Studies On The Investigation, Risk Assessment Of Heavy Metal Contaminated Sites And Assessment Of The Remediation Effect In Red Soil Zone Of South China

At present, attention is increasingly taken to the problem of heavy metal pollution in soil around the world, and it is found that the area of contaminated sites is increasing. The problem of heavy metal pollution is beginning to emerge throughout our country, and the events of heavy metal pollution occurred frequently in recent years. Moreover, the problem is even more serious in the red zone of southern China for the richly non-ferrous metal mines, intense mining and smelting. This thesis is concerned primarily with the heavy metal pollution of soil in China. Investigation of the heavy metal contaminated sites has been taken in red soil region, and its risk assessment and assessment of the remediation effect also been taken, which provides a case study and a reference of site remediation for the related studies in this region.The main contents and results are as follows:(1) A case study was taken in a chromium slag site with red soil in Jiangxi Province. Based on the analysis of the pollution sources, a comprehensive survey of the soil profile was carried out in this contaminated site. Results showed that among33topsoil (30cm) samples, total chromium content not exceeded the national standard (GB15618-1995) in only a sampling point, and the rest of the points has been found exceeding the standard with the multiple of1.25times to104.34times. The general tendency is that the smaller exceeding times appeared up the hillside, the largest ones in northeast area below the hillside, and the exceeding times of middle region and the southeast region between them. The spatial distribution of Cr6+and total Cr content coincided with each other in the contaminated sites, and the contents in the area varied markedly along the gradient. The soil is not exceeded the national standard below the depth of1.2m upon the hillside, below4m in the northeast region down the hillside and all in the depth of5m. Exceeding times and pollution depth of the middle area and the southeast region are between them. The spatial distribution of Cr6+and total Cr content coincided with each other. Through the method of single factor evaluation index on soil chromium content, it is found that most topsoil is severely polluted in the studying site, and the pollution index reached to2.35-122.46. Through the evaluation method of potential ecological risk coefficient for the heavy metals, it is found that Cr in the chromium slag indeces the mild-moderate risk to the environment.(2) According to the evaluation model of contaminated sites risk assessment technical guidelines, Pb and Cr as attented contaminants were selected, and the human health risk were estimated in3exposure routes of the contaminated sites. Results showed that inhalation of soil particles mainly leads to carcinogenic risk contribution, and both the value of factory comprehensive cancer risk (2.92x10-4) and farmland comprehensive cancer risk (2.92x10-4) are much greater than the risk warning value (1×10-6), which have high carcinogenic risk. Through risk analysis, we found that calculation of cancer risk or hazard quotient determines the key pollutant (Cr) needing to repair in the factory, but the key pollutants (Pb and Cr) in the farmland near the factory.(3) We studied the compound pollution sites of Cr and Pb in Jiangxi Province. Through risk assessment analysis, the calculated soil remediation target value of non-carcinogenic risk in all exposure routes are551.58mg/kg, which is lower than the standard value of "Exhibition for oil environmental quality assessment standards"(HJ350-2007). Restored land can be used for the land use types non-directly exposed to the body, e.g. stadium, green land, commercial land and public municipal land. For residential land and other sensitive land type, the calculated soil remediation target value of non-carcinogenic risk in all exposure routes are57.63mg/kg, which is lower than the limit values of "Environmental quality standards of edibling agricultural production in China"(HJ332-2006) at pH<6.5for the land of such as fruit trees, but higher than the limit of vegetable planting. Cr (VI) has carcinogenic risk, and the recommended restoration target values of Cr-contaminated sites for industrial land and for sensitive land such as residence are4.57mg/kg and2.36mg/kg, respectively. They are both far below the Grade I standard limit of "Environmental quality standards of soil in China"(GB15618-1995), which is the natural background levels of the soil. From a technical and economic point of view, the restoration target is not feasible. The target recommended value of Cr for site remediation should be determined by the type of land use of the sites and the technology reachability. (4) A case study was taken in a contaminated site with compound heavy metals (Cd, Pb and Cu) in red soil zone. Through the method of SPLP, leaching contents of heavy metal pollutants were determined for the remediated farmland by stabilization. Results show that the contents of harmful heavy metal pollutants concerned (Cu, Pb, Cd and As) after the treatment are much lower than the standard value according to the national standard "Hazardous waste identification standard in China-identification of leaching toxicity"(GB5085.3-2007). It means that the restored soil has met with safety standards, and the adopted treatment methods and measures are appropriate for this kind of soil.