4.25.2014

Variations in setback distances from sewerage systems: one size doesn't fit all

It's often too easy to assign a "safe" setback distance from a source of contamination when considering the installation of a new drinking water source. In B.C., that setback is set forth in the Sewerage System Regulation, and states that an onsite sewerage system can't be installed closer than 30m from a drinking water supply (unless a hydrogeologist is willing to say that there's no risk to reducing it). This distance of 30m, or 100', is fairly common: it's also used by Maine, Massachusetts, California, etc. In a province as diverse and large as B.C., however, this singular measure does not take into account the soil and climate differences that might prevent wastewater from being adequately treated before it makes it to somebody's tap.

After a Norovirus outbreak in Iceland, which was traced back to contaminated drinking water, a group of researchers set out to identify whether there were some considerations for setback distances that weren't being accounted for. The outbreak in question occurred in late summer at a location frequented by tourists and those who owned summer property, with the drinking water being drawn from a well. Due to a few different factors, which will be discussed further on, the minimum setback required for a 9-log reduction in viral load was nearly 900m. That's 30x the setback in the B.C. legislation!

According to this specific study, approximately 1/3 of waterborne outbreaks in affluent nations are due to sewage contamination of groundwater. We assume that a combination of deep drilled wells and relatively slow-moving soils will allow for adequate filtration of the waste water to ensure the effluent entering the aquifer is treated. However, what this doesn't account for are the aspects of soil chemistry that can affect the filtration rate. For instance, viruses travel longer distances in cold groundwater. The groundwater in the Iceland study was around 5'C, which can lead to an inactivation rate for viruses of 1 order of magnitude lower than ground water at 25'C. The researchers point out that not many studies have focused on the relationship between groundwater temperature and viral inactivation.

To properly identify a safe distance between a sewerage system and a well, it's important to consider more than just soil grain size. It's also necessary to look at groundwater temperature, seepage velocity, and soil acidity to determine how long it will take to adequately neutralize the wastewater. The researchers suggest that looking at travel times (e.g. 50 days) may be more reasonable than simply looking at setback distances.

I mentioned above that a 9-log reduction in viral load would've required a setback of 900m in this specific case study. While B.C. legislation has no requirements for reduction of viruses, the Guidelines for Canadian Drinking Water Quality speak to a minimum 4-log reduction of viruses in treatment systems. Since most groundwater systems don't have subsequent treatment (because of their inherent safety), one could attribute a 4-log reduction in soil to being adequate as well. Given all the various parameters that must be considered to determine viral inactivation, it's nearly impossible to say "this setback distance throughout B.C. will lead to a 4-log reduction in viruses".

The summary of this study was that a) the setback distance between the well and the hotel's septic system was inadequate to provide safe drinking water, and that subsequent treatment was necessary, and b) that a lot more research needs to be done to determine how various soil chemistry factors affect wastewater treatment in vivo. Simply saying "it's 30m away, so it's safe" isn't an adequate means of protecting public health, without considering what happens in those 30m.

Source: Gunnarsdottir, M.J., Gardarsson, S.M., & Andradottir, H.O. (2013). Microbial contamination in groundwater supply in a cold climate and coarse soil: case study of Norovirus outbreak at Lake Mÿvatn, Iceland. Hydrology Research, 44(6), 1114-1128.


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