the literature, such as TEC (Threshold Effect Concentration) and PEC (Probable Effect Concentration), which are widely recognized indicators of risk assessment for benthic organisms [33,34]. The obtained results showed that the concentrations of NAP, DBP, and DEHP did not exceed the TEC values in any case, suggesting a limited toxicological impact on benthic organisms. In the case of PHE and B(a)P, occasional exceedances of TEC were recorded, indicating a potential, though infrequent, threat to benthic fauna. Meanwhile, for B(b)F, the TEC value was exceeded in most analyzed sediments, suggesting the possibility of adverse biological effects. However, none of the analyzed compounds reached the PEC threshold, which would indicate a high probability of harmful effects. In summary, it was found that in the vast majority of cases, sedimenting SPM particles may cause only sporadic effects on benthic organisms (

Table 5. Classification of entrapped sediments based on ecotoxicological criteria [33,34].

	TEC (μg/kg)	PEC (μg/kg)	Station N1				Station N2				Station N3
			SI	SII	SIII	SIV	SI	SII	SIII	SIV	SI	SII	SIII	SIV
NAP	176	561	I	I	I	I	-	-	-	-	-	-	-	-
PHE	204	1170	I	I	I	I	I	I	I	I	I	I	I	II
B(b)F	240	13,400	II	II	II	II	I	II	II	II	I	II	II	II
B(a)P	150	1450	I	I	I	I	I	II	II	I	I	I	II	II
DBP	2200	17,000	I	I	I	I	I	I	I	I	I	I	I	I
DEHP	610	1100	-	-	I	I	-	I	-	I	-	I	I	I
Final assessment			II	II	II	II	I	II	II	II	I	II	II	II

Note(s): (-) concentration < LOD, I—PAH/PAE < TEC (no toxic effect), II—TEC < PAH/PAE < PEC (possible sporadic adverse effects), III PAH/PAE > PEC (concentration of the compound that may cause a toxic effect).

). However, further research should consider not only contaminant concentration analysis but also their bioavailability and potential interactions with other environmental factors. Special attention should be given to B(b)F due to its frequent TEC exceedances and potential toxicological effects. A comparison of the obtained results with previous studies on bottom sediments in Polish dam reservoirs indicates similar patterns in their ecotoxicity. Reservoirs under strong anthropogenic pressure tend to exhibit higher sediment toxicity [49,50,51].

3.3. Deposition Fluxes of SPM, POC, PAHs, and PAEs

Concentrations of PAHs and PAEs in bottom sediments depend not only on point source emissions but also on the sedimentation rate [5]; hence, it is more meaningful to assess changes in terms of deposition fluxes [52].

Temporal variability of SPM, POC, PAH, and PAE fluxes is presented in Figure 4. SPM fluxes ranged from 217 to 618 g/m²d (mean 395 g/m²d) at station N1, from 130 to 333 g/m²d (mean 195 g/m²d) at station N2, and from 78.5 to 248 g/m²d (mean 128 g/m²d) at station N3. The highest sedimentation was observed in June at station N1, followed by a decrease in SPM fluxes in subsequent months. Conversely, the peak SPM fluxes at stations N2 and N3 were observed in July. This indicates high sedimentation in the Nielisz Reservoir. For comparison, in the meso-eutrophic Lian’an Reservoir and eutrophic Liuhuahu Lake in China, SPM flux values ranged from 2.79 to 17.6 g/m²d and from 23.7 to 78.3 g/m²d, respectively [53]. In Viitna Linajärv Lake in Estonia, sedimentation rates ranged from 0.23 to 1.4 g/m²d [54]. In Ríó Grande II Lake in Colombia, flux rates ranged from 229 to 18,573 g/m²d [55]. In the Blizne Reservoir in Poland, sedimentation rates varied from 13.5 to 17.7 g/m²d [56], while in the Maziarnia Reservoir in Poland, they averaged from 8.16 to 119.76 g/m²d [57].

POC fluxes followed the same trend as SPM fluxes. At station N1, POC fluxes ranged from 26.8 to 77.3 g/m²d (mean 49.2 g/m²d), at station N2 from 8.03 to 22.5 g/m²d (mean 12.9 g/m²d), and at station N3 from 1.97 to 6.53 g/m²d (mean 4.53 g/m²d). It is evident that both SPM and POC fluxes decreased along the reservoir from inflows to the dam, and a statistically significant correlation was observed between SPM and POC (r = 0.953, p < 0.05), suggesting that organic matter is a significant component of suspended sediments in the studied environment. Furthermore, a statistically significant relationship between autochthonous matter content and POC in entrapped sediments (r = 0.658, p < 0.05) indicates that higher POC fluxes in the upper compared to the lower parts of the reservoir were likely due to higher primary production in that part of the reservoir.

Fluxes of PAHs in the Nielisz Reservoir varied widely from 47.8 to 458 µg/m²d, with the highest mean flux recorded at station N1 (302 µg/m²d) and the lowest at station N3 (75.6 µg/m²d). At all research stations, the highest proportion of 5-ring PAHs in ∑4PAHs flux and the highest peak flux occurred in July. Compared to other studies listed in

Table 6. Mean values of PAH fluxes in different water ecosystems around the world.

Lake/Reservoir	∑PAHs Flux (μg/m2d)	References
Nielisz Reservoir	172	This study
Lake Biwa (Japan)	2.06	[60]
Lake Windermere (UK)	109.59	[61]
Lake Suwa (Japan)	2.41	[62]
Lian’an Reservoir (China)	2.85	[53]
Liuhuahu Lake (China)	34.1	[53]
Lake Michigan (USA)	1.17	[63]

, PAH fluxes in the Nielisz Reservoir were significantly higher than in other aquatic ecosystems worldwide. This is due to the fact that the variability of PAH fluxes in the aquatic environment is strongly linked to SPM fluxes [58], and these reached very high values in the studied reservoir [59].

PAE fluxes in the Nielisz Reservoir were significantly lower than PAH fluxes, ranging from 1.81 to 17.6 µg/m²d, with an average of 10.8 µg/m²d at station N1, 5.07 µg/m²d at station N2, and 9.06 µg/m²d at station N3. In the majority of cases, DBP contributed more to PAE deposition. At stations N1 and N2, the highest sedimentation rates of PAEs were observed in July, while at station N3, it was toward the end of August. Comparing PAE fluxes with other studies is challenging due to the lack of available literature on this topic.

Examining the spatial distribution of deposition of the analyzed pollutants, it was observed that, similar to SPM and POC, PAH fluxes decreased along the reservoir, from the station located near the inflow to the station near the dam (Figure 5). This trend applied not only to ∑₄PAHs but also to individual congeners. In the case of PAEs, no such trend was observed (Figure 5).

3.4. Factors Controlling the Deposition of PAHs and PAEs

Pearson correlation analysis (

Table 7. The basic statistical parameters for pH, EC, TOC, and Chla in water and POC, δ¹³C-POC, and u_a in entrapped sediments.

	Min	Max	Mean	Median	SD
	Water
pH (-)	8.00	8.86	-	-	-
EC (μS/cm)	384	481	418	417	26
TOC (mg/dm3)	9.18	18.9	14.3	14.8	2.96
Chla (μg/dm3)	42.6	185	90.9	71.3	43.5
	Entrapped sediments
POC (%)	2.26	12.6	7.65	6.81	3.70
δ13C-POC (‰)	−24.1	−17.2	−21.5	−21.9	2.19
ua (-)	0.24	0.73	0.55	0.58	0.16

) for fluxes of SPM, POC, PAHs, and PAEs, along with selected environmental parameters (

Table 8. Correlation coefficients between PAH and PAE fluxes and SPM and POC fluxes, as well as water quality indices and entrapped sediments.

	SPM Flux (g/m2d)	POC Flux (μg/m2d)	Water				Entrapped Sediments
			pH (-)	EC (μS/cm)	TOC (mg/dm3)	Chla (μg/dm3)	POC (%)	δ13C-POC (‰)	ua (-)
Σ4PAHs flux (μg/m2d)	0.900 *	0.922 *	−0.754 *	0.648 *	0.613 *	0.442	0.779 *	−0.497	0.488
2-rings PAHs flux (μg/m2d)	0.889 *	0.976 *	−0.728 *	0.833 *	0.613 *	0.258	0.791 *	−0.591 *	0.585 *
3-rings PAHs flux (μg/m2d)	0.869 *	0.923 *	−0.783 *	0.747 *	0.634 *	0.425	0.868 *	−0.581 *	0.574
5-rings PAHs flux (μg/m2d)	0.894 *	0.907 *	−0.738 *	0.612 *	0.600 *	0.446	0.749 *	−0.469	0.460
Σ2PAEs flux (μg/m2d)	0.480	0.511	−0.294	0.119	0.036	0.154	0.412	−0.316	0.300
DBP flux (μg/m2d)	0.462	0.548	−0.294	0.181	−0.035	−0.081	0.415	−0.375	0.363
DEHP flux (μg/m2d)	0.831 *	0.513	−0.202	0.153	0.775 *	0.717	0.103	0.351	−0.356

Note(s): * Correlation statistically significant, p < 0.05.

), provides significant insights into the interrelationships among these variables.

The conducted analysis revealed statistically significant relationships between SPM and Σ₄PAHs fluxes, as well as between POC and Σ₄PAHs fluxes. The correlation coefficients were r = 0.900 and r = 0.922, respectively, with a significance level of p < 0.05. Strong positive correlations were also found between the deposition fluxes of Σ₄PAHs and total dissolved organic carbon (TOC), as well as particulate organic carbon (POC), with Pearson correlation coefficients of r = 0.613 and r = 0.779, for p < 0.05. The influence of organic carbon was evident for individual PAH congeners, as well (Table 8). These results underscore the crucial role of sedimenting SPM particles in the distribution of PAHs in aquatic environments, consistent with other studies [53]. As demonstrated above, total organic carbon was a significant component of sedimenting SPM particles in the Nielisz Reservoir, considered a key factor influencing sediment sorption capacity [35]. Sediments with finer grain sizes, rich in aromatic and lipid components, exhibit particularly high sorption capacities for PAHs [64].

The analysis of the obtained results also indicates that the source of organic matter may play a significant role in the adsorption of PAHs on sedimenting particulate matter. This is suggested by relationships (some statistically significant) between PAH fluxes and the share of autochthonous organic matter in entrapped sediments, as well as the δ¹³C-POC values. These relationships imply that autochthonous organic matter may enhance the adsorption of PAHs on sedimenting particulate matter.

The conclusions formulated above can also be applied to PAEs. The conducted analysis showed that SPM and POC fluxes are responsible for the transport and deposition of these pollutants in the Nielisz Reservoir. For Σ₂PAEs, although the observed relationships did not reach statistical significance, they exhibited similar trends to PAHs, with Pearson correlation coefficients of r = 0.480 and r = 0.511, respectively, at p < 0.05. A statistically significant relationship was found between SPM and DEHP fluxes (r = 0.831, p < 0.05), confirming that due to its high hydrophobicity, DEHP tends to adsorb onto suspended particles, which act as vectors for its transport to sediments [45]. Therefore, a significant portion of DEHP in the water may potentially be transported with suspended sediments, especially in systems with high concentrations of suspended solids [65].

As shown above, organic carbon promotes the adsorption of PAHs, which highlights its importance in the distribution and accumulation of these substances in the aquatic environment. The analysis of the obtained results confirmed these observations also in the case of PAEs. The obtained correlations between POC streams and Σ₂PAEs, DBP, and DEHP were not statistically significant but showed the role of organic carbon in the deposition of phthalates. Previous studies on the presence of phthalates in bottom sediments also suggested that, in addition to the size of sediment particles and the higher molecular weight of PAEs, organic carbon is a key factor controlling their adsorption [42,47,66,67,68].

The obtained results also confirmed that the role of organic carbon in the distribution and accumulation of micropollutants in bottom sediments is not the only important factor. It was shown that the PAH deposition rate was inversely proportional to pH and directly proportional to electrical conductivity, which reflects water salinity. However, in relation to PAEs, these relationships were less clear and statistically insignificant. A review of the literature confirms the importance of these parameters [64,69,70,71,72,73,74,75]. Higher water salinity reduces the solubility of PAHs and PAEs, thus affecting the adsorption capacity of sediments. In turn, lower pH values favor the adsorption of pollutants by increasing the electrostatic interactions between their particles and the sediment surface [64,70,71,76].

4. Conclusions

In the sedimenting particulate matter of the Nielisz Reservoir, only a subset of the studied PAHs and PAEs was detected, namely naphthalene (NAP), phenanthrene (PHE), benzo(b)fluoranthene (B(b)F), benzo(a)pyrene (B(a)P), di-n-butyl phthalate (DBP), and bis(2-ethylhexyl) phthalate (DEHP). Among PAHs, higher molecular weight compounds predominated, while DBP was the most abundant phthalate. A spatiotemporal analysis revealed concentration differences depending on location and time. Ecotoxicological analysis indicated that concentrations of most PAHs and PAEs did not exceed Threshold Effect Concentrations (TEC), except for B(b)F, which exceeded TEC in most entrapped sediments and could potentially have a negative impact on benthic organisms.

SPM and POC fluxes exhibited a decline along the reservoir, with a significant correlation between them, suggesting that organic matter was a major component of suspended sediments. PAH deposition rates were considerably higher than those of PAEs, although both pollutant groups showed similar temporal and spatial variations to SPM and POC. The analysis of factors controlling PAH and PAE deposition highlighted the significant role of sedimenting particulate matter in the distribution and accumulation of these compounds on the reservoir bed. Key factors influencing adsorption rates and, hence, deposition of PAHs and PAEs included particulate organic carbon, especially of autochthonous origin, as well as pH and water salinity, with these relationships being more pronounced for PAHs.

In summary, the conducted study indicates that the distribution and deposition of PAHs and PAEs in the Nielisz Reservoir result from the complex interplay of physicochemical and hydrological factors. A key role in these processes is played by settling suspended particulate matter (SPM), which not only influences the transport of these pollutants but also determines their accumulation on the reservoir bed. The observed relationships between SPM and POC fluxes and the deposition of PAHs and PAEs highlight the importance of organic matter, particularly of autochthonous origin, as a carrier of these contaminants. The obtained results provide valuable insights into the mechanisms governing the transport and accumulation of organic micropollutants in dam reservoirs, which may be crucial for water quality monitoring and the implementation of remediation measures in polluted aquatic ecosystems.