Soil-water-crop pathways of heavy metal contamination and human health risks in intensive smallholder farms of the Nahavand Plain, Iran.

Heavy metal (HM) contamination in intensively managed agroecosystems of arid regions poses persistent challenges for soil quality, groundwater integrity, and human health. This study presents an integrated assessment of HM concentrations, ecological pressure, and potential human health risks across 150 smallholder farms (< 1 ha) in the Nahavand Plain, Iran, encompassing wheat (45), barley (30), sugar beet (35), and coriander (40) cultivation systems. Seven metals (Cd, Pb, Hg, Cr, Ni, Cu, and Zn) were analyzed in soil and irrigation water samples using a suite of pollution indices (Cf, Igeo, PLI, Cdeg, and PERI) and health risk models (CDI, HQ, HI, and TCR). Across all cropping systems, Zn and Cu exhibited the highest concentrations, while Cd and Pb emerged as the most influential contributors to both ecological and health risk indices. Sugar beet systems consistently showed the highest contamination intensity and cumulative ecological risk, reflecting long-term intensive input management, whereas cereal-based systems displayed intermediate risk profiles. Groundwater metal concentrations were generally lower than those in soils; however, the recurrent presence of Cu, Zn, Cd, and Cr highlights the vulnerability of shallow aquifers to sustained agrochemical inputs and irrigation-related transport processes. Health risk assessment revealed pronounced age-dependent patterns, with children consistently exhibiting higher non-carcinogenic and carcinogenic risk estimates than adults across soil- and water-related exposure scenarios. Although total carcinogenic risk values generally remained within commonly accepted regulatory ranges, localized elevations—particularly in sugar beet- and wheat-dominated systems—underscore the importance of precautionary management. Elevated Pb levels observed in coriander systems indicate sensitivity to surface soil contamination rather than direct evidence of food safety non-compliance. Overall, the findings demonstrate that intensive fertilizer use and crop-specific management practices are key drivers of HM pressure in smallholder farming systems of arid regions. The results highlight the need for improved fertilizer governance, routine monitoring of soil and irrigation water quality, and the adoption of sustainable management practices to reduce long-term exposure risks and protect vulnerable populations. Future research should incorporate multi-season monitoring and refined assessment of soil–water–crop transfer pathways to better characterize cumulative exposure under diverse agricultural management scenarios.