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MT3DMS - mass loading inconsistent with results concentration


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i have a point source with tracer mass loading of 0.17 kg/day. mass input units can be kg, grams or milligrams, as in the GMS unit’s menu.

if i use grams, the mass loading input should be 170 g/day, and respectively for mg.

the problem is with the output concentration units. i can chose between g/lit, mg/lit, ppm or ppb.

if i use concentration units of mg/lit, and change the mass units while updating the source value, i get different values for the solution. 

it seems that the model is ignoring the input units, and take into consideration only the mass value itself.

for example - using 0.17 kg/day doesn't multiple with the correction factor for the output units, and i get very low concentration.


i would appreciate any advice.

Thanks, Yoni 

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well, I think this feature that is not working properly.

Here is an excerpt from MT3D manual, that describes how it handles units:

"The MT3DMS code uses any consistent units for input and output variables. In the input file to the Basic Transport Package, the user decides the units for time, length, and mass. Then, any input variable or constant should be entered in units consistent with the three basic units. ..."  [chapter 6.2 of Zheng, Wang (1998): MT3DMS A modular three-dimensional multispecies transport model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems (Release DoD_3.50.A).]

There is more in the chapter, so please read it. You can contact technical support for GMS and point them to the problem. I would treat the units specifed in GMS as purely informative for the user. I think the value for a given property you specify goes directly into the input files of MT3D. And therefore if you specify values of a mass rate in terms of different units compared to what your input concentrations are, your outputs will most likely be wrong (and cannot simply be rescaled). Also if there is any nonlinear process involved in the simulation the units must be consistent otherwise the results will be wrong.

Edited by Michal
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