Identifying A Suitable Septic System Through Soil Testing
When determining if a building site is suitable to hold a septic system a site investigation must be conducted. This includes evaluating and testing the soils for structure, texture, consistency, depth and permeability.
Imagine that you are ready to build your dream home. You’ve found the ideal plot of land, and you’ve envisioned how your home will overlook forest and field and the lake in the distance. The location you’ve chosen is remote, because space and quiet, away from the crowds of the city, is part of the dream. All of which makes it worth the time, energy and cost of building your dream.
Now, first things first. In British Columbia, if your planned home will need a septic system because there is no public sewer available, you’ll need a septic approval, and the resulting Record of Sewerage System, before a building permit can be issued.
Septic approval means that it will be possible to install a septic system at that location and that there is a plan for a functional system that will work on that specific site. Soil and percolation or “perc” testing, the subject of this article, is the preliminary step in evaluating the suitability of a potential home site for a septic system and determining the most appropriate system design for that particular site.
Soil and percolation testing is all about understanding the makeup of the land on which the septic system is to be installed. Soil is amazing in its diversity and complexity. It’s much more than just “dirt” outside your window. Soil has texture, structure and order, mineral diversity and biological diversity, water-holding capacity and a range of other distinguishing properties. In fact, soil is diverse enough to warrant classification systems to help organize and understand their properties. The diversity of soil means different things for different people. If you’re a farmer, you want soil that is rich in suitable nutrients for crops. If you’re an engineer, you need to know how the structure you’re building will be supported. And if you’re a homeowner, you’ll want to know the type of septic system, and its layout, that will be required for your home, given the soil makeup of the plot. In this case, perc and soil testing answers these questions at the start and informs the design and installation of the septic system.
Now, while we get our hands dirty with soil testing, let’s keep in mind what’s happening with a septic system that we’ve buried in our backyard. Recall from previous posts that the two major components of a septic system are the septic tank and the leaching field. Primary treatment takes place in the septic tank. That’s the primary receptacle for household wastewater, where solids settle to the bottom of the tank and are broken down by bacteria and other microorganisms, while lighter-than-water fats, oils and grease floats on the top as a “scum layer.”
The separated wastewater, or effluent, then flows out of the septic tank and into the leaching field, where it is allowed to percolate through the soil. Here, soil bacteria and other microorganisms break down suspended solids and kill harmful microorganisms in the effluent, so that fully treated water returns to the water cycle, to
groundwater and surface water.
It’s important to keep in mind that the treatment that occurs in the septic tank, while critical to the treatment process, does not result in fully treated effluent. The treatment process that occurs in the leaching field is the critical final step in treating the effluent, and this is why testing and characterizing the soil make-up of a property is so important.
Wastewater Treatment in Soil
Treatment of effluent in soil is dependent on two factors: the time that effluent will spend in the soil, and the available microbial population of the soil that will act to break down material in the effluent, whether it is suspended solids or harmful bacteria. The time factor will depend on the physical makeup of the soil, and the microbial factor will depend on the organic and biological makeup of the soil. Soil and perc testing provides quantifiable measures of each of these factors and is used to design the most suitable septic system for a particular plot of land.
To understand how and why those factors are important, we have to understand a little about soil itself. Soil scientists order and characterize soil just as biologists order and characterize plants and animals. That is, they group them according to similar properties, how they behave, their origins and relationships with each other and their particular place in an ecosystem. Soils are classified by texture and colour. Soil texture comes from the three major components of all soils – sand, silt, and clay – so that soils can be characterized as “sandy clay” or “silty loam” for example. Soil colour gives an indication of its geological origins and mineral make-up.
Soil texture has a big impact on percolation rate – the measure of how quickly water moves through the soil. Remember, for effluent to be treated effectively and efficiently in the soil, it has to move slowly enough to allow for the biological and chemical breakdown of solids and pathogens, but not so slowly that the system can’t keep up with household demand. Goldilocks would understand; percolation rate has to be “just right.”
Physical texture describes the spaces between individual grains of soil that will allow water to flow downwards through the various soil horizons. Think about a container of rice compared to a container of flour. If you pour water over the rice it will quickly flow through to the bottom – that’s like very sandy soil. However, if you pour water over the flour, it will tend to just sit there for quite a while before soaking through – that’s like soils with high in clay content.
This video tutorial shows how we texturize the soils and evaluate the consistency:
That represents the two extremes of percolation rates. In sandy soils, the soil grain is relatively large and the arrangement of grains is more random which means that channels and spaces between the grains are pretty large. So, water percolates or travels through sandy soils pretty quickly. Conversely, clay particles are very fine and stack together fairly closely, so the spaces between are smaller and more sparse, meaning that water travels within and through clay soils very slowly. In fact, clay soils can form a significant barrier to water penetration, leaving soil above it waterlogged.
Aside from the physical structure of soil, the presence of organic matter in the soil is another critical factor in determining the suitability of the soil for a septic system, because it contains the microbial populations responsible for the final treatment of septic effluent. Very sandy soil, for example, has low organic matter content, and so has limited capacity to break down or degrade suspended solids or pathogens in the effluent. Richly organic soil, on the other hand, would efficiently and effectively clean up effluent to a high standard.
To sum things up so far, we need effluent to spend the right amount of time in the soil, with sufficient microbial populations to treat it. That’s the kind of information the soil and perc testing will provide and will determine if there is sufficient treatment potential at the site for a simple gravity-based system, or if alternate systems for supplemental treatment will be required. In either case, the testing will ensure that the most appropriate system for the particular site is installed. In this sense, every septic system is a custom job!
Soil and perc testing follows a systematic approach outlined in the “Sewerage System Standard Practice Manual.” The SPM is used by professionals during the design and installation process to ensure that standards are being met, and to establish appropriate maintenance schedules for a particular system. It’s intended for septic professionals and health officers, but it’s also useful for homeowners to better understand their own system, the reasons for its specific design and how to understand and effectively follow the maintenance plan.
Here is a great video tutorial on how water travels through the soil
Regulations and Soil Science For Septic Sytems
While septic installation “by the book” might make you think of bureaucracy and unthinking adherence to arcane rules, keep in mind that in going by the book we are taking advantage of the experience and expertise of hundreds to thousands of professionals. And from that, we’re getting what has been learned from years of practice and years of development, and learning from other peoples’ mistakes so that we don’t have to reinvent the wheel. So, going “by the book” means that the homeowner can have confidence that their system is going to work right the first time, and with proper care and maintenance work well for years to come. The Sewerage manual is a carefully written guide; not only does it spell out best practices, it has an entire volume devoted to the rationale behind all the regulations and recommendations, which lets the user understand the “why” of the standards, so that there is greater knowledge and motivation to do the “what.”
Now, what exactly are those people doing with their backhoes and clipboards? The first thing is to establish the proposed location of the septic tank and leaching field relative to property boundaries, drinking water sources, water bodies such as ponds and streams, and buried service lines such as power lines or drainage pipes.
They are also determining the overall topography of the land, noting the position and degree of any slopes on the property, because slopes will affect the movement of effluent through the system and therefore how the planned system is to be laid out. Having mapped out all of that, the real fun begins (because who isn’t fascinated by digging holes?)
The digging serves two purposes. The test pit, a hole up to a few meters in depth, allows the septic professionals to view the soil profile in order to characterize soil properties (up to 13 distinct characteristics such as texture, colour, structure, moisture, root depth, etc.). It will also reveal rock formations, impenetrable barriers or constraining soils, as well as the vertical separation between the proposed infiltrative surface (were the effluent leaves the distribution pipes and gravel bed of the leaching field and enters the soil) and the limiting layer, which is the point at which the effluent should be treated to a suitable quality to enter the saturated groundwater zone.
The test pit and an additional two to six holes (depending on the particulars of the site) are then used for the percolation test, to physically measure the rate that water moves through the soil at that specific location.
The hole is filled with water a few times to prepare the soil for the test, then it is filled again to a specified height. The movement of the water soaking into the ground is measured as minutes/inch of water disappearance. This physical testing of water movement through the soil profile provides a real measure of how septic effluent would behave in that location.
Once this data is in hand – the soil logs recording soil characteristics, and the perc test results demonstrating the eventual behaviour of effluent on the site – the pros can get to work designing an appropriate system for the site.
Again, the Standard Practice Manual provides benchmarks for soil types and perc test results that dictate the type of system, and system configurations that would be allowed under the standards, which in effect is determining the system that will work reliably and will last for years with proper care.
Modern septic technology is pretty advance and founded in the knowledge-base of thousands of septic professionals, health officials, soil scientists, hydrologists and other professionals. That means that even if soil and perc test results are not particularly favourable, all is not lost. In that case, they point the way to workable alternative designs for the septic system.
Hopefully, this article has helped you to appreciate the complexity and diversity of soil and its potential as an all natural “bio-reactor” to treat household wastewater. Next time you’re standing in the backyard, enjoying the view of the mountains or of the garden, take a moment to appreciate the complex, dynamic physical and biological systems situated right below your feet!