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Enhanced Attenuation of Septic System Phosphate in Noncalcareous Sediments
Author(s) -
Robertson W.D.
Publication year - 2003
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.2003.tb02567.x
Subject(s) - phosphate , septic tank , infiltration (hvac) , lysimeter , effluent , carbonate , geology , mineralogy , environmental chemistry , chemistry , environmental science , environmental engineering , soil science , soil water , materials science , biochemistry , composite material , organic chemistry
Review of phosphate behavior in four mature septic system plumes on similar textured sand has revealed a strong correlation between carbonate mineral content and phosphate concentrations. A plume on calcareous sand (Cambridge site, 27 wt % CaCO 3 equiv.) has proximal zone PO 4 concentrations (4.8 mg/L P average) that are about 75% of the septic tank effluent value, whereas three plumes on noncalcareous sand (Muskoka, L. Joseph, and Nobel sites, <1 wt % CaCO 3 equiv.) have proximal zone phosphate concentrations (<0.1 mg/L P) that are consistently less than 2% of the effluent values. Phosphate attenuation at the noncalcareous sites appears to be an indirect result of the development of acidic conditions (site average pH 3.5 to 5.9) and elevated Al concentrations (up to 24 mg/L), which subsequently causes the precipitation of Al‐P minerals such as variscite (AlPO 4 . 2H 2 O). This is supported by scanning electron microscope analyses, which show the widespread occurrence of (Al+P)—rich secondary mineral coatings on sand grains below the infiltration beds. All of these septic systems are more than 10 years old, indicating that these attenuation reactions have substantial longevity. A field lysimeter experiment demonstrated that this reaction sequence can be readily incorporated into engineered waste water treatment systems. We feel this important P removal mechanism has not been adequately recognized, particularly for its potential significance in reducing P loading from septic systems in lakeshore environments.