Mass balance and isotope effects during nitrogen transport through septic tank systems with packed-bed (sand) filters

Science of the Total Environment
By: , and 



Septic tank systems are an important source of NO3 to many aquifers, yet characterization of N mass balance and isotope systematics following septic tank effluent discharge into unsaturated sediments has received limited attention. In this study, samples of septic tank effluent before and after transport through single-pass packed-bed filters (sand filters) were evaluated to elucidate mass balance and isotope effects associated with septic tank effluent discharge to unsaturated sediments. Chemical and isotopic data from five newly installed pairs and ten established pairs of septic tanks and packed-bed filters serving single homes in Oregon indicate that aqueous solute concentrations are affected by variations in recharge (precipitation, evapotranspiration), NH4+ sorption (primarily in immature systems), nitrification, and gaseous N loss via NH3 volatilization and(or) N2 or N2O release during nitrification/denitrification. Substantial NH4+ sorption capacity was also observed in laboratory columns with synthetic effluent. Septic tank effluent δ15N–NH4+ values were almost constant and averaged + 4.9‰ ± 0.4‰ (1 σ). In contrast, δ15N values of NO3leaving mature packed-bed filters were variable (+ 0.8 to + 14.4‰) and averaged + 7.2‰ ± 2.6‰. Net N loss in the two networks of packed-bed filters was indicated by average 10–30% decreases in Cl-normalized N concentrations and 2–3‰ increases in δ15N, consistent with fractionation accompanying gaseous N losses and corroborating established links between septic tank effluent and NO3 in a local, shallow aquifer. Values of δ18O–NO3 leaving mature packed-bed filters ranged from − 10.2 to − 2.3‰ (mean − 6.4‰ ± 1.8‰), and were intermediate between a 2/3 H2O–O + 1/3 O2–O conceptualization and a 100% H2O–O conceptualization of δ18O–NO3 generation during nitrification.

Publication type Article
Publication Subtype Journal Article
Title Mass balance and isotope effects during nitrogen transport through septic tank systems with packed-bed (sand) filters
Series title Science of the Total Environment
DOI 10.1016/j.scitotenv.2008.08.036
Volume 407
Issue 1
Year Published 2008
Language English
Publisher Elsevier
Contributing office(s) Toxic Substances Hydrology Program
Description 9 p.
First page 324
Last page 332
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