Identifying the major influences on the microbial composition of roof harvested rainwater and the implications for water quality
The microbiological and chemical quality of tank stored rainwater is impacted directly by roof catchment and subsequent run-off contamination, via direct depositions by birds and small mammals, decay of accumulated organic debris, and atmospheric deposition of airborne micro-organisms and chemical pollutants. Previous literature reports on roof water quality have given little consideration to the relative significance of airborne micro-organisms.
This study involved analyses of direct roof run-off at an urban housing development in Newcastle, on the east coast of Australia. A total of 77 samples were collected by auto-sampler during 11 separate rainfall events spanning a 2 year period. Heterotrophic plate, total coliform, faecal coliform and Pseudomonas spp counts were matched to climatic data corresponding to each of the monitored events. The aim was to investigate the influence of certain meteorological parameters on the bacterial composition of the roof water and indirectly assess the relative contribution of airborne micro-organisms to the total bacterial load.
Both prevailing storm and antecedent dry interval conditions were examined, and strong relationships between wind directions, velocities and several bacterial parameters were found in each case. It is concluded that airborne micro-organisms represent a significant contribution to the bacterial load of roof water at this site, and that the overall contaminant load is influenced by wind velocities, while the profile (composition) of the load varies with wind direction. The implications of these findings to the issues of water quality and health risk analysis, appropriate usage and system design are discussed.
Keywords: rainwater tanks;roof run-off;water quality;bacterial composition;airborne micro-organisms;climatic variables.