that under unsaturated flow conditions, bacteria can be removed

within .9 to 1.2 m of effluent flow through the soil.

     Present site criteria that must be met for a septic tank

system approval include a specified percolation test, and a minimum

of 1.2 m separation between the bottom of the seepage system and

the maximum seasonal elevation of groundwater (Canter and Knox,

1984). This is required so the unsaturated soil has a high ability

to remove Total Suspended Solids, BOD, COD, and soluble organic

carbon with a 75-95% reduction occurring in these concentrations

typically within the first five feet of soil.

     Unfortunately, the unsaturated flow of septic tank effluent

increases the chance of nitrate contamination of groundwater. The

principal sources of nitrogen in wastewater are feces and urine

which contain urea, uric acid, ammonia, undigested proteinaceous

materials and bacterial cells. Typically, 75% of the nitrogen in

septic tank effluent exists in the ammonium form and 25% exists in

the organic form (Canter and Knox, 1986). Most of the ammonium is

biologically converted to nitrate as the wastewater moves through

the unsaturated soil beneath the crust of the soil absorption

system. Walker et. al. (1973) studied five subsurface seepage beds

in which the subcrust contained 19.6% oxygen. They concluded that

nitrification of ammonium to nitrate was essentially complete and

commenced in the unsaturated subcrust soil within about 2 cm of the
crust.    The ammonium levels were relatively high beneath the

seepage beds but decreased to low levels within a few centimeters.

The general increase in NO3-N with depth concurrent with the