As we have written in the past, the design loading rate in terms of gallons per day per square foot (gpd/sq.ft.) is determined by the soil property’s texture, structure and consistence. This combined with an estimate of daily water use determines the area in square feet needed for the soil treatment area.
Loading rates at the infiltrative surface take into account development of the biomat. The term adopted for this loading rate is the Long Term Acceptance Rate (LTAR). Other factors, though, determine how water moves out of and away from the soil treatment area. As systems become larger in size, these factors become more important from both a design and installation perspective. They also have a large impact on the amount of treatment that will occur.
SEPARATION DISTANCE
One factor is simply the separation distance from the infiltrative surface (bottom of the soil treatment trench) to some type of limiting layer. This layer can be bedrock, seasonally saturated soil, dense soil conditions or a significant change in soil texture.
Required separation distance is specified in state and local codes, depending on the level of safety selected to provide for treatment of pathogens. In Minnesota, our separation number is 3 feet, which was based on early research showing pathogen removal within 2 feet of soil if flow was unsaturated and did not exceed a loading rate of about 1.2 gpd/sq.ft.
Soil considered non-limiting between the bottom of the trench and a limiting layer should also be evaluated in terms of estimated LTAR. If there is a layer or horizon below the infiltrative surface of the trench with a slower estimated LTAR, an evaluation should be made as to whether water that leaves the trench will infiltrate into and through this layer without mounding water underneath the trench, thus creating a saturated condition that would interfere with treatment or impact an adjacent trench.
A simple example to consider is a series of trenches on a level site. The soil at an infiltrative surface has an LTAR of 0.6 gal/sq.ft./d and there is a soil layer below that has an LTAR of 0.4 gal/sq.ft./d. For 1 lineal foot (lf) of a 3-foot-wide trench, the total load is 1.8 gal/lf of trench. This means the slower soil will take 1.5 times more area to infiltrate the effluent, so instead of infiltrating 1.8 gallons in 3 feet it will take 4.5 feet.
Water will infiltrate from the trench and be moved down by gravity until it encounters the slower soil, where it will spread out laterally 4.5 feet before it all moves into the layer below. On a level site, water would move out both directions away from the trench because the primary downward force would be gravity.
CONSIDER LANDSCAPE LOCATION
Since most trenches are installed 7 feet on center, there should be enough area under and around the trench to have the water move into the slower layer without impacting the next trench. Now consider a layer that has an LTAR of 0.2 gal/sq.ft./d. In order for the 1.8 gallons to infiltrate, it will take three times the distance; so instead of 4.5 feet it will take 9 feet. Now the trenches are located too close together and the spacing should be changed to 10 feet on center at a minimum to make sure the trenches do not interfere.
When a series of trenches is placed on a slope, landscape location must be considered. This is where the contour loading rate is something that may need to be evaluated. Let’s think about two 3-foot-wide trenches placed 7 feet on center on a slope. Water will move out of the trenches downward due to gravity, but also laterally down the slope. In the area of the trenches with the same 0.6 LTAR soil, the loading rate along the contour is 3.6 gallons per lineal foot. There is now the danger of “stacking” water up along this contour over the limiting layer. For the layer with a 0.2 LTAR, there has to be 18 feet under and downslope from the trenches to infiltrate the water.
If the surface of the slower layer runs parallel to the land surface, this is probably not a problem. But think of the areas you have seen: bedrock layers where it intersects (outcrops) downslope or seasonally perched water that intersects with the toe of the slope. Now it is more critical that the water either infiltrate before it reaches these points in the landscape or has passed through enough unsaturated soil before it exits to ensure treatment.
For these reasons, we always suggest that the soil treatment part of the system be designed and installed as long as possible along the contour. Long and narrow is better than short and wide. For example, let’s look at a narrow bed that is 10 feet wide in the same materials. Now the loading per lineal foot along the contour is 6 gallons. Now we need 30 feet under and downslope from the bed to get the water to move away.
WORK TOGETHER FOR SUCCESS
The bottom line: A designer and installer need to know the conditions below the excavated surface. If landscape factors and loading rates are not accounted for, there can be problems with treatment and having the water stay below the surface.
