Background and Objectives: Water scarcity has become one of the most critical environmental challenges worldwide, particularly in arid and semi-arid regions. Freshwater resources are rapidly depleting due to population growth, climate change, and increasing demands from agriculture and industry. As a result, the reuse of treated wastewater has emerged as an alternative irrigation source to reduce pressure on freshwater supplies. However, such reuse introduces environmental and health concerns associated with the accumulation of heavy metals in soil–crop systems. Heavy metals such as cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) are of particular concern due to their persistence and potential toxicity. Once these metals accumulate in agricultural soils, they can be transferred into crops, ultimately entering the human food chain and posing non-carcinogenic or carcinogenic health risks. Wheat (Triticum aestivum L.) is a globally important cereal crop that provides a substantial portion of human caloric and protein intake. Given its dietary significance, monitoring heavy metal accumulation in wheat irrigated with treated wastewater is essential for ensuring food safety and public health. The present study was conducted to: 1) quantify Cd, Cu, Ni, Pb, and Zn concentrations in soil and wheat irrigated with treated wastewater in Rey County (southern Tehran Province, Iran); 2) assess the contamination and risk level of these heavy metals in soil and wheat grains; and 3) evaluate bioaccumulation and transfer patterns of the studied metals within the soil–wheat system and their potential non-carcinogenic health risks to adults and children.
Materials and Methods: The study area was located in the agricultural lands of Rey County, south of Tehran, characterized by a semi-arid climate with an average annual temperature of 16.7 °C and mean annual precipitation of 212 mm. The wastewater used for irrigation originated from the South Tehran Wastewater Treatment Plant, which employs an activated sludge process with nitrogen removal and supplies irrigation water to parts of the Rey and Varamin plains. Soil and plant sampling were conducted during the wheat harvest period (June 2024). Ten agricultural fields irrigated with treated wastewater were selected, and composite samples of surface soil (0–15 cm) and wheat plants were collected in triplicate from each field. Wastewater samples were also collected from ten points along the open-channel conveyance system. Soil and plant samples were air-dried, sieved (2 mm), and oven-dried (60 °C for 48 h). Wheat plants were divided into roots, stems, and grains, washed with distilled water, and ground for analysis. Total concentrations of Cd, Cu, Ni, Pb, and Zn in soils and plant tissues were determined by acid digestion using Aqua Regia (1:3 HNO₃/HCl) and measured by atomic absorption spectrophotometry (Varian-220AA). Physicochemical parameters of wastewater (pH, NO₃⁻, PO₄³⁻, TSS) were analyzed according to APHA (2005) standard methods. The bio-concentration factor (BCF) and transfer factor (TF) were calculated to evaluate the metal accumulation and translocation capacities of wheat. The estimated daily intake (EDI) and hazard index (HI) for non-carcinogenic risk assessment were calculated following the USEPA (1989, 2011) methodology. Oral Reference Doses (ORD) for Cd, Cu, Ni, Pb, and Zn were 0.001, 0.004, 0.02, 0.0035, and 0.3 mg kg⁻¹ day⁻¹, respectively. An HI > 1 indicates a potential non-carcinogenic health risk. Data analysis and graphing were performed using Microsoft Excel 2021.
Results: Analysis of treated wastewater revealed that concentrations of nitrate (NO₃⁻) and Cd exceeded the permissible limits set by the Iranian Environmental Protection Organization and FAO/WHO standards. Ni concentrations were within the Iranian guideline but above the FAO/WHO limit, indicating that the treated wastewater used in the study area was not fully suitable for agricultural irrigation. Mean concentrations (mg kg⁻¹) of Cd, Cu, Ni, Pb, and Zn in the irrigated soils were 3.16, 47.35, 45.99, 70.75, and 107.29, respectively. Although most metals were within national permissible limits, Cd, Cu, and Ni exceeded recommended levels in some samples. High coefficients of variation (CV) for Cd and Ni suggested non-uniform spatial distribution, likely influenced by anthropogenic sources. Metal concentrations varied among wheat organs, decreasing in the order: roots > stems > grains. The highest mean concentrations (mg kg⁻¹) were found in roots: Cd (71.36), Cu (1.73), Ni (39.25), Pb (8.21), and Zn (34.60). Grain concentrations were Cd (0.95), Cu (11.20), Ni (3.64), Pb (3.12), and Zn (24.27). Compared with the FAO/WHO (1984) limits, all metals except Zn exceeded safe thresholds. The mean BCFs of Cd, Cu, Ni, Pb, and Zn in wheat organs were generally < 1, suggesting that wheat absorbs but does not strongly accumulate metals. Bio-concentration Factor (BCF) values followed roots > grains > stems for Cd, Cu, and Zn, and roots > stems > grains for Ni and Pb. Mean TF values were < 1, showing that metals predominantly accumulated in roots. The overall TF trend was Cd ≈ Zn > Cu > Pb > Ni, with Cd and Zn having relatively higher mobility. The estimated daily intakes (EDI) for all metals were higher in children than in adults due to lower body weight. Cd and Pb exposure levels exceeded reference doses in adults, while all metals exceeded safe limits for children. Hazard index (HI) values for both age groups were > 1, confirming potential non-carcinogenic health risks. The highest hazard quotient (HQ) values were for Cu, while Zn posed the lowest risk.
Conclusion: The study demonstrated that treated wastewater used for irrigation in Rey County contains elevated levels of Cd and Ni, leading to detectable accumulation of heavy metals in both soils and wheat plants. Although overall concentrations in soil were within national limits, Cd, Ni, and Pb levels in wheat grains exceeded FAO/WHO permissible standards, rendering the crop unsuitable for safe human consumption. Wheat exhibited strong metal uptake by roots but limited translocation to grains (BCF and TF < 1). However, the health risk assessment indicated non-carcinogenic risks (HI > 1) for both adults and children, with children being more vulnerable. To safeguard food safety and public health, continuous monitoring of wastewater and soil quality is recommended. The use of treated wastewater for wheat irrigation should be either restricted or improved through the application of advanced treatment technologies. Future research should explore low-accumulating wheat cultivars and bioremediation strategies, such as plant growth-promoting rhizobacteria, to mitigate heavy metal contamination in wastewater-irrigated systems.