I'm using the Transport Observation Package (TOBS) with MT3DMS in a simple GMS model (v10.0.10) for testing purposes. I noticed that when the Transport Observation Package is active, calculated concentrations at observation locations are not being interpolated from its neighboring nodal points. This is in contrast to how GMS reports concentrations when the TOBS package is not activated. I checked the TOBS input file (*.tob) and noticed that the iConcINTP flag which controls how TOBS handles interpolation is set by GMS to zero (no interpolation). The benefit of the TOBS package is that it calculates mass fluxes at groups of source/sink boundary conditions. I do not believe that GMS can do this in post-processing without the TOBS package active. Based on the above, I have two questions: 1) Is there any way to tell GMS to turn on the interpolation in TOBS on so that interpolation is performed? I was able to turn it on manually by saving in GMS, opening the TOBS file and changing the iConcINTP from 0 to 1. 2) I'm wondering if interpolation "on" (iConcINTP = 1) should be the default option in GMS since the output would be more consistent with how GMS does post-processing on concentrations at observation points. Any insight on this would be appreciated. - Rich Carbonaro