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RedR

The short answer is predicting the precise flow of underground flow of effluent would be very difficult if not impossible. It would depend on the steepness of the slope, the rock type and water table level.

If the rock is porous such as sandstone then water moves in the spaces between the grains and will flow fairly uniformly down through the rock until it reaches the water table where the spaces are already saturated. It will then flow laterally with the rest of the water. Broadly speaking the surface of the water table mirrors the ground topography so underground water flows from hills towards valleys just as surface water does. Where the water table intersects the surface of the ground you will find a spring line or seepage into a lake, river or wetland. It is here that the effluent will come out.

In fractured rocks such as limestone and granite the bulk of groundwater flow is along the fractures. Again water will go predominantly downwards under gravity until it reaches the saturated zone but if your seepage pit happens to intersect a fracture that leads to the surface nearby it could pop out anywhere.

Mapping such fractures may be possible with expensive and complicated geophysics but that is not realistic. Here in Zimbabwe they simply make a blanket recommendation that domestic latrines should be more than 30m from wells or other water sources. With the school latrine that you are talking about the volumes will be much greater so a greater separation would be advisable. With the design that you are using does the liquid seepage have to be under the toilets or could it be piped downhill as far as possible towards a safe place which will minimise the risk of contamination?