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I think it's going to be hard to find the 'scientific' justification that Michael is looking for (from the question it looks as though he wants technical data to back up the need for an apron). The answers given already are all sensible, but in terms of contamination problems, you don't necessarily need anything much except a good seal at the top of the annulus around the pipe. This is reflected in standard practice for monitoring boreholes (for which it is also important to avoid contamination from the surface). These emphasise sealing the borehole annulus with bentonite clay and cement grout, and if they mention a surface 'apron' it is small and designed just to shed water away from the borehole pipe (e.g. 3ft square, USEPA "Design and installation of monitoring wells", available on line).

As has already been said, the main reasons for pumped boreholes having large aprons are practical ones: avoiding erosion produced by people and animals walking on wet earth; avoiding erosion from waste water; anchoring the borehole pipe and pump; and providing a firm and clean platform for people to stand on while using the pump. The standard designs for handpump platforms are of course generally for community wells, and if you are working with self-supply then presumably the traffic at and use of the well is much less, there is less waste water and it is also easier to control behaviour (e.g. keeping animals out). Depends how many users you expect...

The other variable in terms of protecting the groundwater quality is of course soil and geology; if you have a sandy soil to the surface, as in many coastal areas of Madagascar such as Maroantsetra area or Fort Dauphin, and if your water table is shallow, then it's more important to take care than if you have clayey soils near the surface.

Sorry - i'm only giving you recommendations here, Michael, but i would say that the principle of self-supply is to provide as good a water source as you can for the resources you have; and for water quality the depth of the well screen, some kind of seal to prevent surface water filtering down the borehole annulus, and a means of draining waste water away are the most important elements. The seal could just be well-placed clay within the annulus; but for durability would ideally include a pad to shed water away from the pipe.