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

Abstract

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.