If there's one thing that interests me, it's the intersection between ecology and human health. There's a fascinating link between our interactions with the world in which we live, and public health outcomes. In the most straightforward sense, these interactions manifest themselves in things like drinking water quality and outdoor air quality. But there can also be considerations for things like freshwater ecology and wildlife health. Environmental health often looks at how changes to the environment can affect public health, but rarely look at how public activities can affect the environment (mostly because it's out of scope).
There's value to looking at ecology and environmental indicators without thinking directly about public health, however. In the early part of the millennium, 80% of creeks surveyed in the USA were found to be contaminated with "environmentally relevant" concentrations of compounds that were indicative of sewage contamination; I would suspect the number to be similarly high in semi-populated areas of Canada (either those within proximity to large communities, or those within proximity to a large number of onsite wastewater systems). Our sewage treatment systems are still designed to remove (mostly) contaminants of biological concern, and are unable to adequately remove contaminants of emerging concern, such as pharmaceuticals and "hormonally active compounds". While the long-term risk of these compounds to human health is relatively unknown, there is certainly an environmental impact to them making their way into lakes and streams.
A study by the North Carolina Water Science Center looked at water quality in a number of small, freshwater streams to determine how effective on-site and centralized wastewater treatment methods are, and to identify potential new indicators for sewage contamination. They looked at a number of sites, and ensured they had representative samples from areas served by municipal wastewater systems and onsite systems, and a control with no houses nearby. Using GC/MS, they looked for a total of 33 pharmaceutical compounds (in addition to normal fecal coliform analysis) to identify sewage contamination of the freshwater streams.
In general, and somewhat surprisingly, the researchers found that "properly functioning onsite wastewater" systems really had no effect on the water quality in freshwater streams. The "properly functioning" part is important, since one of the sites they looked at (which showed "effects of wastewater") was close to a "suspected" sewer line leak. Though they found that some compounds were prevalent in higher concentrations in the sites serviced by onsite wastewater, the site with the lowest concentration was actually one serviced by an onsite system.
Given the small sample size (just 7 sites) , and the relative lack of information provided on what specific "onsite" treatment methods were being utilized, I'm not sure I'd be willing to take this data to the Ministry of Environment as proof that there's no risk to freshwater streams from nearby sewerage systems. However, it is nevertheless interesting that the onsite systems were found to be equally as effective at treating wastewater as the larger, centralized systems. From experience, I know how difficult it is to link stream contamination with a specific sewerage system or event. Unless there's only one in the area, there's always a factor of doubt in your head that any contaminants you find are actually due to some other system. True background data or control sites are exceedingly difficult to find and match to your sample sites, but the researchers here did a decent job of identifying a freshwater stream with a low chance of being impacted by sewage.
Personally, I think that one of the most interesting things that came out of this research was the identification of a potential indicator for wastewater contamination. The researchers state that "optical brighteners" were strongly correlated with the presence of wastewater, and therefore have value in identifying sources of contamination. These are the compounds that you find in your laundry detergent that make your "whites whiter" (and aren't bleach). They're relatively inexpensive to test for, since they fluoresce at a specific wavelength, and could provide a quick and dirty methodology for "yes/no" contamination data. However, it's worth pointing out that other (naturally occurring) compounds fluoresce at the same wavelength, so one wouldn't want to hinge a court case on the presence of optical brighteners identified through fluorescence, unless one first removed any source of that background organic material.
While research like this isn't going to cause a sea change in the way we identify contamination from sewerage, every study helps to change the way we view the interactions between our waste disposal and our environment. It's clear that simply looking at coliform bacteria is no longer an adequate means of assessing whether our wastewater treatment is effective; we must recognize that there are new contaminants in our wastewater, and identifying their prevalence is the first step to determining how to remove them.
Ferrell, G.M., & Grimes, B.H. (2014). Effects of centralized and onsite wastewater treatment on the occurrence of traditional and emerging contaminants in streams. Journal of Environmental Health, 76(6), 18-27.