Near-threshold environmental and health risks of untreated wastewater irrigation under informal water supply networks in the Zabol Plain

Persistent and severe water scarcity in the Zabol Plain of southeastern Iran has increasingly driven farmers to rely on untreated wastewater as an informal irrigation source, raising substantial environmental and public health concerns. This study adopted a cross-sectional mixed-methods approach to assess the impacts of wastewater irrigation on soil properties, groundwater quality, and heavy metal accumulation in wheat grains. Irrigation water, soil samples (0–20 and 20–40 cm depths), wheat grains, and adjacent groundwater were collected from fields irrigated with untreated wastewater and from paired control farms supplied with conventional water sources. Physicochemical characteristics and concentrations of Pb, Cd, Cr, and Ni were determined following standard APHA and EPA protocols. Bioaccumulation (BCF) and translocation (TF) factors were estimated, and potential human health risks were evaluated using USEPA risk assessment models. In parallel, qualitative data obtained through semi-structured interviews were analyzed to identify the socio-economic drivers, governance gaps, and informal water-access arrangements that sustain untreated wastewater use. The qualitative component was used to contextualize untreated wastewater irrigation under conditions of water scarcity, livelihood pressure, weak monitoring, and informal water access, without making criminological claims about organized crime. Results indicated that wastewater irrigation significantly increased soil salinity, sodium adsorption ratio (SAR), and organic matter content, accompanied by elevated heavy metal concentrations in surface soils. Wheat grains harvested from wastewater-irrigated fields exhibited higher levels of Pb, Cd, Cr, and Ni compared to controls. Although most concentrations remained within international guideline limits, their proximity to threshold values suggests that the system is approaching a critical contamination tipping point. Groundwater in the vicinity of wastewater-irrigated farms showed moderately increased nitrate and trace metal concentrations, indicating early-stage vertical migration of contaminants. Human health risk assessment revealed that non-carcinogenic risks associated with Cd exposure remained below the acceptable level (HQ < 1) for both adults (HQ = 0.78) and children (HQ = 0.96), while carcinogenic risks for Cd fell within the upper bound of the acceptable range (10⁻⁶–10⁻⁴). Overall, the findings characterize the agro-environmental system as being in a near-risk yet still manageable condition. This underscores the urgency of implementing preventive management strategies, strengthening environmental monitoring, and addressing the socio-economic constraints that perpetuate dependence on untreated wastewater for agricultural production.