Study of Arsenic in water and soil sources of a contaminated region and its absorption and accumulation rate in the local pasture plants of the area (Case study: Bijar city)

Abstract
Background and objectives: Arsenic, one of the most critical environmental toxins, accumulates in plant and animal organs through root absorption in plants and subsequent ingestion by humans via the food chain or direct consumption of contaminated water. Reports have highlighted the contamination of regional drinking water sources in Bijar city, Kurdistan province, with arsenic and its adverse effects on local residents. Therefore, this research aimed to investigate the distribution of arsenic in the water and soil sources of the region and evaluate the capacity of native pasture plants to absorb and accumulate this element.
Materials and methods: Arsenic concentrations were measured in surface and subsurface waters of a 360 km2 region using the atomic absorption-graphite furnace method. The distribution of arsenic in the contaminated area was mapped using the ArcGIS program. Three sampling areas with varying pollution intensities (A > B > C) were selected. Soil samples and aerial parts of 13 plant species were collected and analyzed for arsenic concentrations, enabling a comparison of different plant species' ability to absorb arsenic.
Results: The intensity of arsenic contamination in surface water, soil, and plants studied in this research exceeded typical concentrations of arsenic in water, soil, and plant sources. Total arsenic concentrations in water samples ranged from 4.5 to 280 μg l-1 and correlated with electrical conductivity, total dissolved solids, and total hardness (p < 0.001). Arsenic concentration in the soil of area A (Ali-Abad village) exceeded 2059 μg g-1 of dry soil, with contamination decreasing with distance from area A. A high correlation was observed between the intensity of water and soil pollution (R2=0.84). Arsenic concentrations in the aerial parts of 13 plant species with three replications varied depending on the plant species and the intensity of soil contamination (0 to 47.8 μg g-1). The intensity of soil contamination was on average 27.66 times higher than the amount of arsenic accumulated in the plant, indicating the high resistance of native pasture plants in the region to arsenic toxicity.
Conclusion: Field monitoring revealed that 45% of water resources in the sampling areas exceeded the national drinking water standard for arsenic (50 μg l-1), reaching 78% according to the World Health Organization standard (10 μg l-1). Concentrations of parent materials containing arsenic compounds were higher in areas A and B than in area C, with decreasing concentrations further from area A. The sampling region significantly influenced the average arsenic concentration in water and soil samples (P < 0.05). Arsenic uptake by the plant samples exceeded typical concentrations (0.1 to 3 μg g-1), with the highest mean arsenic concentrations found in the leaves of Astragalus bisulcatus, Chenopodium album, and Mentha longifolia, accumulating more than 47.8, 35.5, and 22.5 μg g-1, respectively. The study demonstrates heavy contamination of water, soil, and plant resources in the region, with most surface water resources utilized for agricultural irrigation, animal consumption, and occasionally drinking water for village residents. The entry of arsenic into the human body, directly or through the food cycle, poses a severe health risk to the region's residents, necessitating immediate intervention measures.