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The most important is to ensure the apron is not cracked and various apron radii from 1 to 2m from the handpump are suggested. I would suggest a minimum of 1.5m from the edge of the handpump or well cover slab.

The next most important is to keep animals away from the wellhead.

There is, as far as I can see (and I have checked all the usual texts in our resource centre e.g. SPHERE, Env Health Engineering, ITDG books on Hand Dug Wells and Hand Pumps and others on handpump construction) no standard for distance from apron to soakaway. The soakaway here is different from that for pour flush latrines or household waste as the wastewater should be very lightly loaded with pathogens (unless people wash themselves/clothes at the wellhead or water animals). The soil characteristics are the most important in terms of polluting - a chalk or lime rock could allow very quick ingress of wastewater back into the well without filtration through cracks whereas a sand or loam would allow good filtration and so the soakaway could be placed nearby. A clay would allow very little wastewater to soak away and so other means of getting rid of the wastewater is important. The only books I can find with drainage channel lengths are Field Engineering which talks about several metres.

There is a nice design in Engineering in Emergencies p253 that has a 2% slope on the apron and drainage channel. The drainage channel is minimum 6.3m long, leading to a cattle trough of minimum 1.5m long then to a soakaway. This should allow plenty of distance between the soakaway and well, although the main reason for its length must be to keep animals from the well (if animals are watered nearby the well should be fenced).

If you are replacing aprons they really should be pretty strong as they will get plenty of abuse - good deep foundations and steel reinforcements over the well itself. If you want I can send you a design or two, especially if you give some more details of the well and handpumps.

It is often the way that the wastewater irrigates a vegetable garden kept by the handpump caretaker - a useful use for the waste. I have also seen it feeding a duckpond (a very small one). People often have very good strategies themselves to reuse the wastewater from a handpump.