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This article in JEQ

  1. Vol. 41 No. 5, p. 1437-1444
     
    Received: Nov 11, 2011
    Published: September 14, 2012


    * Corresponding author(s): wroberts@sciborg.uwaterloo.ca
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doi:10.2134/jeq2011.0427

Phosphorus Retention in a 20-Year-Old Septic System Filter Bed

  1. W. D. Robertson *
  1. Dep. of Earth and Environmental Sciences, Univ. of Waterloo, Waterloo, ON, Canada N2L 3G1. Assigned to Associate Editor Paul DeLaune

Abstract

Septic systems in lakeshore environments often occur where thin soils overlie bedrock and, consequently, filter beds may be constructed of imported filter sand. The objective of this study was to assess the mobility of wastewater phosphorus (P) in such a potentially vulnerable setting by examining a 20-yr-old domestic septic system located near Parry Sound, ON, Canada, where the filter bed is constructed of imported noncalcareous sand. The groundwater plume is acidic (pH 6.0) and has a zone of elevated PO4–P (up to 3.1 ± 1.7 mg L−1) below the tile lines but no elevated PO4–P is present beyond 5 m from the tile lines. Elevated concentrations of desorbable P (up to 137 mg kg−1) and acid-extractable P (up to 3210 mg kg−1) occur in the filter sand within 1 m below four of seven tile lines present and the total mass of excess acid-extractable P (39 kg) is similar to the estimated total lifetime P loading to the system (33 kg). Microprobe images reveal abundant Fe and Al-rich authigenic mineral coatings on the sand grains that are increasingly P rich (up to 10% w/w P) near the tile lines. Additionally, 6 yr of monitoring data show that groundwater PO4 concentrations are not increasing. This indicates that mineral precipitation, not adsorption, dominates P immobilization at this site. This example of robust long-term P retention opens up the possibility of improving P removal in on-site treatment systems by prescribing specific sand types for filter bed construction.

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Copyright © 2012. Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.

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