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Long-Term Phosphorus Sorption in a Brunisol in Response to Dosed-Effluent Loading¹

ABSTRACT

Phosphorus sorption characteristics of a major soil in the Shawnigan Lake area of Vancouver Island were studied in response to periodic secondary effluent loading. Five soil columns 0.60 m long and 0.139 m in diam were prepared from the B horizon of the Shawnigan gravelly silt loam (classified as a Dystric Brunisol and Typic Dystrochrept according to the Canadian and U.S. Systems of soil classification, respectively). Water table level was maintained at the base of the two partially saturated columns (PSC). The other three columns (USC) remained unsaturated at all times and were subjected to a tension of 6.0 kPa. A single, daily dose of 1 L (6.6 cm) of secondary effluent was applied to all columns except on weekends and holidays.

The effluent P was retained effectively by the USC columns initially, but broke through with time as in case of the PSC columns. The leachate P concentration reached an equilibrium value (EPC) after about 70 pore volumes of effluent had passed through the columns. The EPC changed only slightly beyond 70 pore volumes, but was a function of effluent P concentration and percolation rate. About 60 to 90% of added P was retained by the columns, even after 2.7 yr of leaching. Surface layers were the most effective renovators of effluent P; however, the total amount of P retained by the soil was dependent more on effluent percolation rate and leaching period, and less on water table level.

Key Words: phosphorus movement • septic tank drain field • groundwater pollution

¹ Contribution from Water Management Branch, B.C. Ministry of Environment, Victoria, British Columbia.

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

Received for publication June 9, 1984.