Evaluation and analysis of heavy metals in vegetable field of Beijing

Abstract: Over the last decades, large-scale greenhouse vegetable production has been widely developed in the suburban areas of Beijing. However, a lot of agricultural production materials (such as fertilizers, organic fertilizers, pesticides, and agricultural films) are always input into greenhouse conditions to achieve high vegetable production. Therefore, soil contamination problem especially heavy metal pollution has emerged gradually. Due to the direct or indirect threat to food safety, human health, and its detrimental effects on ecosystem, heavy metal pollution in fields has become an important frontier in environmental research and drawn more and more attentions. There have been several studies on the quality of the agricultural soils, concerning about soil fertility, soil enzyme activities, and available microelements. However, there is little discussion on the present heavy metal condition of vegetable field in Beijing. A survey was conducted to analyze and assess the soil heavy metals’ environmental quality of vegetable field in Beijing. During 2011-2013, a total of 319 topsoil (0-20 cm depth) samples were collected from the main vegetable planting areas in Beijing, which included 8 districts. The sampling sites were randomly collected based on the distribution of local agricultural land use. Soil pH value ranged from 6.11 to 9.03. Most of the soil samples were alkaline, and 80.50% of the samples had the pH value of above 7.50. The mean values of the heavy metal concentrations were (0.24±0.28), (75.71±47.21), (7.71±4.50), (0.08±0.06), (23.80±6.09), (23.48±3.91), (24.96±4.68), and (89.92±27.96) mg/kg for cadmium (Cd), chromium (Cr), arsenic (As), mercury (Hg), lead (Pb), nickel (Ni), copper (Cu), and zinc (Zn), respectively. In addition, the background values of heavy metals in Beijing were also listed for reference (0.12, 29.80, 7.09, 0.07, 24.60, 26.80, 18.70 and 57.50 mg/kg for Cd, Cr, As, Hg, Pb, Ni, Cu and Zn). The concentrations of Zn, Cu, As and Pb, especially those of Cr and Cd in the study area exceeded the background values in Beijing. The coefficient of variation (CV) was calculated by standard deviation divided by mean of each kind of heavy metal. A modified version of the ranking for CV suggested by Phil-Eze (2010) was used in this study: CV≤20%, low variability; 21%≤CV≤50%, moderate variability; 51%≤CV≤100%, high variability; CV≥100%, exceptionally high variability. The CV values varied from 16.75% (for Ni) to 114.11% (for Cd), which decreased in the order of Cd > Hg > Cr > As > Zn > Pb > Cu > Ni. The CV values of Cd, Hg, Cr and As (from 114.11% to 58.37%) were extremely higher than the other elements, suggesting that they had the greatest variation among the studied metals and showed a higher possibility of being influenced by extrinsic factors, such as human activities, agronomic practices, automobile exhaust, and deposition of aerosol particle. Among the studied districts, soil Cd and Cr were enriched in Daxing, Changping, Miyun, Yanqing, Fangshan, Shunyi and Tongzhou. According to Environmental quality standard for soils and Farmland environmental quality evaluation standards for edible agricultural products, the data were analyzed with the single factor index and Nemerow index for the soil environmental quality assessments. Single factor analysis indicated that in some areas (such as Shunyi and Fangshan District), the soil Cd concentrations reached the limited value (0.40 mg/kg) of Farmland environmental quality evaluation standards for edible agricultural products, but below the Ⅱ grade standard (0.60 mg/kg) of soil environmental quality assessment classification. Average single pollution index of Cd, Cr, Cu and Zn in the investigation area was between 0.7 and 1.0, which was at the “relatively clean (alert)” level. Average Nemerow index of the investigation area was 0.65, which reached the I grade standard (≤0.7) of soil environmental quality assessment classification. It is concluded that the soil heavy metals’ environment quality of vegetable field in Beijing is relatively safe, but the potential ecological risk of soil Cd accumulated in greenhouse vegetable field should be paid more attentions.