Clay soil turns to a hardpan when introduced to excessive amounts of sodium. In one year a 4 person household will discharge the equivalent of 56 pounds of salt, which is made of sodium chloride.
Household cleaning and laundry products also contain sodium which makes its way to the drain field.
Discharge of water softener backwash into septic drain fields can also contribute to the problem.
Over time all of this combined will affect the soil's ability to absorb water.
Clay particles are smaller than 0.004mm and are shaped like flakes or plates. This means that when these minute particles come in contact with sodium in wastewater, they stick together and create an impenetrable hardpan layer.
Broken pipes, solid objects in drain lines, roots interfering with pipes.
Presence of sodium bonds clay particles to form a waterproof hardpan barrier.
Flooded conditions kill air-dependant bacteria in the soil, which keep the bio-mat in check.