Survival and Speed of Escherichia coli Infiltration in a Hydromorphic Soil in Wet Tropical Zone

Aims: Untreated wastewater is usually used for crop irrigation in developing countries; however it contains a lot of pathogenic bacteria. This study was carried out to determine the fate of E. coli contained in wastewater in a hydromorphic soil.
Study Design: Environmental microbiology
Place and Duration of the Study: This study was carried out in the experimental field of the Dschang University, during the dry season (November 2011- Mars 2012) and the rainy season (June 2012-September 2012).
Methodology: Six plots of 4 m2 each were tilled in 400 m2 surface area in the dry and in the rainy seasons. Wastewater was collected from the experimental wastewater treatment station in the University of Dschang; it was applied on three plots, and three other plots were used as controls. Once every week, soil samples were taken on the surface (0 - 10 cm), in the medium (20 - 30 cm) and at the water table level (40 - 50 cm). Levels of E. coli in soil samples were determined on “Lactose Tergitol® 7 Agar with TTC” medium, and biochemical confirmation tests were carried out (tests of indol, Simmons citrate, gas production, mobility, fermentation of mannitol, glucose and lactose).
Results: In the rainy season, E. coli was detected on the soil surface until the 112th day, while in the dry season detection did not exceed the 63rd day. E. coli was detected in the deeper layers of the soil (20 - 30 and 40 - 50 cm) from the 14th and the 70th day respectively. This helps to estimate the speed of vertical migration to be between 5 and 18 mm per day.
Conclusion: E. coli bacteria contained in urban wastewater survive for a long-time in hydromorphic soils and reach significant depths, and can consequently pose serious problems of public health.