Rare Earth Elements (REEs) encompass the series of lanthanide elements (La-Lu) along with yttrium (Y) from the periodic table, which exhibit analogous chemical characteristics. In geochemical studies, they are classified into three groups based on atomic weight: Light Rare Earth Elements (LREEs: La-Nd), Middle Rare Earth Elements (MREEs: Sm-Dy), and Heavy Rare Earth Elements (HREEs: Ho-Lu) (Han et al., 2023). Rare Earth Elements (REEs) enter aquatic and sedimentary systems in natural environments through the weathering and erosion of parent rocks and are transported and deposited by erosional agents (Kumar et al., 2019). Since the composition of REEs remains relatively constant during these processes, they serve as powerful tracers for studying geochemical processes in aquatic and sedimentary systems because they exhibit a pronounced sensitivity to fluctuations in environmental parameters (Van Pelt et al., 2018). Rare Earth Elements (REEs), in addition to their geochemical significance, are of interest due to their strategic applications in scientific and technological industries. These elements are extensively employed in the manufacturing of various products, including electronics, phosphate fertilizers, and fuel additives (Olias et al., 2018; Silva et al., 2019; Jiang et al., 2022). Consequently, the worldwide requirement for REEs has experienced a considerable escalation over the past ten years, leading to increased release of these elements into the environment (Dushyantha et al., 2020; Mleczek et al., 2021; Filho et al., 2023). Human uses of REEs can lead to increased concentrations of these elements beyond background levels, as well as alter their natural distribution patterns, resulting in anomalies in REE profiles. Indeed, given the growing demand for REEs in advanced technology applications and modern industries, these elements have been recognized as emerging micropollutants (Adeel et al., 2019). Rare Earth Elements (REEs) penetrate and collect within the environmental matrix via several processes, comprising fluvial contributions, atmospheric deposition, and anthropogenic actions (Gwenzi et al., 2018; Sager and Wiche, 2024). Consequently, REEs serve as effective indicators for identifying inputs resulting from human activities. For example, Samarium (Sm) has been recognized as a significant component in agricultural fertilizers (Liu et al., 2022), whereas gadolinium (Gd), extensively utilized as a contrast medium in diagnostic imaging, is frequently detected at elevated concentrations in urban water bodies and effluents from medical facilities (Kulaksız and Bau, 2013). Positive anomalies of samarium (Sm) are frequently associated with the petrochemical industry, due to the use of samarium-based catalysts in various processes. Furthermore, industrial processes associated with the extraction, purification, and production of rare earth elements (REEs) and REE-derived materials frequently contribute to the contamination of aquatic ecosystems through the release of these elements. The application of sewage sludge (Kaegi et al., 2021) and chemical fertilizers in agriculture (Tommasi et al., 2021), as well as urban runoff contaminated with particles from coal combustion and transportation (Kolawole et al., 2021), also contribute to the pollution of surface waters with REEs. The extensive production and widespread application of REEs have led to the release of significant quantities of these elements into various environments, including atmospheric particulate matter, soil, water, and sediments (Silva et al., 2019). Since the drainage basins of major rivers often pass through areas with concentrated industrial activities, the distribution pattern of REEs in river sediments is influenced by inputs resulting from anthropogenic activities. In other words, these anthropogenic REEs, after entering aquatic environments, lead to increased concentrations, anomalies, and pollution in sediments, consequently causing sediments to act as the main and important reservoirs for these pollutants (Lu et al., 2022). River sediments contaminated with REEs typically exhibit different REE distribution patterns compared to pristine samples. Contamination of riverine sediments by REEs has been documented across numerous rivers worldwide, including rivers located in the Xiangjiang River watershed, China (Fang et al., 2023), the Linggi River, Malaysia (Elias et al., 2019), the Terengganu River basin, Malaysia (Sultan and Shazili, 2014), major rivers around the Yellow Sea, China (Song and Choi., 2009), the Jiulong River estuary, China (Ma and Han, 2024), the Catumbela River, Angola (Silva et al., 2016), and the Kerala River, India (Babu et al., 2024). Changes in environmental conditions in aquatic ecosystems can lead to the re-release of accumulated elements in sediments into the water phase, acting as a secondary source of pollution. This process can potentially create environmental hazards and threats to human health (Liu et al., 2019). The accumulation of REEs in sediments has raised concerns regarding bioaccumulation potential, ecological toxicity, and potential impacts on aquatic species diversity (Lu et al., 2022; Banaee et al., 2025).Therefore, monitoring and assessing the ecological risks posed by REEs in sediments is of great importance. REEs accumulated in sediments can be absorbed by aquatic plants and animals and transferred to humans through the food chain (Adeel et al., 2019). Consequently, long-term exposure to low doses of these elements, or their consumption, can have adverse effects on human health. For instance, REEs have the potential to induce nephrogenic systemic fibrosis, resulting in substantial impairment of the renal system (Gwenzi et al., 2018). Therefore, understanding pollution levels and assessing the potential risks posed by REEs in sediments affected by human activities is essential. The Maroon-Jarahi River basin, located on the southern and southwestern slopes of the central Zagros Mountains, is a sub-basin of the Persian Gulf and the Sea of Oman in Iran, ultimately leading to the Shadegan International Wetland. This hydrological sub-basin constitutes a multifaceted system and is in proximity to industrial, residential, and agricultural zones (Raeisi et al., 2022; Hedayatzadeh et al., 2024a). Increasing human activities in this sub-basin, in addition to natural factors, play a significant role in disrupting the natural cycles of the elements. However, the distribution, sources, and risks of REEs in the Maroon-Jarahi River sub-basin have not been comprehensively investigated, and further research in this area is necessary. Accordingly, the present study, focusing on the Maroon-Jarahi River sub-basin, aims to achieve the following objectives: (1) determine the concentration and investigate the spatial distribution pattern of REEs in surface sediments; (2) identify potential sources of REEs in the surface sediments of this sub-basin using enrichment factor; and (3) evaluate the contamination level and ecological-health risks related to REEs in surface sediments using various indices.

Study Area

The present investigation was executed within the confines of the Maroon-Jarahi sub-basin, a sub-basin of the Persian Gulf and the Sea of Oman, a region with ecological significance and unique hydrographic characteristics. This sub-basin, with an area of 24,307 square kilometers, is located on the southern-southwestern slopes of the central Zagros Mountains and encompasses a diverse topography ranging from mountainous areas (10,331 square kilometers) to plains/foothills (13,976 square kilometers). The Maroon-Jarahi sub-basin is geologically situated within the folded structural zone of the Zagros and politically encompasses parts of the Kohgiluyeh-Boyer-Ahmad, and Khuzestan provinces. The sub-basin is a closed hydrological system, and its ultimate drainage leads to the Shadegan International Wetland. The sub-basin has distinct topographical features. The maximum elevation in the northern part of the basin reaches 3,613 meters above sea level and in the central part reaches 3,102 meters, while the minimum elevation in the Khuzestan plain is almost level with the sea surface (Hedayatzadeh et al., 2024a). From a climatic perspective, the average annual air temperature also varies from around 15°C to over 24°C, indicating a significant temperature gradient across the basin. Based on the hydrological and geographical characteristics, within the Maroon-Jarahi River sub-basin, there are three constituent smaller sub-basins: the Maroon, Allah (Ramhormoz), and Jarahi River sub-basins (Fig. 1).

Field Sampling

To accurately locate the sampling and quality monitoring stations within the study area, a Digital Elevation Model (DEM) possessing a resolution of 30 meters was employed utilizing ArcGIS software. Based on this, the Maroon-Jarahi River Sub-watershed was divided into smaller sub-watersheds and hydrographic units. Sampling stations were located at the outlet points of these sub-basins, with a total of 70 stations (specifically in the Maroon (N = 38), Allah (N = 13), and Jarahi (N = 19) sub-basins) considered for qualitative monitoring of surface sediments in the study area (Fig. 2). To accurately analyze the study results, 3 samples were collected from each station. Surface sediment samples (depth 0–10 cm, n = 70) were collected and immediately placed in labeled zip-lock polyethylene bags containing the sample location, date, and time of collection. The samples were kept cool in an ice chest until analysis in the laboratory. The sediment samples were transferred to the laboratory and, spread on a suitable surface at ambient temperature for drying. After the sediment samples were fully dried, extraneous materials such as stone fragments and plant debris were removed from the samples. Subsequently, the sediment samples were ground using a mortar and pestle and homogenized to achieve uniform sediment consistency.

Analysis of Rare Earth Elements (REEs) Concentrations in Sediments

To prepare sediment samples for Rare Earth Elements (REEs) analysis, digestion was performed using the EPA 3050B method. In the mentioned method, 1 gr of the sediment sample was accurately weighed and transferred to a Falcon tube. Subsequently, acids were added and the mixture was heated to degrade the sample matrix and facilitate the release of REEs for analysis. To remove the interfering organic matter, 2 mL of hydrogen peroxide (H₂O₂) was incorporated into the samples, followed by the sequential addition of 8 mL of nitric acid (HNO₃) and 2 mL of perchloric acid (HClO₄). The tubes containing the samples and acids were placed in a hot water bath (bain-marie) at 40°C for 1 hour, and then at 100°C for 6 hours, to complete the digestion process. After cooling the samples to room temperature, the solution was diluted using deionized water and then passed through a 0.45 µm filter (Idera et al., 2014). The final solution was brought to a volume of 25 mL and stored at 4°C until analysis by ICP-MS. Finally, the concentration of REEs in the digested samples was determined by ICP-MS. To ensure quality control data related to REEs analysis in surface sediments, meticulous control of all sample preparation stages was emphasized in order to prevent the potential loss or contamination of samples. To ensure data quality, measures such as the use of laboratory blank samples for contamination control and the laboratory duplicate samples were analyzed to assess accuracy. In addition, precise instrument calibration and determination of the limit of detection (LOD) for each REE element were considered.

Abundance and distribution of REEs in surface sediments of sub-basins and monitoring stations of the Maroon-Jarahi sub-basin

The concentrations of various REE and Y (REYs) elements in the surface sediments at 70 stations across the entire Maroon-Jarahi River sub-basin system and separately for its sub-watersheds, including the Maroon (N = 38), Allah (N = 13), and Jarahi (N = 19) sub-basins, can be found in Table (3). According to the findings, the concentration of REYs in all the analyzed samples varied from below the detection limit (for Eu, Tb, Ho, Tm, and Lu) to 42.98 mg kg-1 (Ce). The content of REY concentrations was compared spatially among the three sub-basins analyzed including the Maroon, Allah, and Jarahi rivers. The results from the analysis of variance (ANOVA) revealed no statistically significant difference in the concentrations of Y, La, Ce, Nd, Sm, Eu, Tm, and Lu among the sediments from the three investigated sub-basins (p > 0.05), but the concentrations of the other elements Pr (P = 0.022), Gd (P = 0.035), Tb (P = 0.019), Dy (P = 0.001), Ho (P = 0.013), and Yb (P = 0.002) showed statistically significant differences (p < 0.05). Investigation of the amount and presence of REYs in surface sediments of the Maroon-Jarahi River sub-basin is the first report of these elements in the Maroon-Jarahi sub-basin system from the Persian Gulf. The pronounced presence of these elements in both riverine and coastal environments has been extensively examined, leading to the formulation of numerous hypotheses (Armstrong-Altrin et al., 2022). These elements are used as indicators of anthropogenic pollutants input as a result of petroleum catalysis for fossil fuel production (Orris & Grauch, 2002), as indicators of hospital wastewater (Bau and Dulski, 1996), and as indicators of industrial and factory waste (Olmez et al., 1991). The concentration of REYs in all analyzed samples ranged from below the instrument's detection limit (for Eu, Tb, Ho, Tm, and Lu) to 98.42 mg kg^− 1 (Ce). The average ranking of element concentration in surface sediments of the Maroon-Jarahi sub-basin was Ce > Nd > La > Y > Pr > Sm > Gd > Dy > Er > Yb > Eu > Ho > Tb > Tm = Lu, ranging from 53.33 mg kg^− 1 for Ce to 24.0 mg kg^− 1 for Tm = Lu, respectively. Among the REY elements, Ce and Nd were detected at the highest concentrations in all analyzed samples, whereas Ho, Tb, Tm, and Lu exhibited the lowest concentrations in the samples. These concentrations are relatively similar to PAAS values and within the range of concentrations found in other studied rivers globally. For example, research results by Benabdelkader et al. (2019), in the analysis of the source, distribution, and behavior of REEs in bed sediments of the Tafna River basin, Algeria, showed that the order of element concentration is Ce > La > Nd > Pr > Sm > Gd > Dy > Yb > Er > Eu > Ho > Tb > Tm = Lu from 42.22 (µgg^− 1) for Ce to 0.21 (µgg^− 1) for Tm = Lu, which is similar to the present study for all elements (except Nd and Er). In another study in this area by Celis et al. (2022), in presenting the first report of REEs in river sediments throughout Chile, the findings indicated that the rank order of chemical element abundance in the surface sediments of Chilean rivers is as follows: Ba > Nd > Pr > La > Ce > Zr > Rb > Y > Th > Nb. This pattern was slightly different from the enrichment order and abundance reported in the rivers of the Maroon-Jarahi watershed sub-basin system in the present study. de Freitas et al. (2023) also investigated rare earth elements as tracers of sedimentary pollution in a coastal wetland in the state of Rio de Janeiro, Brazil. In this study, several rare earth elements were identified in the surface sediments, which is similar to the current study. The concentration order of these elements was observed as Ce > Y > La > Pr > Sm > Gd > Dy > Er > Yb > Eu > Ho > Tb > Lu > Tm, with only the order of Y and La differing. In another similar study, Fiket et al. (2018) examined the distribution of REEs within the sediments of Lake Mir, located on Dugi Otok in Croatia. The results indicated that REY concentrations varied from below the detection threshold (for Tb, Ho, Tm, and Lu) to 81 mg kg^-1 (for Ce), with Ce and La exhibiting the highest concentrations across all samples, while Tb, Ho, Tm, and Lu revealed the lowest values. A study conducted by Hu et al. (2019) focused on the distribution and origin identification of REEs in the marine sediments of Xiamen Bay, Southeast China, also revealed that the average concentration of REEs in surface sediments of the western Xiamen Bay follows a sequence of Ce > La > Nd > Pr > Sm > Gd > Dy > Yb > Er > Eu > Ho > Tb > Lu > Tm, which adheres to the Oddo-Harkins rule (Zhenggui et al., 2001).

The outcomes of an ANOVA statistical examination revealed a significant difference in the concentrations of Pr, Gd, Tb, Dy, Ho, and Yb elements in the sediments of the three sub-basins (Maroon, Allah, and Jarahi). According to the findings, the highest concentrations of Pr and Tb were recorded in the Jarahi River sub-basin, followed by Allah and Maroon, and for Gd and Yb elements, the Allah and Maroon rivers had the highest levels, respectively. Regarding the Dy and Ho elements, both sub-basins of Allah and Jarahi had the highest amounts (with no statistically significant difference), and the Maroon River had the lowest amount. Therefore, in light of these findings, the distribution of the content of some elements showed a specific order and significant spatial variations, such that, compared to the upstream flow (Maroon River), the concentration of REEs was higher in the middle and especially downstream sections of the Maroon-Jarahi watershed. Given that the Maroon-Jarahi sub-basin system traverses urban, industrial, and agricultural zones, the presence of REEs in river surface sediments can be linked to anthropogenic activities (including industrial and medical wastewater, municipal wastewater, and the application of fertilizers and pesticides) in addition to natural processes (Raeisi et al., 2022; Hedayatzadeh et al., 2024b). In this context, the study by Celis et al. (2022), in presenting the first report of rare earth elements in Chilean river sediments, the findings indicated that among the rivers examined, the sediments of the Loa River in northern Chile exhibited the most elevated average concentrations of Ba, Ce, La, and Nd, while the Turbio River contained the highest average level of Rb, and the sediments from the Marchant River (southern Chile) displayed the peak values of Nb, Pr, Y, and Zr. This research posited that the elevated concentrations of the lanthanide series (Ce, La, Nd, and Pr) alongside Ba in the river sediments across Chile may be correlated with the geological presence of volcanic formations in the regions encompassing the Loa, Turbio, and Marchant rivers (Wulaningsih et al., 2013). Furthermore, there is substantiated evidence indicating that mining operations associated with copper and molybdenum extraction significantly influence the concentrations of rare earth elements within aquatic systems (Munemoto et al., 2020; Tripodi et al., 2019). Gao et al. (2023) also investigated the spatial distribution and origins of rare earth elements within urban riverine environments (Yongding) as an indicator of anthropogenic inputs. The spatial patterns of ΣREEs concentration in the Yongding River showed that the concentration of ΣREE in the mid and downstream sections, as well as in the estuary, surpassed that of the upstream river flows. In this investigation, the comparatively diminished concentrations observed in the estuary (YD-1) were ascribed to salt-induced flocculation, a process that disrupts the colloidal associations of rare earth elements when river waters intermingle with seawater, thereby resulting in a reduced concentration of these elements in the riverine waters (Bayon et al., 2015). Additionally, this research revealed that the ΣREE concentrations in downstream regions, particularly in areas traversing populous urban locales, were significantly elevated. The concentration of rare earth elements in the downstream segment of the Yongding River is intricately linked to urbanization and is likely to escalate with intensified urban development. Moreover, the YD-4, YD-5, and YD-6 sampling sites, characterized by notable concentrations of rare earth elements, were strategically situated in proximity to numerous coal-fired power plants and industrial heating facilities. Researches have indicated that substantial amounts of anthropogenic rare earth elements are contained in the ash produced by coal combustion processes (Wei et al., 2021), and they may enter river water through the wind, leading to an increase in the dissolved REE content in the adjacent river water.

Additionally, a comparative analysis of the average ΣREE in surface sediments across the three sub-basins of the Maroon, Allah, and Jarahi rivers revealed that the Jarahi River sub-basin yielded the highest ΣREE values in its sediment samples, while sediments from the Maroon and Allah River sub-basins showed lower ΣREE levels. The difference in ΣREE levels among these three sub-basins is likely related to the different sources of emission of these elements and the sediment composition of these aquatic systems. For example, the high concentration of ΣREE in the Jarahi River sub-basin compared to other sub-basins may be due to the proximity and establishment of petrochemical industries and factories in Bandar-e Mahshahr County, which use fossil fuels for their operations (de Freitas et al., 2021; Costa et al., 2021). Additionally, atmospheric transport of REEs should also be considered, as these substances can be distributed over extensive distances (de Oliveira et al., 2007), which may affect notable accumulation of rare earth elements even within coarse-grained sediment samples. Given this fact, particles from industrial processes may have been transported through the atmosphere and deposited in the Jarahi River sub-basin, particularly in the Shadegan Wetland. The area surrounding the Jarahi River is currently under extensive development, with a significant increase in small rural areas primarily engaged in agricultural and animal husbandry activities. Therefore, the high concentration of ΣREE in this sub-basin indicates extensive human activities in the study area, including the use of agricultural pesticides enriched with REEs, as well as the result of atmospheric transfers from industrial centers and the Petrochemical Special Economic Zone of Bandar Mahshahr County. Globally, atmospheric dust samples collected from various locations in Nigeria (residential and industrial zones) were compared with surface soil and granite gneiss samples to evaluate the pollution levels in Ibadan, located in southwestern Nigeria. The authors noted significantly elevated levels of rare earth elements in both residential and industrial areas, alongside variations in the light-to-heavy lanthanide ratio values, which are indicators of pollution attributed to their emissions from power plants and vehicle exhausts (Kolawole et al., 2021). Rare earth elements were also detected in the Donetsk region, an industrial area in Ukraine, attributed to atmospheric transport mechanisms. The authors employed various moss species from a natural reserve to determine the levels of rare earth elements in the study region. High concentrations of La, Ce, Nd, Th, and U were identified in two moss species, which were linked to coal combustion emissions carried by atmospheric transport (Sholkovitz, 1993). Comparing the ΣREE levels between different stations in each of the three sub-basins showed that the highest ΣREE levels were at stations M-38 (230.43), M-25 (227.73) in the Maroon sub-basin, A-9 (200.83), A-4 (191.98) in the Allah sub-basin, and J-13 (228.33), J-14 (231.81) in the Jarahi sub-basin, and the lowest levels were observed at stations M-31 (100.79), A-10 (103.64), and J-2 (110.38). The ΣREE content obtained in this study (100.79–231.81) is comparable with the data published for other rivers. Such as the Tafna River in Algeria (Benabdelkader et al., 2019); in this study, the mean ΣREE concentration in river sediments was recorded at 102 µg/g (with a range of 90.27 to 153.39 µg/g) at the T3 and DamB stations, respectively. Vembanad estuary, southwest India (Manoj et al., 2016); in this study, the total REE values (ΣREE) for 17 sampling stations were 157.63 ppm (29.55–229.67). A comparison of the study stations showed that the ΣREE values in Murinpuzha and Thanneermukkom exhibit significant variations compared to other stations and are much lower than the global average. Indeed, these two sampling locations exhibited the lowest values for REE elements, which may indicate a much lower input of lithogenic material or variations in diagenesis processes at these stations. Xiamen Western Bay, southeastern China (Hu et al., 2019); In this study, the concentration of ∑REEs in surface sediments was observed between 189.9 and 297.8 mg/kg, with an average of 245.9 mg/kg. A comparative analysis of the sampling sites revealed that the minimum concentration of ∑REEs was observed at site X4, whereas the maximum concentration was detected at site S10. The Mgoua watershed, Southwest Cameroon (Ndjama et al., 2022): In this research, the concentration of ΣREEs within the sedimentary matrices of the watershed fluctuated between 282.12 and 727.67 ppm, with a mean value established at 550.05 ppm, which approximates 2.5 times greater than the values outlined by PAAS (Taylor and McLennan, 1985). In this study, the change in ∑REEs concentration was considered a function of the change in the dissolved solids, indicating the relationship between TDS and REEs. This observation aligns with the findings of Sholkovitz and Szymczak (2000), who posited that the concentration levels of REEs diminish as salinity increases in tropical river systems situated in coastal regions. A comparison of the data from the aforementioned studies with the outcomes of the current investigation indicates that the average level of ΣREEs in the surface sediments of the Maroon-Jarahi River sub-basin is lower than the levels reported in the sediments of Xiamen Bay, Southeast China, and the Mgoua watershed in southwestern Cameroon, while it is higher than the levels found in the sediments of the Tafna River in Algeria and the Vembanad Estuary.

Figure 3 illustrates the spatial patterns of REE concentrations in surface sediments of different sampling stations in the three sub-basins (Maroon, Allah, and Jarahi). A one-way analysis of variance (ANOVA) statistical test was used to compare the concentration of REEs among different stations in the sub-basins. Results from the ANOVA indicated that in the Maroon River sub-basin, the concentrations of various elements (except Er, Tm, and Lu) differed significantly between stations (P < 0.05). In the Allah River sub-basin, only Ce, Pr, Sm, Tb, Dy, Er, and Yb showed statistically significant differences, and in the Jarahi River sub-basin, all elements except La, Gd, Dy, Tm, and Lu showed statistically significant differences between different stations along this sub-basin. As depicted in Fig. 3, the concentration of REEs across the various sampling stations within the Maroon-Jarahi region showed different spatial and dispersion patterns. In summary, stations such as stations M-38, M-24, M-25, M-35, A-9, J-14, and J-12 exhibited the highest concentrations for various elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Y), while stations such as stations M-31, M-23, A-5, A-1, A-6, J-1, J-2, J-4, J-10, and J-18 had the lowest concentrations. These changes in REEs concentrations are probably related to differences in lithology, weathering, and sediment transport or anthropogenic inputs (Sojka et al., 2021; Yang et al., 2025).

Total concentrations of ∑REE, LREE, MREE, and HREE in surface sediments of the Maroon-Jarahi sub-basin

The average ΣREE in the surface sediments of the entire Maroon-Jarahi sub-basin was 166.83 (100.79-231.81) mg kg^− 1. The values ascertained for the three sub-basins of the Maroon, Allah, and Jarahi rivers were 162.75 (100.79-230.43), 165.39 (103.64-200.83), and 172.35 (110.38-231.81) mg kg^− 1, respectively (Table 4). As is usually presented in the research backgrounds (Sholkovitz, 1995; Romero Freire et al., 2018), REEs include three subgroups: LREEs (from La to Nd), MREEs (from Sm to Ho), and HREEs (from Er to Lu). The average values of LREEs (La–Nd), MREEs (Sm-Ho), and HREEs (Er-Lu and Y) in the surface sediments of the entire Maroon-Jarahi River sub-basin were observed to be 130.21, 17.76, and 18.43 mg kg^− 1, respectively. The values of these elements were also calculated separately for each sub-basin, which were 126.68, 16.84, and 18.48 mg kg^− 1 in the Maroon River sub-basin; 128.96, 18.11, and 18.31 mg kg^− 1 in the Allah River sub-basin; and 134.98, 18.35, and 18.49 mg kg^− 1 in the Jarahi River sub-basin, respectively (Table 4). The spatial patterns of rare earth element concentrations among different stations in all three sub-basins indicated that the Jarahi River, located downstream within the Maroon-Jarahi sub-basin, had the highest amounts of LREE, MREE, and HREE (Fig. 4).

In this study, the normalization of REE values to Post-Archean Australian Shale (PAAS, Taylor and McLennan, 1985) was also used to minimize interference from natural abundance. The Australian Post-Archean Shale (PAAS) has frequently been used in normalization methods and facilitates comparisons with existing data in the research background. In this study, PAAS values (Table 3) were used as standard values for this research section. The results of the normalization of the mean concentration of REEs in the sediments of different stations studied in the Marun-Jarahi sub-basin, as well as separately for the three sub-basins of the Maroon, Allah, and Jarahi rivers, are depicted in Figure (5) using the Post-Archean Australian Shale (PAAS) bedrock reference.

To assess the enrichment of REEs in sediments, the average concentration of REEs in sediments from different stations studied across the Maroon-Jarahi basin, as well as separately for the three sub-basins of the Maroon, Allah, and Jarahi rivers, was normalized using the Post-Archean Australian Shale (PAAS) as a reference bedrock. Based on this normalization, different REE enrichment patterns enrichment were observed in the sediments. The maximum value (28.1) for Gd was obtained in all three sub-basins, while the minimum amount for the elements Tm and Lu was recorded, especially in the Maroon and Allah sub-basins. Based on normalization, the overall pattern and ranking of elements in the sediments indicate a greater enrichment of LREEs and HREEs than of MREEs. The research conducted by Slavković-Beškoski et al. (2024), which examined the environmental and health hazards associated with REEs in coal fly ash (CFA), the results of REY concentration normalization using the UCC reference showed that these values typically ranged between 1 and 3 across the majority of CFA samples in different countries. The maximum value (3.5) for Gd was derived from an average sample originating from China, while the minimum (0.3) was documented for Tm in the CFA10 sample. In the research by Benabdelkader et al. (2019) on REEs in the bed sediments of the Tafna River basin, the normalization of elements using two bedrock references (PAAS and local bedrock) revealed different overall patterns and enrichments; nevertheless, the hierarchical ranking of the stations exhibited similarity. Based on the normalization of REEs in river sediments relative to PAAS, MREE elements were observed to be more enriched than LREE and HREE, but normalization based on local bedrock indicated that LREE and MREE elements were more enriched than HREEs. Overall, the results of this study indicate a higher enrichment of LREEs and, to some extent, MREE elements. In the present study, the enrichment levels of LREE and MREE elements were relatively homogeneous and not significantly elevated, suggesting a natural weathering process as the source (McLennan and Taylor, 2012). However, the extremely high enrichment of LREE and MREE elements in some stations may not rule out the contribution of local pollution (Brito et al., 2018). For instance, within the Tafna basin, there has been no evident mineral exploration; nonetheless, certain dust emanating from Moroccan mines has been recognized as an origin to atmospheric lead deposition within the basin (Benabdelkader et al., 2018). Consequently, the provenance of REEs resulting from dust deposition in river sediments and soil erosion products warrants considerable attention.

The overall average of the I-geo index for REE elements in sub-basins is shown in Figure (6), and at monitoring stations is presented in Table (5). Based on the findings in Figure (6), the average I-geo for Nd, Sm, and Gd elements was only in the classification of 0 < I-geo < 1 corresponds to unpolluted to moderately polluted conditions and the other elements were in the unpolluted class (I-geo ≤ 0). Based on the analytical outcomes of I-geo values at monitoring stations throughout the entire Maroon-Jarahi sub-basin, the elements La (-1.30 to 0.24), Pr (-1.37 to 0.04), Nd (-0.77 to 0.49), Sm (-1.14 to 0.67), Eu (-1.33 to 0.33), Gd (-0.55 to 0.52), and Dy (-2.04 to 0.20) were observed in 0 < I-geo < 1 class, and other elements were observed in the unpolluted category (I-geo ≤ 0). In each of the Maroon, Allah, and Jarahi sub-basins, the average I-geo index for REE elements indicated that based on I-geo, the elements Ce, Tb, Ho, Er, Tm, Yb, Lu, and Y for all monitoring stations in all three sub-basins are distributed in class 1 (uncontaminated: I-geo ≤ 0) and are unpolluted. However, based on the I-geo index for other REE elements, the distribution of monitoring stations within the Maroon-Jarahi sub-basin is characterized in class 1 (unpolluted: I-geo ≤ 0) to class 2 (unpolluted to moderately polluted: 0 < I-geo < 1).

In the investigation of rare earth element contamination in surface sediments of the region, the average I-geo index for REEs in each of the Maroon, Allah, and Jarahi sub-basins within the study area showed that, based on the I-geo of elements Ce, Tb, Ho, Er, Tm, Yb, Lu, and Y, all monitoring stations were distributed in Class 1 (uncontaminated: I-geo ≤ 0) and were free of contamination. However, according to I-geo for other REEs, the monitoring stations within the Maroon-Jarahi sub-basin are classified in category 1 (uncontaminated: I-geo ≤ 0) to 2 (uncontaminated to moderately contaminated: 0 < I-geo < 1). According to Igeo-La, the Maroon and Jarahi sub-basin monitoring stations were in Class 1 and the Allah sub-basin in Class 2 (highest levels at M-13 and M-24), based on Igeo-Pr, the Maroon and Allah sub-basins were in Class 1 and the Jarahi sub-basin in Class 2 (highest level at J-9), based on Igeo-Eu, the Maroon sub-basin was in Class 1 and the Allah and Jarahi sub-basins were in Class 2 (highest levels at A-6 and J-3). Based on the I-geo values for Nd, Sm, Gd, and Dy elements, the monitoring stations of all three sub-basins of Maroon, Allah, and Jarahi are distributed in classes 1 and 2. In the investigation by Atibu et al. (2016), which evaluated trace rare earth element pollution in rivers adjacent to both abandoned and operational mining sites, the findings indicated that, according to I-geo values, except for Mo, Th, U, Eu, Ho, and Tm — classified in category 2 (moderately polluted and/or unpolluted sites) — all elements across various sites were categorized in class 6 (extremely polluted). This research demonstrated that the sediment contamination observed at CT and RL stations could be elucidated not solely by the geological formations of the region but also by the influence of other potential sources. In another study by de Freitas et al. (2023), investigating rare earth elements as tracers of sedimentary pollution in a coastal wetland in Rio de Janeiro State, Brazil, results showed that all elements presented an I-geo between 0 < I-geo ≤ 1 in both surface and core sediments, indicating partial environmental pollution. In this analysis, it was deduced that the presence of REEs might stem from atmospheric transfers originating from the industrial hub of Rio de Janeiro. In another related study by Ma and Han (2024) on the assessment of REE contamination levels in Mekong River sediments in Thailand, the results of the pollution evaluation using the I-geo index indicated that REEs in the samples showed low to moderate contamination. This study suggests that regular fertilization of agricultural areas leads to pollution and environmental hazards for surrounding areas, including rivers, which are a source of LREE enrichment in river sediments.

The EF values of investigated REEs are presented in Figure (7). EF values for elements ranged from 16.2–37.8 (42.4) La, 44.0–83.5 (35.3) Ce, 56.1–36.7 (26.4) Pr, 39.2–30.11 (76.5) Nd, 20.2–88.10 (83.5) Sm, 0.00–81.9 (08.4) Eu, 26.2–43.12 (15.6) Gd, 0.00–51.7 (86.2) Tb, 82.1–64.9 (91.4) Dy, 0.00–93.6 (47.2) Ho, 0.39–83.4 (67.2) Er, 0.00–85.4 (71.0) Tm, 0.00–38.6 (56.2) Yb, 0.00–25.4 (26.0) Lu, and ranged from 1.35–61.5 (90.2) for the Y. Results show that the average EF for Tb, Ho, Er, Tm, Yb, Lu, and Y elements was less than 3 (partial contamination class), for La, Ce, Pr, and Eu elements it was in the moderate contamination class (3 < EF < 5), and for other elements Nd, Sm, and Gd it was in the relatively severe contamination class (5 < EF < 10). A comparative analysis of the EF of total rare earth elements across different monitoring stations in the three sub-basins of the Maroon, Allah, and Jarahi is depicted in Figure (8). As can be seen, the highest enrichment of various elements is associated with the monitoring stations situated in the Jarahi River sub-basin.

The effect of anthropogenic sources on the concentration of REE elements in sediment samples was similarly assessed through the calculation of enrichment factor (EF) values. Results from the elemental enrichment analysis in the surface sediments of the studied area indicate that the average Enrichment Factor (EF) for elements Tb, Ho, Er, Tm, Yb, Lu, and Y was less than 3 (minor enrichment class), for elements La, Ce, Pr, and Eu, it was in the moderate enrichment class (3 < EF < 5), and for elements Nd, Sm, and Gd it was in the relatively severe enrichment class (5 < EF < 10). A comparison of the EF factor of total REEs among different monitoring stations in each of the three sub-basins of the Maroon, Allah, and Jarahi rivers also indicated that the most pronounced enrichment of various elements was associated with the monitoring stations positioned within the Jarahi River sub-basin. In the present study, the EF factor signifies the enrichment of LREEs in relation to HREEs in the surface sediment samples of the study area. According to the findings, the highest enrichment factor (EF) for Nd, Sm, and Gd supports the hypothesis that anthropogenic activities, such as the discharge of urban wastewater (containing hospital wastewater) and agricultural drainage, can act as sources of these elements within the study area. In this context, one may refer to the investigation conducted by Chen et al. (2024) regarding the assessment of rare earth elements within the sediments of the Pearl River Estuary, China, which, based on EF values, minimal enrichment of REE was observed in the sediments of the study area. In another study by Ma and Han (2024) investigating rare earth element pollution in surface sediments of the Jiulong River estuary and adjacent basin, the evaluation of enrichment factors indicated that Gd, Yb, Lu, Tb, and Dy represented the elements with the highest degree of enrichment in the sediments. The higher enrichment of these elements was attributed to human activities such as agricultural fertilizer consumption and coal combustion in the region. Another study by Soroaga et al. (2022), in evaluating the impact of human activities and the distribution of rare earth elements in lake sediments from northeastern Romania, showed that the EF values of rare earth elements for all sampling points were less than 1.5, indicating their natural origin in the region. In the research by Madukwe et al. (2020), which assessed the levels of rare earth elements in river sediments in the Ijero-Ekiti area, southwestern Nigeria, the results indicated that, based on the EF factor, these elements have the lowest enrichment levels and therefore originate naturally from river sediments and geogenic sources resulting from weathering processes in the environment.

The ecological risk index (ERI) calculation results for REEs in the entire Maroon-Jarahi sub-basin indicated a low environmental risk (ERI < 150) with variable values from 23.21 to 112.27 and an average of 63.76 (Fig. 9). A comparison of the ERI averages for the Maroon (RI = 07.84), Allah (RI = 11.64), and Jarahi (RI = 15.91) sub-basins also indicated that the elements in the sediments of the Jarahi River sub-basin had the most significant ecological risk in comparison to the other two sub-basins. Additionally, individual risk index (Er) values for different elements were calculated, which can be seen in Figure (9).

Based on the results presented in Figure (10), the most substantial percentage contribution of each rare earth element (REE) in the potential ecological risk across the entire Maroon-Jarahi sub-basin was attributed to Lu at 17%, followed by Eu at 12%, and Gd = Tb = Tm (9%). By sub-basin, the most significant percentage contribution of these elements was also recorded in the three sub-basins (Maroon (Lu (17%), Eu (13%) and Gd = Tb = Tm (9%)), Allah (Eu (16%), Tb (13%) and Gd = Ho (12%)) and Jarahi (Lu (18%), Eu (12%) and Ho = Tb = Gd (9%)))

The results of the Ecological Risk Assessment (ERI) of rare earth elements in the surface sediments of the Maroon-Jarahi sub-basin, as well as separately for the three sub-basins of Maroon River, Allah, and Jarahi, showed low environmental risk. A comparison of the ecological risk (RI) posed by these elements in the sediments of three sub-basins indicated that the highest RI was associated with elements in the sediments of the Jarahi River sub-basin. The values of the individual risk index (Er) of various elements were also observed in the order Sm > Lu > Eu > Tm > Tb > Gd > Ho > Dy > Pr > Er > Yb > Nd > Y > La > Ce. Specifically, Lu showed the highest amount of Er in the sub-basins (average 14.92), followed by Eu (with an overall average of 10.48), while La (1.07) and Ce (0.82) had the lowest amounts. In an analysis by Chen et al. (2020), the computation of the toxicity coefficient and the ecological risk evaluation of REEs indicated that Lu possessed the highest toxicity coefficient, succeeded by Eu, Tb, Ho, and Tm. The toxicity levels of LREEs such as La and Ce, which are frequently utilized in fertilizers, are comparatively low. Related research has also shown that the toxicity of HREEs is relatively higher than that of LREEs (Jin and Huang, 2013). Furthermore, in comparison to the toxicity coefficient for heavy metals as determined by Hakanson (1980), the toxicity coefficient values for La = 1, Ce = 1, Nd = 2, and Y = 2 are relatively minimal, and the market demand for these four elements constitutes 99.9% of the total (Goodenough et al., 2018), which may be attributed to the low toxicity associated with these rare earth elements (Chen, 2004). In a study by Liu et al. (2023) examining the ecological risks of REEs in the sediments of the "Yellow River-Estuary-Gulf" system, the findings suggested that while Lu exhibited moderate potential ecological risks within the sediments, the other rare earth elements displayed relatively low ecological risks. In this research, the potential ecological risk indices for Y and rare earth elements ranged from 7.78 to 144, and similar to the current study, demonstrated an upward trend from the river's upstream section towards the estuary and the adjacent bay. The results of research by Xia et al. (2023) on the geochemical characteristics and ecological risks of rare earth elements in river sediments in Eastern China indicated that river sediments throughout the region had a low potential ecological risk, and similar to the present study, Eu (Er = 13.05) and Lu (Er = 14.07) showed the highest potential risk. In an investigation by Wang et al. (2021), which assessed the distribution and potential ecological risks of four LREEs in the Anning River, Sichuan Province, China, results indicated that the average Ei values for the elements were in the order of Pr > Nd > Ce > La, and Pr in the sediments posed a high ecological risk (114.62–12.13). Based on the results, the highest potential ecological risk associated with rare earth elements in the Anning River was mainly observed at monitoring stations S4-S6 and S16 of the river. It was proposed that agricultural activities in the middle and lower reaches of the Anning River valley, extensive crop cultivation, and significant soil erosion are among the contributing factors to non-point source pollution, thereby influencing the rare earth element content in the sediments of this region. Examination of the percentage contribution of each REE element to the potential ecological risk index (RI) at the study basin, as well as in each of the Maroon, Allah, and Jarahi sub-basins, showed that the highest percentage contribution was related to the elements Lu, Eu, and subsequently Gd = Tb = Tm. Similar to the findings of the current research, the results of the research by Slavković-Beškoski et al. (2024) in assessing the environmental risks of rare earth elements in coal ash also showed that Lu has the highest contribution (29.2%) to the ecological risk caused by elements in the region, while Pr has the lowest contribution (5.3%).

The adverse health effects of REEs in surface sediments of the Maroon-Jarahi River sub-basin were assessed for children and adult age groups using the USEPA model. This model delineates the potential exposure pathways (ingestion and dermal contact). The average hazard quotient (HQ) values for REEs present in sediments are detailed in Table (6). Figures (11) further illustrates the contribution of various elements to the cumulative non-carcinogenic health risk.

In assessing the non-carcinogenic health risks of REEs in surface sediments of the Maroon-Jarrahi River sub-basin for two age groups, children and adults, the hazard quotient (HQ) values obtained for different elements in both age groups were in the order of: Ce > Nd > La > Y > Sm > Pr > Gd > Dy > Er > Yb > Eu > Ho > Tb > Tm > Lu. In the present investigation, based on the hazard quotient values for REEs, it is concluded that the rare earth elements in the surface sediments of the study area exert a negligible adverse effect on human health. In this research, the analysis of the contribution of various elements to the overall non-carcinogenic health risk revealed that the greatest contribution of rare earth elements to non-carcinogenic health risk was associated with Ce (31.14%), Nd (21.21%), and La (18.99%). In a similar study by Slavković-Beškoski et al. (2024) on the health risk assessment of REEs in coal ash, the obtained HQ values for both age groups were observed in the order Ce > La > Nd > Y > Dy > Pr > Gd > Sm > Er > Yb > Eu > Ho > Tb > Lu > Tm. In this research, similar to the present study, the average HQ values for REYs in the CFA indicated that REYs do not pose a risk to human health. In a study conducted by Ma and Han (2024), which examined the health risks linked to rare earth element exposure in the sediments of the Mekong River in Thailand, Monte Carlo simulations estimated the mean daily dose of total rare earth elements from sediments to be 0.24 µg/kg/day for adults and 0.95 µg/kg/day for children. These levels are found to be below the established health thresholds for humans, However, based on the findings of this research, it appears that children are at a higher risk of exposure to REE. Also, in a study by Xia et al. (2023), investigating the risks of REEs in river sediments in eastern China showed that the average daily dose (ADD) of rare earth elements for children was approximately 2000 µg/(kg·day), which is substantially higher than that for adults.

A carcinogenic health risk assessment of various REEs in the surface sediments of the Maroon-Jarahi sub-basin was conducted. The average CR values for REEs in sediments and ILCR are presented in Table (7). The risk assessment of carcinogenic health risks due to various REEs in the Maroon-Jarahi sub-basin sediments also showed that the average ILCR values for adults and children were 9.91 × 10^− 16 and 6.89 × 10^− 15, respectively. Comparison of CR values for different elements also indicated that Ce had the highest carcinogenic health risk compared with other elements, with an average of 2.19 × 10⁻¹⁵ for children (CRc) and an average of 3.07 × 10⁻¹⁶ for adults (CRa). Following that, the Nd (with averages of 1.50 × 10⁻¹⁵ for CRc and 2.10 × 10⁻¹⁶ for CRa) presented the highest carcinogenic health risk. Other REEs had lower CR values. Overall, based on the ILCR and CR values for REEs, it can be concluded that there is no carcinogenic effect or risk from REEs elements in the sediments of the Maroon-Jarahi sub-basin for human health. In the research by Slavković-Beškoski et al. (2024), the health risk assessment of REEs in coal ash also showed that the non-carcinogenic risk index (HI) and carcinogenic risk (CR) from REYs, although higher for children (HIc = 0.15, CRc = 8.4 × 10^− 6) than for adults (HIc = 0.017, CRc = 3.6 × 10⁶), were significantly lower than the permissible limits (HI = 1, CR = 1 × 10^− 6).

In this section of the study, the concentrations of REEs in the surface sediments of the Maroon-Jarahi River sub-basin are investigated and compared with reported data from other studies for the Jiulong (China), Agbangudu (Nigeria), Miño (Iberia), Tafna (Kenya), Xiamen (China), and Ipojuca (Brazil) rivers, to identify patterns and differences in REE distribution among these river systems (Table 8). Comparison with the findings of other studies showed that the Miño River has the highest ΣREE (472 mg kg^− 1), indicating a significant enrichment of REEs in its sediments, while the Tafna River (Kenya) shows the lowest ΣREE with 102.01 mg kg^− 1. A comparison of the average ΣREE in surface sediments of the Maroon-Jarahi sub-basin with other river systems shows that the Maroon-Jarahi River (165.11 mg kg^− 1) has a relatively moderate ΣREE content, with a lower average ΣREE compared to the Jiulong (China), Miño (Iberia), Xiamen (China), and Agbangudu (Nigeria) rivers, and a higher average compared to the Tafna (Kenya) and Ipojuca (Brazil) rivers. A detailed, element-by-element comparison of REE concentrations in the Maroon-Jarahi River with other river systems globally indicates that this river is generally in a moderate range in terms of REE concentrations. However, slight differences in the concentration of individual elements were observed, which may be due to differences in the composition of parent rocks, degree of weathering, depositional processes, and anthropogenic inputs.

To provide a more comprehensive understanding of the pollution status in the Maroon-Jarahi sub-basin and to assist in the optimal management of pollution sources and environmental protection, future research can be designed to offer more extensive information in this area. It is recommended that, in addition to rare earth elements (REEs), other potential contaminants, such as heavy metals, pesticides, and pharmaceutical substances be investigated in sediments to provide a comprehensive view of the pollution status of the area. To effectively manage pollution, it is essential to quantitatively determine the contribution of each pollution source (such as municipal wastewater, industrial effluent, and agricultural runoff) to the emission of REEs. Additionally, evaluating the effects of different land-uses on REE concentrations in sediments will contribute to a better understanding of the distribution and sources of these elements. Developing mathematical models to examine the effects of water and sediment characteristics, topography, and land use on REE concentrations in the sub-basin can also facilitate better management of pollution sources and enable the evaluation of the potential impacts of control measures.

The present study in the Marun-Jarahi sub-basin showed that the average concentration of rare earth elements (REEs) in surface sediments is in the order Ce > Nd > La > Y > Pr > Sm > Gd > Dy > Er > Yb > Eu > Ho > Tb > Tm = Lu, which is relatively consistent with PAAS values. Based on the geo-accumulation index (I-geo), surface sediments at monitoring stations were classified as either unpolluted or moderately polluted. A study investigating the impact of anthropogenic sources on the concentration of REEs in sediments by determining the enrichment factor (EF) revealed that LREEs are more enriched than HREEs. The highest EF values were observed for Nd, Sm, and Gd, indicating the role of human activities, such as the discharge of urban wastewater (including hospital sewage) and agricultural runoff, are potential sources of these elements in the study area. The ecological risk and health risk assessment of elements in the studied area also showed that REEs have low potential environmental risk and no significant adverse effects on human health. In general, the analysis of the spatial distribution patterns of REEs and the assessment of pollution indices in this study showed that the downstream areas of the sub-basin, specifically the areas adjacent to the Jarahi River and Shadegan Wetland, were identified as pollution-sensitive spots due to the concentration of industrial and agricultural activities, wastewater discharge, and urban development. Therefore, comprehensive management and effective control of pollutant sources are essential to reduce the environmental and health risks caused by REEs in this sub-basin.