Bioavailability And Human Health Risk Assessment Of Heavy Metals In Soil-water Spinach System

Forty two vegetable field soil samples and38water spinach samples were collected synchronously from a typical heavy industry city, Zhuzhou of Hunan province, China. The samples were analyzed to study the pollution status and distribution characteristics of heavy metals. Single step chemical extractions were used to study the phytoavailability of heavy metals in soil-water spinach system and the bioaccessibility of heavy metals through oral ingestion, and to discuss their relationships with soil properties. Finally, the potential health risk of heavy metals was preliminarily assessed focused on two exposure pathways:through consumption of water spinach and through oral ingestion of polluted soils. The results showed that:(1) The soils studied in this area are primarily contaminated by Cd and Pb, followed by Zn, As and Cu. And Cu, Zn, As, Cd and Pb in soils were significantly affected while Cr was less affected by huaman activities. The main pollutants in the edible parts of water spinach were Pb and Cd. Water spinach has an increased propensity for Cd phytoextraction, and atmospheric deposition may have effect on the accumulation of Pb in water spinach. Spatial distribution of heavy metals in soils and water spinach showed that Zn, As, Cd and Pb have consistent distribution characteristics, with the highest content in the Qingshuitang industrial park and spreading to a wide range in the form of diffusion. At present, Hetang area may be more suitable as a local vegetable production base.(2) Bioavailable heavy metals extracted by different methods covered a wide range and in most cases they had significant positive correlations with total heavy metals (p<0.01). And the discrepancies of the relationships between the bioavailable content and the total content for different heavy metals might be influenced by soil properties. The results of simple correlation analysis and multiple regression analysis indicated that bioavailable content have no obvious advantage in predicting the phytoavailability of main heavy metal pollutants in soil-water spinach system, total content in soils could be used directly in predicting the phytoavailability of heavy metals.(3) The high variability of bioaccessibility of soil heavy metals was observed. The rates of bioaccessibility of heavy metals were generally decreased as the order of Cd> Pb> Cu> Zn> As> Cr. The bioaccessibile heavy metals were well predicted by statistical models based on total contents, pH, organic matter and total carbon.(4) After consumption of water spinach, Zn and Cu had little impact on exposure population, and the potential health risks from exposure to Cd, Pb and As were of great concern. Results based on the total metal contents might overestimate the actural human health risks in comparison with oral bioaccessibility adjustment could reduce the risk significantly. After adjustment by bioaccessibility, the average HI and TCR values were reduced71%and74%, respectively. But the HI and TCR values of some samples were still higher than the risk thresholds after adjustment by bioaccessibility and deserved our special attention. Moreover, the health risk of heavy metals to children was obviously higher than those to adults.