Critical Comparison of Soil Pollution Indices for Assessing Contamination with Toxic Metals

Abstract

Aggregation of environmental monitoring data into indices is a common procedure when the objective of the assessment is the evaluation of some environmental criterion for large areas, usually with planning purposes. Two types of aggregation functions are commonly used in the construction of indices: the weighted sum and the constant elasticity of substitution. Several criteria have been proposed for the selection of aggregation functions, namely, (i) ambiguity, which happens when all indicators indicate non-contamination, but the index fails to reflect this observation; (ii) eclipsing, i.e., the index fails to reflect contamination indicated by one of the variables; (iii) rigidity occurs when the introduction of more variables result in increased failure in the classification given by the index, as indicated by a decrease of the index. The first two criteria are easily checked, but the latter is more difficult to evaluate. A method to assess rigidity is here proposed and applied. Two other criteria are also proposed: sensitivity and accuracy. The present study compares and discusses the use of pollution indices for the classification of soils as to heavy metal pollution, with both empirical and real-world data. In the end, some criteria for index selection are indicated, along with their ranking for different practical circumstances. The Nemerow pollution index and the ecological risk index complied with all the fundamental criteria making them good general-use indices.