Relationships between standard plate counts and other parameters in water distribution systems

Standard plate counts (SPCs) of water samples collected from seven small, community systems were tested for correlation with coliform occurrence, turbidity, free chlorine residual, total chlorine residual, and temperature. Significant correlations between SPC and one or more of the other parameters were found for some systems. The significant correlations were sometimes positive and sometimes negative, but no generalizations could be made that would fit data for all systems sampled. Data from an experimental system in which sewage was injected into the treated water showed a strong (α <0.01) positive correlation between SPC and coliform densities. From this, it may be inferred that an increase in SPC for a water distribution system may signal a breakdown in sanitary barriers, but, on the other hand, it may be the result of bacterial contamination by some other mechanism.