Abstract: Three guidelines are set forth for evaluation of soil contamination at the scale of a region or a specific site in view of ambiguity of the definition for soil contamination at present.The three guidelines,mainly based on chemical levels,are upper limit of soil environmental background,Grade Ⅱ criteria of the national soil environmental quality standard(GB 15618) and threshold value of soil contamination.The upper limit of soil environmental background,which corresponds to the local soil environmental background,is a critical value for discriminating between pedogenic source and anthropogenic inputs of chemicals as well as a target value for protecting the soil environment.If a chemical would be higher than this upper limit,measures should be undertaken in exploration and control of its potential anthropogenic sources in order to protect the soil quality from further impact of these sources.The soil environmental quality standard,which is equivalent to Grade Ⅱ criteria of the national soil environment quality standard,is a screening level for distinguishing uncontaminated soils from contaminated soils preliminarily.If the level of a chemical in the soil is lower than this standard,it indicates that the soil is not contaminated.Otherwise,the soil might be contaminated potentially and further investigation and/or risk assessments of the eco-environment and human health should be recommended.The threshold value of soil contamination,which could be acquired through risk assessment of the problem soil,is a critical value for determining whether the soil is contaminated or not.Beyond this value,the soil is contaminated and countermeasures should be undertaken to control the pollutant sources and remedy the contaminated soil.

Key words: soil contamination, soil environmental quality guideline, soil environmental quality standard, risk assessment