How deep does the E. coli go?

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.

Can an inappropriate septic setback affect more than just your health?

Regardless of what your level of education is, you likely know that if your septic system isn't far enough away from your drinking water / house / swimming pool / driveway, you're going to have a bad time. There are other factors to take into consideration, of course, like soil composition, pre-discharge sewage treatment, and potential break-out areas, but in general you want to keep your septic system far away from the rest of your life.

There are certain geographic areas where a traditional on-site sewerage system just isn't going to be feasible because of high water tables, or poor soils, or just small lot sizes. In B.C., the Sewerage System Regulation is fairly outcome based: maintain a 30m setback from a well, hire an "authorized person" to do the work, and you'll be just fine. It's up to the professional expertise of the authorized person to determine appropriate setbacks and siting for the sewage systems. If you're in one of these tough geographic areas, your course of action is typically a) hire an engineer to design you a treatment plant that treats the effluent before it goes into the soil, or b) find an Onsite Wastewater Practitioner who's willing to fudge some data to get your system in the ground.

Apparently, there are considerations besides just public health when you're looking to place your septic system in a less-than-ideal location. In Ohio, only 6.4% of soils have the requisite 4' vertical separation to allow for traditional tank-and-field on-site sewerage systems. As mentioned above, there are alternatives to traditional systems, but those typically cost a fair bit more money and require some professional input (which also doesn't come cheap). A recent study has shown that costs associated with inadequate sewerage systems can include repair costs, human health costs, increased system maintenance costs, and a "loss of property valuation".

The study looked 800 on-site sewerage systems (out of a possible 22000 in Licking County, OH), of which 616 used traditional tank-and-field systems. They only looked at those for which appropriate soils data was available, leaving them with a sample size of 549 properties for the study. Using the hedonic pricing method, which "uses the different characteristics of a traded good, such as real estate, to estimate the value of a non-traded good, such as water or soil quality", the researchers were able to identify the effect that a well-functioning septic system had on property values in the county.

The researchers looked at a number of variables that could have an effect on property value (property size, number of bedrooms, etc.), but their specific hypothesis was looking at the quality of soils with regard to sewage disposal. They found that a property with soils suitable for a traditional leach field system were worth $14,062 more than a comparable property with soils unsuitable for an onsite sewerage system. Properties with soils suitable for a mound system were worth $12,897 more. This correlated to a difference of 6.8% and 6.2%, respectively. Interestingly, these price differences were actually higher than the cost in Ohio of installing a drip irrigation or mound system.

While the research outcome does a fantastic job of tying economic benefits in with good soil profiles, it's worth noting that the median value of a housing unit in the county of study was $110,700. As of January, 2014, the median value of a housing unit in the North Okanagan (where I live) is $238,750, or 2.2x higher. It would be valuable to look at similar statistics in places with higher housing costs (where onsite sewerage is prevalent, unlike Metro Vancouver) to see if similar correlations between quality soils and housing values could be found.


Source: Vedachalam, S., Hitzhusen, F.J., & Mancl, K.M. (2013) Economic analysis of poorly sited septic systems: a hedonic pricing approach. Journal of Environmental Planning and Management, 56(3), 329-344.

Are there better methods for identifying sewage than coliforms?

Typically when health or environment officials are looking for confirmation of sewage contamination of a water source, they'll go with indicator organisms as evidence. By sampling the water and looking for fecal coliforms, you can tell whether it has been contaminated by bacteria that typically reside in the gut of warm-blooded animals. There's also fluorescein dye if you're looking to confirm that sewage isn't staying in the ground, but that's only effective if the septic failure leads to wastewater being discharged to the ground surface. In other words, if the sewage is making its way into an aquifer, you're not going to see the dye.

There are a couple of problems with fecal coliforms as indicator organisms: they're not necessarily confirmation of human sewage (i.e. just confirmation of some sort of fecal contamination) and they're not overly persistent in the soil. If you're just trying to tell a water system operator that they've got some contamination issues and need to issue a public notification, they work just fine. But if you're looking to confirm that some actual sewage is getting into the water, you're going to have a hard time in front of a judge.

Researchers from Ontario looked at a wastewater plume from a septic field serving a campground that had been in existence for around 20 years (which, incidentally, is about the life span of the average on-site sewerage system), and note that typical indicators of contamination (besides coliforms) are not necessarily unique to sewage, and therefore don't make the best indicators. Chemical compounds that are unique (ibuprofen, pseudoestrogens, carbamazapine) haven't been studied enough to give a clear indication of how long they persist in the environment. They suggest that artificial sweeteners might have value as a wastewater indicator, since they're unique to human waste, resistant to breakdown in normal sewage treatment, and persist in groundwater.

By setting up a number of piezometers and trace gas sampling points along the wastewater plume from the campground, the researchers were able to not only sample the groundwater for the contaminants of interest, but were also able to perform tritium/helium age dating to identify the age of the wastewater plume. Unsurprisingly, their study showed that once you got about 50m away from the sewage dispersal field area, nutrients and pathogens normally found in sewage were reduced to non-detectable levels. Of the sweeteners tested for persistence, they found that cyclamate and sacharrin appeared to degrade quite effectively, while acesulfame and sucralose concentrations remained relatively constant regardless of distance from the septic tank.

Since the acesulfame was detected in levels nearly 1000x higher than background concentrations in the wastewater plume, and degradation didn't occur over approximately 20 years of sewage system use, it presents itself as a potentially viable indicator for wastewater contamination. Apparently, acesulfame is also added to some animal feed, so it could be used as an indicator of groundwater contamination from manure spreading as well.

There's still some further work to be done, since this is just one study of one onsite sewerage system. However, it shows great potential for a new way of determining whether aquifers are being impacted by nearby wastewater. It's worth noting that, from a public health perspective, there is always the issue of cost when speaking to new indicators. The current culture sampling for pathogens is relatively inexpensive, and provides a "good enough" method of identifying contamination. Moving to a compound that requires some analytical chemistry for identification may just be simply too expensive for publicly funded environmental health organizations.

Source: Robertson, W.D., Van Stempvoort, D.R., Solomon, D.K., Homewood, J., Brown, S.J., Spoelstra, J., & Schiff, S.L. (2013). Persistence of artificial sweeteners in a 15-year-old septic system plume. Journal of Hydrology, 477, 43-54.