Potential role of water-percolates pH and sand size on the transfer/ retention of Escherichia coli cells through sandy soil

Under natural conditions, water contaminants can be retained during infiltration through sandy soil or a sand column. The interaction between water pH and sand size may influence this process. This study aims to assess the combined effects of these two parameters on the transfer or retention of E. coli cells (used as a model) during the percolation of bacteria-contaminated water through a sand column. Experiments were conducted from August to December 2024, using sand with three different size ranges (0.1-0.3 mm, 0.3-0.5 mm, and 0.5-0.8 mm) poured into PVC pipes. Water at various pH levels (5, 6, 7, 8, and 9) containing E. coli (7×107 CFU/ml) was prepared in Erlenmeyer flasks with a tap connected. Percolation tests of the contaminated water were then carried out through the sand columns. Bacteriological analysis of the percolated water was performed on the first drops and after 2, 4, 6, and 8 hours. The concentrations of cells in the percolated water fluctuated between 0.036×104 and 29.5×104 CFU/ml. The lowest concentration was observed at 4 hours of percolation with water at pH 5, while the highest was noted at 8 hours with water at pH 9. The lowest percentage of retained cells in 1 ml of percolated water was 99.71% for sand size 0.1-0.3 mm, 99.70% for 0.3-0.5 mm, and 99.57% for 0.5-0.8 mm. E. coli cells in acidic percolating water were retained more effectively in the sand column than those in alkaline water. This retention process appears to be more efficient with smaller sand grain sizes. The percentages of retained cells varied over time. Water treatment through sand filtration is a complex process that depends on several factors for its effectiveness. This knowledge could help predict the fate of bacterial contaminants in the environment. Continued field and laboratory experiments are necessary to clarify the individual roles of various microbiological, physicochemical, and hydrological factors involved in the complexities of interactions during the transfer of bacteria-contaminated water through sand columns at the household scale, as well as through sandy soil or subsoil in natural environments.