Vadose Zone Microbial Transport Below At-Grade Distribution of Wastewater Effluent |
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Authors: | Motz Erin C Cey Edwin Ryan M Cathy Chu Angus |
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Institution: | (1) Department of Civil Engineering, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada, T2N 1N4;(2) Department of Geoscience, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada, T2N 1N4; |
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Abstract: | 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. |
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