It is important to understand how to properly handle samples that are classified as non-detects. A non-detect is an analytical sample where the concentration is deemed to be lower than could be detected using the method employed by the laboratory.
Non-detects should never be left out of the data file. They are critically important in determining the spatial extent of the contamination. Furthermore, it is important to understand what it means to have a sample that is not-detected. It is not the same as truly ZERO, or perfectly clean. In some cases samples may be nondetects but the detection limit may be so high that the sample should not be used in your data file.
Non-detects are accommodated using two very important parameters that should be carefully considered. They are the Pre Clip Min and Post Clip Min. These parameters control the clipping parameters for pre and post processing in all of the EVS modules that use read chemistry (.csv, or .gwc) files. The affected modules are Krig_3D, Krig_Z, Krig_Fence, Map_Spheres, and Stat_CSV_GEO.
All of the kriging modules use the Pre Clip Min values as the value to assign to non-detects or any value less than the clipping value. C Tech recommends that:
Pre Clip Min value should be set to a fraction of the detection limit for the selected analyte (chemicals or parameters).
When using "Log processing" the fraction should normally be one-tenth, but may be as high as one-half of the detection limit.
If all of the analytes for a site do not have the same detection limit it is best to krige them separately, using appropriate (fraction of detection limit) clipping values for each.
If all of the samples for any analyte do not have the same detection limit it is best to assign numeric values to your non-detects (fraction of detection limit) values for each non-detect value.
If you have already assigned numeric values to your non-detects be sure to set the Pre Clip Min value less than your lowest assigned value to prevent modifying your assigned values.
Several modules use the Post Clip Min values as the value to assign the final minimum values used in the output field (visualization). C Tech recommends that:
Post Clip Min value should be set to the actual (nominal) detection limit for the analyte. By doing this, you avoid having a model where any values are less than the detection limit. A lower value would not be defensible since your measurement techniques would be unable to verify the lower value.
If all of the analytes for a site do not have the same detection limit it is best to krige them separately, using appropriate detection limit clipping values for each. If you wish to have multiple analytes in the same field, they can be combined later using the combine_comp module. [This note only applies to the kriging modules.]