LTE Lifetime-expectancy vs. Backward-Streamline-Isochrone markers

Hi,
I have a fully confined 3D-model.
When I start backward-streamlines from a 3D-polyline along the outflow-border with isochrone-markers (e.g. 10a distance) the result is completely different to the isolines of the LTE (same distance 10a). Why?
The isolines of the LTE are nearly the same in each layer, no matter what conductivity is set.
The isochronemarkers of the streamlines depend on the conductivity (that is what I expect).

Thanks a lot for any help!

best regards
Stefan

Hi Stefan,

The most likely reason for this is that the LTE isolines are based on a transport calculation, i.e., they include the effects of diffusion and dispersion. If the layers are relatively thin, the default transverse dispersivity will lead to a similar distribution of LTE in the different layers. You can test this by applying the random-walk method as well (=pathlines with dispersion). I'd expect that the result will be somewhat similar to the LTE isolines. Another reason could be that the aquitards consist in only one layer. LTE concentration then can also be passed through the aquitard more easily than realistic.

To know what the 'real' distribution is, you'll have to apply realistic values for dispersivity plus a subdivision of the aquitard(s). Note that using small values for dispersivity may require a fine discritization, especially in vertical direction, in order to obtain a stable transport solution. This is similar to a mass or heat transport simulation.

Cheers,
Peter

Hi Peter,

thank you for this quick reply!

…but still another question:
I built up a simple confined 3D-model (1000 m x 1000 m x 5 m; 5 Layers).
When I set the dispersivity in the field-line-computation-settings for the random-walk tracks as fixed homogeneous value = 0 m, the isochrones-result for the standard streamlines, the random walk tracks AND the LTE are the same.
That is, what I expect for the standard streamlines and RWT.
But: The LTE-isolines don’t change, when I change the dispersivity (material properties/mass transport-lte) within realistic values (< 100m/10m). The LTE-isolines only begin to move if I set the dispersivity > 500m/50m!!
To get nearly the same result with the RWT and LTE I have to set the dispersivity for the RWT (in the field-line-computation-settings) 0,5m/0,05m while the dispersivity for the LTE (in the material properties/mass transport-lte) is set 500m/50m.

So, I'm not sure which method I should choose  to define e.g. a '3-years-travel-time distance' to a well.

Thanks for your help!

Best regards
Stefan

If the transverse dispersivity is relatively large compared to the layer thickness, the travel times may be significantly influenced by boundary effects (particles are reflected at model boundaries), while in the LTE model this fact only leads to perfect vertical mixing. This difference is the reason for the observed behavior in this very thin aquifer.

Thanks a lot!
Good to know about this effect.
I think it is not so unusual to have a thin (quarternary) aquifer with a relatively large extent.

Best regards
Stefan