Because of historic difficulties in the measurement of sewage-borne pathogens, the microbiological quality of stormwater runoff is often characterized on the basis of bacterial indicator species. These species are assumed to derive from a common (sewage) source with pathogens of interest, and to arrive in, survive in, and move through watershed environments in numbers that correlate with health risk from those pathogens. Commonly used indicator species (especially E.coli and Enterococcus spp.), however, may derive from sources other than sewage, and survive in the (non-enteric) environment at rates divergent from those of the pathogens they are presumed to indicate.
In an ongoing effort to model “background” (i.e., of non-sewage origin) discharges of indicator species from stormwater source areas in the Tuscaloosa, AL area, a model for the environmentally relevant survival of indicator species (E. coli and Enterococci) on simulated pervious environmental surfaces is presented.
A full-factorial study (25, Temperature/Moisture/pH/organics/UV-B exposure) of the indicator-species’ environmental survival factors was performed. Pet-feces slurries were applied to sterile sand/silt/clay mixtures, doped with or without organic acid and soluble sugars, and incubated in controlled environmental chambers (refrigerators fitted with BOD-cubators for high temperature control, desiccant or humidifiers for moisture control, and UV-B enhanced fluorescents with Lexan panels to split the chambers into UV-exposed and UV-shielded regions) at conditions encompassing those likely to be found in Tuscaloosa. Most Probable Number (MPN) of surviving E. coli and Enterococci Colony Forming Units, over an extended time period (about two weeks), were measured using IDEXX methods and normalized to inoculation-date (Day 0) MPN for each soil formulation.
The model was developed as log-linear (first-order) segmented (with unknown break points) net decay of MPN with respect to time for each treatment, followed by a factorial analysis of the survival factors’ (and their interactions’) influence on segmental decay constants and timing of inter-segment breakpoints.
The studied species were found to be environmentally persistent on impervious surfaces under many realistic Tuscaloosa conditions, raising questions as to their unqualified use as indicators of sewage-borne pathogens. Risk analysis of stormwater exposures, and efficient search for presumed environmental sewage releases, would be better informed by improved knowledge of background discharges of indicator species from stormwater source areas.