Distribution And Risk Evaluation Of Heavy Metals In Greenhouse Vegetable Soils

Heavy metal is the key research field in soil and environment science. The studies on temporal and spatial distributions of heavy metals in vegetable filed soil and their pollution risks are important for soil and environment safety of greenhouse soil and vegetable safe production. Based on the theories of soil and environment chemistry and the temporal and spatial distributions of heavy metals in vegetable soils, the accumulation, pollution risk, potential ecological risk and health risk were evaluated in this paper on vegetable greenhouse soils as affected by high-intensity human activities in typical region of Shouguang city, Shandong province. The relationship between heavy metal accumulation in vegetables and soil heavy metals in different forms were made clear and the threshold values of soil heavy metals for vegetable safety were brought forward. The evolutionary regularities of soil and environment quality were revealed. Also, the mechanism of soil heavy metals'effects on vegetable safety was disclosed. The whole studies provided the scientific foundation for the control of soil heavy pollution and the safe production of greenhouse vegetables.1. The temporal and spatial distributions of heavy metals in the soils of greenhouse vegetable fields were made clear. The results indicted that the total concentrations of 8 heavy metals or in available forms were increased with the increase of planting time, the activation rates of which had a decreasing tendency. The soil heavy metals were prone to accumulate in top layer of soil profile. With the increase of depth, the concentrations of total metals decreased due to human activites.2. The spatial variations of heavy metals in the soils of greenhouse vegetable fields were made definite. The CVs of soil heavy metals were 10.5%~87.7% in town scale. The theoretical models of 8 heavy metals fitted by half-variance function were as follows: exponential model for As and Pb, spherical model or Cd, Hg and Zn, Gauss model for Cr, Cu and Ni. The spatial distributions of soil heavy metal in the region were directly described using the method of associating Kringe interpolation and GIS, ie., the distributions of different elements were patchiness, stripped and scattered.3. The heavy metals in the soils of greenhouse vegetable fields were evaluated using accumulation index and pollution index. Except for As and Pb, the single accumulation index of Cd, Cr, Cu, Hg, Ni and Zn were above 1. The distribution of maximum accumulation index for Cd, Hg and Cu were 10.78, 7.00 and 5.05, respectively, which stands for high accumulation. Percentage of surpassing background value was in the order of Zn > Cd, Cr, Ni > Cu,Hg > As, Pb. The contributions rate of heavy metals were Zn, Ni > Cd, Cr > Hg > Cu > Pb > As. The pollution risk evaluation of soil heavy metals showed that the above-norm percentage of Cd,Cu,Ni were 3.31%, 1.32% and 22.52%, respectively. Among all the 151 experimental sites, the percentage of light, medium and heavy pollution were 31.0 %, 4.0 % and 0.7%, respectively, ie., experimental sites of 35.7% in the risk of soil pollution.4. The potential ecological risk single factors of greenhouse vegetable soils were averagely at low level following an order of Cd > Hg > Ni > As > Cu > Pb > Cr > Zn, 2.65% among which were above the standard of Grade-â…¢. The composite ecological pollution index of greenhouse vegetable soils were at low-value level (Grade-A), 3.31% among which were above the medium risk rank. Single potential ecological risk factors had the function of risk warning, the probability order of which was As,Pb > Hg > Cu > Cd > Zn > Cr > Ni. Composite index of 97.4% among experimental sites were in the rank of risk-free and that of 2.6% in the risk rank.5. Cadmium and Pb in soil and culture solution had significant effects on the growth of lettuce and turnip. With the increase of Cd and Pb concentrations, the biomass of lettuce and turnip increased first and then decreased. The low concentrations of Cd and Pb could enhance the growth of vegetables. The added Cd and Pb increased the concentrations of Cd and Pb in lettuce and turnip significantly. The exponential curve was suitable to describe the relationship between Cd concentration in vegetables and in soil or culture solution, the same for Pb in culture solution. However, the relationship between Pb concentration in vegetables and in soil should be described only by linear curve.6. The method of using total heavy content in soil to evaluate the soil heavy metal pollution was one-sided. The results indicated that CaCl2-extractable Cd in soil was a good indicator to evaluate the Cd pollution of lettuce. For the Cd pollution evaluation of turnip, water-extractable Cd in soil was more suitable. However, to evaluate the Pb pollutions of lettuce and turnip, carbonate-Pb in soil should be used respectively.