Sampling Selecting Sampling Sites

Selecting Sampling Sites

Selecting sampling sites

As the 5 random point sampling design in each GTN grid cell of 160x160 km is the most cost- and time-effective, and was used in the FOREGS Geochemical Mapping of Europe, the procedure for selecting the sampling sites will be described below, and it can be similarly used in the other recommended sampling design of 8 random points in each GTN grid cell.  

1. Selection of small drainage basins in each GTN grid cell:


The randomly selected points are used to select the five (5) nearest small drainage basins of <100 km2 in area.  From the small drainage basin the following samples are collected:
  •  Filtered and unfiltered water
  •  Active stream sediment
  •  Residual soil (top- and sub-soil)
  •  Overbank sediment (top and bottom), and
  •  Rock.
Soil samples should represent the dominant residual soil type of the small catchment basin.  Hence, the 1:50,000 soil map should be studied.
If the randomly selected points happen to fall in the sea or lake or in an unreachable area the nearest useable catchment basin should be used.
Although ''The Blue Book'' (Darnley et al., 1995), does not recommend sampling of rocks, the updated field manual includes the collection of rock samples.  Rock samples should represent the dominant rock type of the small catchment basin.  Hence, the 1:50,000 geological map should be studied.

The optimum small drainage is selected, after studying the 1:50,000 soil and geological maps, using the aforementioned criteria.


2. Selection of large drainage basins in each GTN grid cell:


The floodplain sediment sample (top and bottom) is collected from the lowermost point of the larger drainage basin (area 1000 - 6000 km2) to which the small catchment is connected.  If no suitable size drainage basin is available, the floodplain sediment can be taken from a smaller drainage basin (>500 km2 and <1000 km2.   If the size of the largest related drainage basin is less than 500 km2, the floodplain sample should not be taken.  Justified exceptions can also be made to this recommendation, especially on small islands and small area countries.