Ecological Risk Assessment Of The Typical High Cadmium Background In Guangxi

High cadmium background exists in Guangxi karst region, according to the preliminary investigation found that, natural and geographical environment in some areas, soil Cd concentration of 20mg/kg., 2013 cadmium rice event, pollution of heavy metals in the soil into the problem has been paid more and more attention. In this paper, using correlation analysis method, health risk assessment method, the method of potential ecological risk index, the cumulative index method of Guangxi typical high background areas of cadmium total cadmium and effective cadmium, Cadmium Forms in soil, cadmium content of crops and on the ecological environment were studied by analysis and evaluation. The conclusions are as follows:(1) The study area 74 soil sampling point average cadmium content 4.08mg/kg, beyond the national soil environmental quality standard value of 0.3 mg / kg of 13.6 times; 74 soil samples, with 66 samples of soil cadmium content of more than 0.3 mg/kg, exceeding the rate was 89.19%; cadmium content in the highest value for 21.88 mg/kg, super abroad standard 72.9 times. The average cadmium content of high Cd background, middle Cd background and low Cd background was 7.56mg/kg, 3.38mg/kg and 0.88mg/kg, which exceeded the national standard 25 times, 11 times and 2.9 times respectively. Soil property is steady, the soil organic matter content in the 2.10g/kg-59.49g/kg, average value for 38.55g/kg, belonging to the mineral soil; soil pH range between 4.32-7.13 average 6.08, the soil is acidic, pH variation coefficient was 10.45%, indicating that the difference of soil pH in the study area is not big. The correlation between total cadmium content and soil pH was weak, the correlation between total Cd content and soil organic matter content was weak, and the correlation between soil pH and soil organic matter was weak. The high cadmium content in the study area is due to the high natural background. 74 points in the study area, cadmium, heavy pollution points accounted for 60.8%; specific to high cadmium, cadmium, low cadmium research area, cadmium and heavy pollution points accounted for 93%, 80%, 11.54%. Cadmium pollution is the most serious in high cadmium research area.(2) The content of available cadmium in soil is between 0.04mg/kg-4.13mg/kg, with an average of 0.78 mg/kg, and the average proportion of available cadmium is 26%. Available cadmium content, high cadmium research area > middle cadmium research area > low cadmium research area. Available cadmium content accounted for the percentage of total cadmium, low cadmium research area> middle cadmium research area > high cadmium research area. The correlation degree between the available cadmium and total cadmium in the study area, high cadmium research area > middle cadmium research area > low cadmium research area. The content of available Cd in the soil of the study area was very high, and the content of total Cd was very high. The correlation among the three variables of high cadmium, cadmium, low cadmium three of the study area effective state of soil cadmium, soil pH, soil organic matter, are very poor and related degree from the point of view of the average level, showing weak correlation, basically.(3) The content of Cd in the soil of the study area, five kinds of forms, high cadmium research area: Fe Mn oxidation state > residue > exchangeable > organic bound state. In the study area of middle Cd background and low Cd background: the residual state> the exchangeable state> the oxidation state of Fe and Mn> and the organic bound state. Fe Mn oxidation state accounted for 38%, and the remaining four species accounted for of the total cadmium ratio: residue state 24%, exchangeable state 20%, carbonate state 9%, organic bound state 8%. The content of cadmium and organic cadmium in carbonate is very close. Correlation coefficients of soil cadmium five forms and soil p H, carbonate state > residual > Fe Mn oxide bound > organic bound > exchangeable; exchangeable Cd and soil pH is negative correlation and carbonate Cd and soil pH highest correlation in high cadmium and cadmium in the study area. Higher cadmium background, soil cadmium five forms and soil organic matter correlation better; to the study area of high cadmium and cadmium soil five kinds of forms and soil organic matter present close to highly correlated; in the cadmium in the study area, the part of cadmium fractions in soils began with soil organic matter showed negative correlation; the study area of low cadmium, cadmium soil five kinds of forms and soil organic matter show negative correlation and related degree is low, for low to moderate negative correlation. Tessier extraction method has a certain relationship between the five forms of Cd and soil particle size. The Cd content of Fe Mn oxidation state, residual state and organic bound Cd content decreases with the decrease of soil particle size. With the increase of soil particle size, the content of Cd in carbonate was increased slowly. The content of exchangeable Cd decreased firstly and then increased with the decreasing of soil particle size. Between the large, medium and small soil particle size of various chemical forms of Cd are exhibited a highly correlated relationship, the total cadmium and iron and manganese oxidation state had the highest correlation with residual fraction and other forms of relatively poor correlations; and residual correlation best is total cadmium, and organic combination of states of the best is the iron and manganese oxidation state.(4) The cane, corn seeds, peanut fruit, rice, fruit, cassava fruit average cadmium content exceeding, exceeding the rate were 64.1%, 34.5% and 41.2%, 33.3%, 75%; exceeding standard multiple were 1.8 times, 1.1 times, 1.002 times, 1.05 times, 1.88 times. The content of cadmium in different parts of crops generally showed a root > stem > leaf > fruit. Excessive crops are mainly concentrated in the research area of high cadmium.(5) In the study area, the risk of carcinogenic and non carcinogenic risk caused by three kinds of exposure to the human body is very small. The crops in the study area and the low cadmium area of the high cadmium area will have a health risk to the human body, and the risk is greater than the risk to the adult.(6) From the overall perspective of the study area, two kinds of ecological risk assessment methods, the study area of about half of the area belongs to cadmium extremely serious pollution, and mainly concentrated in the high cadmium research area. From the point of view of the degree of pollution, the research area of high cadmium?low cadmium research area. From the research area, the cadmium pollution in high cadmium research area is very serious, and the pollution degree is moderate.