EXPERIENCE WITH THE REPLICATION OF REGIONAL SURVEY OF SOIL POLLUTION

Protecting the soil from pollution requires reliable estimates of conc entrations of contaminants and, in particular, of heavy metals. Labora tory procedures have been standardized to achieve this, but field samp ling and statistical procedures for regional mapping have not. In part icular, we found no published experience about the precision with whic h estimates can be reproduced from different surveys of a region. We h ave compared results from two independent surveys in which the topsoil of a 10-km(2) region of the Swiss plateau was sampled for potentially toxic metals. The concentrations of copper and zinc were measured in hot 2 M HNO3 extracts in different laboratories and afterwards correct ed to a common standard by linear regression. The corrected data had s imilar means, variances and skewness. Experimental variograms were com puted and fitted by bounded models with ranges between 1400 and 2000 m . Then, the concentrations were estimated by block kriging to produce maps. The general structures revealed by the two sets of data were sim ilar, but local details differed substantially. To test the reproducib ility of local predictions the values at the locations of the second s urvey were estimated by ordinary kriging from the first survey and com pared with the measured values. An 'uncertainty' interval of twice the kriging error was constructed so that, theoretically, 95% of the meas ured values should lie within it. For copper, only 85% were found in t he interval, and for zinc 92% lay within it. In both cases the actual variation of the underlying process was underestimated by the kriging variances. This seems to result from underestimates of the variances i n the region. (C) 1998 Elsevier Science Ltd. All rights reserved.