Careful Siting of the Soil Treatment Area Protects the Environment

Our reader asked: “Where does the sewage go?” This question reminds us of the important role installers and designers play in protection of groundwater.

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We had an interesting conversation with one of our local service providers and he made a comment we didn’t not think much about at first. He said: “We do not always think about where our sewage ends up.” The more we thought about this, the more important the comment became. It fits with our overall goals to have our systems not only accept the amount of wastewater the building generates but also provide treatment to protect the environment and human health.

We spend a lot of time focusing on the infiltrative surface of the soil treatment and dispersal area, and sometimes the area right beneath the system. However, more than the infiltrative surface and development of the biomat, limiting infiltration needs to be considered when siting and installing an on-site system.

As we have stated numerous times, soil is an effective media to treat and disperse septic tank effluent. In general, if three feet of unsaturated soil is available beneath the infiltrative surface, BOD and pathogens will be removed and many of the nutrients tied up. We always characterize these treatment activities as happening before effluent is released elsewhere into the environment, such as ground water or surface waters. One major nutrient exception is nitrogen. 

In the case of nitrogen, we rely at least somewhat on dilution as well as treatment mechanisms in the soil to reduce levels below 10 mg/L (human health standard) before reaching a groundwater source. We are particularly concerned, where space is limited, and there are large numbers of systems in a small area, or where soils are uniform coarse sandy over shallow water tables or bedrock. In these situations, additional pretreatment is required before discharge into the soil treatment area.

The siting process

As effluent travels downward and laterally through the soil, it is important to consider in the design and siting process where the water ultimately discharges. If the discharge is into a water table, the treatment processes must be complete enough so there is no concentration of constituents in excess of water quality standards. It is also important to understand the location of the system on the landscape to avoid interference with water moving away from the system.

To begin, the soil treatment and dispersal area should be located away from areas where surface water flows converge. Any additions of water over the surface or subsurface provide the potential to hydraulically overload the treatment area, causing failure. The most suitable locations are on ridgelines or high on the landscape in areas with convex slopes that naturally shed surface water so it does not collect over the treatment area. 

Often, soil permeability is lower deeper in the profile. This is one reason it’s important to keep systems shallow. It takes advantage of higher rates of water and oxygen infiltration as well as increased biological activity to assist in treatment. This change in permeability is factored into the distance required between treatment trenches. We usually express this in terms of distance from the center of one trench to the center of the next trench. 

On center, distance needs to be evaluated to make sure effluent delivered to one trench in the sequence does not interfere any of the others. On sloping sites this is termed the contour loading rate. This assumes that the soil is deep enough, there is not another type of limiting condition near the surface, and water will move vertically to a regional water table.

The double whammy

What if there is a more restrictive layer closer to the infiltrative surface? Examples of these would be either a regional or perched watertable, dense more slowly permeable soil layer or bedrock. In these cases, there is the potential to mound water above these layers. This is termed groundwater mounding. 

When water is mounded into the unsaturated zone beneath a trench infiltrative surface, the treatment capacity of the soil is reduced. There is less ability of oxygen to enter around the system and the biomat will develop more extensively. This creates a double whammy for the system of reducing infiltration rates less than design numbers and reducing the ability for aerobic soil organisms to survive and assist in the treatment process.

In addition, water can move laterally away from the mounded area on top of the limiting layer — pushing the water out to the surface — if the restrictive layer meets the ground surface lower in the landscape. Partially treated effluent at the surface creates a nuisance and a threat to public health. Proper trench spacing and attention to the contour loading rate on sloping sites will help prevent or mitigate this condition; but it must be factored into the design from the start.

There are sites where the limiting layer occurs below our typical site evaluation 5- to 8-foot depths in borings or pits. It is important that treatment has occurred because often these layers do intercept the ground surface at some point or a drainageway or stream. Having a knowledge of the geology in the area will help the site evaluator/designer make proper system choices. In some areas, we see additional site evaluation requirements in state or local codes to address these possibilities. It is due to a recognition that where the sewage goes after it infiltrates is, in fact, important. 


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