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 impervious environmental surfaces is presented.
A full-factorial study (23, Temperature/Moisture/UV-B exposure) of the indicator-species’ environmental survival factors was performed. Pet-feces slurries were applied to paving blocks 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.
The model was developed as a log-linear (first-order) segmented (with unknown break points) net decay of MPN with respect to time of the entire dataset, followed by a multiple linear regression of the decay constant of each segment with respect to survival factors.
The studied species were found to be environmentally persistent on impervious surfaces under 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.