Introduction
Wetlands are among the most productive and ecologically valuable ecosystems on Earth, providing a wide range of ecosystem services that are essential for environmental sustainability and human well-being. They support exceptionally high levels of biodiversity, serve as habitats for numerous aquatic and terrestrial species, regulate hydrological processes by mitigating floods and storing water, recharge groundwater resources, improve water quality through the retention and transformation of pollutants, sequester carbon, and contribute significantly to climate regulation. In addition, wetlands provide important socioeconomic benefits by supporting fisheries, agriculture, tourism, and the livelihoods of local communities. Despite their ecological and economic importance, wetlands throughout the world have experienced extensive degradation during recent decades owing to rapid urbanization, industrialization, agricultural expansion, population growth, and climate change. These pressures have substantially increased the discharge of industrial effluents, municipal wastewater, agricultural runoff, petroleum products, and other contaminants into aquatic ecosystems.The Hoor Al-Azim Wetland, located in southwestern Iran along the Iran–Iraq border, is the largest permanent freshwater wetland in the country and is recognized as one of the most valuable wetlands in the Middle East. The wetland plays a fundamental role in maintaining regional biodiversity, supporting migratory birds, sustaining fisheries, regulating the local climate, and preserving ecological balance. However, the ecosystem has undergone severe environmental degradation during the past several decades. Intensive oil exploration and extraction, expansion of the Yadavaran and Azadegan oil fields, construction of roads, pipelines and drilling platforms, repeated drought events, upstream dam construction, and reduced freshwater inflow have significantly altered the wetland's hydrological regime and ecological condition. These anthropogenic activities have increased the risk of contaminant accumulation, particularly heavy metals, within wetland sediments.Heavy metals are among the most persistent environmental pollutants because they are non-biodegradable, resistant to natural degradation processes, toxic even at relatively low concentrations, and capable of bioaccumulation and biomagnification through aquatic food webs. Sediments function both as long-term sinks and potential secondary sources of heavy metals, releasing contaminants back into the water column under changing environmental conditions. Consequently, sediment quality assessment provides valuable information regarding the long-term pollution status of aquatic ecosystems and the potential ecological risks posed to benthic organisms and higher trophic levels. Therefore, the present study aimed to determine the spatial distribution of selected heavy metals in surface sediments of the Hoor Al-Azim Wetland and to evaluate sediment contamination using internationally recognized sediment pollution indices and sediment quality guidelines.
Materials and Methods
Twenty-seven sampling stations were selected throughout the Hoor Al-Azim Wetland to provide representative spatial coverage of different ecological zones while considering potential anthropogenic pollution sources, including oil drilling sites, production facilities, transportation pipelines, industrial infrastructures, inflowing waterways, and relatively undisturbed reference areas. Surface sediment samples (0–5 cm depth) were collected using standard sediment sampling procedures during the study period. The collected samples were placed in clean polyethylene containers, transported to the laboratory under appropriate conditions, air-dried, homogenized, sieved, and prepared for chemical analysis. Concentrations of iron (Fe), copper (Cu), zinc (Zn), nickel (Ni), vanadium (V), lead (Pb), cadmium (Cd), and chromium (Cr) were determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), which provides high analytical sensitivity and precision for trace metal determination. Quality assurance and quality control procedures included the analysis of reagent blanks, duplicate samples, certified reference materials, and recovery tests. To evaluate sediment contamination, measured concentrations were compared with international Sediment Quality Guidelines (SQGs), including Threshold Effect Level (TEL), Probable Effect Level (PEL), Effect Range Low (ERL), and Effect Range Median (ERM) values where applicable. In addition, the Geoaccumulation Index (Igeo) was calculated to evaluate contamination intensity relative to background concentrations, whereas the Combination Pollution Index (CPI) was employed to determine the overall pollution status at each sampling station. Statistical analyses were performed using one-way analysis of variance (ANOVA) to identify significant spatial differences among sampling stations at a significance level of P < 0.05.
Results and Discussion
Heavy metal concentrations exhibited considerable spatial variability throughout the wetland, indicating heterogeneous contamination patterns associated with both natural processes and anthropogenic activities. Mean concentrations ranged from 0.26 to 106.49 mg kg⁻¹ dry weight depending on the element analyzed. Statistical analysis demonstrated significant differences among sampling stations for all investigated metals (P < 0.05), confirming substantial spatial heterogeneity in sediment contamination. The highest concentrations of most heavy metals were consistently observed at Stations 2 and 19, which are located adjacent to intensive oil drilling operations, production facilities, oil transportation pipelines, and associated industrial infrastructure. These observations strongly suggest that petroleum-related activities constitute the dominant source of localized heavy metal contamination in the study area. Additional contributions from industrial wastewater discharge, surface runoff, and atmospheric deposition associated with petroleum production cannot be excluded. Comparison of measured concentrations with international Sediment Quality Guidelines demonstrated that nickel and chromium represent the most environmentally significant contaminants in the wetland sediments. Their concentrations exceeded guideline values at several locations, indicating the potential for adverse biological effects on benthic organisms and other sediment-associated aquatic fauna. Because benthic organisms constitute an important component of aquatic food webs, prolonged exposure to elevated concentrations of these metals may influence ecosystem structure and ecological functioning.The Geoaccumulation Index classified sediment quality from unpolluted to moderately polluted, with Igeo values ranging between −2.81 and 2.11. Similarly, the Combination Pollution Index identified polluted conditions only at Stations 2, 3, 10, and 19 (CPI > 1), whereas the remaining sampling stations exhibited CPI values below one, indicating generally low contamination throughout most areas of the wetland. Collectively, these findings demonstrate that although widespread severe contamination has not yet occurred, localized pollution hotspots are closely associated with oil production activities and require continuous environmental surveillance.
Conclusions
This study provides a comprehensive assessment of heavy metal contamination in the surface sediments of the Hoor Al-Azim Wetland and offers valuable baseline information for future environmental monitoring programs. The results demonstrate that oil exploration, drilling operations, industrial wastewater discharge, and associated petroleum infrastructure are the principal sources of localized heavy metal contamination. Although most sampling stations exhibited relatively low contamination levels, Stations 2 and 19 showed markedly elevated concentrations of nickel, vanadium, chromium, and lead, reflecting the influence of nearby petroleum-related activities. The sediment quality assessment further indicated that contamination generally ranges from unpolluted to moderately polluted according to the Geoaccumulation Index and Combination Pollution Index. However, nickel and chromium were identified as the metals posing the greatest ecological risk because of their elevated concentrations relative to international sediment quality guidelines and their potential adverse effects on benthic organisms. These findings emphasize the necessity for continuous environmental monitoring, stricter environmental management of petroleum exploration and industrial activities, implementation of effective pollution mitigation measures, and development of sustainable conservation strategies to preserve the ecological integrity, biodiversity, and ecological functions of the Hoor Al-Azim Wetland. The results of this study can also provide a scientific basis for environmental decision-making and long-term management of protected wetland ecosystems affected by industrial development.