What causes clay soil to turn to hardpan?

A household of 3 to 4 people using an on-site septic system will discharge the equivalent of about 56 pounds of table salt (sodium chloride) into the drain field soil each year. Within 4-10 years, this sodium discharge begins to affect the ability of drain field soils to treat and absorb domestic wastewater.

Sodium found in household laundry, kitchen, bath and cleaning products makes its way to drain field and is a primary source of soil failure. Additionally, discharge of water softener backwash in to septic drain fields and sodium content in local water supplies can also contribute to the problem.

Research pinpoints an old problem

A ten-year study by Dr. Robert Patterson contains a very thorough study of the contribution of sodium to septic system soil failure. Dr. Patterson's work sheds new light on the influence of modern products on septic system drainage soils. The detailed records and scientific laboratory evaluations provided in this outstanding scientific work give us clear insights into problems long noted by leading scientists over the years. After ten years of thoroughly documented research, Dr. Patterson concludes: "The inevitable consequence of continual addition of sodium in septic tank effluent is a decrease in the soil's hydraulic conductivity leading, in many cases, to drain field failure."

If a grain of sand were the size of a basketball, then a piece of silt would be the size of a marble, and a particle of clay would be a pinpoint. Clay particles are tiny, less than one 12,500th of an inch. These tiny specks are "surface active" with contaminants found in wastewater, but the problem is that they are shaped like plates or flakes. When sodium is present in wastewater passing through these tiny clay particles, they tend to stick together forming hardpan conditions in the soil.

Three causes of septic system soil failure

Septic system drain field failure may be physical, chemical or biological.

1- Physical failure can be caused by crushed or broken pipes; tree roots blocking pipes or solid objects in the lines.

2- Chemical failure can occur when sodium causes fine clay particles found in the soil to bond into a waterproof barrier, which in turn causes the physical flooding and blockage of soil passages.

3- Biological death of air-dependent cleaning organisms (aerobic bacteria) in the soil occurs when soils flood.

Agricultural soil and wastewater scientists have long recognized that in time, sodium in irrigation waters will cause finer soil particles to bond together into impermeable layers. In agriculture, this chemical change causes physical or structural changes in the soil, which ultimately leads to loss of biological uptake of plant nutrients.

As far back as the 1950s many field studies of septic system chemistry and soil structure have documented the effects of high sodium levels in domestic wastewater and the negative impact on septic system soils.