Duplicate field sampling
Each participating country must collect duplicate field samples of all recommended sample types. Duplicate field samples in the Global Geochemical Baselines project are normally collected at a rate of 1 to 30 drainage basins, i.e., approximately 3% duplication. From your country's GTN grid cells list, randomly select the ones for duplicate field sampling.
Duplicate field samples of each material are collected from one geologically representative small catchment (either 1, 2, 3, 4 or 5) of the selected GTN grid cell in the case of the cost-effective 5 random point sampling design. In case the more costly 8 random point sampling design is used, duplicate field samples of each material are taken from one geologically representative small catchment (either 1, 2, 3, 4, 5, 6, 7 or 8) of the selected GTN grid cell.
Countries with less than 6 GTN grid cells, i.e., 30 drainage basins, should randomly select 2 drainage basins for duplicate field sampling, which must be in different GTN grid cells.
How to select the drainage basin? The random small drainage basin for duplicate field sampling is situated in a GTN grid cell, where in total there are 5 drainage basins for sampling. The 1:50,000 geological and soil maps are studied, and the small drainage basin for duplicate field sampling should be the one which is lithologically and pedologically more representative of the lithology and soil type of the whole GTN grid cell. Of course, duplicate floodplain sediment samples are collected from the adjoining large, third order, river.
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