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Seasonal and diel variation in greenhouse gas emissions from septic system leach fields

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Approximately one-fifth of US households rely on septic systems for wastewater treatment. Despite their prevalence, only a handful of studies quantify greenhouse gas (GHG) emissions from residential septic systems. Recent work demonstrated that carbon dioxide (CO2) emissions from a soakaway dispersal system vary seasonally and diurnally. Similar work does not exist for other GHGs or for GHG emissions from leach field dispersal systems. This study asks the question: Do nitrous oxide (N2O), methane (CH4), and CO2 emissions from septic system leach fields vary seasonally or diurnally, and if so, do these variations impact current flux estimates? N2O emissions from the leach field were significantly greater than from control lawn for most of the summertime (June–August; p < 0.05 for all sampling dates, except mid-July p = 0.067), but dropped to a level comparable to the control lawn emissions in the fall (September–December; p > 0.05 for all sampling dates). The time of sampling also impacted CH4 and N2O emission levels. CH4 leach field emissions became significantly greater than control lawn emissions at the 22:00 sampling time (p = 0.042). N2O leach field emissions were comparable to control lawn emissions at night (p > 0.05 at all night sampling times). CO2 leach field emissions were never significantly different from control lawn emissions. These results indicate that annual emissions of N2O and CH4 from septic system leach fields cannot be calculated using one constant flux rate, as is current practice.

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We thank K. K. Cantilina, C. Fernandez-Baca, E. Gardner, S. K. Leung, N. R. Morse, B. G. Rahm, R. E. Richardson, Y. Seo, S. A. Sternberg, and M. Y. Szeto for their help with experimental design, fieldwork, analysis, and editing the manuscript. Special appreciation is extended to all the homeowners who volunteered to be a part of this study.

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Correspondence to A. M. Truhlar.

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This study was prepared for NYS WRI and the NYSDEC HREP, with support from the NYS Environmental Protection Fund, under Service Contract Number C009452.

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Editorial responsibility: Josef Trögl.

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Truhlar, A.M., Ortega, K.L. & Walter, M.T. Seasonal and diel variation in greenhouse gas emissions from septic system leach fields. Int. J. Environ. Sci. Technol. 16, 6043–6052 (2019).

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