Phosphorus Contributions from OSWS

Phosphorus in septic tank effluent is derived from organic molecules and detergents and dishwashing powders. Most of the P contributions from OSWS are in the form of orthophosphate ion (PO₄^3-). Wihelm et al. (1992) reported that approximately 76% of the total P in the septic tank effluent was in the form of PO₄^3-. Once in the soil PO₄^3- removal occurs by adsorption of PO₄^3- onto hydrous oxides of iron, aluminum, manganese, and carbonate surfaces on soil particles (see Figure 2). Plant root and microbial uptake of PO₄^3- is another non-permanent way of removing P. Adsorption processes for P are intricately related to precipitation reactions. These reactions are believed to be the main cause of PO₄^3- removal in soils as complexation or precipitation occurs with very low concentration of Al³⁺, Ca²⁺, and Fe³⁺. These precipitates are known as varisite, hydroxylapatite, and strengite respectively. These precipitates are formed in oxidizing conditions, that is, when oxygen is present in the soil pores.

Phosphorus concentrations in shallow groundwater are normally low with values ranging from 0.005 mg/l to 0.1 mg/l (Reneau et al. 1989). Septic tank effluent can contain total P concentrations ranging between 7 mg/l - 15 mg/l (Wihelm et al., 1994). Regardless of these high contributions from OSWS, many studies have documented high degree of PO₄^3- sorption and complexation within the first few meters downgradient from the drainfield (Reneau et al., 1989; Weiskel and Howes, 1992; Robertson et al., 1991; Wilhelm et al., 1994).

A study performed by Reneau et al. (1989) discussed the mechanisms that drive P removal in soils based on collected evidence from different OSWS studies. Reneau et al. (1989) argued that in acid soils P complexation occurs with Fe³⁺ and Al³⁺ whereas in basic calcareous soils complexation of P occurs with Ca²⁺. This study referred to Reneau and Pettry (1976) who studied an OSWS that had been in use for 15 years. This system was built in an acid Plinthic Paleudult. This study found that compared to a control profile this soil showed large increases in P associated with aluminum and ferrous complexes. Complexation occurred within the first 0.15 m from the system. The authors reported that Al³⁺-P and Fe³⁺-P fractions increased with time after long-term exposure of the soil to the OSWS effluent.

Robertson et al. (1991) reported rapid PO₄^3- removal at two study sites in Cambridge and Muskoka, Ontario. At Cambridge, soils were described as a sand layer of glaciolacustrine and outwash deposits overlaying low permeable silt till. Phosphate concentration at the tile effluent was 8 mg/l [PO₄^3--P], this amount was reduced to 5 mg/l [PO₄^3--P] and very rapidly to less than 0.05 mg/l [PO₄^3--P] in approximately 10 m downgradient. Attenuation in the unsaturated zone was little compared to that in the groundwater zone. The unsaturated zone was oversaturated with hydroxylapatite as documented by Wilhelm et al. (1994). Thus, PO₄^3- had very few adsorption places available. The PO₄^3- attenuation zone was a zone of increasing pH and high Ca²⁺ concentrations of 88 mg/l. The authors concluded that these conditions favored hydroxylapatite precipitation. At the Muskoka sites soils were fine fluvial sands occurring to a depth of more that 10 m and overlying a granitic bedrock layer. The tile effluent was 13 mg/l [PO₄^3--P] but before the effluent reached the groundwater the plume exhibited PO₄^3- concentrations of <0.1 mg/l [PO₄^3--P]. the unsaturated layer was 3 m and a 99% reduction was observed within the first 2 m. The calcium concentration in this soils was not very high, only 44 mg/l and the pH was 5.1. Robertson et al. (1991) argued that precipitation of strengite or varisite are the driving processes in PO₄^3- removal. Background concentrations of iron and aluminum ions were not reported.

Reneau et al. (1989) suggested that the enhanced ability of P removal in such short distances is caused by soil regeneration of Al³⁺, Fe³⁺, and Ca²⁺. This suggestion was based on evidence from several studies that found an enhanced sorption capacity of the soil (Ellis and Erickson, 1969; Sawhney and Hill, 1975, Uebler, 1984). In addition, Reneau et al. (1989) argued that field data that suggested an enhanced P removal was in discrepancy with laboratory data on sorption maximums as evidenced by Whelan and Barrow (1984). Whelan and Barrow (1984) observed that P sorption characteristics obtained in the laboratory agreed to the amount of P present in the field around an OSWS. However, the quantity of P accumulated in the soil was higher than the one predicted from the equilibrium isotherms for the septic tank plume. Laboratory sorption experiments predicted P horizontal movement of 5 m while only 135 cm was observed in the field.

Saturated conditions might dissolve PO₄^3- precipitates formed in acid soils. Thus PO₄^3- ion desorbs and becomes available for subsequent adsorption and precipitation. Studies such as Carlile et al. (1981) and Cogger and Carlile (1984) support this theory. These studies monitored total P in seasonally to continuously saturated soils in Craven, Hyde, and New Hanover counties in the lower Coastal Plain of North Carolina. The soils found in the study sites were of coarse, silty , fine, and organic texture. Seasonally saturated OSWS exhibited lower P concentrations than saturated systems. Saturated systems P concentrations exceeded 1.0 mg/l [P] for wells located 5 ft away from the drainfield. However there was no difference in P concentrations between seasonally saturated and continuously saturated systems for wells located at 25 ft and 50 ft away from the drainfield.

Please address any questions to Dr. David Lindbo.

This page (http://www.ces.ncsu.edu/plymouth/septic/98cardonaphos.html) created by
Vera MacConnell, Research Technician, I on March 1, 1999.
Last Updated on 6/29/00 by Roland O. Coburn,Research Tech. I.