Revision history [back]
I agree with Martin's comments, but would add that the
rule-of-thumb distances between toilets and water sources/points tend to assume a worst case scenario
(and can lead to expensive solutions and/or unnecessary costs ... all in the name of keeping things
simple). In addition, you should be aware that pathogens need a transport medium (usually water), can
be filtered out by the soil (depending on their size), and have a limited life span outside the body
(which means that some die off before reaching a water source/point).
The amount of water entering the seepage pit is a critical parameter. Dry (non-flush) VIP latrines tend to have a relatively small static head to force water into the unsaturated zone around the pit, which means that most of the pathogens remain within the pit. But the depth to the water table is also important.
The BGS Guidelines for Assessing the Risk of Groundwater from On-Site Sanitation (ARGOSS) suggest that you calculate the "number of travel days" = soil porosity x horizontal distance/(permeability x hydraulic gradient).
The information you provided would suggest a "very low risk" or microbiological contamination, largely because the soil porosity and permeability in a loam/clay soil are low (resulting in a long travel time for the pathogens). VIP latrines are generally considered low risk because of the relatively low static head likely in the pit ... but this all depends on the pit sizing, amount of water entering the pit, and whether (as Martin notes above) there are any local soil characteristics that encourage faster groundwater flows (e.g. layers of sand, rock fractures, voids etc).
I will forward on a useful WELL factsheet on Microbiological Contamination of Water Suppies (Steve Sugden, 2006) which outlines the main issues to consider, and explains how to do some of the simple calculations, and hope that this provides you with some appropriate guidance? But it sounds to me that your second site is preferable (as flatter and further away from water sources), and that it would be worth doing some sort of simple infiltration test at this site to check the soil permeability (and to help size the leach pit)?
Finally, I wouldn't worry too much about contaminating the farm fields, as - if the effluent reaches that far - the nutrients in the excreta (particularly the urine) should be beneficial?
Good luck - andy