A soil sampling strategy for mapping trace element concentrations in a test area

A sampling strategy, based on previous knowledge of a test area, was compar ed with a Reference Sampling in the frame of the EU-funded CEEM project. Th e aim was a comparison of sampling strategies for the drafting of concentra tion maps for a set of trace elements (Cd, Pb, Cu and Zn), with three-dimen sion information that can be used for risk assessment. The main constraint was that only 15 soil samples were to be analysed. The test area tin Dornac h, Switzerland) included different soil types and land uses (forest, arable land and permanent grasslands). The sampling strategy proposed defined str ata on the basis of land use, then applying a systematic regular sampling w ithin each stratum. A composite sample was formed from each individual samp le. In forests, H and Ah horizons were taken separately. In arable lands, a depth within the ploughed layer (0-15 cm) was analysed. In grasslands, sam ples were taken at two depths (0-10 and 10-20 cm) to provide information on the vertical variability. The comparison of the results obtained with thos e derived from the Reference Sampling allowed us to estimate if the informa tion obtained from the proposed sampling strategy was sufficiently represen tative for an assessment of the level of elements considered. It was conclu ded that recommendations on sampling strategy depend on the interaction pat tern of the element. In this case, for mobile trace elements, such as Cd, t he information derived from sampling of two soil layers was not as importan t as that for the rest of metals studied that accumulated in the upper laye r. If the H horizons in forests was not considered this led to unreliable p redictions of the concentration levels, since the concentrations in H and A h horizons differed irrespective of whether they were expressed in mg kg(-1 ) (the H horizon was enriched in those trace elements, except for Cd) and i n mg m(-2) (the low bulk density of the H horizon led to a lower trace elem ent level than in Ah horizon). Finally, in undisturbed systems such as gras slands, sampling 0-10 cm instead of 0-20 cm is recommended to prevent dilut ion of the concentration of trace elements. (C) 2001 Elsevier Science B.V. All rights reserved.