Bacterial indicator organisms in various classes of cisterns in the U.S. Virgin Islands

Abstract

Because cisterns are exposed to the environment via their direct attachment to catchment surfaces, cistern water quality is often poor. In the U.S. Virgin Islands cisterns are involved in supplying the water needs for private homes, hotels, businesses and public housing, making them subject to the mandates of the Federal Safe Drinking Water Act. This study compares the results of the occurrence of various water quality indicators in Private Residential, Public Housing, and Hotels and Guest House cisterns in the U.S. Virgin Islands in a side-by-side analysis. Forty-seven separate independent variables were created and tested for correlation, of which only the rate at which chlorination was applied was significant. A high degree of correlation was found to exist between total and fecal coliforms. A moderate, but still significant correlation between the coliform and the heterotrophic levels in the cistern was also noted. There was no correlation between any of the bacterial indicators studied and the opportunistic pathogen Pseudomonas aeruginosa. The results show, depending upon on the type and amount of chlorination received, different bacterial indicators will predominate and that there is a logical succession from one to the next with chlorination. On the unchlorinated end of the scale, normally associated with privateresidential homes, there is a correlation between the heterotrophic bacteria and total coliform. On the other end where chlorination is constant, normally associated with public water supply systems, correlation is between total coliform and fecal coliform. The proportion at which these indicators occur is a function of the diligence at which chlorination takes place. Regression analysis showed that the heterotrophs and total coliforms were the most sensitive to chlorination while the streptococci and fecal coliform were the most resistant. Thus, what is seen is fecal coliform unmasking based upon chlorination pattern. Depending upon whether the cistern is a private or public water supply which is mandated to be treated, an appropriate indicator system based upon the likelihood of treatment can be suggested. For unchlorinated private residential home cisterns the prime indicator should be the heterotrophic plate count and for chlorinated systems the take-action organism should be the fecal coliform. P. aeruginosa should be tested for in both domestic cistern water supplies and public cistern water supply systems.