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.
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
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.
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
Clay particles magnified by an
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.
causes of septic system soil failure
system drain field failure may be physical, chemical
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
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.