Risk Assessment In The Hilly Region Of Sichuan Central Section

Shuangqiao township of Neijiang city which belongs to the Sichuan basin hills is located in center of the Chengdu-Chongqing economic circle. The farming activity with a high population density in the region is frequent, and the social economy industry develops well. Consequently, the prediction on the heavy metal geochemical behavior and the ecological risk has important practical significance.In this article, by the means of the field investigation, analysis test indoor, making full use of the research results in the chemical, agricultural geology, forestry and other disciplines in this area, the author proceeded quantitative statistical analysis on the background value of soil heavy metals, the vertical occurrence characteristics of heavy metals in soil profile, soil and crop interface occurrence characteristics of heavy metals and the movement and accumulation of elements in rocks-soil-plant system. In addition, the ecological risk assessment and the ecology risk early warning evaluation were tested in this paper. The conclusions are as follows:1) The geometric mean is adopted to obtain soil heavy metal background value which closed to real. As:5.235 mg·kg-1、Cd:0.355 mg·kg-1、Cr:82.311 mg·kg-1、Cu:31.785mg·kg-1、 Hg:0.031 mg·kg-1、Ni:41.132 mg·kg-1、Pb:30.235 mg·kg-1、Zn:98.719 mg·kg-1. We have concluded that the Cd’s background value is very high, and its relative abundance reaches 3.660. The cause may due to the differences where compared to before on investigating background value of Cd and the content determination method and sampling method (sample point).2) The paper has made clear vertical occurrence characteristics of the soil of heavy metals in the soil profile. In eight kinds of heavy metal elements, As, Cd, Cu, Hg and Pb are demonstrated cumulative trend in the A layer of soil (0～20cm). Element Cr has accumulation trend by the B layer (20～40cm), and Ni enrich in the D layer (60～80cm). Correlation analysis showed that:In the different soil layers, the strongest correlation of each layers is Ni, the correlation between the adjacent layers are up significantly; input source of each heavy metals in layer C is the most similar; Pb in different soil layers are independent of each other; Only in the layer B, correlation of Cr and Zn appear to be significant in the same soil layer.3) This article has found out the occurrence characteristics about heavy metals on the soil-crop interface. On this interface, there is a significant synergy between the content of Pb and Hg, and the possibility of homologous input is very large. However, the content variation of Zn compared with As, Cr, Ni, Pb is independent, and its source should be further analyzed. In combined feature, Cr, Hg and Pb, As and Cd, Ni and Cr are closely related.4) The element’s movement and accumulation in rock-soil-plant system is important ecological processes in ecosystem. It is the core content of material flow and information flow. In the four major crops in study area (rice, corn, citrus and peanuts), heavy element’s movement general rule and cohesive performance as follows:In rice, Cr and Ni, most prone to migration, the two elements in the role of planting soil interface migration has similar figures, followed by the As element. Pb, Cu, Zn, Cd, Hg have a lot of similar figures, and their migration is more difficult; In corne, Hg and Pb are prone to migration, followed: Cr>Cu>Ni>Ad>As>Zn; in orange, the element migration sequence:Cr>Ni>Hg>Cu>Zn>Pb> As>Cd; in peanuts, migration sequence:Cr>Ni>As>Pb>Zn>Cu>Cd>Hg.5) Soil heavy metals in the study area were evaluated which using the coefficient of heavy metal accumulation and pollution index. A single cumulative index of soil heavy metals were greater than 1, showing that eight heavy metals content expressed as the cumulative characteristics compared to the local background values. Hg maximum distribution of accumulated index is 7.934, showing a high degree of accumulation characteristics.The sharing rate descending order of heavy metals:Hg>As>Cd>Ni>Zn>Cu> Cr>Pb. Soil heavy metals pollution risk assessment with single pollution index show that:in accordance with the national standard (the natural background value), the biggest risk of contamination were Cd, Ni, and Zn, followed by Cu, Cr, Pb, Hg, and As was no pollution risk. According to the national secondary standards (pH> 7.5), the single pollution index of soil heavy metals were less than 1, and the result is located in the pollution level using probabilistic risk assessment of single-factor pollution. Nemerow index evaluation criteria to evaluate the risk of contamination, the results show:According to the national standard (the natural background value), As is in the class Ⅰ level (clean); Cr and Pb are in the class Ⅱ level (relatively clean); Cu, Hg, Ni, and Zn have reached the Class Ⅲ level (light pollution); Cd has reached IV class moderately polluted. according to the national secondary standards (pH> 7.5), eight kinds of heavy metals are below the level of the class Ⅱ,and the heavy metals are in the within security zone of warning limits.6) Single factor ecological risk index is calculated and evaluated using the national standard (the natural background value). The potential risks of the single factor, from highest to lowest order:Cd>Hg>Ni>Cu>Pb>As>Cr>Zn. Cd of high levels not only constitute environmental pollution but also pose a serious ecological hazard. At the same time, the maximum value of Ei is 79.20, and the individual feature points has reached the considerable Ⅲ grade level. Composite ecological risk assessment shows that:only 2.88 percent at the middle level,97.12% at a low level. According to the ecological risk and early warning, the order of early warning risk probability which using individual ecological risk index: Cd>Ni>Zn>Cu>Cr>Pb>Hg, in results, As is no police level, and the probability of Cd warning level is 82.52%, and the light police level is 17.48%. In the determination of sample points,IER of heavy metals are not reach the police level or above. From the composite index,95.06% of the sample points are in the level of non-police which this points are in the risk of early-warning level, and 4.12% of the ones is at the lowest ecological risk, and only 0.82% of the sample point is in the police-level risk.7) This article takes full account of the actual situation of the study area, and simulates the soil library elements balance equation, and then selects six contents (Input:weathering of rocks, atmospheric fallout, irrigation water, fertilizer; output:soil erosion, crop harvest) about heavy metal input and output. Finally, the article predicts the process of ecological security. The forecast results showing:Under natural conditions, As, Cu, Ni, Cr, Cd, Pb and Zn need a long time to achieve the environmental self-purification; Hg cannot achieve the environmental self-purification, and through the human management to eliminate the pollution.