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Effect of Soil and Effluent Characteristics on Phosphorus Sorption in Dosed Columns¹

ABSTRACT

The effects of soil and effluent loading characteristics on long-term P retention and movement in soil columns were investigated. Six soil columns, 0.120 to 0.160 m deep and 0.20 m diam, were obtained from < 2-mm fraction of Rosewall gravelly sandy loam (Inceptisols) and Qualicum gravelly loamy sand (Inceptisols). The columns were subjected to an instantaneous daily dose, except on weedends and holidays, of three effluent treatments over a period of 609 d. The three effluent treatments varied in P concentration and hydraulic loading levels; low (7.8 mg P/L, 0.0158 m/d), moderate (15.7 mg P/L, 0.0158 m/d) and high (15.7 mg P/L, 0.0315 m/d). The soils in the columns were kept unsaturated during the experiment. Continued effluent loading resulted in P breakthrough and a decrease in P sorption by soils. In five of six columns, P sorption by soils reached equilibrium. Both pore volume at P breakthrugh (PVBT) and P sorption at equilibrium (PSEQ) appeared to be affected by preferential movement of effluent through macropores. Irrespective of the mode of effluent flow, an increase in effluent P concentration and hydraulic loading decreased PSEQ; the effect of hydraulic loading on PSEQ was, however, more pronounced. Doubling the P loading through an increase in effluent P concentration decreased PSEQ by 2 to 11%; however, a similar increase in P loading obtained by a twofold increase in hydraulic loading, decreased PSEQ by 12 and 34% in Rosewall and Qualicum soils, respectively. Strong influence of effluent flow rate on PVBT, masked the effect, if any, of soil type and effluent loading on PVBT. The implications of these results with respect to wastewater renovation capability of a septic tank drainfieid were discussed.

Key Words: phosphorus • P sorption • P movement • septic tank drainfield • wastewater leaching

¹ Contribution from Water Management Branch, and Surveys and Resource Mapping Branch, B.C. Ministry of Environment, Victoria, BC.

² Soil Specialist, Ministry of Environment, Parliament Buildings, Victoria, BC, Canada, V8V 1X5.

Received for publication July 10, 1985.