Dietary intake risks of trace metal(loid)s via maize–animal–human pathway and derivation of bioavailability-based soil criteria in Hezhang county, China

Abstract

Historically metal-polluted agricultural soils are an important source of human exposure to toxic metals such as arsenic (As), cadmium (Cd), and lead (Pb) through food chains, especially in remote rural areas like Hezhang county, China. This paper aimed to investigate the processes by which a staple food crop, maize (Zea mays L.), became contaminated, identify the dominant exposure pathway and dietary intake risks, identify soils that were unsafe for growing maize, and share findings with farmers and decision-makers. A greenhouse experiment was performed using 20 historically metal-polluted soils with different soil types, properties, and contamination levels. Metal(loid) concentrations in maize grains, soil porewater (in situ), and DTPA (0.005 M) extracts of rooting-zone soils were measured to investigate the factors that influence the availability and the uptake of metal(loid)s. An investigation was performed to identify the main exposure pathway. Regression models describing metal(loid)s transfer from soils to maize grains were inversely used to derive bioavailability-based soil criteria. Soil results indicated that 28% of total Cd and 17% of total Pb in soils were potentially plant-available whereas the labile fractions in soil porewater were negligible due to high soil pH (> 7.5). For As in soils, approximately 2% was the non-specifically adsorbed fraction. Grain metal(loid)s (i.e., As, Cd, and Pb) levels were satisfactorily predicted using porewater concentrations and key properties such as pH and dissolved organic carbon contents (adj. R2 0.814 - 0.887). The bioaccumulation of As in maize was very limited. However, the level of Cd in 73% and Pb in 25% of grain samples exceeded Chinese food quality standard, thereby threatening food safety. Metal(loid)s transfer through maize → animal → human pathway would result in a substantial dietary intake of metals, e.g., 79.7 μg Pb day−1 and 217.9 μg Cd day−1. A more implementable soil criterion for DTPA-extractable Cd (0.6 mg kg−1) was derived allowing identification of soils that were unsafe for growing maize. Hezhang residents experience dietary intake risks of Pb and Cd. Excessive Cd and Pb in maize are the result of soil pollution. Soil with DTPA-extractable Cd concentration < 0.6 mg kg−1 can ensure a zero exceedance of Cd and other trace metals as well in maize. Findings are of importance to farmers and policy-makers to lower the risks of exceedance of Cd and Pb in food crops by soil management.