Calculating the Average Contaminant Plume Concentration in Surfer 14

I recently ran across an article that was written by an experienced Surfer user, Joseph A. Ricker. The article, A Practical Method to Evaluate Ground Water Contaminant Plume Stability, provides a workflow for determining the average contaminant plume concentration that I thought our Surfer community may find interesting. Below, I've summarized the workflow outlined in Mr. Ricker's article and provided the exact steps required to duplicate this in Surfer 14.

First, grid the contaminant concentration data using Surfer. In Mr. Ricker's example the values range from non-detectable (0) to 9400 µg/L so applying a Log, save as linear Z transform would be recommended. This prevents the areas of high concentration from being ignored, or from having too large an effect on the surrounding area. Since we are gridding concentration data, a minimum Z of 0 can also be defined to prevent the grid z values from becoming negative. To grid the data with these options:

1. Click Home | Grid Data | Grid Data.
2. In the Grid Data dialog,
   1. In the Grid Z Limits section, set the Minimum to Custom and enter 0 into the box.
   2. Set the Z Transform to Log, save as linear.
   3. Set any other options you'd like, and then click OK to create the grid.

Next, calculate the volume of the plume where the concentration is above the accepted safe level.

1. Click the Grids | Calculate | Volume command.
2. In the Grid Volume dialog, select the contaminant concentration grid file that you just created in the Upper Surface section.
3. Toggle Constant in the Lower Surface section and enter the accepted safe concentration.
4. Click OK to generate a Grid Volume report.

Finally, divide the volume (in units of µg/L*m²) by the planar area (in units of m²) to get the average plume concentration above the cutoff or safe concentration.

1. In the Grid Volume report, make note of the Positive Volume [Cut], which provides the volume above the lower surface (safe concentration).
2. Make note of the Positive Planar Area [Cut], which provides the area of intersection between the lower surface and upper surface where the concentration is above the defined value.
3. Divide the Positive Volume by the Positive Planar Area and then add the cutoff concentration to determine the average concentration of the entire plume.

This workflow is a great example of the many ways Surfer's gridding algorithms and grid-based calculations are used in the field.