The Distribution And Its Risk Assessment Of Heavy Metals In Farmland Soil Irrigated By Yellow River Water In Baiyin, China

Silong town, the main irrigation area of Baiyin city, is one of the major grain production bases. This region’s agricultural products are supplied for local resident consumption, and also transported to Baiyin and Lanzhou for consumption demand. So the security of food in this place concerns many people’s health. Baiyin city is a major non-ferrous metal mining and smelting base, and long-term industrial activities lead to environmental pollution. At the same time, that the long history of sewage irrigation increases the level of soil heavy metals content contributes to the grave consequence that some farmlands are seriously contaminated Crops could take in toxic heavy metals from farmland soil, and then though food chain, these toxic heavy metals finally do harm to body health. So the damage brought by farmland soil heavy metals pollution and its potential risk are extremely high. The pollution of heavy metals in Baiyin farmland may have an adverse influence on local animals, plants and residents.In this study, soil samples were collected from different soil layers and topsoil of different sites. Then the concentrations of Cd, Cu, Pb and Zn are detected, and the detected heavy metals concentrations are measured against soil environmental background value of Gansu province and national soil environmental quality standard (GB15618-1995). Different assessment methods are employed to analyze the distribution, pollution as well as ecological risk of Baiyin farmland soil. The results are as follows:1. The levels of heavy metals Pb and Zn in topsoil are almost equal to the background values of Gansu province, respectively. Meanwhile, the level of Zn in topsoil is slight higher than the background value, and the level of Cd is exceeding standard near from two to six times. The bioavailability fractions of Cu, Pb and Zn are relatively low. However the residual fraction of Cd is low, which indicates the mobility and bioavailability of Cd in Baiyin farmland soil.2. Overall, soil samples from Silong town present higher heavy metal concentrations than those of Beiwan town, and the highest levels of heavy metals are found in the site Minlecun (0#) which is the nearest to the sewage drain exit. And the heavy metals concentration sequence is: Layer1(0-20cm)> Layer2(20-40cm)> Layer5(80-100cm)> Layer4(60-80cm)> Layer3(40-60cm).3. The synthetic pollution index shows the soil is moderately polluted. The single pollution indexes (Pi) of Cu, Pb and Zn are all below the0.7, presenting no pollution. Except for few sites, the topsoil of Baiyin farmlands is polluted by Cd, and the pollution levels of most sites are moderate or above. Accompanied by the increasing depth of soil, the level of synthetic pollution is gradually low, and the soil below soil surface40cm is only slightly polluted. All in all, the contamination degree of Cd is serious.4. Whether in the topsoil of different sites or in the soil of different depths, the risk index and potential hazard of Cd are both extremely high. However, the potential hazard of the other three heavy metals is relatively low, such as Er<40. Considering that Cd contributes greatly to the integrated ecological risk which is approximately more than90%. Therefore, it may cause damage on local crops and residents. Otherwise, the potential ecological hazard of the other heavy metals is extremely low. Therefore, in theory, the other three heavy metals will not have such harmful effect.In conclusion, the situation of these four heavy metals in local farmland soil are different, among which soil Cd presents a high level, and displays a relatively high percent of bioavailability. So the pollution and risk of soil Cd are both high, and may do harm to local organisms. However, both Cu, Pb and Zn in surface layer and deep layer soil are not high, and non-bioavailability fractions are very high. Therefore, their mobility is low and those three heavy metals almost have no bad impact. Therefore, regarding the current pollution of heavy metals in the soil of Baiyin farmlands, it is suggested that the local government should attach much more importance to the supervision and management of industry sewage. Meanwhile, viable measures should be carried out to reduce the polluted soil, lowering the concentration and toxicity of Cd. Clean and safe production of agricultural products should be ensured. In these ways local residents are able to lead a healthy a life.