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 NO₃⁻leaving 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.