Bacterial Movement in Subsurface Soil during Winter Irrigation of Reclaimed Wastewater

Processes to remove and inactivate Escherichia coli from wastewater effluents and drainage are complex and interrelated. The objective of this study was to determine if irrigation of undisinfected wastewater effluents in the winter moves bacteria to surface water through subsurface drainage, posing a public health risk. The central Ohio study site, an open meadow constructed in the 1970s, is irrigated with lagoon effluents each summer. The irrigated area has subsurface drainage that collects for discharge in one spot. Undisinfected wastewater from a stabilization pond was irrigated for the first time in the winter of 2013/2014. E. coli was measured in the subsurface discharge during the irrigated winter season and compared to the non-irrigated previous winter season. Soil temperature and moisture were also monitored. E. coli moved to subsurface drains when the water table was above the drain. E. coli also moved to subsurface drains when the shallow soil temperature dropped to near freezing. With less winter sunlight and minimal evapotranspiration, the soil stayed moist near field capacity. Temperature appears to be the most important factor in limiting natural inactivation in subsurface soil and allowing the movement of E. coli in undisinfected wastewater effluents to the subsurface drainage systems. The results show that winter reuse of undisinfected wastewater does pose a public health risk to surface water through subsurface drainage. Therefore, disinfection of wastewater effluents used for irrigation is strongly recommended.