Distribution Characteristics Of Cadmium In Soil-Water-Plant System And Its Risk Assessment

China Geological Survey conducted investigations in 9 provinces and municipalities along the Yangtze River such as Sichuan, Chongqing, Hubei and so on, and found that there was abnormal belt of cadmium and other heavy metals with width of tens to several hundred kilometers through the whole drainage basin, from the Tuotuo River to the Yangtze River estuary and in the plains along the Yangtze River. As"the first of five evils", cadmium is a dangerous harmful substance, it migrates actively in the environment system, is one of the highly enriched heavy metals in crops. It not only leads to decreased crop yields and quality, but also comes into the human body through the food chain, which is a serious threat to human health.Huangshi City located in the southeast of Hubei Province and the south bank of the midstream of the Yangtze River, is an important raw material industrial base in Central China and the open city approved by State Council, an important fulcrum of economic development zone along the Yangtze River, deputy central city of Wuhan city circle and economic leader of eastern regional of Hubei province. In alluvial farmland along the Yangtze River in the outskirts of Huangshi City, the main crops are vegetables and grains. Reports about if the soils and growing crops (mainly vegetables) are subject to Cd contamination and meet the national food hygiene standard are relatively little, especially the adsorption characteristics of cadmium in the soil of the area, the content of cadmium in soil water and cadmium bioavailability of the soil are hardly researched. Therefore, taking Huangshi City as an example in this paper, we studied distribution characteristics of cadmium in soil-water-crop system, explored and discussed the bioavailability of Cd in the soil, provided scientific basis for citizens in Huangshi City on rational layout of planting structure, pollution-free vegetables construction and cadmium contamination prevention.Through field investigation, in farmlands of typical section along the Yangtze River and Yang Jiafan vegetable production base of Huangshi City, we collected 36 surface soil samples with a rust-free steel blade, measured soil water content using soil moisture meter (MPM-160, Australia) at the same time; digged 7 soil profiles with a twist drill, sampled the soil at the depth of 20,40,60,80,120,160 cm from the soil surface respectively; embedded 28(two for one site) homemade soil water samplers of pottery clay head type at the corresponding 14 sites collected surface soils, sampled soil water with vacuum extraction regularly in field; and collected mature crops like sorghums, maizes, hot peppers, etc, at the corresponding sites collected surface soils, which were divided into roots, stems, edible parts (here mainly refer to the fruits). Indoor cadium contents of all soil samples, soil water samples and crop samples were measured and analyzed; with tessier sequential extraction method, the contents of different speciations of cadmium of 22 surface soil samples were determined, with conventional analysis methods their basic physical and chemical properties, such as soil pH, organic matter, particle composition, cation exchange capacity, etc, were determined and analyzed; and selected four soil samples as representative for the cadmium adsorption-desorption experiment. Levels of cadmium accumulation and contamination were assessed according to national soil environmental quality standard and the background value of Cd in A layer of the soil near Daye Nonferrous Metal Smelter; studied distribution characteristics of speciations of cadmium in the soil and the relationship with the soil properties; analyzed static adsorption-desorption characteristics of cadmium of the soils and their influence factors; discussed about the absorption and accumulation of the cadmium of different crops and distribution characteristics of the cadmium in different parts of them; explored and discussed about Cd bioavailability in the soils; evaluated the potential risk of these crops to local people health according to the limited value of cadmium in national food hygiene standard, maximum acceptable level of risk recommended by International Commission Radiological Protection and the average consumption structure of these agricultural products of Chinese residents, preliminaryly putted forward some preventive measures against cadmium harm for the study area. The main conclusions are as follows:1.The cadmium contents of 36 soil samples in the study area are all above second-level standard value of the national soil environment standard, appeare mild to severe contamination (abnormity). Cadmium is significantly accumulated in surface soils (0-20cm) of Yang Jiafan vegetable production base in Huangshi City, with the depth increase, the content of cadmium decrease, when the depth is more than 2 meters, the content of cadmium is lower than 1mg/kg.2.The average contents of different speciations of cadmium in the soils of the study area follow the order of residual>Fe-Mn oxides bound>carbonate bound>organic bound>exchangeable. The contents of speciations of cadmium which are unavailable and potentially available to biology are relatively high, while that of the available speciation is very low. Exchangeable Cd and organic bound Cd are closely related, with the increasing in total cadmium of the soil, the content of exchangeable cadmium increases to a certain extent, and it significantly influences the contents of residual cadmium and iron-manganese oxides bound cadmium. The environmental factors that significantly influence the content of total cadmium and different speciations of cadmium are mainly soil pH, organic matter content and clay content.3.Cd adsorption isotherms of the soils in the study area all can be described by Langmuir equation, Freundlich equation and Temkin equation, overall Langmuir equation fits better. With the quantity of adsorbed cadmium increased,desorbed cadmium is also increased. Under the experimental conditions, Cd adsorption and desorption process can be divided into two stages, the fast reaction stage before 40min and the slow stage after 60min. The content of Cd adsorbed by the soil are mianly controlled by organic matter content, cation exchange capacity, soil clay and silt. The potential ability to adsorb Cd of the soil in the study area is relatively strong.4.Accumulation ability of cadmium of different crops are different, that of different parts of the same crop are also quite different. Water spinaches and eggplants, hot peppers and other fruit vegetables are easy to accumulate cadmium, while sorghums, corns and other grain crops and legumes are not. In general, the contents of Cd in different parts of crops follow the order of roots>stems>edible parts. The overall migration ability of cadmium in crops are roots-stems> stems-edible parts. Water spinaches, peppers and eggplants in the study area, have been contaminated, which have slight health risk, other vegetables and grain crops are relatively safe, having no health risk.5.The content of cadmium in soil water of the study area is very low, basically less than the limited value 5ug/L of national agriculture irrigation standard. Cadmium in soil water appear that the cadmium content of the deep is slightly higher than the shallow; in dry autumn higher than in rainy summer; in Yang Jiafan vegetable production base higher (for instance SW0314 7.67μg/L) than in farmlands along the Yangtze River (for instance SW0307 0.60μg/L).6.The contents of cadmium absorbed by crop roots and stems are closely related to that of exchangeable cadmium, iron-manganese oxides bound cadmium, carbonate bound cadmium and total cadmium in the soil, while the contents of cadmium in edible parts are not that related to all the speciations of cadmium in the soil, but are significantly positively correlated to cadmium in soil water. It showes that cadmium in soil water has a relatively high bioavailability, total cadmium content of the soil is just initial indicator of the cadmium bioavailability, exchangeable cadmium, iron and manganese oxides bound cadmium and carbonate bound cadmium are important indicators of cadmium bioavailability in the study area.7. Cadmium abnormity in the soil of the study area has influenced the safety of crops to some degree, it is recommended that relevant departments pay high attention to and take effective countermeasures to control the food safety in the area, such as setting up strict emission standards for industrial and mining enterprises, encouraging farmers to use more organic fertilizer and less wastewater for irrigation and so on.