Septic tanks – out of sight out of mind?
It’s common for rural landowners to dispose of wastewater via septic tanks or similar wastewater treatment systems, largely due to the distance to reticulated wastewater networks in rural locations. Both blackwater from the toilet and greywater from showers, sinks and laundry are disposed of via these systems.
A lot of people know very little about their systems, to the point that they are unaware of the way the systems work, the location of the discharges from their systems or (in some cases) even that they have a system!
These types of systems discharge wastewater directly to ground following primary and/or secondary treatment. In older systems the discharge is often directly into gravel trenches or boulder holes, through perforated pipe. Newer systems have a network of pipes which discharge waste within mounded soils or into specifically designed sand and river gravel trenches which are effective barriers to microbial contamination entering the environment.
Research has shown that these types of sediments, along with soils, the unsaturated zone and aquifers are effective barriers to contamination as a result of various microbial removal processes. The sediments do not remove nitrates, which are simply diluted, in most cases, by water flowing through the aquifer system.
An “out of sight out of mind” attitude towards septic tanks is common, and its root lies in the fact that visible onsite issues rarely occur. However, should a system not be functioning correctly (and recent research has shown that possibly 30% of existing systems in New Zealand may have issues), then concentrations of contaminants entering the environment, and groundwater in particular, may be more significant than expected. This is especially concerning if systems are installed within close proximity to drinking water supply bores, as greater concentrations of microbial contaminants (bacteria and viruses) may reach the bores than would be expected.
In terms of installing new systems, the first consideration should be the location of the septic tank discharge compared with drinking water bores, and the depth of the source water bore. In Canterbury, groundwater broadly flows from the Southern Alps to the coast and loosely parallel to the region’s rivers. Placement of septic tanks down gradient of drinking water bores is recommended. If this is not possible, then maximising the distance between the bore and the discharge location (both laterally and vertically) is important.
New systems are only being consented after a thorough investigation into the potential effects of the system on the environment and drinking water sources. This may require a site survey to assess the suitability of the discharge location, including both above and below ground characteristics (soil, unsaturated and saturated zones) to assess the potential of these sediments to act as barriers to contamination, and therefore the suitability of the site for the proposed system. If a proposed system is in an area where there is a public supply bore, or in an area where nitrate concentrations are already elevated, then the level of scrutiny will be higher than for areas where there is less concern about impacts.
Once in place, regular servicing of systems is recommended. System providers will often have recommended servicing regimes to maintain the effectiveness of systems, and these should be seen as necessary, not optional. Any obvious ponding of effluent at the land surface needs to be addressed promptly.
There are many septic tank systems out there, and the overall cumulative effect is somewhat unknown. To protect human health and the environment, it’s important to ensure that your system is working effectively and is located optimally.
