Survival of pathogenic and indicator organisms in groundwater and landfill leachate through coupling bacterial enumeration with tracer tests

We reported on the transit and survival of 6 potentially pathogenic bacterial populations in a schist aquifer beneath the Etueffont landfill (France). Total coliforms, Escherichia coli, Enterococci, Pseudomonas aeruginosa, Salmonella and Staphylococcus aureus were monitored for 15 months in groundwater and leachate and coupled to tracer tests in an attempt to identify the source of contamination. The results showed the absence of S. aureus and Salmonella. The monitoring of piezometer 30 (PZ30) located downstream from the landfill highlighted leachate infiltrations into the substrate. Groundwater analysis showed high levels of faecal bacteria in the underground environment (20,000 CFU 100 mL− 1 for total coliforms, 15,199 CFU 100 mL− 1 for E. coli and 3290 CFU 100 mL− 1 for Enterococci). Data from tracer tests indicated that bacteria originated from the septic tank of the transfer station and part of these bacteria transited through waste. Bacterial density was lower in leachates than in groundwater sampled from PZ30, except for P. aeruginosa which seemed to take advantage of adverse environmental conditions. The landfill, closed since 2002, was not a source of faecal bacteria which appeared to be able to survive in the schist substrate, and may be considered as good markers of recent faecal contamination.