STATISTICAL EVALUATION OF THE INFLUENCE OF SEVERAL SAMPLE PRETREATMENT METHODS ON THE MERCURY CONTENT DETECTABLE BY CHEMICAL-ANALYSIS OF CONTAMINATED SOIL SAMPLES UNDER PRACTICAL CONDITIONS

The estimation of the environmental risk of contaminated sites caused by hazardous components may be obtained, for instance, by means of a s oil survey. There unavoidable errors by sampling, sample preparation a nd chemical analysis occur. Furthermore, in case of mercury contaminat ions, the mercury content detectable by chemical analysis can be falsi fied, if between sampling, on the one hand, and sample preparation and sample decomposition for chemical analysis, on the other hand, volati le components or elementary metallic mercury escape from the sample. T hus, in these cases, handling of samples such as air drying, storing i n plastic bags or thermal evaporation, generally termed sample pretrea tment, is a further source of error in evaluating a material. However, the measuring results are influenced not only by sampling, sample pre treatment, sample preparation by homogenization and splitting, and che mical analysis; they must also reflect the intrinsic properties of the soil sample subject to both global fluctuations and local heterogenei ties. The present work shows by example of a non-uniformly contaminate d site to what extent the analytically delectable mercury content is c hanged by the method of handling of soil samples in the period between sampling and chemical analysis. A hierarchical experimental design wa s realized in order to separately quantify the different sources of va riation of the measured mercury contents, which are caused by global v ariations, local heterogeneities, sample preparation, sample pretreatm ent as well as chemical analysis. As turned out by variance analysis, the variance portion contributed to the total variance by sample pretr eatment is highly significant and lies in the same order of magnitude as the variance caused by local heterogeneities of the soil. That mean s that the type of sample pretreatment influences the analytical resul ts essentially. In order to quantify the effect of a definite pretreat ment method in comparison with the mercury content of the unchanged or iginal soil sample, the probable systematic error of a method was intr oduced. Investigations were only carried out at two sampling locations of the contaminated site because of the relatively high labour; the m ean values and variances obtained cannot be immediately transferred to other sites. However, the general knowledge can be used as methodical basis for further investigations. Particularly, the consequence arise s that the regulations existing for the treatment of mercury-contamina ted samples between sampling and chemical analysis must be revised to obtain comparable criteria of evaluation.