The B.C. Sewerage System Regulation (B.C. Reg. 326/2004) restricts individuals from placing sewerage systems within 30m from a well supplying a domestic water system, unless a "professional" provides written confirmation that doing so would not cause a health hazard. While setting a minimum setback level gives sewage practitioners and homeowners a baseline for ensuring the safety of their water supply (and that of their neighbors), it doesn't take into account the multitude of factors that can limit or extend the transport of disease-causing bacteria from a sewage system to a water system.
While "professionals" (essentially engineers) can reduce this setback distance, it's not clear how much research and information gathering goes into doing so. In my experience, professionals will provide support for a reduced setback because of secondary or tertiary treatment methods, or because they've looked at the soils and feel that lateral transport is going to be minimal. A study in last month's issue of the Journal of Environmental Health looked at the ability of E. coli to move through sandy loam soil (specifically that from North Carolina) to a water table in situ. They looked at 30cm, 45cm, and 60cm vertical separation between ground level and the water table, and took samples daily to identify whether those individual soil separation distances were sufficient to reduce the load of E. coli prior to the effluent coming into contact with the water.
Using a 64% sand / 30% silt / 6% clay mixture of soil (that was obtained in the field by the researchers), the researchers applied 200mL of an artificial wastewater solution to each soil column daily. The artificial wastewater (which contained nutrients like potassium, sodium, and phosphate) was spiked with E. coli that was obtained from human urine. With this application, they found that 30cm and 45cm of vertical separation were inadequate to mitigate the bacterial content to acceptable levels, with 45cm reducing the levels somewhat, but not below the recommended standard of 200cfm/100mL. 60cm of vertical separation, on the other hand, was found to be most efficient at removing E. coli to levels where the water table would not be adversely affected.
While this research was done with a specific soil type, and did not include extenuating factors that could impact the amount of bacteria that was able to move through the soil, it does provide some great data on what sort of vertical separation is required when designing onsite sewerage systems. It's the type of relatively low-cost research that should be done more often by both regulators and practitioners to ensure that the best sewerage systems (and those that are most cost effective) are being installed in areas that are potentially sensitive. When "professionals" (as defined in the Regulation) are determining whether or not to reduce the minimum setback to a well, this is the type of data that should be consulted prior to making that determination. One can use mathematical models to determine rates of flow, but they might not adequately determine the actual decrease in bacterial concentrations. Having actual in situ research that shows how wastewater moves through a specific soil composition, and how effective it is at reducing bacteria, would give professionals the confidence to reduce minimum setbacks without putting themselves at any professional liability risk.
Source: Amoozegar, A., Graves, A., Lindbo, D., Rashash, D., & Stall, C. (2014). Transport of E. coli in a sandy soil as impacted by depth to water table. Journal of Environmental Health, 76(6), 92-100.