Recommended septic tank soil-absorption-system densities for the principal valley-fill aquifer, Sanpete Valley, Sanpete County, Utah

The mocement and transformation of nitrogen under seven septic tank installations was studied. Samples of the effluent and of the soil solution beneath the discharge systems were taken and analysed for ammonium, nitrate and pH. Most of the nitrogen came from household toilets, and after passage through a septic tank, the nitrogen was mostly in the ammonium form. It remained in this form while it was in the ponded effluent above the slime layer in the absorption systems. Once it had passed through the slime layer into the unsaturated, aerobic soil below, it was usually oxidised to nitrate within 0.5 m. This was accompanied by a drop in pH of about two units. However, in one case in which the distance between the top of the slime layer and the water table was less than 0.8 m, ammonium was not oxidized to nitrate. This was taken to indicate that the saturated zone extending below the absorption system overlapped the capillary fringe and consequently the soil was anaerobic. Accumulation of organic nitrogen was restricted to the 0.5 m slime layer which accumulated on the sandy base of the soak wells and leach drains. The low cation exchange capacity of the soil and the high rate of conversion of ammonium in the effluent to nitrate in the unsaturated soil resulted in almost all of the nitrogen from the septic tanks entering the groundwater except that lost to plant uptake.

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Role of saturated and unsaturated zone in soil disposal of septic tank effluent

Canadian Journal of Civil Engineering ◽

10.1139/l88-091 ◽

1988 ◽

Vol 15 (4) ◽

pp. 709-716 ◽

Cited By ~ 3

Author(s):

K. R. Johnson ◽

J. W. Atwater

Keyword(s):

Oxygen Demand ◽

Ground Surface ◽

Pilot Scale ◽

Scientific Basis ◽

Separation Distance ◽

Saturated Soil ◽

Fecal Coliforms ◽

Septic Tank ◽

Absorption System ◽

Septic Tank Effluent

The guidelines for the use of the septic tank – soil absorption system (ST–SAS) in the Province of British Columbia are very specific in regard to the separation distance between the ground surface and the groundwater table (minimum 1.2 m), and between the tile field and perimeter drains or ditches (minimum 3.0 m). A pilot-scale experiment employing waterproof channels filled with a saturated soil was used to evaluate the scientific basis for these guidelines. Septic tank effluent was applied to unsaturated columns and the inlet end of the channels and samples were taken at different points in the channels. Measurements were made of total and fecal coliforms, chemical oxygen demand, ammonia, nitrate, and orthophosphate. Coliform reductions within the channels were at least 30 000-fold with total coliform numbers generally lying below 200 coliforms/100 mL and fecal coliforms generally less than 50 coliforms/100 mL. Varying degrees of nitrification occurred in the unsaturated columns, resulting in relatively high concentrations of nitrate in some of the channel sections (1–7 mg/L in channels filled with sand and 0.1–1.0 mg/L in channels filled with loamy sand). The removal of orthophosphate was greater than 90% in all of the channel sections, independent of the saturated or unsaturated zones. Reductions in measured influent parameters were substantial in all of the channels. This suggests that the guidelines may be conservative with respect to these particular soils. Of concern are the high nitrate values observed in some of the channels, therefore consideration of nitrification potential should be made in conjunction with the potential for reduction of other contaminants. Key words: septic tank, soil absorption system, saturated soil, unsaturated soil, coliforms, nutrients.

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