A small Thai house walled on 3 sides with a septic tank system. At the beginning of the rainy season (early June in Udon ) we have underground water sipping to the surface with a foul smell.  During the dry season we have very little rising water. We checked the toilet-septic tank piping system to detect leaks, using coloring agents but all seem to be ok… The septic tank was inspected by the blue pumping truck staff and declared leak free. My understanding is that the grey water from the septic tank penetrates a very wet soil under the septic tank, the soil unable to absorb more liquid forces it to rise to the surface. Is installing a sump pump the only possible solution ? Any good water-management engineer around ? Thanks all.

Believe you mean "seeping" is it water or gas that is seeping, do you have a vent line on the tank?

Install a bio septic tank, no more seepage or smells.