By Sara Heger, Ph.D.
Maintaining natural soil structure is critical when installing a soil treatment area for an onsite system. All excavation to the infiltrative surface or surface preparation must be done so that the original soil structure is not smeared or compacted.
Generally, the soil located at or near the soil surface is the best soil for treatment and dispersal due to its structure and oxygen-transfer potential. In addition, evapotranspiration and natural biological activity is greatest near the surface. Compacted soil has reduced void space; therefore, compacted sites are less permeable and are more likely to have problems with water movement.
To limit compaction in the STA, do not drive excavation equipment or other vehicles on the infiltrative surface. Foot traffic on these areas must also be minimized to reduce the risk of compaction. Raking sidewalls of trenches and beds may help expose the natural soil structure which may have been smeared by the bucket. From upslope of the system, place the media using low ground pressure equipment to avoid compaction. Working upslope is beneficial as you keep the equipment off the downslope portion of the system. Take materials directly from the dump truck to the system to limit material contamination.
If a site is compacted there is no guaranteed method to rectify the problem. If possible, move the location of the STA to an area that has not been compacted. If that isn’t an option, discuss the options with the system designer and permitting authority. Various methods have been tried after severity of the compaction has been determined. For shallow depths of compaction the natural freeze/thaw cycling, root activity and weathering may help reduce the impacts of the compaction. When the compaction is more severe, experimental methods include making the STA larger, mechanically fracturing the soil, deep plowing and ripping, and complete removal and backfilling with appropriate fill material.
Clay soils that are too dry when worked may be smeared by the bucket. When excessively dry soil is manipulated and exposed, fine soil particles can be separated from each other and stratified into layers or even become airborne. Stratification can negatively influence the movement of wastewater in the STA. Avoid blowing soil across a job site because some may be deposited, adding unwanted fine material to media on the site.
To determine if a soil is too dry, start by feeling it. If it is a loam or silt soil and feels like talcum powder, it is likely too dry. If a dry clay soil gets shiny or glazes when being excavated, it is also likely too dry. If the site is too dry, you can either wait for rain or apply a limited amount of water over the area. Application of water should be done very carefully to make sure the site doesn’t get too wet.