Metal bioaccessibility in a wastewater irrigated soil-wheat system and associated human health risks: Implications for regional thresholds

Abstract

Accumulation of metal pollutants in soils and crops is putting increasing pressure on ecological function and human health. In the present study, the physiologically-based extraction test (PBET) was executed to determine bioaccessibility of Cd, Cu, Pb, Zn, Cr and Ni in soils and grains collected from a wastewater irrigated soil-wheat system. Furthermore, regional Cd thresholds in the agroecosystem were deduced based on the relationships among soil Cd, grain Cd and associated health risks. The results indicate that metal bioaccessibility vary among elements, with a mean range of 27.16–89.51% in soils and 18.53–87.96% in grains, respectively. Bioaccessibility of Cd, Cu, Pb and Zn in gastric phase (GB) are significantly greater than those in intestinal phase (IB) (p < 0.05); however, those of Cr and Ni vary with sample type (soils: GB > IB; grains: GB < IB). Linear regression analysis shows that strong correlations between total metals and bioaccessible metals are found for Cd, Cu, Zn and Ni in soils (R2 = 0.50–0.99) and Cd and Zn in grains (R2 = 0.85–0.99), while no significant correlations were found for other tested metals. Human health risks from bioaccessible metals via soil ingestion are negligible, while those via grain consumption are high, especially for Cd which accounts for 84.66% of the cumulative risks. To prevent associated health risks via grain consumption, values of 0.50 and 0.15 mg/kg are identified as regional Cd thresholds in the soils and grains, respectively. Our data provide an actual assessment of human health risks based on the PBET method, which facilitates the establishment of regional thresholds in the wastewater irrigated soil-wheat system.