Site Location and Sampling Media

The primary global Geochemical Reference Network, according to Darnley et al. (1995) was to be based on approximately 5000 grid cells of 160x160 km distributed over the Earth’s land surface (see map showing grid cell boundaries). The original 5000 grid cells have now been updated to 7356 by adding all missing grid cells.

• Sample sites should be located in several (minimum of 5, 8 are preferred) 20x20 km, or 40x40 km, or 80x80 km subcells within each 160x160 km grid cell to permit the application of statistical analysis of variance techniques; access and cost considerations will determine whether the subcells are randomly selected along one or two sampling profiles or road corridors, or randomly distributed in any part of a 160x160 km grid cell; in undeveloped regions helicopter-borne sampling profiles are recommended.

Several materials are to be sampled from within each drainage basin.

From the small, second order drainage basin of <100 km² in area the following sample types should be collected:

• 0-25 cm depth residual soil,

• C-horizon soil at a depth range between 50 and 200 cm,

• Surface humus, if present,

• Stream sediment or lake sediment,

• Stream water, if present,

• Surface overbank sediment (0-25 cm), and

• Deep overbank sediment (depth range depending on site conditions).

NOTE:  Each stream sediment sample should be a composite of a minimum of 5 subsamples; this composite material must be thoroughly homogenised.

             Overbank sediment (top and bottom) should be collected from a single site and a single layer.

Residual soil samples should be taken from a suitable site within the same 100 km² catchment basin, and over the dominant rock and soil type within the catchment basin.

• Each residual soil sample should be a composite from three adjacent subsites at a maximum distance of 5 m;  the condition is that the same soil horizon must be sampled.  As this condition is difficult to meet, using the experience of soil sampling in different morphoclimatic environments, it is recommended to collect the residual soil sample from a single site, as this technique is cost- and time-effective. 

From the large river, third order, or greater, of 1000 to 6000 km² in area the following sample types should be collected:

• Floodplain sediment (top and bottom) from a single site and a single layer.

Gamma ray spectrometer readings should be taken at each site for in-situ radioelement determination; the acquisition of carborne or airborne gamma ray spectrometry in transit between sample collection sites is recommended to establish a linked data network.