Vadose Zone Microbial Transport Below At-Grade Distribution of Wastewater Effluent

The attenuation of Escherichia coli and total coliform from secondary treated wastewater effluent under two “at-grade” effluent distribution systems was evaluated in a sandy silt vadose zone in a cold climate. The two at-grade distribution lines had different designs and hydraulic loading rates. Effluent transport was examined using chloride as a tracer. Coliform fate was evaluated relative to the chloride using a combination of in situ pore water sampling and destructive soil sampling, combined with the observation of a dye tracer along excavation sidewalls. Although bacteria attenuation in the subsoil appeared to decrease during colder, winter temperatures (likely due to decreased viability and decreased predation), the subsoil provided about a four log reduction in E. coli over 90 cm of vertical transport. Horizontal transport of bacteria (up to 1.5 m from the line) was likely aided by flow on top of a microbial biomat observed at the soil surface. Both the subsurface dye patterns and the E. coli sampling suggested less preferential flow occurred below the lower loading rate design. At-grade distribution of secondary treated wastewater appears to be a viable alternative to conventional distribution fields at sites with similar climate and soils.