Surfer does not currently offer a center of mass calculation in the grid report. If you would like this feature added the Surfer grid report, please let us know.
In the meantime, Joseph A. Ricker outlines a workflow for calculating the average contaminant plume concentration and center of mass in the article A Practical Method to Evaluate Ground Water Contaminant Plume Stability. Step by step instructions for calculating the average contaminant plume concentration as outlined in this article can be found here.
Updated October 24, 2018