Determining the loading rate for an onsite system

November 04, 2021

By Sara Heger

The interaction among the factors discussed previously in the past articles on topography, color, texture, structure and consistence determines, in part, the loading rate for the onsite system. Loading rate is the quantity of effluent applied to the soil treatment area expressed as volume per area per unit time (e.g., gallons per square foot per day).

A site with finer texture, poor structure, poor consistence, horizons that restrict water movement, or shallow depth to a limiting condition should be assigned a lower loading rate. This will limit the amount of water that can be accepted into the soil, and thus a larger soil treatment area will be needed to accept the wastewater generated at the site. 

The ultimate loading rate for a soil treatment area is based on soil and site conditions at several locations below or adjacent to it, as shown in the figure below. The first zone of consideration (Zone 1) is the trench bottom or infiltrative surface. The amount of water this zone can accept is generally the long term acceptance rate. This rate is determined by the most limiting soil conditions adjacent to the infiltrative surface. 

Zone 2 considers additional limiting soil conditions below the footprint and the water balance including rainfall and evapotranspiration of the entire soil treatment area. The amount of water this zone accepts is often called the areal loading rate. 

The last zone evaluated considers the landscape conditions to determine the lateral or contour loading rate. More specifically, the contour loading rate is the cumulative amount of wastewater applied to the soil profile at the down gradient end of the soil treatment area installed on a slope, expressed as volume per unit length per unit time along the contour (e.g., gpd/foot). The ultimate loading rate for a site is the lowest of these three rates. 

Contour loading rate must be considered more closely on sloping sites because effluent moves laterally down hill. Because of the topography, the degree of water movement down the slope must be considered in the design. Stacking of trenches at sites with a slope greater than a few degrees could result in excessive wetness along the trenches at the downslope side of the soil treatment area. This is due to potential lateral flow above the least permeable layer in the unsaturated zone.

Where natural drainage exists, every attempt should be made to make the soil treatment area as long as possible along the drainage system. Additionally, equipment and foot traffic should be minimized in areas downslope from the soil treatment area because the resulting compaction can adversely affect permeability of the soil in that area.