Relative die-off rates of two faecal indicator systems in sea water

Abstract

Die-off rates in sea water for faecal coliforms and enterococci were compared, to determine whether differences in survival characteristics could account for differences in the strength of the relationship between indicator concentration and swimming-related illnesses, found in epidemiological investigations overseas. Die-off was measured under field conditions using a simulated in situ technique, and in containers on shore exposed to natural and artificial sources of radiation.

Die-off patterns for both indicators were non-linear over an extended time period. However, linear regression analysis of rate differences applied to the initial phase of the curves showed that rate differences for the same indicator between experiments were of much greater magnitude than differences between the two indicators for individual experiments. The latter were non-significant (p> 0.1) on most occasions.

Reduction in die-off rates for both indicators with increasing water depth, and the variation in rates between experiments, emphasised the importance of solar radiation for die-off. However, multiple regression analysis indicated the potential importance of other factors, and provided a useful technique for investigating their interrelationships, and relative influence on die-off.

Results did not support a switch from faecal coliform to enterococci standards for marine recreational waters on the basis of superior survival characteristics for the latter. Accurate prediction of the impacts of outfalls on the bacteriological quality of recreational waters would require site-specific measurements of a number of factors under a range of conditions.