DB Doc


NGS Geochemistry by County

This section of the report presents maps of several elements on a county-by-county basis in the conterminous states. Included are elements of importance to environmental studies and major elements. The mean and standard deviation of each element was calculated for each county. This was done on the basis of gridded maps (see separate description), which are interpolations of the original point data. The calculations were done using the "zonal statistics" function of the Spatial Analyst extension to ESRI's ArcMap 9.2 program, which averaged the concentration of each element at each pixel of the geochemical grid contained within each county.  Ideally, the statistic that should have been used here would be the median instead of the mean.  This was not possible with software available at the time this report was prepared.  However, a customized routine to calculate more sophisticated statistics has now been developed, and the results will appear in future releases of this report.  

The results of the calculations are displayed as a series of index maps, where counties are shaded according to the relative concentrations of the elements. Clicking on an index map will zoom to that area of the country. Holding the mouse over any county in the zoomed map will reveal the mean concentration of the element in the county. Clicking on the county will allow the actual data within the county to be examined and downloaded.

View the maps:

(by AA)

(by AA)

(by AA)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

(by ICP40)

[These are the only
elements available
at this time]

Download a file of all county-by-county averages (30 Sept 2008; 1.71 mb):

  1. The calculation that was done does NOT simply average the concentrations of samples that plot within a given county. Pixels in a gridded map are influenced by all sample-points in their vicinity, without regard to county borders (i.e., the grid points are interpolated). For this reason, averages are given for some counties that do not actually contain samples in the NGS. In addition, some averages given for large counties may be based on data that only covers a fraction of the area and may not be representative of the entire county. It is left as an exercise for the user to evaluate how well a particular county average represents the entire area by examining the actual point-data for the county of interest and surrounding regions.
  2. Geochemical trends are not influenced by county or other political borders.  They are mainly the product of natural processes, in some places overprinted by anthropogenic contamination.  Large counties almost always span several heterogeneous geochemical provinces, and thus a county-wide average for an element may not actually represent the concentration of that element anywhere in the county.  The user should use caution, and, again, should always examine the actual point data to gain a better understanding of local variability.
  3. All sample media have been combined to make these calculations. In some areas, there may be statistical differences between media for some elements.  Point data should be examined to evaluate this possibility in any area of interest to the user.