Relative reductions in counts of enteric bacteria and bacteriophages in surface and groundwaters

Abstract

Reductions in counts of enteric indicator bacteria and bacteriophages were compared in surface and groundwaters. In surface waters, inactivation rates were determined in sunlight-exposed and dark chambers of fresh and saline waters, seeded with sewage or waste stabilisation pond (WSP) effluent. All sunlight inactivation (kD) rates were markedly higher than dark (ks) rates. Seawater seeded with sewage gave a ks ranking (over one day) of: faecal coliforms > enterococci > F-RNA phages > somatic coliphages. In fresh water seeded with WSP effluent, the one day ks ranking was: enterococci> faecal coliforms (including E. coli) > somatic coliphages > F-RNA phages. Over 2 days, these patterns continued for the phages and WSP bacteria, but there was evidence of photoreactivation of sewage faecal coliforms (although not enterococci). All inactivation rates increased with increasing salinity. All indicators suffered (photooxidative) damage from a wide solar wavelength range. However, faecal coliforms, and particularly somatic coliphages, were more susceptible to (photobiological) damage from shorter wavelengths, which penetrate further into fresh water than seawater. The results suggest that, although sewage enterococci are initially more sunlight-resistant than faecal coliforms, they are weakened by (apparently irreparable) sunlight damage in a WSP (enterococci ks rates were higher in summer than winter). In contrast, faecal coliforms that survive WSP treatment have their repair mechanisms fully activated, and thereafter survive better than WSP enterococci. Thus, enterococci are less suitable as indicators of WSP discharges than faecal coliforms. Phage survival was not affected by sunlight exposure in the WSP. Assuming both phages exhibit similar sunlight inactivation rates to pathogenic viruses, they will be better models of enteric virus survival in surface waters than faecal coliforms or enterococci. A "safety margin" may be provided by somatic coliphages in seawater, and by F-RNA phages in fresh water.

In groundwaters, reductions in bacterial and bacteriophage counts during transport exceeded inactivation rates in static microcosms, showing the effects of processes such as filtration, sedimentation and adsorption. Comparative transport patterns in saturated gravels were consistent with the theory of pore size exclusion (PSE), i.e., larger particles travel faster, because they can only move through larger, interconnected pores, where water velocity is higher. In field tracing experiments in an uncontaminated gravel aquifer, a velocity ranking of E. coli > F-RNA phage MS2 > rhodamine WT dye was obtained (the opposite occurred for retardation), which is consistent with PSE in terms of the particle sizes of the tracers. In a gravel aquifer contaminated by WSP effluent, a velocity ranking of F-RNA phages > E. coli > rhodamine WT was obtained. This remains consistent with PSE theory, if it is assumed that most effluent phages were attached to effluent particles larger than the bacteria. In an 8 m column of 1 cm diameter gravel, tracing experiments at 16 and 32 L/Hr confirmed the field results, i.e. pure culture retardation values were: rhodamine WT > F-RNA phage MS2 > E. coli, whereas for sewage they were: bromide> E. coli > F-RNA phages. Because enteric bacteria and viruses will normally enter groundwater in effluent, data from effluent-based experiments are likely to better reflect relative bacterial and phage transport rates than pure culture experiments.